Haemophilia A
Haemophilia A is an X-linked recessive bleeding disorder caused by deficiency or dysfunction of clotting factor VIII, leading to impaired intrinsic coagulation and prolonged or spontaneous hemorrhage.
Haemophilia A
Haemophilia A (also spelled "Hemophilia A") is an X-linked recessive inherited bleeding disorder caused by a deficiency or dysfunction of coagulation Factor VIII (FVIII) [1][2][3].
Breaking down the name:
- "Haemo-" (Greek haima) = blood
- "-philia" (Greek philia) = love/tendency toward
- So literally "a love of bleeding" — the blood has a tendency to keep flowing because the coagulation cascade cannot form a stable fibrin clot.
Haemophilia A accounts for approximately 85% of all haemophilia cases, with Haemophilia B (Factor IX deficiency, also called "Christmas disease") accounting for the remaining ~15% [1][2]. They are clinically indistinguishable — the only way to differentiate them is by measuring specific factor levels.
Key Conceptual Point
Haemophilia A is a disorder of secondary haemostasis (the coagulation cascade), NOT primary haemostasis (platelet plug formation). This is why the bleeding pattern is deep-seated (joints, muscles, retroperitoneum) rather than mucocutaneous (petechiae, gum bleeding). The platelet plug forms normally but cannot be reinforced by a fibrin mesh, so it breaks down and rebleeding occurs after initial cessation.
Acquired haemophilia refers to an acquired coagulation factor deficiency caused by an autoantibody (most commonly directed against Factor VIII) — this is a distinct entity from inherited Haemophilia A [1][2]. Classical associations include:
- Pregnancy / post-partum period
- Autoimmune conditions (e.g. SLE, RA)
- Underlying malignancy (e.g. CA lung, CA prostate, CLL)
- Drug-induced (e.g. Penicillin, Sulphonamides, Phenytoin) [1]
2. Epidemiology and Risk Factors
- Haemophilia A: incidence approximately 1 in 4,000–5,000 live male births [2]
- Of these, approximately 1/2 to 2/3 have severe disease [2]
- Haemophilia A is the most common severe inherited bleeding disorder worldwide
- Prevalence is roughly equal across all ethnic groups and geographic regions — there is no strong racial predilection
- X-linked recessive (XLR) — the F8 gene is located on the long arm of the X chromosome at Xq28 [2]
- This means:
- Males (XY) are affected — they only have one X chromosome, so a single defective copy causes disease
- Females (XX) are usually carriers — the normal X compensates. However, due to lyonization (random X-inactivation), some female carriers can have reduced FVIII levels (typically 30–70%) and may experience mild bleeding symptoms, especially during surgery or trauma
- An affected father will pass the defective X to all daughters (who become obligate carriers) but no sons (who receive his Y)
- A carrier mother has a 50% chance of passing the defective X to each son (affected) or each daughter (carrier)
- Approximately 55% of severe haemophilia A cases and 30% of mild/moderate cases are sporadic (i.e. no family history — new mutations) [2]
- This is a critical exam point: absence of family history does NOT exclude haemophilia
- The F8 gene is one of the largest genes in the human genome (~186 kb, 26 exons)
- The most common genetic defect causing severe Haemophilia A is an intron 22 inversion (accounts for ~45% of severe cases) — this large inversion disrupts the gene and abolishes FVIII production
- Other mutations include intron 1 inversions (~5% of severe), point mutations, small deletions/insertions, and large deletions
- Mild/moderate disease is more commonly caused by missense point mutations that allow some residual FVIII function
- Male sex (XLR inheritance)
- Family history of haemophilia (but remember ~50% of severe cases are sporadic)
- Consanguinity — increases risk of homozygous female haemophilia (extremely rare)
3. Anatomy and Function: Factor VIII in the Coagulation Cascade
Understanding Haemophilia A requires understanding where Factor VIII sits in the coagulation cascade and why its deficiency causes the specific clinical pattern observed.
The coagulation cascade is traditionally divided into:
- Extrinsic pathway (tissue factor → Factor VII → Factor X) — measured by PT (Prothrombin Time)
- Intrinsic pathway (Contact activation → Factor XII → XI → IX → X, with Factor VIII as a cofactor) — measured by APTT (Activated Partial Thromboplastin Time)
- Common pathway (Factor X → Factor V + Prothrombin → Thrombin → Fibrinogen → Fibrin) — measured by both PT and APTT
In the modern cell-based model of coagulation, the process occurs in three phases:
- Initiation (on tissue factor-bearing cells): Tissue factor + Factor VIIa generate small amounts of thrombin
- Amplification (on platelet surface): This small amount of thrombin activates platelets and activates cofactors including Factor VIII → Factor VIIIa
- Propagation (on activated platelet surface): Factor VIIIa acts as a cofactor for Factor IXa to form the "tenase complex" (FVIIIa–FIXa), which activates Factor X at a rate ~50-fold greater than Factor IXa alone — this massive amplification generates the "thrombin burst" needed for stable fibrin clot formation
Without Factor VIII:
- The initial haemostatic plug (primary haemostasis via platelets + von Willebrand Factor) forms normally → this is why PT is normal, platelets are normal, and bleeding time may be normal
- However, the tenase complex cannot form properly → Factor X activation is dramatically reduced → insufficient thrombin generation → the fibrin mesh that should reinforce and stabilize the platelet plug is inadequate
- The platelet plug is fragile and breaks down → delayed rebleeding (classically, bleeding stops initially then restarts hours later)
- This explains the deep-seated bleeding pattern: joints, muscles, and enclosed spaces where the platelet plug alone cannot withstand the mechanical forces
- In circulation, Factor VIII is bound to von Willebrand Factor (vWF) in a non-covalent complex [4]
- vWF serves as a carrier protein that:
- Protects Factor VIII from premature proteolytic degradation (especially by activated Protein C and Factor Xa)
- Stabilizes Factor VIII and extends its half-life from ~2 hours (free) to ~8–12 hours (bound to vWF)
- Concentrates Factor VIII at sites of vascular injury where vWF is already binding to exposed collagen
- This is why von Willebrand Disease (vWD) can cause a secondary reduction in Factor VIII levels — if vWF is deficient, Factor VIII is unprotected and rapidly cleared from circulation
Exam Pearl: Distinguishing Haemophilia A from vWD
Both can cause ↑APTT and ↓Factor VIII levels. The key distinguishing feature is:
- Haemophilia A: ↓Factor VIII, normal vWF:Ag and normal vWF:RCo (ristocetin cofactor activity)
- vWD (Type 1/2): ↓Factor VIII, ↓vWF:Ag and/or ↓vWF:RCo
This is exactly what was tested in the Haematology Interactive Tutorial Case 2: Factor VIII 0.13, vWF:Ag 1.11, vWF:RCo 1.22 — the normal vWF levels with low Factor VIII confirm Haemophilia A, not vWD [5].
- Factor VIII is a large glycoprotein synthesized primarily in hepatic sinusoidal endothelial cells (and to a lesser extent in other endothelial cells and the reticuloendothelial system)
- It circulates as a heterodimer (heavy chain + light chain) bound to vWF
- When thrombin cleaves Factor VIII at specific sites, it becomes Factor VIIIa — the active cofactor
- Factor VIIIa is rapidly inactivated by Activated Protein C (APC) and spontaneous dissociation of the A2 domain
4. Aetiology (with Hong Kong Focus)
- Caused by mutations in the F8 gene on Xq28 as described above
- In Hong Kong, haemophilia A is the most common inherited severe bleeding disorder, consistent with worldwide epidemiology
- The Hong Kong Haemophilia Registry (maintained through Queen Mary Hospital and other HA hospitals) tracks patients — there are an estimated ~400 patients with Haemophilia A in Hong Kong
- No specific ethnic predilection within the Hong Kong Chinese population
- Much rarer (incidence ~1.5 per million/year), but important because it presents in previously well individuals (typically elderly) with sudden-onset severe bleeding
- In Hong Kong, as elsewhere, it is associated with:
- Post-partum state
- Autoimmune diseases (SLE, RA — relatively common in Hong Kong)
- Malignancy (lung cancer is extremely common in HK)
- Drug reactions
- Idiopathic (~50% of cases)
- HK has a well-established haemophilia comprehensive care programme
- Recombinant factor concentrates are standard of care (plasma-derived products used less frequently)
- Emicizumab (a bispecific antibody mimicking Factor VIIIa cofactor function) is increasingly used for prophylaxis, especially in patients with inhibitors
- Gene therapy (e.g. valoctocogene roxaparvovec) was approved by EMA/FDA in 2022–2023 and is on the horizon for HK
5. Pathophysiology
- Primary haemostasis: Vascular injury → vasoconstriction + platelet adhesion (via vWF–GPIb) → platelet activation and aggregation (via GPIIb/IIIa) → unstable platelet plug
- Secondary haemostasis: Coagulation cascade activation → thrombin generation → fibrin formation → stable fibrin-reinforced clot
- Fibrinolysis: After wound healing, plasmin degrades the fibrin clot → tissue remodeling
- Primary haemostasis is INTACT — vWF, platelets, and the vessel wall function normally
- Secondary haemostasis is DEFECTIVE specifically at the intrinsic tenase complex (FVIIIa–FIXa):
- The extrinsic pathway (TF–FVIIa) generates a tiny initial amount of thrombin — enough to activate platelets and initiate clotting
- But without Factor VIII, the amplification loop via the tenase complex is crippled → Factor X activation is inadequate → thrombin burst does not occur → fibrin formation is insufficient
- The clot that does form is friable and easily disrupted by normal mechanical forces
- Fibrinolysis may overwhelm the weak clot → this contributes to re-bleeding
| Site | Pathophysiological Explanation |
|---|---|
| Joints (haemarthrosis) | Synovial membrane is highly vascular with thin-walled capillaries. Normal joint movement creates shear stress. Without stable fibrin reinforcement, minor capillary bleeds in the synovium cannot be contained → blood accumulates in the joint space |
| Muscles | Large muscle groups undergo constant contraction/relaxation creating mechanical stress on intramuscular vessels. Without adequate fibrin, small bleeds expand into haematomas |
| Retroperitoneum | Rich vascular supply, minimal tissue resistance to haematoma expansion, no bony compartment to tamponade bleeding |
| ICH | Cerebral vessels are thin-walled and exposed to arterial pressure; even minor trauma can initiate bleeding that cannot be contained |
| Post-surgical / post-dental | Surgical wounds depend on fibrin clot for haemostasis; inadequate fibrin → delayed bleeding hours after the procedure once the initial platelet plug breaks down |
This is one of the most important long-term consequences and deserves detailed explanation:
- Acute haemarthrosis → blood in the joint space
- Blood contains iron (from haemoglobin degradation)
- Iron is directly toxic to synovial cells and chondrocytes:
- Iron catalyzes Fenton reaction → generation of reactive oxygen species (ROS)
- ROS cause oxidative damage to cartilage matrix and synovial tissue
- The synovium responds with hypertrophy and neovascularization (proliferative synovitis)
- These new blood vessels are fragile → predispose to further bleeding (vicious cycle)
- This creates the concept of a "target joint" — a prior bleed results in joint damage and inflammation → predisposes to further bleeding [2]
- Inflammatory mediators (IL-1, IL-6, TNF-α) are released → cartilage degradation
- Over time: cartilage destruction → subchondral bone changes → secondary osteoarthritis → joint contractures and deformity
- Haemophilic arthropathy occurs in up to 50% of patients with severe haemophilia [2]
- Typically presents as joint pain, stiffness, and contractures developing in adolescence [2]
6. Classification
Classification is based on residual or baseline factor activity level, which corresponds with the degree of bleeding symptoms [1][2]:
| Severity | Factor VIII Level | Clinical Phenotype |
|---|---|---|
| Severe (~70% of diagnosed cases) | < 1% ( < 0.01 IU/mL) | Spontaneous bleeding into joints (haemarthrosis) and muscles; bleeding 1–2×/week |
| Moderate | 1–5% (0.01–0.05 IU/mL) | Bleeding with minor trauma; may have joint bleeding; bleeding ~1×/month |
| Mild | > 5% and < 40% (> 0.05–0.40 IU/mL) | Bleeding only with major trauma or after surgery; rarely has joint bleeding; may never have a bleeding problem |
- Normal Factor VIII range = 55–150% of normal (0.55–1.50 IU/mL) [1]
- Factor level ≥ 50% is considered normal and not expected to have clinical bleeding [1]
- Individuals may also be classified as having mild haemophilia despite having factor level ≥ 40% if they share a genetic variant in the relevant factor with a family member who has haemophilia [1]
Why 70% of Cases Are Severe
This seems paradoxical — you'd expect more mild cases. The reason is ascertainment bias: severe cases present early and dramatically (spontaneous bleeds in infancy), so they are almost always diagnosed. Many mild cases go undiagnosed until challenged by surgery or trauma, so the diagnosed population is skewed toward severe disease.
- Haemophilia A — Factor VIII deficiency (85% of all haemophilia)
- Haemophilia B — Factor IX deficiency (15%)
- Haemophilia C — Factor XI deficiency; autosomal recessive; especially common in Ashkenazi Jews [1]
- Inherited (vast majority — XLR for A and B)
- Acquired (autoantibodies against Factor VIII)
- Without inhibitors — responds normally to factor replacement
- With inhibitors — alloantibodies (inhibitory alloantibodies against Factor VIII) develop in response to factor infusion [1]
- Occurs in approximately 25–30% of severe Haemophilia A patients
- These antibodies neutralize infused Factor VIII → treatment becomes extremely challenging
- Inhibitor titre measured in Bethesda Units (BU):
- Low-responding ( < 5 BU): may still respond to high-dose factor
- High-responding (≥ 5 BU): require bypassing agents
7. Clinical Features
A. Symptoms (with Pathophysiological Basis)
The bleeding pattern in haemophilia is characteristically deep-seated, in contrast to the mucocutaneous pattern of platelet/vWF disorders [3][5]:
| Feature | Platelet/vWF Disorder | Coagulation Disorder (Haemophilia) |
|---|---|---|
| Petechiae | Common | Absent |
| Ecchymoses | Small, superficial, multiple | Large, deep, few |
| Haemarthrosis | Rare | Very common (up to 80%) |
| Muscle haematoma | Rare | Common |
| Bleeding after cuts | Prolonged | May restart hours later |
| Post-surgical bleeding | Immediate, oozing | Delayed (hours) — rebleeding after initial cessation |
| Epistaxis | Common | Uncommon (unless severe) |
Why is the bleeding delayed? Because primary haemostasis (platelet plug) works fine — it initially stops the bleeding. But without Factor VIII, the fibrin reinforcement fails, and the platelet plug breaks down hours later → rebleeding [3].
