Limb pain is an unpleasant sensory experience in the upper or lower extremities arising from musculoskeletal, vascular, neurological, or referred causes that warrants systematic evaluation to identify the underlying etiology.

Limb Pain

Epidemiology

Risk Factors

Risk Factor	Mechanism / Details
Smoking	Strongest RF, 3–6× risk for IC ^[1]. Endothelial damage → accelerated atherosclerosis, platelet activation, vasoconstriction
Diabetes mellitus	2× increased risk, 26% increased risk of PAD for every 1% increase in HbA1c ^[1]. Hyperglycaemia → advanced glycation end products (AGEs) → endothelial dysfunction + small vessel disease. Should be screened for PAD (by ABI) every 5 years ^[1]
Hypertension	Shear stress → intimal damage → atherosclerosis
Hyperlipidaemia	LDL oxidation → foam cell formation → plaque
Pre-existing arterial disease	Coronary artery disease, stroke/TIA, carotid disease ^[1] — these all share the same atherosclerotic pathology
Family history of vascular disease ^[1]	Genetic predisposition to dyslipidaemia, endothelial dysfunction
Age > 65 ^[2]	Cumulative vascular damage over time

Anatomy and Function

Understanding the anatomy is absolutely essential because the site of pathology determines the clinical presentation, and the obstruction is typically "one joint above the claudicating muscle" ^[1].

Key clinical correlations:

Arterial Segment Occluded	Claudication Site	Key Clinical Feature
Aortoiliac (30%)	Bilateral buttocks, thighs, calves	Impotence (LeRiche's syndrome) ^[1]
Iliac	Unilateral thigh, calf ± buttock	Absent femoral pulse on that side
Femoropopliteal (70% of PAD)	Unilateral calf	Commonest — 70% of PAD affects the superficial femoral artery ^[1]
Tibioperoneal	Unilateral calf/foot	Often seen in diabetics (preferentially affects distal small vessels)

LeRiche's Syndrome

Triad of LeRiche syndrome: (1) Buttock claudication, (2) Absent or diminished femoral pulses, (3) Erectile dysfunction ^[1]. This occurs because the aortic bifurcation is occluded, cutting off blood supply to both lower limbs AND the internal iliac arteries (which supply the pelvic organs including the corpus cavernosum via the internal pudendal artery).

Etiology (Focus on Hong Kong)

The aetiology of limb pain is vast. I'll organise it by system, with emphasis on what is common and clinically important in Hong Kong.

A. Vascular Causes

Definition: A sudden decrease in limb perfusion that threatens the viability of the limb, presenting within 2 weeks of the acute event ^[2]^[11]

Irreversible damage occurs 4–6 hours after onset if adequate collateral circulation is absent — skeletal muscles can only withstand up to 6 hours of ischaemia ^[2]. Patients with pre-existing peripheral vascular disease can tolerate longer due to establishment of collaterals ^[2].

Causes ^[2]^[11]:

Cause	Frequency	Details
Acute thrombotic occlusion of pre-existing stenotic arterial segment (acute-on-chronic PVD)	60% (most common)	Acute plaque rupture in a patient with background atherosclerotic PVD
Embolism	30%	Usually lodge at branching points — femoral bifurcation (most common), popliteal trifurcation (2nd most common) ^[11]
Arterial trauma	~10%	Iatrogenic (catheterisation), penetrating, blunt

Sources of emboli ^[11]:

Cardiac origin (80%) — atrial fibrillation (most common), LV mural thrombus post-MI, valvular heart disease/prosthetic valves
AAA (→ blue toe syndrome / trash foot)
Aortic dissection

Acute Limb + Chest Pain

Acute limb pain combined with chest pain should make you think of ^[11]:

MI → mural thrombus / new-onset AF → embolism
Aortic dissection → direct arterial occlusion This is an emergency combination — don't treat the limb in isolation!

Distinguishing embolism vs thrombosis-in-situ is critically important because the investigations and management differ ^[1]^[11]:

Feature	Embolism	Thrombosis-in-situ
Common causes	Cardiac origin (80%) — AF, MI (LV mural thrombus), VHD/prosthesis; AAA	Atherosclerosis (acute plaque rupture in PVD), aortic dissection, hypercoagulability (malignancy, APLS, sepsis), Buerger's disease
Severity	Complete ischaemia (no collaterals)	Incomplete ischaemia (collaterals)
Onset	Hyperacute (seconds to minutes)	Acute (hours or days)
History and PE	Embolic source identifiable (e.g. AF); contralateral limb pulses present; absent bruits	Previous claudication; PVD in contralateral limb (absent pulse); present bruits
Angiography	Minimal atherosclerosis; sharp cut-off with few collaterals	Diffuse atherosclerosis; irregular cut-off with well-developed collaterals
Management	Embolectomy, anticoagulation	Medical, bypass, thrombolysis

B. Neurological Causes

This is a critical differential for vascular claudication:

Feature	Vascular claudication	Neurogenic claudication
Cause	Chronic arterial insufficiency → exercise-induced muscle ischaemia	Prolapsed IVD or OA spine → spinal stenosis → compression on spinal arteries → lumbosacral root ischaemia
Nature	LL discomfort on exertion	LL discomfort on exertion
Precipitation	Claudication distance constant	Claudication distance variable
Relief	Shop window to shop window (relief upon standing still)	Park bench to park bench (relief upon flexion of spine)
Going downstairs	—	Going downstairs > upstairs
Pain at rest	None	May be present (prefer to stand in slight flexion)
Other complaints	Nil	Paraesthesia, numbness, weakness

C. Musculoskeletal Causes

Classification

Fontaine Stage	Description
Stage I	Asymptomatic
Stage IIa	Intermittent claudication, claudication distance > 200 m
Stage IIb	Intermittent claudication, claudication distance < 200 m
Stage III	Ischaemic rest pain / nocturnal pain
Stage IV	Ulceration (necrosis/tissue death) or gangrene

Stages III and IV = Critical limb ischaemia (CLI) — limb viability is threatened.

Rutherford Grade	Category	Description
0	0	Asymptomatic
I	1	Mild claudication
I	2	Moderate claudication
I	3	Severe claudication
II	4	Ischaemic rest pain
III	5	Minor tissue loss
III	6	Major tissue loss

Category	Sensory loss	Muscle weakness	Arterial signal	Venous signal	Treatment
Viable (I)	None	None	Audible	Audible	Imaging
Marginally threatened (IIa)	Minimal (toes)	None	Inaudible	Audible	Urgent revascularisation
Immediately threatened (IIb)	Beyond toes	Partial	Inaudible	Audible	Emergency revascularisation
Non-viable (III)	Completely anaesthetic	Completely paralysed	Inaudible	Inaudible	Amputation

Understanding the Rutherford Acute Ischaemia Classification

The logic is simple: nerves are the most sensitive structure to ischaemia (pain/paraesthesia appear first), then muscles (weakness/paralysis appear later). As ischaemia worsens, you lose Doppler signals — first arterial (because the arterial flow is blocked), then venous (because there is no inflow to generate outflow). Once both arterial AND venous signals are inaudible and the limb is completely anaesthetic and paralysed = non-viable = amputation territory.