- Most common bleeding manifestation (up to 80% of bleeding episodes in severe haemophilia) [2]
- Sites: most commonly major weight-bearing joints — ankles, knees, elbows [2]
- Also hips, shoulders, wrists (less common)
- Small joints of hands/feet are rarely affected
- Why weight-bearing joints? These joints experience the greatest mechanical stress during daily activities; even normal walking creates shear forces on the synovial vasculature that cannot be contained without adequate fibrin
- Symptoms: prodromal stiffness ± warmness ("aura" of a bleed) followed by acute pain and swelling [2]
- Patients (especially those with severe disease) learn to recognize this prodrome and should be treated immediately
- As the joint fills with blood: increasing pain, restricted range of motion, joint held in flexion (position of maximum capsular volume)
- Resolution over days to weeks with treatment
- Common, most commonly in calf and psoas (iliopsoas) muscles [2]
- Calf haematoma → risk of compartment syndrome:
- Ischaemia → necrosis → fibrosis → subsequent contraction of Achilles tendon [2]
- (Because the calf is a closed fascial compartment; expanding haematoma raises compartment pressure → compresses vessels → ischaemia)
- Psoas (iliopsoas) haematoma:
- Femoral nerve compression in large psoas bleed [2] — presents with hip flexion, groin/anterior thigh pain, weakness of knee extension, loss of knee jerk, anterior thigh numbness
- Can mimic appendicitis or renal colic
- Bleeding into abdominal wall can produce intense pain → mimic acute abdomen [2]
- Haemophilic pseudotumour: large encapsulated haematoma [2]
- Sites: large muscle groups in pelvis/lower limbs and bone (long bone, pelvis, cranium) [2]
- Pathology: repeated bleeding with bone involvement → progressive cystic swelling with bone turnover and new bone formation [2]
- Can grow to massive sizes and may require surgical excision
- On imaging, can be mistaken for malignant bone tumour
- Oropharyngeal bleeding: may occur with minor trauma or dental procedures [2]
- Cough/vomiting can induce bleeding into posterior pharynx → aspiration or upper airway obstruction [2] — this is a life-threatening emergency
- GI bleeding and bowel wall haematoma → obstruction, risk of intussusception [2]
- Haematuria: common in severe haemophilia but not associated with decreased renal function [2]
- Usually painless, gross haematuria
- Thought to arise from renal papillary vessels
- Prolonged bleeding after wisdom tooth extraction — the classic exam stem presentation [5]
- Increased risk of cephalhaematoma and ICH during delivery (in affected neonates) [2]
- ICH is the leading cause of haemophilia-related death in the current era
- Can be spontaneous (in severe disease) or post-traumatic
- Low threshold for brain imaging in any haemophilia patient with headache or head trauma
- Average age of diagnosis: 1 month for severe, 8 months for moderate, 36 months for mild haemophilia [2]
- Severe: may present at circumcision, heel-prick test, or with excessive bruising when starting to crawl/walk
- Mild: may not present until adolescence or adulthood, often after dental extraction or surgery
- Classical presentation is retroperitoneal haematoma due to poor mobilization [2]
- Unlike inherited haemophilia, predominantly affects the elderly (median age 60–70 years)
- Often presents with extensive soft tissue bleeding and ecchymoses rather than haemarthrosis (because these patients haven't had lifelong joint damage)
B. Signs (with Pathophysiological Basis)
- Joint swelling and warmth — blood in the joint capsule causes distension; inflammatory response to blood → warmth
- Joint held in flexion — the position that maximizes intra-articular volume and minimizes pressure/pain
- Muscle swelling — expanding haematoma within fascial compartments
- Ecchymoses — deep, large bruises (not petechiae) — because the bleeding is from vessels larger than capillaries, into deep tissue planes
- Bruising over shins is classic in the history [5] — shins have minimal subcutaneous tissue so even minor trauma causes visible bruising
Late complications [2]:
- Haemophilic arthropathy (up to 50% of severe haemophilia patients):
- Joint deformity — especially flexion contractures of knees and elbows
- Muscle wasting around affected joints (disuse atrophy)
- Limited range of motion with crepitus (secondary OA changes)
- Chronic pain and disability
- Typically presents as joint pain, stiffness, contractures developing in adolescence [2]
- Limb-length discrepancy — chronic haemarthrosis in growing children can stimulate epiphyseal growth plates → overgrowth of the affected limb
- Short stature — if multiple joints are affected and mobility is severely limited, overall growth may be impaired
- Bloodborne infections: HIV, HBV, HCV [2] — particularly in patients treated with plasma-derived products before viral inactivation techniques (pre-1985 for HIV, pre-1990s for HCV)
- In Hong Kong, there is a cohort of older haemophilia patients with HCV co-infection
- Decreased risk with use of recombinant factors [2]
- Signs of iron overload (in patients receiving repeated blood transfusions) — skin hyperpigmentation, hepatomegaly, cardiomyopathy
- Signs of inhibitor development — failure to respond to standard factor replacement, escalating bleeding episodes despite treatment
When examining a patient with suspected haemophilia [3][5]:
- Cutaneous: Look for ecchymoses (deep, large), note distribution — dependent areas vs. trauma-prone areas. Initially red/purple and later becoming yellow as haemoglobin is degraded [3]
- Joints: Joint effusion (haemarthrosis) and deformities (chronic haemarthrosis) [3] — systematically examine all major joints
- Muscles: Palpate for haematomas, check for compartment syndrome signs (pain on passive stretch, tense compartment)
- Mouth: Check for gum bleeding, sublingual haematoma
- Abdomen: Hepatosplenomegaly (in liver disease from HCV, or from extramedullary haematopoiesis in chronic disease) [3]
- Fundoscopy: Retinal bleeding indicates increased risk for CNS bleeding [3]
Clinical Approach: Distinguishing Platelet vs Coagulation Bleeding
Even at the bedside without talking to the patient, you can clinically determine whether the patient's bleeding tendency is due to platelet or coagulation problems [4]:
- Petechiae = platelet disorder (never seen in pure coagulation factor deficiency)
- Haemarthrosis / deep muscle haematoma = coagulation factor disorder
- Delayed rebleeding = coagulation factor disorder
- Both can cause intracranial haemorrhages — cannot say one is more deadly than the other [4]
- Duration of bleeding tendency — long-standing → inherited; acute → acquired (drugs, e.g. aspirin, warfarin) [5]
- Pattern of bleeding — mucocutaneous vs. deep-seated [5]
- Previous bleeding challenges — dental procedures, circumcision, surgery, labour, injury [3]
- Other bleeding history — head-to-toe: ICH, epistaxis, gum bleeding, easy bruising, joint/muscle haematoma, GI bleeding, menstrual history [3]
- Family history — pattern of inheritance (X-linked: maternal uncles, brothers affected; no male-to-male transmission) [6]
- Age of onset [6]
- Drug history — especially anticoagulants, antiplatelets [3]
- Transfusion history and complications [6]
- Symptoms of haemolysis (relevant if chronic haemolytic complication) [6]
Haemophilia A/B: No change in PT, Increase in APTT [4]
This is because:
- PT measures the extrinsic pathway (Factor VII → Factor X → common pathway) — Factor VIII is NOT in this pathway → PT is normal
- APTT measures the intrinsic pathway (Factor XII → XI → IX → VIII → X → common pathway) — Factor VIII is IN this pathway → APTT is prolonged
- Platelet count is normal (this is not a platelet disorder)
- Bleeding time / PFA-100 is normal (platelet function is intact)
| Test | Result in Haemophilia A | Reason |
|---|---|---|
| PT | Normal | Extrinsic pathway intact |
| APTT | Prolonged | Factor VIII is in intrinsic pathway |
| Platelet count | Normal | Not a platelet disorder |
| Fibrinogen | Normal | Common pathway intact |
| Bleeding time | Normal | Primary haemostasis intact |
| Factor VIII level | Decreased | The fundamental defect |
| vWF:Ag | Normal | Distinguishes from vWD |
| vWF:RCo | Normal | Distinguishes from vWD |
The Mixing Study — How It Works
To differentiate Factor VIII deficiency (haemophilia A or vWD) from Factor VIII inhibitor (lupus anticoagulant or acquired haemophilia A), perform a mixing study [7]:
Mix 50% patient plasma with 50% normal plasma, then re-measure APTT:
- In von Willebrand disease / haemophilia A (deficiency): The normal plasma supplies 50% Factor VIII → this is sufficient to correct the APTT → APTT corrects
- In lupus anticoagulant: Antibodies in the patient's plasma immediately destroy the phospholipids in the normal plasma → APTT does NOT correct (immediate non-correction)
- In acquired haemophilia A (inhibitor): The inhibitor is time- and temperature-dependent → APTT may initially partially correct but fails to correct after incubation at 37°C for 1–2 hours (delayed non-correction)
The word "immediately" is key — lupus anticoagulant acts immediately, while Factor VIII inhibitors in acquired haemophilia are typically time-dependent [7].
| Feature | Severe ( < 1%) | Moderate (1–5%) | Mild ( > 5%– < 40%) |
|---|---|---|---|
| Spontaneous bleeding | Yes (frequent) | Rare | No |
| Haemarthrosis | Common, recurrent → arthropathy | Occasional | Rare |
| Muscle haematomas | Common | Occasional | Rare |
| Post-surgical bleeding | Severe | Moderate | Mild, may be only presentation |
| ICH risk | Highest | Moderate | Low |
| Age at diagnosis | ~1 month | ~8 months | ~36 months |
| Joint damage | Up to 50% develop arthropathy | Less common | Unusual |
High Yield Summary
- Haemophilia A = X-linked recessive deficiency of Factor VIII (Xq28); accounts for 85% of all haemophilia
- Incidence: 1 in 4,000–5,000 live male births; ~55% of severe cases are sporadic (no family history)
- Pathophysiology: Defective intrinsic tenase complex (FVIIIa–FIXa) → inadequate thrombin burst → insufficient fibrin formation → friable clot → delayed rebleeding
- Primary haemostasis is INTACT → no petechiae; PT normal; bleeding time normal
- Isolated prolonged APTT with normal PT is the hallmark lab finding
- Severity classification: Severe ( < 1%), Moderate (1–5%), Mild (5–40%) — based on residual factor level
- Clinical pattern: Deep-seated bleeding — haemarthrosis (80%, weight-bearing joints), muscle haematomas (calf, psoas), delayed post-surgical/post-dental bleeding, ICH
- Haemophilic arthropathy: Iron toxicity → synovial hypertrophy → neovascularization → "target joint" → cartilage destruction → secondary OA → contractures
- Distinguished from vWD by normal vWF:Ag and vWF:RCo with low Factor VIII
- Mixing study: Corrects in deficiency (haemophilia A/vWD); does NOT correct with inhibitors (lupus anticoagulant = immediate; acquired haemophilia = delayed)
- Acquired haemophilia A: Autoantibodies to FVIII; elderly, post-partum, autoimmune, malignancy; presents with retroperitoneal haematoma, massive soft tissue bleeding
- Inhibitor development: Occurs in ~25–30% of severe haemophilia A; alloantibodies against infused exogenous Factor VIII; measured in Bethesda Units
Active Recall - Haemophilia A (Definition to Clinical Features)
[1] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai) — Haematological Diseases, Hemophilia A/B section [2] Senior notes: Ryan Ho Haemtology — Section 4.3.2 Haemophilia A and B [3] Senior notes: Ryan Ho Fundamentals — Section 3.6.5.2 Approach to Bleeding Disorders [4] Senior notes: Block A - Abnormal bleeding after tooth extraction — Patterns of Bleeding and PT/APTT interpretation [5] Senior notes: Block A - Hematology Interactive Tutorial — Case 2 [6] Senior notes: Block A - Family history of anaemia — History and Physical Examination for inherited anaemia [7] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting) — Mixing study for isolated prolonged APTT
Differential Diagnosis of Haemophilia A
The differential diagnosis of Haemophilia A is best approached systematically by considering two clinical scenarios: (1) the patient presenting with a bleeding tendency (the broad DDx of "why is this patient bleeding?"), and (2) the patient presenting with the specific laboratory finding of an isolated prolonged APTT (the narrow DDx of "what causes this clotting pattern?"). We then layer on the clinical pattern (deep-seated vs mucocutaneous bleeding) and additional factor-level results to arrive at the final diagnosis.
The starting point is almost always the clotting profile. The combination of PT, APTT, platelet count, and bleeding pattern creates a diagnostic matrix that narrows the field efficiently.
The Felix PY Lai Paediatric notes provide an extremely high-yield DDx table organized by PT/APTT pattern [1]:
| PT | APTT | Inherited Causes | Acquired Causes |
|---|---|---|---|
| ↑ | Normal | Factor VII deficiency | Warfarin; Vitamin K deficiency (mild: affects Factor 7 first as shortest half-life); Liver disease (mild); Lupus anticoagulant; Inhibitors of Factor VII |
| Normal | ↑ | Factor VIII deficiency (Haemophilia A); Factor IX deficiency (Haemophilia B); Factor XI deficiency (Haemophilia C); Factor XII deficiency | Heparin (unfractionated); Von Willebrand disease; Lupus anticoagulant; Inhibitors of Factor XII, XI, IX, VIII |
| ↑ | ↑ | Factor V deficiency; Factor X deficiency; Prothrombin (Factor II) deficiency; Fibrinogen deficiency; Combined factor deficiency | Combined warfarin + heparin; Direct thrombin inhibitor; Direct Factor Xa inhibitor; Supratherapeutic anticoagulant dose; Vitamin K deficiency (severe: affects Factors 2, 7, 9, 10); Liver disease (severe/failure); DIC; Inhibitors of Factor V, X, prothrombin, fibrinogen |
| Normal | Normal | Platelet disorders | Platelet disorders |
GC High Yield — Haemarthrosis DDx
From GC 075. Pain red joint [8]:
Haemarthrosis — Common causes:
- Trauma
- Haemophilia (Congenital VIII and IX deficiency)
- Acquired haemophilia (autoantibodies)
- Drugs (warfarin or heparin)
Symptoms: Pain, swelling, stiffness Diagnosis: Arthrocentesis Treatment: Treat the "treatable" underlying causes
This slide is especially high yield because it frames haemophilia within the monoarthritis / haemarthrosis differential — a very common exam stem.
B. Differential Diagnosis of Isolated Prolonged APTT (Normal PT)
This is the classic laboratory pattern of Haemophilia A. When you see this, your differential list should be:
| Condition | Factor Deficient | Inheritance | Key Distinguishing Features |
|---|---|---|---|
| Haemophilia A | Factor VIII | X-linked recessive | Low FVIII, normal vWF:Ag and vWF:RCo; deep-seated bleeding pattern; male patients; ~85% of haemophilia [2][3] |
| Haemophilia B (Christmas disease) | Factor IX | X-linked recessive | Clinically indistinguishable from Haemophilia A — can ONLY be differentiated by specific factor assays; ~15% of haemophilia [2] |
| Haemophilia C (Rosenthal syndrome) | Factor XI | Autosomal recessive | Especially common in Ashkenazi Jews [2]; bleeding tendency is unpredictable — 50% will bleed, 50% will not bleed [7]; typically milder than A or B |
| Factor XII deficiency | Factor XII | Autosomal recessive | Causes prolonged APTT but does NOT cause clinical bleeding — Factor XII (Hageman factor) is part of the contact activation system used in vitro but is not essential for in vivo haemostasis. This is a classic exam trick: do not transfuse these patients for a prolonged APTT! |
Why does Factor XII deficiency not cause bleeding? Because in vivo, coagulation is initiated by the tissue factor (extrinsic) pathway, not the contact activation (intrinsic) pathway. Factor XII is needed for the APTT reagent-based test but not for physiological haemostasis. In fact, Factor XII deficiency may paradoxically be associated with thrombosis.
- vWF carries Factor VIII in circulation and protects it from premature degradation [7] — therefore severe vWD (especially Type 3) can cause secondary FVIII deficiency → prolonged APTT
- Key distinction: In vWD, vWF:Ag and/or vWF:RCo will be reduced alongside low FVIII. In Haemophilia A, vWF:Ag and vWF:RCo are normal [5]
- vWD is much more common than haemophilia (prevalence ~1%) but is usually milder
- vWD typically causes a mixed bleeding pattern — both mucocutaneous (because vWF is needed for platelet adhesion) AND deep-seated (because secondary FVIII deficiency impairs coagulation)
- Inheritance: Autosomal dominant (Types 1 and 2) or autosomal recessive (Type 3) — affects both sexes equally, unlike Haemophilia A
| Condition | Mechanism | Key Distinguishing Features |
|---|---|---|
| Unfractionated heparin (UFH) | Activates antithrombin → inactivates thrombin-induced activation of Factors V, VIII, and XI → prolongs APTT [4] | Drug history! PT usually normal (unless very high dose). No change in PT, increase in APTT [4] |
| Lupus anticoagulant (LAC) | Autoantibodies against phospholipids — the APTT test is phospholipid-dependent, so these antibodies interfere with the test in vitro | Despite prolonged APTT, lupus anticoagulant actually causes a thrombotic tendency, NOT bleeding [7] — this is a classic exam point. Confirmed by DRVVT (Dilute Russell's Viper Venom Time) [9]. Mixing study: immediate non-correction [7] |
| Acquired Haemophilia A (Factor VIII inhibitor) | Autoantibodies directed against Factor VIII | Elderly patients, post-partum, autoimmune disease, malignancy; classical presentation is retroperitoneal haematoma [2]; mixing study: delayed non-correction after incubation |
| Acquired Factor inhibitors (other) | Autoantibodies against Factors XII, XI, or IX | Rare; diagnosed by mixing study + specific factor assays |
The Mixing Study — Exam-Critical Interpretation
Mix 50% patient plasma with 50% healthy plasma and re-measure APTT [7]:
- vWD / Haemophilia A (deficiency): Normal plasma supplies the missing factor → 50% Factor VIII is sufficient → APTT corrects
- Lupus anticoagulant: Antibodies immediately destroy the phospholipids in normal plasma → APTT does NOT correct (immediate non-correction) [7]
- Acquired haemophilia A (FVIII inhibitor): Inhibitor is time- and temperature-dependent → APTT may initially partially correct but fails after incubation at 37°C for 1–2 hours (delayed non-correction) [7]
"Immediately" is key — lupus anticoagulant acts immediately; other autoimmune inhibitors show delayed non-correction following incubation [7]
- Platelet disorders cause NO change in PT and NO change in APTT [4]
- They present with a mucocutaneous bleeding pattern (petechiae, purpura, epistaxis, gum bleeding, menorrhagia) — NOT haemarthrosis or deep muscle bleeding
- Therefore, if APTT is prolonged, this is NOT a platelet problem
C. Differential Diagnosis by Clinical Presentation
From GC 075. Pain red joint [8] and Ryan Ho Rheumatology [6]:
Common causes of haemarthrosis [8]:
- Trauma (intra-articular fractures, dislocations, ligamentous sprain/tear, meniscal injury) [6]
- Haemophilia (Congenital VIII and IX deficiency) [8]
- Acquired haemophilia (autoantibodies) [8]
- Drugs (warfarin or heparin) [8]
Other uncommon causes [6]:
- Intra-articular tumours (e.g. pigmented villonodular synovitis/PVNS)
- Coagulopathy with haemarthrosis [6] — any coagulopathy, not just haemophilia
Critical distinction from other causes of acute monoarthritis [6]:
| Condition | Key Distinguishing Features |
|---|---|
| Septic arthritis | Fever, very warm/red joint, leucocytosis, elevated CRP/ESR; synovial fluid WCC usually > 50,000 with >75% neutrophils; positive Gram stain/culture |
| Crystal arthritis (gout) | Acute severe pain, classically 1st MTP; negatively birefringent needle-shaped crystals (MSU) on polarized microscopy of synovial fluid; raised urate |
| Crystal arthritis (pseudogout/CPPD) | Knee most common; positively birefringent rhomboid crystals on polarized microscopy; chondrocalcinosis on XR |
| Traumatic haemarthrosis | Clear trauma history; onset in seconds to minutes |
| Haemophilia | Recurrent episodes in weight-bearing joints, FHx (may be absent), prolonged APTT, deep-seated bleeding pattern, male patient |
The diagnosis of haemarthrosis is by arthrocentesis [8] — the synovial fluid will be uniformly bloody (haemorrhagic) rather than turbid/purulent (septic) or inflammatory.
When a young male presents with deep-seated bleeding (haemarthrosis, muscle haematomas, prolonged post-procedural bleeding), the differential is essentially:
- Haemophilia A (most common, ~85%)
- Haemophilia B (clinically identical, ~15%)
- Severe von Willebrand Disease (Type 3) — extremely rare; would expect mucocutaneous features too + ↓vWF
- Factor XI deficiency — milder, unpredictable bleeding
- Acquired haemophilia — if elderly/post-partum/autoimmune → different demographic
From Haematology Interactive Tutorial Case 2: "A 20-year-old man presented with prolonged bleeding after wisdom tooth extraction" [5]:
The approach to this presentation:
-
Duration of bleeding tendency [5]:
- Long-standing → inherited
- Acute → acquired (drugs → aspirin, warfarin) [5]
-
Pattern of bleeding [5]:
- Mucocutaneous vs. Deep-seated
- Mucocutaneous → platelet/vWF disorder
- Deep-seated → coagulation factor disorder
-
Check clotting profile:
- Normal PT + prolonged APTT → Haemophilia A/B, vWD, Factor XI/XII deficiency, heparin, lupus anticoagulant, acquired inhibitor
- Then specific factor assays + vWF studies to differentiate
| Feature | Haemophilia A | Haemophilia B | vWD (Type 1) | vWD (Type 3) | Acquired Haemophilia A | Lupus Anticoagulant |
|---|---|---|---|---|---|---|
| Inheritance | XLR | XLR | AD | AR | Acquired | Acquired |
| Sex affected | Males | Males | Both | Both | Both (elderly, post-partum) | Both |
| PT | Normal | Normal | Normal | Normal | Normal | Normal |
| APTT | Prolonged | Prolonged | Normal or mildly ↑ | Prolonged | Prolonged | Prolonged |
| Factor VIII | ↓↓↓ | Normal | Mildly ↓ | ↓↓ | ↓↓ | Normal |
| Factor IX | Normal | ↓↓↓ | Normal | Normal | Normal | Normal |
| vWF:Ag | Normal | Normal | ↓ | ↓↓↓ | Normal | Normal |
| vWF:RCo | Normal | Normal | ↓ | ↓↓↓ | Normal | Normal |
| Mixing study | Corrects | Corrects | Corrects | Corrects | Delayed non-correction | Immediate non-correction |
| Bleeding pattern | Deep-seated | Deep-seated | Mucocutaneous | Mixed | Soft tissue, retroperitoneal | Thrombosis, NOT bleeding |
| Bleeding time / PFA-100 | Normal | Normal | Prolonged | Prolonged | Normal | Normal |
Why Lupus Anticoagulant Is a DDx Despite Causing Thrombosis, Not Bleeding
This is one of the most counter-intuitive concepts in haematology. Lupus anticoagulant causes a prolonged APTT but results in a thrombotic tendency, not a bleeding tendency [7][9]. The reason is:
- The APTT test uses phospholipids in vitro as a reagent surface for coagulation — anti-phospholipid antibodies interfere with this in vitro step → prolonged APTT
- In vivo, these antibodies actually promote thrombosis through mechanisms including endothelial activation, complement activation, and interference with natural anticoagulants (protein C, annexin V)
- So a prolonged APTT does not always mean bleeding risk — you must consider lupus anticoagulant and exclude it before attributing the APTT prolongation to a factor deficiency
F. Special DDx Considerations
- Important DDx in patients with myeloproliferative neoplasms, especially essential thrombocythaemia (ET) with extremely high platelets (usually > 1000 × 10⁹/L) [10]
- Excess platelets consume excess vWF → acquired vWF deficiency → presents with bleeding tendency [10]
- Lab pattern mimics vWD: ↓vWF:Ag, ↓vWF:RCo, ± ↓FVIII
- Distinguished from inherited vWD by: late onset, no family history, underlying MPN
- Triad of aortic stenosis, GI bleeding, and acquired von Willebrand syndrome [11]
- High shear stress across the stenotic aortic valve causes conformational changes in vWF → early degradation by ADAMTS13 → loss of high-molecular-weight vWF multimers → acquired vWD
- Relevant because it can mimic any vWF/FVIII-related bleeding disorder
- This is a sneaky DDx because PT and APTT are both normal (Factor XIII cross-links fibrin after the cascade is complete)
- Presents with delayed bleeding, poor wound healing, umbilical stump bleeding in neonates
- Diagnosed by urea clot solubility test or Factor XIII activity assay
- Important to remember when PT/APTT are normal but there is still a clear bleeding tendency
- Hyperfibrinolysis (e.g. from α2-antiplasmin deficiency or PAI-1 deficiency) can cause delayed rebleeding similar to haemophilia
- PT and APTT are typically normal
- Euglobulin clot lysis time is shortened
High Yield Summary — Differential Diagnosis of Haemophilia A
- Isolated prolonged APTT (normal PT) narrows the DDx to: Haemophilia A, Haemophilia B, Haemophilia C, Factor XII deficiency, vWD, heparin, lupus anticoagulant, acquired factor inhibitors
- Mixing study is the key next step: Correction → deficiency; Immediate non-correction → lupus anticoagulant; Delayed non-correction → acquired inhibitor
- Specific factor assays + vWF studies differentiate Haemophilia A (↓FVIII, normal vWF) from vWD (↓FVIII + ↓vWF) and Haemophilia B (normal FVIII, ↓FIX)
- Haemophilia A vs B are clinically indistinguishable — must measure factor levels
- Factor XII deficiency prolongs APTT but causes NO clinical bleeding — do not transfuse
- Lupus anticoagulant prolongs APTT but causes thrombosis, not bleeding — confirm with DRVVT
- Haemarthrosis DDx (GC 075): Trauma, Haemophilia, Acquired haemophilia, Drugs (warfarin/heparin)
- Deep-seated bleeding pattern = coagulation factor disorder; Mucocutaneous pattern = platelet/vWF disorder
- Acquired haemophilia A: Elderly, post-partum, autoimmune, malignancy; retroperitoneal haematoma; delayed non-correction on mixing study
- Acquired vWD in MPN with platelets > 1000: excess platelets consume vWF
Active Recall - Haemophilia A Differential Diagnosis
References
[1] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai) — Haemophilia A/B, Etiology, DDx table (p.592) [2] Senior notes: Ryan Ho Haemtology — Section 4.3.2 Haemophilia A and B (p.123–124) [3] Senior notes: Ryan Ho Fundamentals — Section 3.6.5.2 Approach to Bleeding Disorders (p.404) [4] Senior notes: Block A - Abnormal bleeding after tooth extraction — PT/APTT interpretation [5] Senior notes: Block A - Hematology Interactive Tutorial — Case 2 [6] Senior notes: Ryan Ho Rheumatology — Section 2.1 Approach to Acute Monoarthritis (p.28) [7] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting) — Mixing study, Factor XII, DRVVT (p.19, 22–23) [8] Lecture slides: GC 075. Pain red joint (p.60) — Haemarthrosis [9] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting) — DRVVT and lupus anticoagulant (p.23) [10] Senior notes: Block A - Splenomegaly — Essential Thrombocythaemia, acquired vWD (p.29) [11] Senior notes: Block A - Coffee ground vomitus tarry stool upper GI bleeding — Heyde syndrome (p.8)
Diagnostic Criteria, Diagnostic Algorithm and Investigation Modalities
A. Diagnostic Criteria for Haemophilia A
There is no single "diagnostic criteria checklist" for Haemophilia A analogous to, say, the Jones criteria for rheumatic fever. Instead, the diagnosis is made through a stepwise laboratory-confirmed approach endorsed by the World Federation of Hemophilia (WFH) and the International Society on Thrombosis and Haemostasis (ISTH). The diagnosis rests on three pillars:
- Male patient with a deep-seated bleeding pattern (haemarthrosis, muscle haematomas, delayed post-procedural bleeding)
- OR a positive family history with X-linked pattern (maternal uncles/brothers affected, no male-to-male transmission)
- OR incidental finding of isolated prolonged APTT in pre-operative screening
Haemophilia A / B: No change in PT, Increase in APTT [4]
| Parameter | Expected Finding | Rationale |
|---|---|---|
| PT | Normal | Extrinsic pathway (Factor VII → X → common) is intact; FVIII is not in this pathway |
| APTT | Prolonged | FVIII is part of the intrinsic pathway; its deficiency slows intrinsic tenase complex assembly |
| Platelet count | Normal | Not a platelet disorder |
| Fibrinogen | Normal | Common pathway intact |
| Thrombin time | Normal | Fibrinogen → fibrin conversion is unaffected |
Note: APTT may be normal in some mild haemophilia patients [2] — this is a critical caveat. A patient with Factor VIII at 30–40% may have an APTT that falls just within the upper end of the reference range. Therefore, a normal APTT does NOT exclude mild haemophilia if clinical suspicion is high.