Pattern	Examples
Hyperacute (seconds–minutes)	Arterial embolism, fracture, compartment syndrome
Acute (hours–days)	Arterial thrombosis, DVT, septic arthritis, gout
Subacute (days–weeks)	Cellulitis, osteomyelitis, inflammatory arthritis
Chronic (weeks–months)	PAD/claudication, OA, neuropathy, CVI

Clinical Features

A. Symptoms

B. Signs

Inspection:

Trophic changes: hair loss, shiny thin skin, thickened dystrophic nails, muscle wasting — all due to chronic tissue hypoperfusion
Colour changes: pallor (ischaemia), cyanosis (deoxygenated blood in sluggish capillary beds)
Tissue loss: non-healing ulcers (typically at pressure points — toes, heel, lateral malleolus), gangrene (dry = mummified dead tissue; wet = infected dead tissue)
Dependent rubor: when the leg is dependent, gravity pools deoxygenated blood in maximally dilated (due to chronic ischaemia) capillary beds → the foot looks deceptively red/purple

Palpation:

Temperature: cool/cold limb compared to contralateral side
Pulses: diminished or absent at and distal to the level of obstruction
- Femoral, popliteal, dorsalis pedis, posterior tibial
Capillary refill time: prolonged ( > 2 seconds)

Special tests:

Buerger's test: lift both legs slowly for pallor (Buerger's angle) → swing legs down for reactive hyperaemia ^[2]
- Buerger's angle: the angle at which the elevated leg becomes pale. Normal > 90°. In severe ischaemia it can be as low as 20°. The lower the angle, the worse the disease.
- Reactive hyperaemia: on lowering the leg, blood rushes into dilated capillary beds → red/purple colour. The time to refill and the intensity of redness correlates with severity.
- Why does this work? Elevating the limb removes the gravitational assist. In a normal limb, arterial pressure is more than enough to perfuse the foot even when raised. In a severely ischaemic limb, the already low perfusion pressure cannot overcome gravity → pallor.
Bedside Doppler for pulses, measure ABI on both sides ^[2]
Examine abdomen for AAA / RAS ^[2]
Cardiovascular examination for other CV risk factors (e.g. BP/P, AF) ^[2]
Urine multistix for glucose ^[2]

Ankle-Brachial Pressure Index (ABPI/ABI) ^[2]:

To confirm diagnosis and quantify severity
ABI = highest ankle systolic pressure ÷ highest brachial systolic pressure
Interpretation:

ABI	Interpretation
> 1.3	Non-compressible (calcified) — common in diabetics. Unreliable.
0.9–1.3	Normal
0.5–0.9	Intermittent claudication
< 0.5	Critical limb ischaemia
< 0.3	Severe, rest pain/tissue loss likely

ABI in Diabetics

In diabetic patients, medial arterial calcification (Mönckeberg sclerosis) makes arteries non-compressible → falsely elevated ABI ( > 1.3). In these patients, use toe-brachial index (TBI) instead, because digital arteries are less prone to calcification. A TBI < 0.7 suggests PAD.

Clinical Feature	Pathophysiological Mechanism
Intermittent claudication	Exercise-induced demand exceeds supply through stenotic artery → anaerobic metabolism → lactic acid → muscle pain
Rest pain	Basal metabolic demand of skin/subcutaneous tissue not met → tissue ischaemia at rest
Rest pain relieved by dependency	Gravity augments arterial perfusion pressure to foot
Pallor (acute ischaemia)	No arterial inflow → empty capillary bed
Fixed mottling	Capillary bed thrombosis → irreversible tissue death
Dependent rubor	Chronic ischaemia → maximal capillary dilatation → pooling of deoxygenated blood when dependent
Hair loss/thin skin/dystrophic nails	Chronic hypoperfusion → insufficient nutrient delivery for skin appendage maintenance
DVT pain + swelling	Venous obstruction → elevated hydrostatic pressure → transudation of fluid → oedema → fascial stretching
Neuropathic pain (burning)	Damaged small unmyelinated C-fibres → ectopic discharge → perceived as burning/tingling
Glove-and-stocking pattern	Longest axons affected first (dying-back neuropathy)
Compartment syndrome pain with passive stretch	Stretching ischaemic muscle increases its metabolic demand → worsens mismatch
Venous ulcer at medial malleolus	Highest venous pressure at medial ankle (site of perforating veins) → skin breakdown

High Yield Summary

Key Concepts for Limb Pain:

Acute vs Chronic arterial ischaemia: 2-week cutoff. Acute = limb-threatening emergency (6-hour window). Chronic = progressive atherosclerosis with collateral formation.
Fontaine classification: I (asymptomatic) → IIa/b (claudication) → III (rest pain) → IV (tissue loss). Stages III–IV = critical limb ischaemia.
6 Ps of acute limb ischaemia: Pain, Pallor, Perishingly cold, Pulseless, Paraesthesia, Paralysis. Pain = early (nerves first), Paralysis = late (muscle infarction).
Embolism vs Thrombosis: Different history, different examination, different management. Emboli = acute/complete/cardiac source. Thrombus = subacute/incomplete/background PVD.
Vascular vs Neurogenic claudication: Constant distance + relief by standing = vascular. Variable distance + relief by bending = neurogenic.
Rest pain vs Neuropathic pain: Rest pain = unilateral, toes/forefoot, relieved by dependency. Neuropathic = bilateral, glove-and-stocking, position-independent.
ABI: 0.9–1.3 = normal, < 0.9 = PAD, < 0.5 = critical ischaemia. Falsely elevated in diabetics (calcification).
Buerger's test: Elevation pallor (Buerger's angle) + dependent reactive hyperaemia.
Compartment syndrome: Pain out of proportion + pain with passive stretch. Don't wait for absent pulses!
Risk factors: Smoking (strongest for PAD), DM (26% risk per 1% HbA1c), plus standard CV risk factors.
LeRiche's syndrome triad: Buttock claudication + absent femoral pulses + erectile dysfunction (aortoiliac occlusion).
Hot, swollen joint = septic arthritis until proven otherwise — rheumatological emergency.

Active Recall - Limb Pain: Definition to Clinical Features

Differential Diagnosis of Limb Pain

The differential diagnosis of limb pain is vast, and the key to mastering it is having a systematic framework rather than memorising a random list. Think of it as peeling an onion: first decide which system is responsible, then narrow down within that system.

The overarching principle from Murtagh's diagnostic strategy is to categorise differentials into ^[15]:

Probability diagnoses — the common, everyday causes
Serious disorders not to be missed — the ones that kill or maim if delayed
Pitfalls (often missed) — conditions that mimic common causes and trip you up
Masquerades — systemic diseases presenting as limb pain (e.g. diabetes, drugs, spinal dysfunction)

I'll organise this by anatomical region first, then provide a unifying system-based framework.

A. Region-Specific Differential Diagnoses

This is the most critical differential to get right because vascular causes are time-sensitive emergencies.

DDx of acute limb ischaemia ^[2]^[11]:

Acute extremity compartment syndrome — can cause extrinsic compression of arteries leading to ischaemic symptoms ^[2]
DVT with superficial vein thrombosis — known as phlegmasia cerulea dolens (i.e. = painful blue oedema) — venous pressure is increased to an extent that extremity perfusion is impaired ^[2]^[11]

Why are these the key differentials? Because all three (acute arterial ischaemia, compartment syndrome, phlegmasia cerulea dolens) can present with a painful, pale/dusky, swollen limb — but the management is radically different:

Acute arterial ischaemia → revascularisation (embolectomy or thrombolysis)
Compartment syndrome → fasciotomy
Phlegmasia cerulea dolens → anticoagulation ± venous thrombectomy

DVT itself has important differentials ^[7]:

DVT Clinical Feature	Major Differentials
Leg vein DVT: Unilateral leg swelling, pain, heat	Cellulitis (look for predisposing factors), Lymphoedema / other oedema, Haematoma from trauma, Ruptured Baker's cyst (only in pre-existing arthritis)
Axillary vein DVT: Acute onset UL pain, swelling, heat in young athletic male	UL oedema of other causes (e.g. lymphoedema), Arterial or neurogenic thoracic outlet syndrome

Why does a ruptured Baker's cyst mimic DVT? A Baker's cyst is a popliteal synovial cyst (a distension of the gastrocnemius-semimembranosus bursa, usually secondary to knee joint pathology like OA or RA). When it ruptures, synovial fluid dissects into the calf → acute calf swelling, pain, and tenderness — exactly like a DVT. You need Doppler ultrasound to differentiate.

When a patient complains of leg pain on walking, the differential is not just arterial disease:

Cause	Nature	Claudication Distance	Relief	Other Features
Vascular claudication ^[1]	Cramping muscle pain	Constant	Standing still (shop window to shop window)	Absent pulses, reduced ABI
Neurogenic claudication ^[1]	Aching, heaviness, numbness	Variable	Bending forward / sitting (park bench to park bench)	Paraesthesia, numbness, weakness; going downstairs > upstairs
Venous claudication ^[1]	Bursting, heavy sensation	Relatively constant	Raising leg	History of DVT, signs of CVI
Hip/knee OA	Aching joint pain	Variable	Rest	Localised to joint, crepitus, reduced ROM

Exam Pearl: Venous vs Arterial Claudication

The key distinguishing feature is the effect of limb position: arterial claudication is relieved by standing still (reducing demand), venous claudication is relieved by raising the leg (reducing venous congestion). Neurogenic claudication requires spinal flexion. If you remember nothing else, remember: shop window (arterial) vs park bench (neurogenic) vs leg up (venous).

Acute monoarthritis deserves special emphasis because it includes a rheumatological emergency (septic arthritis) ^[12]^[16].

Common causes ^[16]:

Septic arthritis: bacterial, mycobacterial, Lyme
Crystal-induced arthritis: gout, pseudogout, hydroxyapatite deposition, Ca oxalate deposition
Trauma: fracture, internal derangement, haemarthrosis
Osteoarthritis
Polyarthritis with monoarticular onset: RA, JRA, viral arthritis, spondyloarthritis

Uncommon causes ^[16]:

Erythema nodosum, PVNS, avascular necrosis, coagulopathy with haemarthrosis, joint foreign body reaction, leukaemia, osteomyelitis, synovial metastasis

Cause	Distinguishing Features
Septic arthritis	Hot, swollen tender joint = septic arthritis until proven otherwise ^[12]. Fever (may be absent in elderly). S. aureus most common in adults. Joint aspirate: WBC > 50,000/μL, > 75% neutrophils, +ve Gram stain/culture
Gout	> 50% affects 1st MTP (podagra), rapid onset (peak ≤ 12–24h), often at night/early morning, severe pain, erythema, resolves spontaneously 5–14 days ^[5]. Joint aspirate: needle-shaped, negatively birefringent MSU crystals ^[16]
Pseudogout	Most commonly affects knee, acute presentation mimics gout, most commonly in elderly women ^[16]. Joint aspirate: pleomorphic/rhomboid-shaped, weakly positively birefringent CPP crystals ^[16]
Trauma	History of trauma, typically seconds to minutes after injury
Haemarthrosis	Trauma or coagulopathy (e.g. haemophilia), bloody aspirate
Spondyloarthritis	Classically asymmetrical oligoarthritis, usually in men < 45 years ^[17]. Look for: enthesitis, dactylitis, anterior uveitis, psoriasis, IBD, HLA-B27 ^[17]

Gout vs Pseudogout vs Septic Arthritis

These three are the classic "hot joint" mimics. All can present with fever, raised WBC, and raised CRP. The only way to definitively differentiate is joint aspiration and synovial fluid analysis — this is why every acute monoarthritis should be aspirated before treatment (unless overlying cellulitis makes it unsafe). Remember:

MSU crystals = needle-shaped, strong negative birefringence (Yellow when parallel to polariser — mnemonic: Yellow = Negative = Urate)
CPP crystals = rhomboid, weak positive birefringence (Blue when parallel — Blue = Positive = Pseudogout)
Septic = organisms on Gram stain ± culture, WBC typically > 50,000 with > 75% neutrophils

When limb pain involves multiple joints ^[16]^[17]:

Common causes ^[16]:

Rheumatoid arthritis
SLE-associated arthritis
Viral polyarthritis
Spondyloarthritis: AS, IBD-associated SpA, PsA
Osteoarthritis

Uncommon causes ^[16]:

Other CTD: polymyalgia rheumatica, systemic vasculitis, systemic sclerosis, polymyositis/dermatomyositis
Juvenile idiopathic arthritis and Adult-onset Still's disease
Crystal-induced: polyarticular gout, pseudogout
Infection-related: bacterial endocarditis, rheumatic fever, Lyme

Key distinguishing features ^[16]:

Diagnosis	Pattern
RA	Symmetrical small joint polyarthritis (MCPJs, PIPJs, wrists, MTPJs), morning stiffness > 30 min, spares DIPs
SLE arthritis	Symmetrical small joint polyarthritis but usually NOT associated with evidence of synovitis, non-deforming, non-erosive
OA	Usually symmetrical polyarthralgia affecting DIP, PIP, 1st CMCJ, weight-bearing joints and neck, Heberden's (DIP) and Bouchard's (PIP) nodes
SpA	Asymmetrical oligoarthritis (LL > UL), does not spare DIPJ, enthesitis, dactylitis, anterior uveitis ^[17]
Polyarticular gout	Distal > proximal joints, migratory/simultaneous, usually with palpable tophi in later disease

Understanding the neurological differential requires recognising the pattern of involvement ^[9]:

Pattern	Likely Site	Key Features
Polyneuropathy	Peripheral nerves (diffuse)	Generalised glove-and-stocking sensory loss + distal weakness
Mononeuropathy	Single peripheral nerve	Single nerve territory sensory/motor loss (e.g. median = CTS, ulnar = cubital tunnel)
Mononeuritis multiplex	Multiple non-contiguous nerves	Sequential sensory/motor loss in different nerve territories → think vasculitis (PAN, RA), DM, leprosy
Radiculopathy	Nerve root	Segmental LMN weakness + sensory loss, severe shooting/burning dermatomal pain, increased by movement, straining or coughing ^[9]
Plexopathy	Brachial or lumbosacral plexus	Multi-segmental LMN weakness + sensory loss
Spinal cord lesion	Spinal cord	UMN signs below level, sensory level, sphincter disturbance

Mononeuritis Multiplex — Think Vasculitis

Mononeuritis multiplex is caused by nerve infarction from small-to-medium arterial disease ^[9]. The classic causes are: Wegener's (GPA), Amyloidosis, Rheumatoid arthritis, Diabetes mellitus, SLE, Polyarteritis nodosa, Leprosy, Carcinomatosis, Churg-Strauss (EGPA). When you see sequential, asymmetric neuropathies, always check vasculitic screen (ANCA, ANA, complement, cryoglobulins).