Specific factor assay: ↓activity ( < 40%) in Factor VIII [2]
| Severity | Factor VIII Level | ISTH / WFH Classification |
|---|---|---|
| Severe | < 1% ( < 0.01 IU/mL) | Spontaneous bleeds, haemarthrosis |
| Moderate | 1–5% (0.01–0.05 IU/mL) | Bleeding with minor trauma |
| Mild | > 5% to < 40% (0.05–0.40 IU/mL) | Bleeding with major trauma/surgery |
Plus the crucial distinction from von Willebrand disease:
- vWF:Ag — Normal (distinguishes from vWD where vWF:Ag is low)
- vWF:RCo — Normal (distinguishes from vWD where ristocetin cofactor activity is low)
From the GC Interactive Tutorial (Haem Case 2) — the index exam case [5][12]:
Factor VIII level 0.13 u/mL, vWF:Ag 1.11 u/mL, vWF:RCo 1.22 u/mL
Interpretation: Factor VIII is reduced (~13% of normal = mild haemophilia A). vWF:Ag and vWF:RCo are both normal, ruling out von Willebrand disease. APTT was 60 seconds (markedly prolonged; reference 21–35s). PT was 10.5 seconds (normal).
Formal Diagnostic Summary — Haemophilia A Is Confirmed When
- Clinical: Deep-seated bleeding pattern in a male patient (± family history)
- Screening: Isolated prolonged APTT with normal PT, normal platelet count
- Mixing study: APTT corrects on 1:1 mixing with normal plasma (excludes inhibitor)
- Confirmatory: Factor VIII activity < 40% (with normal vWF:Ag and vWF:RCo)
- Severity grading: Based on residual Factor VIII level (severe < 1%, moderate 1–5%, mild > 5%– < 40%)
- Optional but recommended: Genetic testing for F8 mutation to confirm and enable family screening
The algorithm below integrates all the investigation steps from initial screening through to confirmatory diagnosis, inhibitor testing, and genetic workup.
C. Investigation Modalities — Detailed Breakdown
Step 1: Initial Screening Tests
| Parameter | Expected in Haemophilia A | Why |
|---|---|---|
| Haemoglobin | Usually normal (unless acute/chronic blood loss) | Haemophilia does not intrinsically cause anaemia; but recurrent bleeds can deplete iron stores over time |
| WBC | Normal | Not a marrow disorder |
| Platelet count | Normal | Primary haemostasis is intact; this is purely a coagulation cascade defect |
| MCV | Normal (unless concurrent iron deficiency from chronic bleeding) | — |
From the tutorial case: Hb 14.5 g/dL, WCC 9.5 × 10⁹/L, Platelet count 260 × 10⁹/L — all normal [5][12]. This is the classic haemophilia pattern: the CBC is completely unremarkable.
The clotting profile is the first-line screening investigation for any patient with a bleeding tendency [3][4][13].
| Test | What It Measures | Result in Haemophilia A | Interpretation |
|---|---|---|---|
| PT (Prothrombin Time) | Extrinsic + common pathway (Factors VII, X, V, II, fibrinogen) | Normal | Factor VIII is not in the extrinsic pathway |
| APTT (Activated Partial Thromboplastin Time) | Intrinsic + common pathway (Factors XII, XI, IX, VIII, X, V, II, fibrinogen) | Prolonged | Factor VIII deficiency → intrinsic tenase complex dysfunction → delayed Factor X activation |
| Fibrinogen | Functional fibrinogen (common pathway endpoint) | Normal | Common pathway intact |
| TT (Thrombin Time) | Fibrinogen → fibrin conversion | Normal | Not affected by upstream factor deficiencies |
Why does APTT specifically measure the intrinsic pathway?
- The APTT reagent provides a phospholipid surface + a contact activator (e.g. kaolin, silica) — this activates Factor XII, which initiates the intrinsic cascade: XII → XI → IX → VIII (cofactor) → X → common pathway
- Without adequate Factor VIII, Factor X activation is slow → the time to clot formation is prolonged
- The PT reagent provides tissue factor (thromboplastin), which directly activates Factor VII and bypasses the intrinsic pathway entirely — so PT is unaffected
The Maksim Medicine Notes provide a clean interpretation framework [13]:
Normal PT + ↑APTT → Intrinsic pathway defect (Factors 8, 9, 11, 12)
- Improved after mixing study (1:1 mix with normal plasma): haemophilia, vWD
- Unchanged after mixing study: heparin use, lupus anticoagulant, other acquired inhibitors
Exam Pitfall: Normal APTT Does Not Exclude Mild Haemophilia
APTT may be normal in some mild haemophilia patients [2]. This occurs because:
- The APTT reagent system has a "threshold" — Factor VIII levels as low as ~30–35% may still generate enough thrombin to produce a clot within the normal APTT time range
- Therefore, if a patient has Factor VIII at 25–38%, their APTT might be borderline-normal or only very slightly prolonged
- Clinical rule: If clinical suspicion is high (e.g. post-surgical bleeding with family history), proceed directly to specific Factor VIII assay even with a normal APTT
- Usually unremarkable in haemophilia A (no morphological red cell or platelet abnormalities expected)
- Useful to exclude other diagnoses: schistocytes (DIC/TMA), giant platelets (Bernard-Soulier), platelet clumping (pseudothrombocytopenia from EDTA)
- If the patient has chronic bleeding → may see features of iron deficiency (microcytic hypochromic cells, pencil cells)
This is the pivotal next step after identifying an isolated prolonged APTT. It answers the fundamental question: Is the APTT prolonged because of a factor DEFICIENCY, or because of an INHIBITOR? [5][7][13]
How it works:
- Mix 50% patient plasma with 50% normal pooled plasma
- Normal plasma contains 100% of all clotting factors
- After mixing, the resultant plasma should have ≥ 50% of each factor (the patient contributes 0%, normal contributes 100%, average = 50%)
- Factor levels ≥ 50% are sufficient to normalize the APTT — because the coagulation cascade has significant "reserve capacity"
- Re-measure APTT on the mixed sample, both immediately and after incubation at 37°C for 1–2 hours
Interpretation:
| Result | Diagnosis | Mechanism |
|---|---|---|
| APTT corrects immediately and stays corrected | Factor deficiency (Haemophilia A, B, vWD, Factor XI/XII deficiency) [7][13] | Normal plasma supplies the missing factor → 50% level is sufficient to normalize APTT |
| APTT does NOT correct immediately | Lupus anticoagulant [7] | Anti-phospholipid antibodies immediately neutralize the phospholipid reagent in both patient and normal plasma |
| APTT corrects initially but fails after 37°C incubation | Time-dependent inhibitor (Acquired Haemophilia A) [7] | The anti-FVIII autoantibody is time- and temperature-dependent → it gradually neutralizes the FVIII in the normal plasma over the incubation period |
From the Haematology Interactive Tutorial [5]:
1:1 mixing test to differentiate between deficiency and inhibition
Why 50% Factor Level Is Enough to Correct APTT
The coagulation cascade is a series of enzymatic amplification steps — each activated factor can activate many molecules of the next factor in the cascade. Therefore, the system has built-in redundancy: you only need about 25–30% of most factors to generate sufficient thrombin for a normal clotting time. By mixing 50:50 with normal plasma, you achieve ~50% of the deficient factor, which is well above the threshold for normal APTT.
Step 3: Specific Factor Assays
Once the mixing study confirms a factor deficiency, the next step is to measure individual factor levels to identify which factor is deficient.
- Method: One-stage clotting assay (most commonly used) or chromogenic assay
- One-stage: Patient plasma is mixed with Factor VIII-deficient plasma and the APTT of the mixture is measured — the shorter the APTT, the more Factor VIII the patient has (calibrated against a standard curve)
- Chromogenic: Uses a two-stage reaction where Factor VIII cofactor activity leads to Factor Xa generation, which then cleaves a chromogenic substrate — more specific and less affected by lupus anticoagulant or heparin
- Result in Haemophilia A: Factor VIII activity < 40% (0.40 IU/mL) [2]
- The level determines severity grading (see classification above)
- Performed simultaneously to exclude Haemophilia B (clinically indistinguishable from A)
- Normal in Haemophilia A; decreased in Haemophilia B [2]
This is perhaps the single most high-yield investigation concept for Haemophilia A [5][12]:
| Test | What It Measures | Result in Haemophilia A | Result in vWD |
|---|---|---|---|
| vWF:Ag | Quantitative amount of vWF protein in plasma | Normal | Decreased |
| vWF:RCo (Ristocetin Cofactor Activity) | Functional ability of vWF to bind platelet GPIb (measured using ristocetin as an agonist) | Normal | Decreased |
| vWF:CB (Collagen Binding) | Functional ability of vWF to bind collagen | Normal | Decreased in some subtypes |
Why is this distinction so important?
- vWF is the carrier protein for Factor VIII in circulation — it protects FVIII from premature degradation
- In vWD (especially Type 2N "Normandy" and Type 3), reduced vWF leads to secondary FVIII reduction → the lab picture can mimic haemophilia A (↓FVIII + ↑APTT)
- But in vWD, the primary defect is in vWF → vWF:Ag and/or vWF:RCo will be reduced
- In Haemophilia A, the primary defect is in the F8 gene → FVIII is low, but vWF is completely normal
The GC Interactive Tutorial (Haem Case 2) tests exactly this [5][12]:
Factor VIII level 0.13 u/mL (low — ~13%) vWF:Ag 1.11 u/mL (normal) vWF:RCo 1.22 u/mL (normal) → This pattern confirms Haemophilia A, not vWD.
Special Note: vWD Type 2N (Normandy)
This is an exam trap. vWD Type 2N has a mutation in the FVIII-binding domain of vWF. As a result:
- vWF:Ag is normal (the vWF protein is present)
- vWF:RCo is normal (the platelet-binding function is intact)
- But FVIII is low (because vWF cannot carry FVIII → rapid clearance)
- This perfectly mimics Haemophilia A on standard testing!
- Differentiated by: vWF:FVIII binding assay (abnormal in Type 2N, normal in Haemophilia A), or genetic testing
- Inheritance is autosomal recessive (affects both sexes) — this is a clue if a female patient appears to have "haemophilia A"
Step 4: Inhibitor Screening — Bethesda Assay
This step is essential for all newly diagnosed haemophilia A patients and for any patient who shows unexpected poor response to factor replacement therapy.
- Inhibitor development occurs in ~30% of severe Haemophilia A and 3–5% of severe Haemophilia B [2]
- These are alloantibodies (IgG) directed against exogenous Factor VIII — the immune system recognizes the infused factor as "foreign" because the patient's body has never produced native FVIII
- Inhibitors neutralize the infused factor → treatment failure → life-threatening bleeding
- Principle: Patient plasma (containing potential inhibitor) is mixed with normal pooled plasma (containing a known amount of FVIII) and incubated at 37°C for 2 hours
- Residual FVIII activity in the mixture is measured
- One Bethesda Unit (BU) is defined as the amount of inhibitor that neutralizes 50% of FVIII activity in the mixture
- Classification:
- Low titre: < 5 BU/mL — may still respond to high-dose factor replacement
- High titre: ≥ 5 BU/mL — requires bypassing agents (e.g. activated prothrombin complex concentrate, recombinant FVIIa) or emicizumab
- At diagnosis (baseline)
- Before and after any period of intensive factor replacement (e.g. surgery)
- When factor replacement fails to achieve expected haemostatic response
- Regularly during the first 50 exposure days (highest risk period for inhibitor development)
Step 5: Genetic Testing
Genetic testing: appropriate → identify the mutation that can be used for relative screening [2]
- The F8 gene is located at Xq28 and is one of the largest genes in the genome (~186 kb, 26 exons)
- First-line genetic test in severe Haemophilia A: Screen for intron 22 inversion (accounts for ~45% of severe cases) and intron 1 inversion (~5% of severe cases) — these can be detected by specific PCR-based assays (inverse-shifting PCR or long-range PCR)
- If negative: full gene sequencing (Sanger or next-generation sequencing) to identify point mutations, small deletions/insertions
- If negative again: MLPA (Multiplex Ligation-dependent Probe Amplification) for large deletions/duplications
- Confirms the diagnosis — especially in mild cases where factor levels are borderline
- Enables carrier detection in female relatives (sisters, daughters, maternal aunts)
- Enables prenatal diagnosis in future pregnancies of known carriers
- Predicts inhibitor risk — certain mutation types (large deletions, intron 22 inversions, nonsense mutations) are associated with higher inhibitor risk because the patient produces no endogenous FVIII at all (null mutations), so the immune system is more likely to recognize exogenous FVIII as completely foreign
- Genetic counselling — essential for family planning
Step 6: Family Screening and Carrier Detection
Further investigations: Check APTT and Factor VIII in family members (e.g. siblings), genetic test [5]
- An obligate carrier is a woman who has an affected father, or who has two or more affected sons, or one affected son plus another affected male relative in the maternal line
- The mother of an affected boy must be an obligate carrier [5] (unless the son has a de novo mutation, which occurs in ~55% of severe cases)
- Factor VIII levels in carriers: Due to lyonization (random X-inactivation), carriers have variable FVIII levels — typically 30–70%, but can range from very low (skewed inactivation) to normal
- A low FVIII level in a female makes carrier status more likely but does not confirm it
- A normal FVIII level does NOT exclude carrier status
- Genetic testing is the definitive method for carrier detection — test for the specific F8 mutation identified in the proband
- Prenatal diagnosis: often presumed based on sex on USG (invasive investigations do not change management) [2]
- If a male fetus is identified and the mother is a known carrier → 50% chance the male is affected
- Options:
- Chorionic villus sampling (CVS) at 10–12 weeks — genetic analysis of fetal DNA
- Amniocentesis at 15–18 weeks — less commonly used now
- Non-invasive prenatal testing (NIPT) — cell-free fetal DNA in maternal blood can determine fetal sex and potentially test for known familial mutations (emerging technology, not yet universally available)
- Cordocentesis (fetal blood sampling) at ≥ 18 weeks — can directly measure fetal FVIII level, but carries higher procedural risk
- Practical note [2]: In many centres, invasive testing does not change obstetric management because even if the fetus is affected, vaginal delivery is still usually possible (just with precautions). The main benefit is preparedness at delivery.