Category	Condition	Why it must not be missed
Vascular emergency	Acute limb ischaemia	Irreversible damage in 6 hours
Vascular emergency	Aortic dissection	Mortality increases 1–2% per hour untreated
Vascular emergency	DVT / PE	PE is fatal in massive embolism
Surgical emergency	Compartment syndrome	Fasciotomy needed within hours
Surgical emergency	Cauda equina syndrome	Permanent bladder/bowel dysfunction if delayed
Rheumatological emergency	Septic arthritis	Joint destruction in days
Oncological	Bone metastases / primary bone tumour	Pathological fracture, cord compression
Cardiac	MI presenting as arm pain	Myocardial death if untreated
Infectious	Necrotising fasciitis	Rapidly fatal soft tissue infection

High Yield Summary — Differential Diagnosis of Limb Pain

System-based approach: Vascular → Neurological → Musculoskeletal → Referred/Systemic
Acute limb ischaemia DDx: compartment syndrome and phlegmasia cerulea dolens
DVT DDx: cellulitis, lymphoedema, haematoma, ruptured Baker's cyst
Claudication DDx: vascular (constant distance, standing relief) vs neurogenic (variable distance, flexion relief) vs venous (leg elevation relief)
Hot joint DDx: septic arthritis, gout, pseudogout — always aspirate the joint!
Joint aspirate crystals: MSU = needle, strong negative birefringence; CPP = rhomboid, weak positive birefringence
Polyarthritis pattern: RA = symmetrical small joints (spares DIP); OA = DIP/PIP/weight-bearing; SpA = asymmetrical oligoarthritis LL > UL + enthesitis/dactylitis
Neuropathy pattern: polyneuropathy = glove-and-stocking; mononeuritis multiplex = vasculitis; radiculopathy = dermatomal
Red flags in back/limb pain: continuous day-and-night pain (cancer/infection); cauda equina signs; fever + immunosuppression; chronic steroids (fracture)
Masquerades: diabetes, drugs (statins, fluoroquinolones), spinal dysfunction, malignancy
Referred pain traps: hip → knee (obturator nerve); cardiac → left arm (T1-T4); spine → limb (radiculopathy)
Murtagh's rules: Continuous pain = cancer/infection; Rest pain relieved by activity = inflammatory; Activity pain relieved by rest = mechanical

Active Recall - Differential Diagnosis of Limb Pain

References

^[1] Senior notes: Ryan Ho Cardiology.pdf (pages 205–207, 212) ^[2] Senior notes: felixlai.md (Acute arterial insufficiency, Chronic arterial insufficiency sections) ^[5] Senior notes: Ryan Ho Rheumatology.pdf (pages 35–37, Crystal-Induced Arthritis / Gout) ^[7] Senior notes: Ryan Ho Haemtology.pdf (page 131, VTE) ^[9] Senior notes: Ryan Ho Neurology.pdf (pages 45, 54, 72, 179–180) ^[11] Senior notes: maxim.md (Acute limb ischaemia section) ^[12] Senior notes: Ryan Ho Rheumatology.pdf (page 67, Septic arthritis) ^[13] Senior notes: maxim.md (Shoulder pain, Achilles tendinopathy, Plantar fasciitis, Approach to spine diseases sections) ^[15] Lecture slides: murtagh merge.pdf (Knee pain p64, Foot and ankle pain p56, Arm and hand pain p19, Back pain lower p22–24, Neck pain p69) ^[16] Senior notes: Ryan Ho Rheumatology.pdf (pages 28–31, Approach to Acute Monoarthritis / Polyarthritis); Senior notes: Ryan Ho Fundamentals.pdf (page 406) ^[17] Senior notes: Ryan Ho Rheumatology.pdf (pages 57, 63, Spondyloarthritis) ^[18] Senior notes: Ryan Ho Endocrine.pdf (page 53, Paget's Disease of Bone)

Diagnostic Criteria, Algorithm and Investigations for Limb Pain

Because "limb pain" is a symptom complex rather than a single disease, there is no single diagnostic criterion. Instead, we apply condition-specific diagnostic criteria depending on which differential we are pursuing, and use a systematic algorithmic approach to narrow down the aetiology. This section covers the diagnostic framework, the key investigations, and how to interpret them — all from first principles.

B. Vascular Investigations

ABI: to confirm diagnosis and quantify severity ^[1]^[11]

Definition ^[1]:

$\text{ABI} = \frac{\text{Higher ipsilateral ankle systolic BP (PT or DP)}}{\text{Higher arm systolic BP (left or right)}}$

Note ^[1]:

Brachial pressure taken as the higher reading among two arms
Ankle pressure taken as the higher reading among dorsalis pedis and posterior tibial
Doppler probe used instead of stethoscope

Interpretation ^[1]^[11]:

ABI Value	Interpretation	Clinical Correlate
0.90–1.30	Normal	No significant PAD
≤ 0.9	Arterial occlusive disease (diagnostic)	Confirms PAD
0.40–0.90	Claudication	Non-critical limb ischaemia
< 0.4	Rest pain, tissue loss	Critical limb ischaemia ^[1]; Maxim notes use < 0.4 as "severe" ^[11]
> 1.30	Calcified arteries	May occur in DM or ESRD. Should use toe-brachial index (TBI) instead ^[1]

Why does ABI work? In a normal person, ankle systolic pressure is slightly higher than brachial pressure (due to pulse wave amplification in the periphery). Any arterial stenosis proximal to the ankle cuff reduces the downstream pressure → ABI falls below 0.9. The worse the stenosis, the lower the ABI.

Exercise testing ^[1]:

Indication: ABI normal but symptomatic
Process: ask patient to exercise on treadmill → stop when experience pain → > 0.2 decrease in ABI equivalent to claudication
Why? At rest, collateral blood flow may maintain adequate ankle pressure. Exercise increases demand, and the stenotic artery cannot keep up → post-exercise ABI drops. This unmasks significant disease that is subclinical at rest.

TBI in Diabetics

TBI allows assessment of small vessel disease which is a more reliable indicator of limb perfusion in DM patients. Small vessels are also less frequently calcified and thus avoid a factitiously high value as in ABI ^[1]. A TBI < 0.7 is considered abnormal.

First-line imaging for all PAD patients ^[1]

Duplex USG = B-mode USG + Doppler ^[11]

Use ^[1]:

Detect flow abnormalities (e.g. turbulence) by Doppler
Locate level of occlusion by B-mode

Advantages ^[11]: Non-invasive, no radiation Limitations ^[11]: Operator-dependent, poor image for aortoiliac segment

Arterial flow waveform ^[1]:

Waveform	Description	Interpretation
Triphasic (normal)	Forward flow (systole) + reverse then forward flow (diastole)	Normal arterial flow
Biphasic	Forward flow (systole) + reverse flow (diastole)	Single-level arterial occlusion
Monophasic	Forward flow alone	Multi-level occlusion

Why triphasic? In a normal high-resistance arterial bed (like the resting lower limb), systolic forward flow is followed by brief diastolic reversal (due to elastic recoil of vessel walls against the closed aortic valve) and then a small forward wave (from elastic recoil of the distended aorta). Stenosis eliminates the high-resistance pattern — the vessel distal to a stenosis is maximally dilated, creating a low-resistance bed that only allows forward flow (monophasic).

DSA: gold standard ^[11]

Indicated only for patients with planned intervention (angioplasty / stenting) ^[11]
Inject radio-opaque dye into arterial tree (usually via femoral artery) → imaging and digitalized by computer ^[11]
Can be done intra-operatively: guide endovascular intervention ^[11]
Risks ^[11]: allergy, contrast nephropathy, arterial injury (e.g. dissection, embolism, pseudoaneurysm)

Key angiographic findings ^[1]:

Finding	Significance
Sharp cut-off with few collaterals	Embolism
Irregular cut-off with well-developed collaterals	Thrombosis (chronic)
Diffuse atherosclerosis	Chronic PAD
Corkscrew arteries, tree-root collaterals	Buerger's disease

Rutherford (SVS/ISCVS) classification ^[1] — this is both a diagnostic classification AND a management guide:

Category	Sensory loss	Muscle weakness	Arterial signal	Venous signal	Treatment
Viable (I)	None	None	Audible	Audible	Imaging
Marginally threatened (IIa)	Minimal (toes)	None	Inaudible	Audible	Urgent revascularisation
Immediately threatened (IIb)	Beyond toes	Partial	Inaudible	Audible	Emergency revascularisation
Non-viable (III)	Completely anaesthetic	Completely paralysed	Inaudible	Inaudible	Amputation

Bedside Doppler is Essential in Acute Ischaemia

The handheld Doppler is your most valuable bedside tool. The presence or absence of arterial and venous Doppler signals directly determines the Rutherford category and therefore the urgency and type of intervention. A pulse that is not palpable may still have detectable flow on Doppler ^[2] — so always use the Doppler before concluding "pulseless."