Additional Investigations in Specific Contexts
| Clinical Scenario | Investigation | Rationale |
|---|---|---|
| Suspected haemarthrosis | Ultrasound or MRI of the joint | US can detect effusion; MRI better for evaluating synovial hypertrophy and cartilage damage (haemophilic arthropathy) |
| Suspected ICH | Urgent CT brain (non-contrast) | ICH is a medical emergency; CT is fast and sensitive for acute haemorrhage |
| Suspected psoas/retroperitoneal haematoma | CT abdomen with contrast [2] | High index of suspicion → consider CT abdomen if initial endoscopy for GI bleed negative |
| Suspected compartment syndrome | Clinical diagnosis (measure compartment pressure if needed) | Tense compartment, pain on passive stretch, paraesthesia |
| Haemophilic pseudotumour | XR → CT/MRI | XR shows expanding bony lesion; MRI delineates soft tissue extent and differentiates from malignancy |
- X-ray: Shows late changes — joint space narrowing, subchondral cysts, secondary OA changes, irregularity of articular surfaces
- MRI: Gold standard for evaluating early changes — synovial hypertrophy, haemosiderin deposition (low signal on all sequences), cartilage erosion, effusions
- Ultrasound: Point-of-care assessment of joint effusion and synovial thickness; increasingly used in haemophilia comprehensive care clinics (HEAD-US protocol)
- Contraindicated in severe haemophilia — severe bleeding disorders (severe haemophilia, DIC) are an absolute contraindication to bone marrow examination [3][14]
- This is because the procedure involves penetrating the periosteum of the iliac crest, which can cause significant haemorrhage in a patient who cannot form a stable clot
- If bone marrow examination is absolutely necessary, adequate FVIII replacement must be given before the procedure to raise levels to ≥ 50%
- All haemophilia patients should be screened for HIV, HBV, HCV — especially those treated with plasma-derived products before the era of viral inactivation [2]
- In HK, this is particularly relevant for older patients diagnosed before ~1990
| Step | Investigation | Key Finding in Haemophilia A | Purpose |
|---|---|---|---|
| 1 | CBC | Normal Hb, WCC, Platelets | Exclude thrombocytopenia, leukaemia |
| 2 | PT | Normal | Exclude extrinsic/common pathway defects |
| 3 | APTT | Prolonged (but may be normal in mild) | Screen for intrinsic pathway defect |
| 4 | Mixing study | APTT corrects | Exclude inhibitor; confirm factor deficiency |
| 5 | Factor VIII assay | < 40% (< 0.40 IU/mL) | Confirm and quantify FVIII deficiency |
| 6 | Factor IX assay | Normal | Exclude Haemophilia B |
| 7 | vWF:Ag | Normal | Exclude von Willebrand disease |
| 8 | vWF:RCo | Normal | Exclude von Willebrand disease |
| 9 | Bethesda assay | Negative at diagnosis (if no prior factor exposure) | Screen for inhibitor |
| 10 | F8 genetic testing | Identifies causative mutation | Confirm diagnosis, carrier detection, family screening |
High Yield Summary — Diagnosis of Haemophilia A
- Screening: CBC (normal) + Clotting profile (isolated ↑APTT with normal PT) — the single most important screening pattern
- APTT may be normal in mild haemophilia — do not be falsely reassured; proceed to factor assay if clinical suspicion is high
- Mixing study: Correction = deficiency (haemophilia, vWD); Immediate non-correction = lupus anticoagulant; Delayed non-correction = acquired inhibitor
- Factor VIII assay: < 40% confirms diagnosis; level determines severity (severe < 1%, moderate 1–5%, mild > 5%– < 40%)
- vWF:Ag and vWF:RCo must be normal to distinguish from vWD — the GC Interactive Tutorial Haem Case 2 tests exactly this
- Beware vWD Type 2N — mimics haemophilia A perfectly on standard tests (normal vWF:Ag, normal vWF:RCo, low FVIII); need vWF:FVIII binding assay to differentiate
- Inhibitor screening (Bethesda assay): Essential in all patients; occurs in ~30% of severe Haemophilia A; < 5 BU = low titre, ≥ 5 BU = high titre
- Genetic testing: Confirms diagnosis, enables carrier detection and prenatal diagnosis; intron 22 inversion is the most common severe mutation (~45%)
- Bone marrow examination is absolutely contraindicated in severe haemophilia without factor cover
- Family screening: Check APTT + Factor VIII in siblings; genetic testing for carrier detection in female relatives
Active Recall - Haemophilia A Diagnosis and Investigations
References
[2] Senior notes: Ryan Ho Haemtology — Section 4.3.2 Haemophilia A and B (p.123–124) [3] Senior notes: Ryan Ho Fundamentals — Section on Marrow examination and Approach to Bleeding Disorders (p.391, 404) [4] Senior notes: Block A - Abnormal bleeding after tooth extraction — PT/APTT interpretation [5] Senior notes: Block A - Hematology Interactive Tutorial — Case 2 [7] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting) — Mixing study and DRVVT [12] Lecture slides: GC_Interactive tutorial (Haem case 2) student copy — Case scenario and lab findings [13] Senior notes: Maksim Medicine Notes — Clotting cascade interpretation (p.161) [14] Senior notes: Ryan Ho Haemtology — Marrow examination contraindications (p.48)
Management Algorithm and Treatment Modalities
The management of Haemophilia A is built around a simple conceptual framework: the patient lacks Factor VIII, so the goal is to either replace it, augment its release, or bypass the need for it. Everything else — lifestyle modification, joint care, inhibitor management, gene therapy — supports this central aim.
Treatment of coagulation disorders as a cause of haemostatic disorder [4]:
- Specific factors / factor concentrates (haemophilia, vWD)
- DDAVP (vWD)
- Fresh frozen plasma
- Vitamin K (jaundice, liver disease)
From the Haematology Interactive Tutorial — Management of Haemophilia A [5]:
- Lifestyle changes (avoid trivial trauma)
- Desmopressin for surgery (DDAVP binds to V2 receptor in endothelial cells and platelets — release Factor VIII (synthesized by and stored in endothelial cells and platelets) into circulation); or Factor VIII replacement
GC High Yield — Haemarthrosis Management
From GC 075. Pain red joint [8]:
Treatment of haemarthrosis: Treat the "treatable" underlying causes
This is the fundamental principle — haemarthrosis in haemophilia is treated by restoring Factor VIII, not by draining the joint (arthrocentesis is for diagnosis, not routine treatment).
These are the foundation of haemophilia care, regardless of severity. They reduce the frequency and severity of bleeding events.
- Avoid invasive procedures: smallest gauge for vaccination, minimize blood taking/IMI (intramuscular injections) — because each needle puncture is a potential bleeding site in a patient who cannot form stable clots
- Dental hygiene to decrease need for dental interventions — dental procedures are a common trigger for bleeding; prevention is always better than treatment
- Exercise: regular exercise, prefer non-contact sports, e.g. swimming, cycling, tennis — weight-bearing exercise strengthens muscles around joints (providing "natural splinting"), but contact sports (rugby, boxing, martial arts) risk joint/muscle trauma
- Avoid antiplatelets/anticoagulants — these would further impair an already defective haemostatic system
- NSAIDs are particularly dangerous because they inhibit platelet function AND can cause GI ulceration → double hit for bleeding risk
- When analgesia is needed for haemarthrosis: use COX-2 inhibitors or paracetamol — avoid traditional NSAIDs [2]
Additional general measures:
- Hepatitis A and B vaccination — all haemophilia patients should be vaccinated (preferably subcutaneously rather than intramuscularly, with firm pressure post-injection); historically high risk of bloodborne infection from factor concentrates
- Medic-Alert bracelet/card — so emergency personnel know to treat with factor replacement before any intervention
- Comprehensive care centre registration — multidisciplinary team (haematologist, orthopaedic surgeon, physiotherapist, dentist, nurse specialist, psychosocial support)
- Genetic counselling and family screening — carrier detection in female relatives, prenatal counselling [5]
D. Specific Treatment Modalities
"DDAVP" stands for 1-deamino-8-D-arginine vasopressin — a synthetic analogue of vasopressin (antidiuretic hormone, ADH).
Trial of DDAVP for mild haemophilia A → 47% responsive [2][15]
Mechanism of Action:
- DDAVP binds to V2 receptors on endothelial cells [5]
- This triggers the release of pre-formed Factor VIII and vWF from Weibel-Palade bodies (storage granules within endothelial cells) into the circulation
- The released vWF also stabilises the released Factor VIII, extending its half-life
- Net effect: transient 2–5 fold rise in both FVIII and vWF levels, peaking at approximately 30–60 minutes
Pre-requisite: usually test dose followed by re-measurement of Factor VIII [2][15]:
- For responsive individuals, Factor VIII activity will rise to 3× baseline with peak at 30 minutes [2][15]
- If the patient's baseline FVIII is 15%, a 3× rise would bring it to 45% — sufficient for minor haemostasis
- If baseline FVIII is very low ( < 5%), even a 3× rise may be insufficient → DDAVP is generally not useful in moderate or severe haemophilia
Use: for minor bleeding or minor invasive procedures [2][15]
| Aspect | Detail |
|---|---|
| Route | Intravenous (0.3 mcg/kg in 50mL NS over 20–30 min), subcutaneous, or intranasal (Stimate® — 150 mcg/spray, 1 spray per nostril) |
| Onset | 30–60 minutes IV/SC; 60–90 minutes intranasal |
| Duration | 6–8 hours — typically sufficient for dental procedures or minor surgery |
| Indications | Mild Haemophilia A (DDAVP-responsive), minor dental/surgical procedures, minor soft tissue bleeds |
| Contraindications | Severe haemophilia (insufficient baseline stores), patients < 2 years old (risk of hyponatraemia/seizures due to immature renal concentrating ability), unstable angina/CVD (rare vasopressor effects) |
| Tachyphylaxis | Repeated doses within 24–48 hours deplete endothelial stores → diminishing response ("tachyphylaxis"). Limit to 2–3 doses in succession; if ongoing haemostasis needed beyond this, switch to factor replacement |
| Side effects | Facial flushing, headache, tachycardia (vasomotor); hyponatraemia (ADH effect → water retention) — monitor fluid intake and serum Na⁺; restrict free water |
Why DDAVP Works in Mild but Not Severe Haemophilia A
DDAVP releases pre-formed FVIII from endothelial storage pools. In mild haemophilia, the patient CAN synthesize FVIII (just at reduced levels) — so the Weibel-Palade bodies contain some FVIII that can be released. In severe haemophilia, the F8 gene mutation is so disabling that essentially no FVIII is synthesized or stored — there is nothing to release, so DDAVP has no effect.
2. Factor VIII Replacement Therapy — The Cornerstone
This is the mainstay of treatment for moderate and severe haemophilia A, and for mild haemophilia A when DDAVP is inadequate.
| Type | Description | Advantages | Disadvantages |
|---|---|---|---|
| Recombinant FVIII (rFVIII) | Produced by recombinant DNA technology in mammalian cell lines (CHO or BHK cells) | No risk of bloodborne infection [2]; consistent potency | Cost; immunogenicity (inhibitor risk ~25–30% in severe) |
| Plasma-derived FVIII (pdFVIII) | Purified from pooled human plasma; undergoes viral inactivation (solvent-detergent, nanofiltration) | Slightly lower inhibitor risk than rFVIII (possibly because trace vWF in pdFVIII provides immune tolerance); cheaper in some settings | Theoretical residual viral risk (prion); requires donor plasma |
| Extended half-life rFVIII (EHL-rFVIII) | rFVIII modified by PEGylation, Fc fusion, or single-chain technology to slow clearance | Longer half-life (~18–19h vs ~8–12h for standard rFVIII) → less frequent infusions (e.g. every 3–5 days instead of every other day) | Cost; not yet widely available in all settings including HK |
The dose of Factor VIII is calculated based on the principle that 1 IU/kg of body weight of FVIII raises the plasma FVIII level by approximately 2% (or 0.02 IU/mL).
Formula:
Dose (IU) = Desired FVIII rise (%) × Body weight (kg) × 0.5
For example, if a 70 kg patient with severe Haemophilia A (baseline FVIII 0%) needs to reach 50% for a minor procedure:
- Dose = 50 × 70 × 0.5 = 1,750 IU
The half-life of infused FVIII is approximately 8–12 hours (standard products) or ~18–19 hours (EHL products), so repeat dosing is needed to maintain haemostatic levels.
| Clinical Scenario | Target FVIII Level | Duration |
|---|---|---|
| Minor bleed (early haemarthrosis, minor soft tissue) | 30–50% | 1–2 days |
| Moderate bleed (muscle haematoma, oral bleeding, haematuria) | 50–70% | 2–5 days |
| Major bleed (ICH, retroperitoneal, GI, pharyngeal) | 80–100% initially, then 50–80% | 7–14 days |
| Minor surgery (dental extraction, minor procedures) | 50–80% initially, then 30–50% | 1–5 days |
| Major surgery (orthopaedic, abdominal) | 80–100% initially, then 50–80% | 7–14 days |
Recombinant factor replacement: can be administered in different strategies [2][15]:
A. On-Demand Therapy
- Only given during bleeding and invasive procedures [2][15]
- Patient recognises the prodrome of a bleed (stiffness, warmth in a joint) → self-infuses at home
- Treats the bleed but does NOT prevent future bleeds or long-term joint damage
B. Prophylactic Therapy
- To be given in the absence of bleeding [2][15]
- Continuous prophylaxis: regular replacement [2][15]:
- Primary prophylaxis: started before age 3 years (or after the first joint bleed but before joint damage) — the gold standard for severe haemophilia
- Secondary prophylaxis: started after ≥ 2 large joint bleeds [2][15]
- Tertiary prophylaxis: started after arthropathy has developed — aims to slow further damage [2][15]
- Intermittent prophylaxis: given for several weeks/months then discontinued → given to interrupt bleeding cycle in those with repeated bleeding especially into target joints [2][15]
- Typical prophylaxis regimen: Standard rFVIII 25–50 IU/kg every other day (or 3× per week); EHL-rFVIII every 3–5 days
| Prophylactic Therapy | On-Demand Therapy | |
|---|---|---|
| Advantages | ↓bleeding risk and ↑overall QoL; ↓risk of life-threatening bleeding, e.g. ICH; ↓hospitalization and absenteeism; ↓long-term complications, especially chronic arthropathy | ↓cost (recombinant factor very expensive and not readily available); No need for regular injections; No need for CVC placement; ↓replacement-related complications, e.g. infection |
| Usual indications | Severe haemophilia (primary prophylaxis); Prior bleeding episodes (e.g. ≥ 2 large joint bleeds, i.e. secondary prophylaxis); Evidence of joint damage on PE or imaging (tertiary prophylaxis) | Resource-limited settings; Mild/moderate haemophilia with limited burden |
Why Primary Prophylaxis Is the Gold Standard
The landmark Swedish prophylaxis studies showed that children started on primary prophylaxis before age 3 had near-normal joint function at age 20, compared to devastating arthropathy in those on on-demand therapy alone. The rationale is that maintaining FVIII > 1% at all times converts a "severe" phenotype into a "moderate" one — dramatically reducing spontaneous bleeds and joint damage. This has been adopted as the standard of care by WFH/ISTH.
Emicizumab (emi = "emicizumab"; -cizu- = chimeric/humanized; -mab = monoclonal antibody) is a bispecific monoclonal antibody that mimics the cofactor function of Factor VIIIa by bridging Factor IXa and Factor X.
Breaking down how it works:
- In normal coagulation, Factor VIIIa acts as a cofactor by bringing Factor IXa and Factor X into close proximity on the phospholipid surface → this is the "tenase complex"
- Emicizumab has one arm that binds Factor IXa and another arm that binds Factor X → it physically bridges them, just like Factor VIIIa would
- Critically, emicizumab is NOT Factor VIII — so it is not neutralised by Factor VIII inhibitors
| Aspect | Detail |
|---|---|
| Route | Subcutaneous injection (huge advantage over IV factor infusion) |
| Frequency | Weekly, biweekly, or every 4 weeks (after loading) |
| Indication | Prophylaxis in Haemophilia A — originally for patients with inhibitors, now also approved for patients without inhibitors |
| Efficacy | HAVEN trials: 87% reduction in treated bleeds vs no prophylaxis; >95% reduction vs on-demand bypassing agents |
| Advantages | Subcutaneous route, infrequent dosing, not affected by FVIII inhibitors, no risk of FVIII inhibitor development (it's not FVIII) |
| Limitations | Does NOT treat acute bleeds (still need factor replacement or bypassing agents for breakthrough bleeds); thrombotic microangiopathy risk when combined with aPCC (FEIBA) at high doses; expensive |
| Lab caution | Emicizumab shortens APTT (because it performs the same bridging function as FVIIIa in the APTT test) → the APTT becomes unreliable for monitoring; one-stage FVIII assay also unreliable. Use chromogenic (bovine-based) FVIII assay to measure true endogenous FVIII levels |
4. Bypassing Agents — For Patients with Inhibitors
When a patient has high-titre inhibitors (≥ 5 BU), standard Factor VIII replacement is neutralised and ineffective. The strategy is to bypass the need for Factor VIII altogether.
- Mechanism: Supraphysiological doses of Factor VIIa directly activate Factor X on the surface of activated platelets, bypassing the need for the Factor VIIIa-IXa tenase complex entirely
- Dose: 90–120 mcg/kg IV, repeated every 2–3 hours until haemostasis achieved
- Advantage: No risk of anamnestic response (does not contain FVIII)
- Disadvantage: Short half-life (~2.5 hours), frequent dosing, expensive, theoretical thrombotic risk
- "FEIBA" = Factor Eight Inhibitor Bypassing Activity
- Content: Contains Factors II, VII, IX, X in both activated and non-activated forms
- Mechanism: The activated factors bypass the FVIII-dependent step; generates thrombin through the extrinsic and common pathways
- Dose: 50–100 IU/kg IV, every 8–12 hours (max 200 IU/kg/day)
- Advantage: Somewhat longer duration of action than rFVIIa
- Disadvantage: Contains trace amounts of FVIII → can trigger anamnestic response; thrombotic risk, especially when combined with emicizumab → thrombotic microangiopathy (TMA) reported in HAVEN-1 trial; should NOT exceed 100 IU/kg/day in patients on emicizumab
Bypassing products: recombinant Factor VIIa, aPCC (e.g. FEIBA) [2]
- Recombinant porcine Factor VIII [2] — porcine FVIII is structurally different enough from human FVIII that many human anti-FVIII antibodies do not cross-react
- Primarily used in acquired haemophilia A (where the autoantibody targets human FVIII epitopes)
- Not first-line for congenital haemophilia with inhibitors (cross-reactivity is possible)
Immune tolerance induction (ITI) → curative treatment to eradicate inhibitor [2]:
- Method: repeated doses of Factor VIII (± immunosuppression) to induce immune tolerance [2]
- The concept: by continuously exposing the immune system to high-dose FVIII over months to years, the immune system eventually "accepts" FVIII and stops producing inhibitory antibodies
- Success rate: ~60–80% in Haemophilia A
- Factors predicting success: Low baseline inhibitor titre ( < 10 BU), starting ITI when titre < 10 BU, younger age, shorter time since inhibitor development
- Regimen: High-dose FVIII (100–200 IU/kg/day) until inhibitor is undetectable and FVIII recovery/half-life normalises — can take 6–24 months or longer
- Immunosuppression adjuncts: Rituximab (anti-CD20), cyclophosphamide, or steroids may be added for refractory cases
6. Adjunctive Therapies
Mucosal bleeding: add antifibrinolytic agents, e.g. tranexamic acid, aminocaproic acid [2]
- Mechanism: Tranexamic acid ("trans" + "amic acid") — a synthetic lysine analogue that competitively blocks the lysine-binding sites on plasminogen, preventing plasminogen from binding to fibrin and being activated to plasmin → inhibits fibrinolysis → the clot that forms is stabilised and not prematurely dissolved
- Indications: Mucosal bleeding (oral, nasal, GI, menorrhagia), dental procedures — mucous membranes are rich in fibrinolytic activity due to high tissue plasminogen activator (tPA) concentration
- Contraindication: Haematuria — antifibrinolytics must NOT be used for haematuria because stabilised clots in the urinary tract can cause obstruction (clot retention → ureteric or urethral obstruction → hydronephrosis)
- Dose: Tranexamic acid 1g TDS (oral) or 10 mg/kg IV
Haematuria: treat with forced diuresis if not severe [2]
- Forced diuresis (generous hydration) helps flush blood from the urinary tract
- Do NOT use antifibrinolytics (risk of clot obstruction as above)
- Factor replacement if haematuria is severe or persistent
Haemarthrosis: analgesics (e.g. COX-2 inhibitors, Panadol → avoid NSAIDs), RICE as needed [2]
- Why avoid traditional NSAIDs? NSAIDs (ibuprofen, naproxen, diclofenac) irreversibly or reversibly inhibit COX-1 → reduced thromboxane A2 → impaired platelet aggregation → worsened bleeding tendency
- COX-2 selective inhibitors (celecoxib, etoricoxib) spare platelet COX-1 → analgesic/anti-inflammatory effect without impairing platelet function
- Paracetamol has no significant effect on platelet function → safe
- RICE = Rest, Ice, Compression, Elevation — reduces swelling and limits haematoma expansion
- Aspiration of haemarthrosis is NOT routinely performed — only if tense haemarthrosis with severe pain and confirmed by imaging, performed under factor cover
Muscle haematoma: consider surgical decompression if failed medical therapy [2]
- For compartment syndrome (most commonly calf) that does not respond to factor replacement and conservative measures
- Must be performed under adequate factor cover (target FVIII > 80%)
7. Blood Products in Haemophilia A
While specific FVIII concentrates are the standard, other blood products have a role in specific situations:
FFP indication: PT/aPTT > 1.5× control with active bleeding or before invasive procedures PLUS single/multiple clotting factor deficiency other than haemophilia A/B [16]
This is a critical point: FFP is generally NOT the treatment of choice for Haemophilia A/B because:
- The amount of FVIII in FFP is low (~1 IU/mL) → to achieve meaningful FVIII levels, enormous volumes would be needed → fluid overload
- Specific factor concentrates provide far more FVIII in far less volume
- FFP is reserved for situations where specific concentrates are unavailable, or for other concurrent coagulopathies
Cryoprecipitate: 1 unit contains fibrinogen (150–300 mg), Factor VIII (80–120 U), vWF (80–120 U) [16]
- Historically used as a source of FVIII before specific concentrates were available
- Indication for vWD if desmopressin or factor concentrate inappropriate [16]
- NOT first-line for Haemophilia A in developed settings — use specific FVIII concentrate instead
- May be used in resource-limited settings where FVIII concentrates are unavailable
Gene therapy represents the potential cure for Haemophilia A. The concept is elegant: deliver a functional copy of the F8 gene to the patient's liver cells so they can produce their own FVIII.
| Aspect | Detail |
|---|---|
| Vector | Adeno-associated virus (AAV) serotype 5 — liver-tropic; non-integrating (episomal) |
| Product | Valoctocogene roxaparvovec (Roctavian®) — AAV5 carrying a codon-optimised B-domain-deleted F8 transgene |
| Approval | EMA (2022), FDA (2023) for adults with severe Haemophilia A without inhibitors and without pre-existing AAV5 antibodies |
| Efficacy | Mean FVIII levels ~20–40% at 1–2 years post-infusion; significant reduction in bleeding events and factor use |
| Limitations | FVIII levels decline over time (durability uncertain — transgene is episomal, not integrated); cannot be re-dosed (immune response to AAV capsid); requires steroid courses for transaminitis; excluded if pre-existing AAV5 antibodies; liver toxicity concern; very expensive (~$2.5M USD per dose) |
| Hong Kong status | Not yet in routine use but under evaluation; patients may access through clinical trials or named-patient import |
These therapies rebalance haemostasis by targeting natural anticoagulant pathways rather than replacing the missing factor:
| Agent | Mechanism | Status |
|---|---|---|
| Fitusiran (anti-antithrombin siRNA) | Small interfering RNA that knocks down antithrombin production in the liver → reduces anticoagulant tone → tips the haemostatic balance toward coagulation | Approved (FDA 2024) for prophylaxis in Haemophilia A and B ± inhibitors; monthly SC injection |
| Concizumab (anti-TFPI mAb) | Monoclonal antibody against tissue factor pathway inhibitor → removes inhibition of TF-FVIIa complex and FXa → enhances coagulation initiation | Approved (EMA 2024) for prophylaxis in Haemophilia A/B with inhibitors; daily SC injection |
| Marstacimab (anti-TFPI mAb) | Similar mechanism to concizumab | Phase III trials |
Why are these exciting? Because they work regardless of whether Factor VIII or IX is the missing factor, and regardless of inhibitor status — they are "factor-mimetic independent."
Treatment of haemophilia-related bleeding: factor replacement as above plus [2]:
| Scenario | Factor Target | Additional Measures |
|---|---|---|
| Severe, life-threatening bleeding | Immediate replacement (treat upon suspicion, not diagnosis) [2] — target FVIII 80–100% | ICU admission, imaging, surgical consult as needed |
| Haemarthrosis | 30–50% | Analgesics (COX-2, Panadol → avoid NSAIDs), RICE [2]; early physiotherapy once acute phase resolves |
| Muscle haematoma | 50–80% | Consider surgical decompression if failed medical therapy [2]; monitor for compartment syndrome |
| Haematuria | 30–50% | Treat with forced diuresis if not severe [2]; do NOT use antifibrinolytics |
| Mucosal bleeding | 30–50% | Add antifibrinolytic agents, e.g. tranexamic acid, aminocaproic acid [2] |
| ICH | 80–100% immediately, maintain > 50% for 14+ days | Treat empirically upon suspicion before imaging; neurosurgical consult |
| Peri-operative | 80–100% pre-op, then 50–80% until wound healing | Plan with haemophilia centre; daily FVIII monitoring; antifibrinolytics for oral surgery |
Critical Rule: Treat Life-Threatening Bleeds on Suspicion, Not Diagnosis
Severe, life-threatening bleeding: immediate replacement — treat upon suspicion, not diagnosis [2]. If a haemophilia patient presents with headache after minor head trauma, infuse Factor VIII immediately before CT scan results are available. Waiting for imaging confirmation of ICH wastes precious time during which irreversible brain damage occurs.
Inhibitors in haemophilia: usually presents as poor response to factor replacement [2]
| Component | Detail |
|---|---|
| Occurrence | 20–30% in Haemophilia A, 1.5–3% in Haemophilia B [2] |
| Risk factors | Generally highest in those with severe disease during first 50 days of exposure to factor [2] |
| Detection | Require regular screening, in pre-operative setting and when response suboptimal ( < 75%) [2]; Bethesda assay: positive if ≥ 0.6 BU/mL, high titre if ≥ 5 BU/mL [2] |
| Management of acute bleeding | Alternative products: recombinant porcine Factor VIII; Bypassing products e.g. recombinant Factor VIIa, aPCC (e.g. FEIBA); Plasmapheresis + transient use of factor replacement to decrease inhibitor titre; High-dose factor infusion for low-titre patients [2] |
| Prophylaxis | Emicizumab, DDAVP for prophylaxis in Haemophilia A [2] |
| Curative | Immune tolerance induction (ITI) → curative treatment to eradicate inhibitor; Method: repeated doses of factor (± immunosuppression) to induce immune tolerance [2] |
Prognosis: life expectancy 63 years for severe disease, 75 years for mild/moderate disease [2]
Cause of mortality: 33% liver failure, 15% (severe) / 11% (mild) haemorrhage [2]
- The high proportion of liver failure deaths reflects the legacy of HCV infection from contaminated plasma-derived products
- With current recombinant products, viral inactivation techniques, and emicizumab/gene therapy, life expectancy is expected to improve significantly for the current generation
High Yield Summary — Management of Haemophilia A
- General measures: Avoid trauma, non-contact sports, dental hygiene, avoid antiplatelets/NSAIDs, smallest gauge needles, vaccination, genetic counselling
- DDAVP: First-line for mild Haemophilia A (47% responsive); works by releasing stored FVIII from endothelial Weibel-Palade bodies; requires test dose; tachyphylaxis after 2–3 doses; contraindicated in severe haemophilia and children < 2 years
- Factor VIII replacement: Cornerstone of moderate/severe disease; recombinant preferred over plasma-derived; 1 IU/kg raises FVIII by 2%
- Prophylaxis: Primary (before age 3), secondary (after ≥ 2 joint bleeds), tertiary (after arthropathy) — gold standard for severe disease
- Emicizumab: Bispecific antibody mimicking FVIIIa; subcutaneous; not neutralised by FVIII inhibitors; approved for prophylaxis ± inhibitors
- Bypassing agents (rFVIIa, aPCC/FEIBA): For high-titre inhibitor patients with acute bleeds
- ITI: Curative approach to eradicate inhibitors — high-dose FVIII ± immunosuppression; 60–80% success
- Antifibrinolytics (tranexamic acid): Adjunct for mucosal bleeding; contraindicated in haematuria
- Treat life-threatening bleeds empirically on suspicion — do not wait for imaging
- Gene therapy (valoctocogene roxaparvovec): Approved 2022–2023; AAV5-delivered F8 transgene; achieves ~20–40% FVIII; durability uncertain
- FFP is NOT first-line for Haemophilia A — insufficient FVIII concentration; volume overload risk
Active Recall - Haemophilia A Management
References
[2] Senior notes: Ryan Ho Haemtology — Section 4.3.2 Haemophilia A and B, Management principles (p.125–127) [4] Senior notes: Block A - Abnormal bleeding after tooth extraction — Treatment options in haemostatic disorders [5] Senior notes: Block A - Hematology Interactive Tutorial — Case 2, Management [8] Lecture slides: GC 075. Pain red joint — Haemarthrosis treatment [15] Senior notes: Adrian Lui Pediatrics Notes — Haemophilia management principles (p.392) [16] Senior notes: Ryan Ho Haemtology — FFP, Cryoprecipitate, PCC (p.144)
Complications of Haemophilia A
The complications of Haemophilia A can be systematically divided into those arising from the disease itself (i.e. consequences of recurrent bleeding) and those arising from treatment (i.e. consequences of factor replacement and blood product exposure). Understanding the "why" behind each complication reinforces the underlying pathophysiology and helps you anticipate, prevent, and manage them.