D. Musculoskeletal and Joint Investigations

Indications ^[16]:

Suspicious of septic arthritis
Suspicious of crystal-induced arthritis
Suspicious of haemarthrosis
Differentiating inflammatory vs non-inflammatory arthritis

Send for ^[16]:

Macroscopic: colour, viscosity, turbidity
Microscopy: wet films, WBC count/differential, crystal microscopy
Microbiology (if suspect septic arthritis): Gram stain (urgent if septic arthritis suspected), bacterial culture, AFB smear and culture, fungal stain (if indicated)

Joint fluid WBC count interpretation ^[16]:

Category	WBC Count (/μL)	Appearance	Examples
Normal	< 200	Clear, straw-coloured, high viscosity	—
Non-inflammatory	200–2,000	Clear, yellow, high viscosity	OA, trauma
Inflammatory	2,000–50,000	Translucent–opaque, yellow-green, low viscosity	RA, gout, pseudogout, SpA
Septic	> 50,000 (often > 100,000)	Opaque, purulent, very low viscosity	Bacterial septic arthritis
Haemorrhagic	Variable (RBCs dominant)	Red/bloody	Trauma, coagulopathy, tumour

Crystal microscopy ^[16]:

Gout: slender and needle-shaped, strongly negative birefringence under polarised light
Pseudogout: pleomorphic or rhomboid-shaped, weakly positive birefringence

Why does birefringence matter? A polarising microscope has two filters. Birefringent crystals rotate the plane of polarised light, so they appear bright against a dark background. The type of birefringence (positive or negative) depends on how the crystal aligns relative to the polariser's axis. Negative birefringence means the crystal appears yellow when parallel to the compensator axis (mnemonic: Yellow = Negative = Urate). Positive birefringence means blue when parallel (Blue = Positive = Pseudogout/CPPD).

Exam Pearl: Always Aspirate a Hot Joint

Hot, swollen tender joint = septic arthritis until proven otherwise, even without fever, increased WBC, increased ESR/CRP ^[12]. The only exception to aspiration is overlying cellulitis (risk of introducing bacteria into a sterile joint). Gout and septic arthritis can coexist — finding crystals does NOT rule out infection. Always send for both crystal microscopy AND Gram stain/culture.

Plain XR ± USG/MRI of affected joint ^[16]:

Traumatic origin: fractures, loose body in joint
Joint effusion: fat pad sign in elbow/knee
Arthritic changes: joint space narrowing, periarticular erosion, chondrocalcinosis (pseudogout), tophaceous erosion

Specific XR features by condition:

Condition	XR Findings
OA	Joint space narrowing, subchondral sclerosis, osteophytes, subchondral cysts
RA	Periarticular erosions, periarticular osteopenia, joint space narrowing, soft tissue swelling
Gout	Punched-out erosions with overhanging edges ("rat-bite"), preserved joint space (initially), tophi
Pseudogout	Chondrocalcinosis (calcification of cartilage, especially menisci and triangular fibrocartilage of wrist)
Septic arthritis	Soft tissue swelling, joint effusion (early); joint destruction, periosteal reaction (late)
SpA	Sacroiliitis (erosion → sclerosis → fusion), syndesmophytes, "bamboo spine" (late AS)

Useful plain XR findings in spinal cord lesions ^[9]:

Pedicle erosion → extradural metastases
Vertebral body collapse
Narrow disc space, osteophytes, hypertrophic facet joints → spondylosis
Expansion of intervertebral foramina → neurofibroma

E. Neurological Investigations

Electrodiagnostic studies (EDx): use of electrophysiological techniques to assess function of peripheral nervous system ^[9]

NCS: assessment of large fibre conduction function by analysis of neuronal signals generated by electrical activation ^[9] EMG: analysis of bioelectric activities of muscles ^[9]

Useful for ^[9]:

Differentiation between focal and multifocal neuropathy
Differentiation between demyelinating and axonal neuropathy
Assessment of severity of neuropathy
Monitor progress of neuropathy or myopathy
Prognostication after nerve trauma and guidance of treatment

NOT useful for ^[9]:

Exclude or confirm neuropathy or myopathy (clinical findings should suffice)
Define aetiology of peripheral neuropathy (cannot identify aetiology)
Cervical myelopathy (does NOT assess CNS function)

NCS Finding	Demyelinating	Axonal
Conduction velocity	Markedly reduced	Normal or mildly reduced
Amplitude	Normal (early)	Reduced
Distal latency	Prolonged	Normal
Conduction block	Present	Absent

Why does this distinction matter? Usually only demyelinating neuropathies are susceptible to treatment ^[9] (e.g. GBS, CIDP respond to IVIg/plasmapheresis). Axonal damage generally implies permanent structural loss.

H. Condition-Specific Diagnostic Criteria

Clinical Scenario	First-Line Investigation	Key Finding
Suspected PAD / claudication	ABI + Duplex USG	ABI ≤ 0.9; triphasic → biphasic/monophasic waveform
ABI normal but symptomatic	Exercise ABI testing	> 0.2 drop post-exercise
Diabetic with ABI > 1.3	Toe-brachial index	TBI < 0.7
Acute limb ischaemia	Bedside Doppler + Rutherford grading	Arterial/venous signal presence
Pre-operative PAD planning	CTA or DSA (gold standard)	Anatomical mapping for intervention
Suspected DVT	Wells score → D-dimer / Duplex USG	Non-compressible vein
Hot swollen joint	Joint aspiration (MOST IMPORTANT)	Crystals, WBC count, Gram stain
Suspected neuropathy	NCS/EMG + bloods (HbA1c, B12, TFT, SPE)	Axonal vs demyelinating pattern
Radiculopathy / cord compression	MRI spine (urgent if cauda equina)	Disc prolapse, stenosis, cord compression
Back/limb pain with red flags	FBE, ESR/CRP, ALP, PSA, plain XR	Screen for cancer, infection, Paget's
Suspected compartment syndrome	Clinical diagnosis ± compartment pressure	Pressure ≥ 30 mmHg or within 30 mmHg of diastolic BP
Venous incompetence	Handheld Doppler → Duplex USG	Biphasic signal = reflux

High Yield Summary — Diagnostics for Limb Pain

ABI ≤ 0.9 = diagnostic for PAD. Use exercise testing if ABI normal but symptomatic. Use TBI if ABI > 1.3 (calcified/diabetic).
Duplex USG = first-line imaging for all PAD. Normal flow is triphasic; single-level disease → biphasic; multi-level → monophasic.
DSA = gold standard for arterial imaging but reserved for planned intervention only (invasive + risks).
Acute limb ischaemia is a clinical diagnosis graded by Rutherford classification using clinical exam + bedside Doppler.
Joint aspiration = MOST IMPORTANT TEST for acute monoarthritis. Always send crystals + Gram stain + culture.
Gout crystals: needle-shaped, strong negative birefringence (yellow parallel). CPPD crystals: rhomboid, weak positive birefringence (blue parallel).
DVT diagnosis: Wells score → D-dimer (low probability) or Duplex USG (high probability). D-dimer: sensitive but not specific.
NCS/EMG: differentiates axonal (reduced amplitude) from demyelinating (reduced velocity) neuropathy — only demyelinating is typically treatable.
MRI spine: investigation of choice for cord compression, radiculopathy, cauda equina — order urgently for cauda equina!
Murtagh's screening bloods: FBE, ESR/CRP, urinalysis, ALP, PSA (males 50–75). XR if red flags. CT/MRI reserved for serious disease.
Compartment syndrome: clinical diagnosis. Pressure measurement if in doubt (delta P ≤ 30 mmHg of diastolic BP). Do NOT delay fasciotomy for measurement if clinically obvious.