I. Complications of the Disease Itself (Bleeding-Related)
1. Haemophilic Arthropathy — The Defining Long-Term Complication
Complications: haemophilic arthropathy [5]
Haemophilic arthropathy: occurs in up to 50% in severe haemophilia [2][15]
This is the single most important chronic complication and the primary driver of morbidity and disability in haemophilia. It deserves detailed mechanistic understanding.
- Acute haemarthrosis: Deficient FVIII → fragile clots in highly vascular synovial membrane → blood accumulates in joint space
- Iron deposition: Haemoglobin from extravasated red cells is degraded → iron (haemosiderin) deposited in synovial tissue and cartilage
- Iron-mediated oxidative damage: Iron catalyses the Fenton reaction (Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + OH•) → generates reactive oxygen species (ROS) → direct cytotoxicity to synoviocytes and chondrocytes
- Proliferative synovitis: Damaged synovium responds with hypertrophy and neovascularisation — the thickened, hypervascular synovium is even more prone to bleeding
- Target joint: a prior bleed results in joint damage and inflammation → predisposes to further bleeding [2][15] — this is the critical vicious cycle that defines the concept of a "target joint" (defined as ≥ 3 spontaneous bleeds into the same joint within a consecutive 6-month period)
- Cartilage degradation: Pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) activated by iron and synovial inflammation → upregulation of matrix metalloproteinases (MMPs) → enzymatic destruction of the collagen-proteoglycan cartilage matrix
- Secondary osteoarthritis: Pathology: multifactorial due to synovial hypertrophy, cartilage destruction, secondary OA [2][15] → subchondral bone remodelling, osteophyte formation, joint space loss
Presentation: typically as joint pain, stiffness, contractures developing in adolescence [2][15]
| Feature | Mechanism |
|---|---|
| Chronic joint pain | Cartilage loss → bone-on-bone contact; chronic synovitis with ongoing low-grade inflammation |
| Joint stiffness | Synovial fibrosis and capsular thickening restrict joint motion |
| Flexion contractures | Chronic haemarthrosis → joint held in flexion (position of maximum volume/minimum pain) → if sustained, periarticular soft tissues and joint capsule shorten permanently |
| Muscle wasting | Disuse atrophy around painful joints; also direct compression of motor nerves by adjacent haematomas |
| Limb-length discrepancy | In growing children, chronic synovial inflammation stimulates adjacent epiphyseal growth plates → overgrowth of the affected limb |
| Joint deformity | Late-stage — valgus/varus deformity, subluxation |
Most commonly major weight-bearing joints — ankles, knees (45%), elbows [2][15]
Why these joints? They experience the greatest repetitive mechanical stress during daily activities. Even normal movement creates shear forces on the thin-walled synovial capillaries, and without adequate fibrin reinforcement, these capillaries bleed repeatedly.
- Prevention is the single most important strategy — primary prophylaxis starting before age 3 dramatically reduces arthropathy incidence (Swedish prophylaxis data: near-normal joints at age 20 with prophylaxis vs devastating arthropathy without)
- Physiotherapy: Maintain range of motion and muscle strength; gentle range-of-motion exercises after acute bleed resolves
- Synovectomy: For chronic synovitis with recurrent bleeds into a target joint despite adequate factor prophylaxis
- Radiosynovectomy (intra-articular injection of radioactive isotope, e.g. yttrium-90) — destroys hypertrophied synovium; less invasive, preferred first-line
- Arthroscopic synovectomy — surgical removal under factor cover; for refractory cases
- Joint replacement: For end-stage arthropathy with severe pain and disability; requires intensive factor coverage peri-operatively
The Target Joint Concept — Exam Favourite
A "target joint" is defined as a joint that has had ≥ 3 spontaneous bleeds within 6 months. Once established, the vicious cycle (bleed → synovitis → neovascularisation → more bleeds) is self-perpetuating. Breaking this cycle with intensified prophylaxis or synovectomy is critical to preventing arthropathy.
↑risk of cephalhaematoma and ICH during delivery [2][15]
ICH is the leading cause of haemophilia-related death in the current era, accounting for a significant proportion of haemorrhage-related mortality.
| Aspect | Detail |
|---|---|
| Incidence | 2–8% lifetime risk; much higher in neonates during delivery |
| Types | Subdural, epidural, subarachnoid, intraparenchymal |
| Neonatal risk | ↑risk of cephalhaematoma and ICH during delivery [2][15]; insufficient evidence to recommend routine caesarean section [2][15] — but should avoid head trauma + brain imaging ≤ 24 hours of life [2][15] for suspected affected neonates |
| Mechanism | Cerebral vessels are thin-walled and subjected to arterial pressures. Even minor trauma (or spontaneous bleeding in severe disease) leads to haemorrhage that cannot be contained due to inadequate fibrin formation |
| Presentation | Headache, vomiting, altered consciousness, focal neurological deficits, seizures — in neonates: irritability, bulging fontanelle, poor feeding, seizures |
| Management | Severe, life-threatening bleeding: immediate replacement — treat upon suspicion, not diagnosis [2]; target FVIII 80–100% immediately; maintain > 50% for ≥ 14 days; urgent CT brain; neurosurgical consultation |
| Outcome | Mortality ~20% even with treatment; significant neurological morbidity in survivors (cognitive impairment, epilepsy, motor deficits) |
Why is it so dangerous? The skull is a closed box — expanding haematoma rapidly raises intracranial pressure (ICP), compresses brain tissue, and can cause transtentorial herniation within hours. The standard platelet plug cannot withstand arterial pressure without fibrin reinforcement.
Compartment syndrome: can occur in acute bleeding into calf muscles → ischaemia, necrosis, fibrosis → subsequent contraction of Achilles tendon [2][15]
Pathophysiology in detail:
- Acute haematoma expands within the closed fascial compartment of the calf (or forearm, less commonly)
- Compartment pressure rises → exceeds capillary perfusion pressure → ischaemia of muscles and nerves within the compartment
- Muscle ischaemia → necrosis → replacement by fibrosis (Volkmann's ischaemic contracture equivalent)
- Fibrotic muscle shortens → subsequent contraction of Achilles tendon → equinus deformity of the foot
- Nerve ischaemia → permanent sensory/motor deficit (e.g. peroneal nerve palsy → foot drop)
Management: Factor replacement to target FVIII 80–100% + surgical fasciotomy if pressure > 30 mmHg or clinical signs progress despite factor therapy [2]
Femoral nerve compression in large psoas bleed [2][15]
The iliopsoas muscle lies in the retroperitoneum, adjacent to the femoral nerve (L2–L4). A large haematoma within the muscle compresses the nerve, causing:
- Hip flexion (antalgic posture — the hip is held flexed to reduce stretch on the psoas and nerve)
- Groin pain and anterior thigh pain (femoral nerve sensory territory)
- Weakness of knee extension (femoral nerve → quadriceps)
- Loss of knee jerk (L3–L4 reflex arc disrupted)
- Anterior thigh numbness (femoral nerve sensory distribution)
This can mimic appendicitis (right-sided), renal colic, or hip pathology — a high index of suspicion is needed in any haemophilia patient with groin/hip pain.
Haemophilic pseudotumour: large encapsulated haematoma [2][15]
- Site: large muscle groups in pelvis/lower limbs and bone (long bone, pelvis, cranium) [2][15]
- Pathology: repeated bleeding with bone involvement → progressive cystic swelling with bone turnover and new bone formation [2][15]
Why does this happen? Recurrent haemorrhages into the same area are not fully resorbed. The body walls off the haematoma with a fibrous capsule. Over time, the encapsulated haematoma enlarges with each new bleed, eroding into adjacent bone through pressure necrosis. The bone responds with reactive new bone formation (periosteal reaction), creating a "tumour-like" mass.
- Can grow to massive size (> 10 cm)
- On imaging (XR/CT), mimics a bone tumour (expansile lytic lesion with periosteal reaction) → biopsy should be avoided (risk of uncontrollable haemorrhage)
- Management: Factor cover + surgical excision if symptomatic; radiotherapy has been used for inoperable cases
Oropharyngeal bleeding: may occur with minor trauma or dental procedures [2][15] Note that cough/vomiting can induce bleeding into posterior pharynx → aspiration or upper airway obstruction [2][15]
This is a life-threatening emergency because:
- The posterior pharynx is a shared space for the airway and oesophagus
- An expanding haematoma can compress the airway → stridor, dyspnoea, asphyxiation
- Blood can be aspirated into the lungs → aspiration pneumonitis, hypoxia
- Management: Immediate FVIII replacement, secure airway (intubation may be needed), ENT consultation
GI bleeding and bowel wall haematoma → obstruction, risk of intussusception [2][15]
- GI bleeding: Can occur from any site; upper GI bleed presents with haematemesis/melaena; lower GI with haematochezia
- Bowel wall haematoma: Bleeding into the wall of the small bowel causes swelling → luminal narrowing → mechanical obstruction (presents with colicky abdominal pain, vomiting, obstipation)
- Intussusception: A bowel wall haematoma can act as a "lead point" — the haematoma-bearing segment telescopes into the adjacent bowel → intussusception → ischaemia → necrosis if not reduced
Haematuria: common in severe haemophilia but not associated with ↓ renal function [2][15]
- Usually painless gross haematuria arising from renal papillary vessels
- Generally self-limiting and benign — renal function is preserved long-term
- Do NOT use antifibrinolytics (tranexamic acid) — clots in the urinary tract cannot be lysed → ureteric/urethral obstruction → hydronephrosis
- Manage with forced diuresis (generous hydration) + factor replacement if persistent
II. Complications of Treatment
1. Inhibitor Development — The Most Feared Treatment Complication
Inhibitor development: develop alloantibodies vs exogenous factor due to congenital lack [2][15]
This is the single most important treatment complication because it renders standard factor replacement therapy ineffective.
- In severe haemophilia A, the patient produces zero endogenous FVIII (null mutation)
- When exogenous recombinant FVIII is infused, the immune system encounters a protein it has never seen before
- CD4⁺ T-helper cells recognise FVIII epitopes presented on MHC class II → activate B cells → production of IgG alloantibodies specific to FVIII
- These neutralising antibodies bind to functional domains of FVIII (especially the A2 and C2 domains) → prevent FVIII from functioning as a cofactor in the tenase complex
- Net result: infused FVIII is rapidly neutralised → no haemostatic benefit → bleeding continues despite treatment
| Factor | Explanation |
|---|---|
| Severe haemophilia (highest risk) | Null mutations → no endogenous FVIII → immune system has no tolerance to FVIII |
| Mutation type | Large deletions, intron 22 inversions, nonsense mutations → higher risk than missense mutations (some residual FVIII provides partial immune tolerance) |
| Family history of inhibitors | Genetic predisposition in HLA and immune-regulatory genes |
| African descent | Higher incidence — likely multifactorial (genetic, socioeconomic) |
| Intensive early treatment | High-dose FVIII given during first 50 exposure days (e.g. for surgery) may trigger immune response |
| Type of product | Debated — some evidence that plasma-derived FVIII (containing vWF) has lower inhibitor risk than recombinant FVIII, possibly because vWF shields immunogenic FVIII epitopes (SIPPET trial) |
- Signs/symptoms: ↑risk of bleeding due to ↓response to factor infusion, anaphylaxis in some individuals [2][15]
- Clinically: Factor replacement that previously controlled bleeds no longer works → bleeds become more frequent, more severe, and harder to stop
- Laboratory: FVIII recovery after infusion is lower than expected; FVIII half-life is shortened
- Acute bleeds: bypassing agents (rFVIIa, aPCC/FEIBA), recombinant porcine FVIII
- Prophylaxis: emicizumab
- Curative: immune tolerance induction (ITI)
Why Haemophilia B Has Lower Inhibitor Rates than Haemophilia A
Factor IX is a smaller, simpler protein than Factor VIII. Additionally, many Haemophilia B patients have missense mutations that allow production of some (dysfunctional) FIX protein — providing partial immune tolerance. In contrast, ~45% of severe Haemophilia A patients have intron 22 inversions that completely abolish FVIII production. However, when Haemophilia B patients DO develop inhibitors, they are more likely to have anaphylactic reactions to FIX infusion (possibly because the immune response is more broadly directed against the completely absent protein).
Bloodborne infections, e.g. HIV, HBV, HCV → ↓risk with use of recombinant factors [2][15]
This is primarily a historical complication from the era before effective viral screening and inactivation of plasma-derived products (pre-1985 for HIV, pre-1990s for HCV).
| Infection | Historical Context | Current Status |
|---|---|---|
| HIV | Devastating epidemic in the 1980s — up to 60–70% of severe haemophilia patients in some countries were infected through contaminated factor concentrates | Virtually eliminated with recombinant products and nucleic acid testing (NAT) of donor plasma |
| HCV | Most common chronic infection in haemophilia patients; up to 80% of those treated with plasma-derived products before 1990 were infected | Curable with direct-acting antivirals (DAAs) — all haemophilia patients with HCV should be treated. In HK, an older cohort of haemophilia patients remain HCV-positive |
| HBV | Less common than HCV but significant historically | All haemophilia patients should receive Hepatitis B vaccination; current products virtually risk-free |
| Parvovirus B19 | Small non-enveloped virus that resists standard viral inactivation methods (solvent-detergent) | Can still theoretically occur with plasma-derived products; causes transient aplastic crisis in patients with chronic haemolytic states |
| Prion diseases (vCJD) | Theoretical risk with plasma-derived products | No proven cases in haemophilia; nanofiltration provides some protection |
Why were haemophilia patients so vulnerable? Because plasma-derived FVIII concentrates were made from pooled plasma from thousands of donors — a single infected donor could contaminate the entire batch. Before viral inactivation techniques were introduced, this was an enormous risk.
Cause of mortality: 33% liver failure [2] — this figure reflects the legacy of HCV-related cirrhosis in the haemophilia population. HCV causes progressive liver fibrosis over decades → cirrhosis → hepatocellular carcinoma and/or liver failure.
In patients receiving repeated red cell transfusions (not factor concentrates — factor concentrates do not contain red cells) for chronic anaemia secondary to recurrent bleeding:
- Each unit of packed red cells contains ~200–250 mg of iron [5]
- The human body has no physiological mechanism for active iron excretion
- Repeated transfusions → progressive iron accumulation → transfusional haemosiderosis
| Organ | Consequence of Iron Overload | Mechanism |
|---|---|---|
| Heart | Dilated cardiomyopathy, arrhythmias, heart failure | Iron deposits in myocytes → ROS generation → myocyte damage and fibrosis |
| Liver | Hepatic fibrosis → cirrhosis | Iron in hepatocytes and Kupffer cells → chronic inflammation → fibrosis |
| Endocrine | Diabetes mellitus, hypogonadism, hypothyroidism, growth failure | Iron deposits in pancreatic β-cells, pituitary, thyroid → glandular destruction |
| Skin | Bronze hyperpigmentation | Iron + melanin deposition in dermis |
- Management: Iron chelation therapy — Desferrioxamine (DFO), deferiprone (DFP), and deferasirox [5]
- These agents bind free iron and promote excretion via urine and faeces
- Monitoring: Serum ferritin (screening), cardiac MRI T2* (gold standard for cardiac iron), liver MRI R2/T2* (liver iron concentration)
- This complication is more relevant to thalassaemia patients on chronic transfusion programmes, but can occur in haemophilia patients with severe recurrent bleeds requiring repeated transfusion
Many children with severe haemophilia require central venous access devices (e.g. Port-a-Cath, Hickman line) for regular IV factor infusions, especially before they develop adequate peripheral venous access.
| Complication | Mechanism |
|---|---|
| Catheter-related bloodstream infection (CRBSI) | Bacterial colonisation of the catheter (especially coagulase-negative staphylococci, S. aureus) → bacteraemia → sepsis |
| Catheter-related thrombosis | Foreign body in the vein → endothelial activation → fibrin sheath formation → thrombosis (paradoxical, given the bleeding disorder — but the foreign body provides a potent thrombotic stimulus) |
| Mechanical complications | Catheter fracture, migration, occlusion |
These complications are part of why subcutaneous emicizumab and extended half-life products (requiring less frequent IV infusions) have been such game-changers — they reduce or eliminate the need for CVCs.
Often overlooked but profoundly important:
| Issue | Detail |
|---|---|
| Chronic pain | From arthropathy — can be severe and debilitating; risk of opioid dependence |
| Physical disability | Joint contractures, limb-length discrepancy, limited mobility → loss of independence |
| School/work absenteeism | Recurrent bleeds, hospitalisations, clinic visits → disrupted education and career |
| Psychological burden | Anxiety (fear of bleeding), depression (chronic pain, disability), social isolation |
| Financial burden | Factor concentrates are extremely expensive; recurrent hospitalisations; loss of productive working days |
| Relationship and reproductive concerns | Carrier status implications for female relatives; genetic counselling; family planning decisions |
| Category | Complication | Frequency/Severity | Key Pathophysiology |
|---|---|---|---|
| Musculoskeletal | Haemophilic arthropathy | Up to 50% in severe | Iron toxicity → synovitis → neovascularisation → target joint → cartilage destruction → secondary OA |
| Musculoskeletal | Compartment syndrome | Uncommon but serious | Haematoma in closed fascial compartment → ↑pressure → ischaemia → necrosis → fibrosis |
| Musculoskeletal | Haemophilic pseudotumour | Rare | Encapsulated chronic haematoma → bone erosion → expansile mass |
| Neurological | ICH | 2–8% lifetime; leading cause of death | Inadequate fibrin → uncontrolled cerebral haemorrhage → ↑ICP |
| Neurological | Femoral nerve palsy | With psoas haematoma | Nerve compression by expanding haematoma |
| GI | GI bleeding, bowel wall haematoma | Uncommon | Intramural bleeding → obstruction, intussusception |
| Airway | Oropharyngeal haemorrhage | Emergency | Posterior pharyngeal haematoma → airway compromise |
| Renal | Haematuria | Common in severe; benign | Renal papillary bleeding; renal function preserved |
| Treatment | Inhibitor development | ~30% in severe Haem A | Alloantibodies against exogenous FVIII |
| Treatment | Bloodborne infections | Historical; reduced with recombinant products | HIV, HCV, HBV from contaminated plasma-derived products |
| Treatment | Iron overload | With chronic transfusion | No excretory mechanism; iron accumulates in heart, liver, endocrine |
| Treatment | CVC-related | With indwelling venous catheters | Infection, thrombosis, mechanical failure |
| Psychosocial | Chronic pain, disability, depression | Very common | Multifactorial — pain, disability, social, financial |
Prognosis: life expectancy 63 years for severe disease, 75 years for mild/moderate disease [2]
Cause of mortality: 33% liver failure, 15% (severe) / 11% (mild) haemorrhage [2]
| Cause of Death | Proportion | Comment |
|---|---|---|
| Liver failure | 33% | Reflects legacy of HCV cirrhosis from contaminated plasma-derived products |
| Haemorrhage | 15% (severe) / 11% (mild) | Primarily ICH; also GI and retroperitoneal haemorrhage |
| Other | Includes cardiovascular disease, malignancy, other infections | As life expectancy improves, haemophilia patients increasingly die from same causes as the general population |
With modern comprehensive care (prophylaxis, emicizumab, gene therapy, DAAs for HCV), life expectancy is expected to approach near-normal for the current generation of patients.
High Yield Summary — Complications of Haemophilia A
- Haemophilic arthropathy: Most important chronic complication; up to 50% of severe patients; iron-mediated oxidative damage → synovial hypertrophy → neovascularisation → "target joint" vicious cycle → cartilage destruction → secondary OA → contractures in adolescence
- ICH: Leading cause of haemophilia-related death; treat empirically on suspicion; target FVIII 80–100%
- Compartment syndrome: Calf haematoma → closed compartment → ischaemia → necrosis → fibrosis → Achilles tendon contracture
- Femoral nerve palsy: Large psoas haematoma compresses L2–L4 nerve → hip flexion, anterior thigh pain/numbness, loss of knee jerk, weak knee extension
- Haemophilic pseudotumour: Encapsulated chronic haematoma eroding bone; mimics bone tumour on imaging
- Oropharyngeal haemorrhage: Cough/vomiting can trigger → airway obstruction or aspiration — life-threatening
- GI complications: Bowel wall haematoma → obstruction or intussusception
- Inhibitor development: ~30% of severe Haemophilia A; alloantibodies neutralise infused FVIII; highest risk during first 50 exposure days; detected by Bethesda assay; managed with bypassing agents, emicizumab, ITI
- Bloodborne infections: Historical — HIV, HCV, HBV from plasma-derived products; HCV-related liver failure accounts for 33% of haemophilia deaths
- Prognosis: Life expectancy 63y (severe) / 75y (mild/moderate); improving with modern therapies
Active Recall - Complications of Haemophilia A
References
[2] Senior notes: Ryan Ho Haemtology — Section 4.3.2 Haemophilia A and B, Clinical presentation, Late complications, Inhibitors, Prognosis (p.124–127) [5] Senior notes: Block A - Hematology Interactive Tutorial — Case 2, Complications and iron overload discussion [15] Senior notes: Adrian Lui Pediatrics Notes — Haemophilia clinical presentation and late complications (p.391)
High Yield Summary
- Haemophilia A = X-linked recessive deficiency of Factor VIII (Xq28); accounts for 85% of all haemophilia
- Incidence: 1 in 4,000–5,000 live male births; ~55% of severe cases are sporadic (no family history)
- Pathophysiology: Defective intrinsic tenase complex (FVIIIa–FIXa) → inadequate thrombin burst → insufficient fibrin formation → friable clot → delayed rebleeding
- Primary haemostasis is INTACT → no petechiae; PT normal; bleeding time normal
- Isolated prolonged APTT with normal PT is the hallmark lab finding
- Severity classification: Severe ( < 1%), Moderate (1–5%), Mild (5–40%) — based on residual factor level
- Clinical pattern: Deep-seated bleeding — haemarthrosis (80%, weight-bearing joints), muscle haematomas (calf, psoas), delayed post-surgical/post-dental bleeding, ICH
- Haemophilic arthropathy: Iron toxicity → synovial hypertrophy → neovascularization → "target joint" → cartilage destruction → secondary OA → contractures
- Distinguished from vWD by normal vWF:Ag and vWF:RCo with low Factor VIII
- Mixing study: Corrects in deficiency (haemophilia A/vWD); does NOT correct with inhibitors (lupus anticoagulant = immediate; acquired haemophilia = delayed)
- Acquired haemophilia A: Autoantibodies to FVIII; elderly, post-partum, autoimmune, malignancy; presents with retroperitoneal haematoma, massive soft tissue bleeding
- Inhibitor development: Occurs in ~25–30% of severe haemophilia A; alloantibodies against infused exogenous Factor VIII; measured in Bethesda Units
High Yield Summary — Differential Diagnosis of Haemophilia A
- Isolated prolonged APTT (normal PT) narrows the DDx to: Haemophilia A, Haemophilia B, Haemophilia C, Factor XII deficiency, vWD, heparin, lupus anticoagulant, acquired factor inhibitors
- Mixing study is the key next step: Correction → deficiency; Immediate non-correction → lupus anticoagulant; Delayed non-correction → acquired inhibitor
- Specific factor assays + vWF studies differentiate Haemophilia A (↓FVIII, normal vWF) from vWD (↓FVIII + ↓vWF) and Haemophilia B (normal FVIII, ↓FIX)
- Haemophilia A vs B are clinically indistinguishable — must measure factor levels
- Factor XII deficiency prolongs APTT but causes NO clinical bleeding — do not transfuse
- Lupus anticoagulant prolongs APTT but causes thrombosis, not bleeding — confirm with DRVVT
- Haemarthrosis DDx (GC 075): Trauma, Haemophilia, Acquired haemophilia, Drugs (warfarin/heparin)
- Deep-seated bleeding pattern = coagulation factor disorder; Mucocutaneous pattern = platelet/vWF disorder
- Acquired haemophilia A: Elderly, post-partum, autoimmune, malignancy; retroperitoneal haematoma; delayed non-correction on mixing study
- Acquired vWD in MPN with platelets > 1000: excess platelets consume vWF
High Yield Summary — Diagnosis of Haemophilia A
- Screening: CBC (normal) + Clotting profile (isolated ↑APTT with normal PT) — the single most important screening pattern
- APTT may be normal in mild haemophilia — do not be falsely reassured; proceed to factor assay if clinical suspicion is high
- Mixing study: Correction = deficiency (haemophilia, vWD); Immediate non-correction = lupus anticoagulant; Delayed non-correction = acquired inhibitor
- Factor VIII assay: < 40% confirms diagnosis; level determines severity (severe < 1%, moderate 1–5%, mild > 5%– < 40%)
- vWF:Ag and vWF:RCo must be normal to distinguish from vWD — the GC Interactive Tutorial Haem Case 2 tests exactly this
- Beware vWD Type 2N — mimics haemophilia A perfectly on standard tests (normal vWF:Ag, normal vWF:RCo, low FVIII); need vWF:FVIII binding assay to differentiate
- Inhibitor screening (Bethesda assay): Essential in all patients; occurs in ~30% of severe Haemophilia A; < 5 BU = low titre, ≥ 5 BU = high titre
- Genetic testing: Confirms diagnosis, enables carrier detection and prenatal diagnosis; intron 22 inversion is the most common severe mutation (~45%)
- Bone marrow examination is absolutely contraindicated in severe haemophilia without factor cover
- Family screening: Check APTT + Factor VIII in siblings; genetic testing for carrier detection in female relatives
High Yield Summary — Management of Haemophilia A
- General measures: Avoid trauma, non-contact sports, dental hygiene, avoid antiplatelets/NSAIDs, smallest gauge needles, vaccination, genetic counselling
- DDAVP: First-line for mild Haemophilia A (47% responsive); works by releasing stored FVIII from endothelial Weibel-Palade bodies; requires test dose; tachyphylaxis after 2–3 doses; contraindicated in severe haemophilia and children < 2 years
- Factor VIII replacement: Cornerstone of moderate/severe disease; recombinant preferred over plasma-derived; 1 IU/kg raises FVIII by 2%
- Prophylaxis: Primary (before age 3), secondary (after ≥ 2 joint bleeds), tertiary (after arthropathy) — gold standard for severe disease
- Emicizumab: Bispecific antibody mimicking FVIIIa; subcutaneous; not neutralised by FVIII inhibitors; approved for prophylaxis ± inhibitors
- Bypassing agents (rFVIIa, aPCC/FEIBA): For high-titre inhibitor patients with acute bleeds
- ITI: Curative approach to eradicate inhibitors — high-dose FVIII ± immunosuppression; 60–80% success
- Antifibrinolytics (tranexamic acid): Adjunct for mucosal bleeding; contraindicated in haematuria
- Treat life-threatening bleeds empirically on suspicion — do not wait for imaging
- Gene therapy (valoctocogene roxaparvovec): Approved 2022–2023; AAV5-delivered F8 transgene; achieves ~20–40% FVIII; durability uncertain
- FFP is NOT first-line for Haemophilia A — insufficient FVIII concentration; volume overload risk
High Yield Summary — Complications of Haemophilia A
- Haemophilic arthropathy: Most important chronic complication; up to 50% of severe patients; iron-mediated oxidative damage → synovial hypertrophy → neovascularisation → "target joint" vicious cycle → cartilage destruction → secondary OA → contractures in adolescence
- ICH: Leading cause of haemophilia-related death; treat empirically on suspicion; target FVIII 80–100%
- Compartment syndrome: Calf haematoma → closed compartment → ischaemia → necrosis → fibrosis → Achilles tendon contracture
- Femoral nerve palsy: Large psoas haematoma compresses L2–L4 nerve → hip flexion, anterior thigh pain/numbness, loss of knee jerk, weak knee extension
- Haemophilic pseudotumour: Encapsulated chronic haematoma eroding bone; mimics bone tumour on imaging
- Oropharyngeal haemorrhage: Cough/vomiting can trigger → airway obstruction or aspiration — life-threatening
- GI complications: Bowel wall haematoma → obstruction or intussusception
- Inhibitor development: ~30% of severe Haemophilia A; alloantibodies neutralise infused FVIII; highest risk during first 50 exposure days; detected by Bethesda assay; managed with bypassing agents, emicizumab, ITI
- Bloodborne infections: Historical — HIV, HCV, HBV from plasma-derived products; HCV-related liver failure accounts for 33% of haemophilia deaths
- Prognosis: Life expectancy 63y (severe) / 75y (mild/moderate); improving with modern therapies
Disseminated Intravascular Coagulation (DIC)
A life-threatening condition characterized by widespread activation of the coagulation cascade leading to diffuse microvascular thrombi formation with simultaneous consumption of clotting factors and platelets, resulting in paradoxical thrombosis and hemorrhage.
Immune Thrombocytopenia (ITP)
Immune thrombocytopenia is an autoimmune disorder characterized by antibody-mediated platelet destruction and impaired platelet production, resulting in isolated thrombocytopenia and increased bleeding risk.