Active Recall - Diagnostics for Limb Pain

References

^[1] Senior notes: Ryan Ho Cardiology.pdf (pages 206–207, 210, 214, 237) ^[2] Senior notes: felixlai.md (Acute arterial insufficiency section) ^[7] Senior notes: Ryan Ho Haemtology.pdf (page 131, VTE diagnostic evaluation) ^[9] Senior notes: Ryan Ho Neurology.pdf (pages 38, 65, 169, 178, 183) ^[11] Senior notes: maxim.md (Chronic limb ischaemia investigations, Acute limb ischaemia, Spinal stenosis) ^[12] Senior notes: Ryan Ho Rheumatology.pdf (page 67, Septic arthritis) ^[15] Lecture slides: murtagh merge.pdf (Back pain p24, Foot and ankle pain p57, Arm and hand pain p20, Paraesthesia p77) ^[16] Senior notes: Ryan Ho Fundamentals.pdf (pages 407, 410); Senior notes: Ryan Ho Rheumatology.pdf (pages 29–30) ^[17] Senior notes: Ryan Ho Rheumatology.pdf (pages 57, 60, Spondyloarthritis) ^[19] Senior notes: Ryan Ho Neurology.pdf (page 178, Approach to generalised weakness investigations)

Management of Limb Pain

The management of limb pain is entirely dictated by the underlying aetiology. There is no "one-size-fits-all" approach. What I'll do here is walk through the management of the major causes systematically — starting with the time-critical emergencies, then moving to chronic conditions. The guiding principle: treat the cause, not just the symptom.

A. Management of Acute Limb Ischaemia

This is a surgical emergency. Irreversible damage begins at 6 hours without collateral circulation ^[2].

Rutherford Grade	Management
Viable (I) and Marginally threatened (IIa)	Conservative: heparin IV bolus 3000–5000 U + continuous infusion (limit propagation of thrombus and protect collaterals). Monitor APTT (60–80s) ^[11]
Immediately threatened (IIb) or IIa failing conservative	Surgical / endovascular revascularisation ^[11]
Non-viable (III)	Amputation ^[2]^[11]

Why Not Revascularise a Non-Viable Limb?

Delays in amputation of a non-viable extremity can result in infection, myoglobinuria, acute renal failure and hyperkalaemia ^[2]. Reperfusing dead muscle releases potassium (→ cardiac arrest), myoglobin (→ renal tubular necrosis), and lactic acid (→ metabolic acidosis). Arteriography is NOT necessary since level of amputation is determined by clinical findings and by viability of tissues at time of surgery ^[2].

Embolic Cause	Thrombotic Cause
Above knee: open embolectomy with Fogarty catheter ± prophylactic fasciotomy (if ischaemia > 6h) ^[11]	Urgent CT angiogram for pre-op planning
The Fogarty catheter is a balloon-tipped catheter passed through an arteriotomy beyond the embolus, inflated, then withdrawn — pulling the clot out	Local thrombolysis / bypass ^[11]
Below knee: local thrombolysis / bypass ^[11]	Blue toe syndrome: manage medically e.g. statins ^[11]

Overview of treatment modalities ^[2]:

	Surgical Revascularisation	Endovascular Revascularisation
Modalities	Embolectomy (OR), Bypass grafting, ± Prophylactic fasciotomy	IA Thrombolysis, Angioplasty and stenting
Indications	Immediately threatened extremities; Recent occlusion < 2 weeks	Viable or marginally threatened extremities; Prolonged occlusion > 2 weeks

B. Management of Chronic Limb Ischaemia

The management of chronic limb ischaemia follows a stepwise approach: conservative first → intervention if fails or critical.

CV risk factor modification ^[11]:

Lifestyle: smoking cessation, weight reduction, supervised exercise programme (to increase O2 extraction, collateral circulation and improve muscle metabolism)
DM / HT / HL control (statin — regardless of lipid level for overall CVS protection) ^[11]

Why supervised exercise? During exercise, muscles that are ischaemic activate anaerobic pathways. With regular training, several adaptations occur: (1) increased mitochondrial density → better O2 extraction; (2) angiogenesis → new collateral vessels; (3) improved endothelial function → better nitric oxide-mediated vasodilation; (4) improved walking biomechanics → reduced metabolic demand per step. Evidence shows supervised programmes increase pain-free walking distance by 50–200%.

Pharmacological ^[11]:

Low dose aspirin (72–325 mg): secondary prevention of coronary heart disease and stroke — aspirin irreversibly inhibits cyclooxygenase-1 (COX-1) in platelets → blocks thromboxane A2 synthesis → reduces platelet aggregation
Cilostazol (PDE3 inhibitor with antiplatelet and vasodilatory effect) — phosphodiesterase 3 ("phospho-di-esterase" = enzyme that breaks down cyclic nucleotides) inhibition → increased intracellular cAMP → vasodilation + platelet inhibition. C/I: CHF (because PDE3 inhibitors can worsen heart failure — historical precedent from milrinone trials) ^[11]
- Alternatives: naftidrofuryl (5-HT2 antagonist: reduce platelet aggregation), pentoxifylline / Trental (PDE inhibitor), prostaglandins ^[11]

Why NOT Surgery First for Intermittent Claudication?

Rationale of not going for surgery in IC patients: (1) Failed surgery has a poorer outcome than no surgery; (2) Second surgery is less likely to succeed ^[1]. The natural history of intermittent claudication is actually relatively benign — at 5 years: stable claudication (70–80%), worsening claudication (10–20%), critical CLI (1–2%) ^[1]. Conservative management addresses the much bigger killer: cardiovascular mortality (non-fatal MI/stroke 20%, death 15–30% at 5 years ^[1]).

Based on overall success rates of treating the lesion using endovascular/surgical means based on the anatomy of occlusion/stenosis ^[1]:

TASC Type	Lesion	Preferred Treatment
A	Short and focal	Excellent result with endovascular therapy
B	Moderate	Prefer endovascular Tx but can perform surgery
C	Extensive	Better results with open revascularisation but endovascular Tx can be used if high risk
D	Diffuse/complex	Usually prefer surgery as primary Tx for low-to-moderate risk patients

Management principles (NICE 2015) ^[7]:

Indications for anticoagulation: only in proximal DVT and PE
- Distal DVT: low sensitivity on duplex USG, low embolic risk, usually asymptomatic → often opt not to treat

Anticoagulation therapy ^[7]:

Standard regimen: LMWH (e.g. enoxaparin) or fondaparinux as initial treatment → bridge to oral anticoagulant (warfarin with target INR 2–3) or use DOAC (rivaroxaban, apixaban) from the start
Cancer patients: prefer LMWH (or DOAC) over warfarin, continue > 6 months if active cancer ^[7]
Pregnancy: switch to LMWH when 1st trimester (decreased teratogenicity) and > 36 weeks (avoid PPH) → cover up to 6 weeks postpartum (highest risk as blood returns from uterus) ^[7]
- Problem: warfarin crosses placenta → risk of fetal ICH and teratogenicity ^[7]
HIT: heparin C/I → start parenteral non-heparin anticoagulants → bridge to warfarin ^[7]
Antiphospholipid syndrome: heparin followed by indefinite warfarin ^[7]

Duration of anticoagulation:

Scenario	Duration
First provoked DVT (clear trigger: surgery, immobilisation)	3 months
First unprovoked DVT	≥ 3 months, consider extended if low bleeding risk
Recurrent VTE / permanent RF (e.g. APS, cancer)	Indefinite

Additional measures:

Early mobilisation (bed rest is NOT recommended)
Graduated compression stockings (now controversial; NICE 2020 does not recommend routinely for post-thrombotic syndrome prevention, though still used in practice)
IVC filter: only if anticoagulation absolutely contraindicated (e.g. active bleeding)

F. Management of Neuropathic Limb Pain

Gabapentinoids (gabapentin/pregabalin) and antidepressants (amitriptyline) for pain ^[3]^[9]

Drug Class	Example	Mechanism	Notes
Gabapentinoids	Pregabalin (Lyrica), Gabapentin (Neurontin)	Binds α2δ subunit of presynaptic voltage-dependent Ca2+ channels → blocks release of excitatory neurotransmitters e.g. glutamate ^[20]	S/E: somnolence, dizziness, increased appetite, mood changes ^[20]. First-line for neuropathic pain
Tricyclic antidepressants	Amitriptyline	Blocks reuptake of noradrenaline and serotonin → enhances descending inhibitory pain pathways from periaqueductal grey → dorsal horn	Low doses (10–25 mg nocte). S/E: anticholinergic (dry mouth, constipation, urinary retention), sedation, cardiac conduction defects. C/I: recent MI, arrhythmia
SNRIs	Duloxetine	Serotonin + noradrenaline reuptake inhibition → similar mechanism to TCAs but cleaner S/E profile	2nd line. S/E: nausea, insomnia
Topical	Capsaicin cream, Lidocaine patches	Capsaicin depletes substance P from C-fibres; lidocaine blocks Na+ channels locally	Adjunctive, useful for localised neuropathic pain

G. Management of Musculoskeletal Causes

H. Management of Spinal/Radicular Limb Pain

I. Management of Varicose Veins / Chronic Venous Insufficiency

For any cause of limb pain, adequate analgesia is essential. Use the WHO analgesic ladder as a framework, but tailor to the specific condition:

Step	Drugs	Notes
1	Paracetamol ± NSAIDs	First-line for mild-moderate pain. NSAIDs C/I: renal impairment, PUD, heart failure, aspirin-sensitive asthma
2	Weak opioids (codeine, tramadol)	Add to Step 1 if insufficient
3	Strong opioids (morphine, oxycodone)	For severe pain e.g. rest pain, post-operative. S/E: respiratory depression, constipation, nausea, dependence
Adjuvants	Gabapentinoids, TCAs, SNRIs	Specifically for neuropathic pain. Also useful: local anaesthetic blocks

For ischaemic rest pain specifically: opioid analgesia is usually required, often IV morphine. The definitive treatment of rest pain is revascularisation — analgesia alone is insufficient.

Condition	First-Line	Second-Line	Surgery/Intervention
Intermittent claudication	Smoking cessation, supervised exercise, aspirin, statin, cilostazol	Endovascular PTA ± stenting	Bypass if TASC C/D or failed endovascular
Critical limb ischaemia	IV heparin, O2, analgesia	Endovascular or surgical revascularisation	Amputation if non-viable
Acute limb ischaemia (embolism)	IV heparin	—	Open embolectomy ± fasciotomy
Acute limb ischaemia (thrombosis)	IV heparin	IA thrombolysis	Bypass surgery
DVT	DOAC (rivaroxaban/apixaban) or LMWH → warfarin	—	IVC filter if anticoagulation C/I
Compartment syndrome	—	—	Emergency fasciotomy
Septic arthritis	IV antibiotics + joint aspiration	—	Arthroscopic washout / open drainage
Gout (acute)	NSAIDs or colchicine	Corticosteroids	—
Gout (chronic)	Allopurinol (check HLA-B*5801)	Febuxostat, probenecid	—
RA	Methotrexate + NSAIDs	Biologics (anti-TNF, etc.)	Joint replacement
SpA	NSAIDs (continuous)	Anti-TNF / anti-IL-17A	Joint surgery if severe
Neuropathic pain	Gabapentin / pregabalin / amitriptyline	Duloxetine	Treat underlying cause
Disc prolapse	Physiotherapy, NSAIDs	Nerve root injection	Microdiscectomy
Varicose veins	Compression stockings, venotonics	EVLA / RFA / sclerotherapy	Trendelenburg + stripping

High Yield Summary — Management of Limb Pain

Acute limb ischaemia: ABC → IV heparin (ALL patients) → assess Rutherford grade → revascularise (IIa/IIb) or amputate (III). Differentiate embolism (embolectomy) vs thrombosis (thrombolysis/bypass).
Chronic limb ischaemia: Conservative FIRST for intermittent claudication (smoking cessation, supervised exercise, aspirin, statin, cilostazol). Surgery only if failed 6 months conservative or critical ischaemia.
TASC II: A/B → endovascular preferred; C/D → surgery preferred.
Thrombolysis C/I: recent stroke, ICH, intracranial malignancy, active bleeding, major surgery/trauma, uncontrolled HTN.
Cilostazol: PDE3 inhibitor. C/I in CHF.
DVT: proximal DVT → anticoagulate (DOAC or LMWH → warfarin). Cancer → LMWH/DOAC > warfarin. Pregnancy → LMWH. HIT → non-heparin anticoagulant.
Compartment syndrome: emergency fasciotomy. Do not delay for pressure measurement if clinical picture is obvious.
Septic arthritis: aspirate → IV antibiotics → joint washout. Emergency!
Neuropathic pain: gabapentinoids (pregabalin/gabapentin) or amitriptyline first-line. Treat underlying cause.
Gout: NSAIDs/colchicine for acute flare; allopurinol for long-term ULT (check HLA-B*5801 in Chinese patients). Do NOT start allopurinol during acute flare.
RA: early DMARDs (methotrexate first-line); biologics if inadequate response. Surgery: LL before UL; forefoot → knee → hip.
SpA: NSAIDs first-line (continuous use); biologics if BASDAI ≥ 4 despite 2–3 NSAIDs tried.
Surgery rationale for IC: failed surgery is worse than no surgery; second surgery less likely to succeed. Conservative Mx addresses the bigger killer (cardiovascular mortality).

Active Recall - Management of Limb Pain

References

^[1] Senior notes: Ryan Ho Cardiology.pdf (pages 212, 216, 239) ^[2] Senior notes: felixlai.md (Acute arterial insufficiency management sections) ^[3] Senior notes: Ryan Ho Endocrine.pdf (pages 97–99, Diabetic neuropathy management) ^[7] Senior notes: Ryan Ho Haemtology.pdf (page 132, DVT management) ^[9] Senior notes: Ryan Ho Neurology.pdf (pages 82, 182–183, Neuropathy and GBS management) ^[11] Senior notes: maxim.md (Acute limb ischaemia management, Chronic limb ischaemia management, Varicose veins management, Compartment syndrome, Fracture management) ^[12] Senior notes: Ryan Ho Rheumatology.pdf (page 67, Septic arthritis) ^[13] Senior notes: maxim.md (Disc prolapse management, Spinal stenosis) ^[17] Senior notes: Ryan Ho Rheumatology.pdf (pages 56, 62, 77, 92, RA surgery, SpA management, SLE joint pain) ^[20] Senior notes: Ryan Ho Psychiatry.pdf (page 60, Gabapentinoids)

Complications of Limb Pain and Its Underlying Causes

Complications in the context of limb pain arise from two broad categories: (1) complications of the underlying disease itself (e.g. tissue death from ischaemia, joint destruction from infection) and (2) complications of the treatment (e.g. reperfusion injury after revascularisation, bleeding from anticoagulation). Understanding the pathophysiology from first principles is the key to predicting, recognising, and managing these complications.

A. Complications of Acute Limb Ischaemia

These are the most critical and examinable complications. They fall into two groups: complications from the ischaemia itself and complications from reperfusion after revascularisation.

Compartment syndrome ^[1]^[2]^[11]:

Mechanism: prolonged ischaemia (≥ 6h) + delayed revascularisation → fluid leaks out via damaged cell membrane → oedema → secondary ischaemia when pressure ≥ 30 mmHg or within 30 mmHg of diastolic BP ^[1]

The pathophysiology in detail ^[2]:

Muscle deprived of arterial blood flow becomes ischaemic → cell membrane integrity breaks down → intracellular contents leak into the interstitium
Upon reperfusion, oxygen-free radicals are generated → these directly damage capillary walls → increased vascular permeability → further fluid leak into the closed fascial compartment
The fascial compartment is non-distensible (fascia does not stretch) → rising intracompartmental pressure → compression of capillaries → secondary ischaemia (a vicious cycle)
When intracompartmental pressure exceeds capillary perfusion pressure, nutrient flow to muscle and nerve ceases completely despite axial blood flow having been restored

Clinical features ^[1]^[11]:

Pain out of proportion to clinical situation (earliest symptom) ^[1]
Pain with passive stretch (most sensitive sign) ^[1] — Why? Stretching the ischaemic muscle increases its metabolic demand, widening the supply-demand mismatch
Numbness in distribution of nerves running within the compartment ^[11]
Tense compartment on passive toe dorsiflexion and plantarflexion ^[11]
Pulses can be present (SBP >> intracompartmental pressure) ^[11] — this is a classic exam trap; do NOT wait for pulselessness

Site ^[1]^[2]:

Commonly calf, especially anterior tibial compartment ^[1]
Anterior compartment is most commonly affected whereas involvement of posterior compartment is the most functionally devastating ^[2] — Why? The deep posterior compartment contains the tibial nerve and posterior tibial artery, whose damage leads to loss of plantar sensation and foot blood supply

Numbness in the web space between first and second toes is suggestive ^[2] — because this is the territory of the deep peroneal nerve, which runs through the anterior compartment

Management ^[1]^[11]:

Emergent fasciotomy → lay open for oedema to resolve, then close after a few days ^[1]
Consider prophylactic fasciotomy in OT if drastic ischaemia ^[1]
Check compartment pressure ( < 20 mmHg of diastolic BP) if in doubt ^[1]
Urgent fasciotomy via medial + lateral incisions ^[11]
Remove constrictive dressings, limb elevation at heart level
Post-op: wound left open, re-inspect at 48h, ± remove necrotic tissue ^[13]

Volkmann's Contracture

Volkmann's contracture is the dreaded late complication of untreated compartment syndrome, commonly found in supracondylar fractures of the elbow in children ^[13]. The sequence is: compartment syndrome → prolonged ischaemia → muscle necrosis → fibrosis → fixed flexion contracture of the forearm and hand. The classic posture is wrist flexion + MCP hyperextension + IP flexion + thumb adduction. This is an irreversible deformity — prevention by early fasciotomy is the only effective strategy.

Rhabdomyolysis ^[1]^[2]^[11]:

Mechanism: reperfusion → release of K+, lactic acid, myoglobin, CK from damaged muscle ^[1]

From first principles: skeletal muscle cells contain huge amounts of potassium (intracellular K+ ≈ 150 mEq/L), myoglobin (for oxygen storage), creatine kinase (for ATP regeneration), and lactic acid (from anaerobic metabolism during ischaemia). When these cells die and their membranes lose integrity, all these contents spill into the systemic circulation simultaneously.

Consequences ^[1]^[2]:

Acute renal failure — myoglobin is freely filtered by the glomerulus but precipitates in the renal tubules (especially in acidic urine, forming myoglobin casts) → acute tubular necrosis (ATN). Additionally, free myoglobin generates reactive oxygen species in tubular cells → direct tubular toxicity.
Cardiac arrhythmia — hyperkalaemia depolarises the resting membrane potential of cardiomyocytes → widened QRS, peaked T waves → eventually VF/asystole
ARDS — systemic inflammatory response from massive muscle necrosis → capillary leak in lungs
Metabolic acidosis — release of lactic acid + failure of renal acid excretion

Risk factors ^[1]: poor background renal function

Management ^[1]^[2]^[11]:

Aggressive hydration ± IV bicarbonate (decreases acidosis, decreases myoglobin cast formation) ± dialysis ^[1]
Diuresis with mannitol ^[11] — osmotic diuretic to maintain tubular flow and prevent cast formation
Post-op: monitor BP/P, UO, APTT, RFT, cardiac monitor ^[11]
Correct hyperkalaemia: IV calcium gluconate (membrane stabiliser), insulin + dextrose (intracellular K+ shift), sodium bicarbonate, calcium resonium, dialysis if refractory

Why IV Bicarbonate for Rhabdomyolysis?

Myoglobin precipitates in acidic urine to form obstructive casts. By alkalinising the urine (target urine pH > 6.5), you keep myoglobin in a soluble form and reduce its nephrotoxicity. Bicarbonate also directly counteracts the systemic metabolic acidosis from lactic acid release. This is why bicarbonate is given alongside aggressive IV fluid resuscitation.

B. Complications of Chronic Limb Ischaemia

Progressive ischaemia → tissue death → ulceration at pressure points (toes, heel, lateral malleolus) or gangrene.

Type	Dry Gangrene ^[2]	Wet Gangrene ^[2]
Texture	Hard, dry, shrunken	Soft, moist, swollen
Infection	Non-infected	Infected
Demarcation	Clear line between viable and necrotic tissue	No clear line
Management	Safe to allow self-amputation after demarcation with precautions against infection	Emergency: surgical debridement or amputation

Why does wet gangrene require emergency treatment? Because the liquefactive necrosis provides an excellent medium for bacterial growth → sepsis → multiorgan failure → death. Dry gangrene, by contrast, is coagulative necrosis — the dead tissue desiccates and essentially mummifies, creating a barrier against infection.

D. Complications of DVT / Venous Thromboembolism

E. Complications of Musculoskeletal Causes

F. Complications of Neuropathic Conditions

Condition	Key Complications	Why
Acute limb ischaemia	Compartment syndrome, rhabdomyolysis, reperfusion injury	Ischaemic cell death → membrane leak; reperfusion → free radicals
Chronic limb ischaemia	Tissue loss (ulcer/gangrene), MI/stroke, amputation	Progressive atherosclerosis, systemic disease
DVT	PE, post-thrombotic syndrome, bleeding from Tx	Embolisation, valvular destruction, anticoagulant S/E
Septic arthritis	Joint destruction, osteomyelitis, sepsis	Bacterial proteolytic enzymes
Gout	Tophaceous deposition, joint destruction, urate nephropathy	Chronic crystal accumulation
Fracture/trauma	Compartment syndrome, DVT/PE, fat embolism, CRPS, Volkmann's	Bleeding/oedema, immobility, nerve injury
Diabetic neuropathy	Foot ulcers, Charcot arthropathy, infections	Loss of protective sensation + deformity + vascular disease
Amputation	Phantom limb pain, stump neuroma, infection, DVT	Nerve transection, immobility

High Yield Summary — Complications of Limb Pain

Compartment syndrome: mechanism = ischaemia → membrane damage → oedema in non-distensible compartment → secondary ischaemia. Earliest symptom = pain out of proportion. Most sensitive sign = pain with passive stretch. Pulses may be PRESENT. Anterior tibial compartment most common. Tx = emergency fasciotomy.
Rhabdomyolysis: K+ → arrhythmia; myoglobin → AKI (ATN); lactic acid → metabolic acidosis. Tx = aggressive hydration + IV bicarbonate + mannitol ± dialysis.
Reperfusion injury: oxygen free radicals → capillary damage → oedema → no-reflow. Thrombolysis has less reperfusion injury than open embolectomy (more gradual reperfusion).
Gangrene: Dry = non-infected, clear demarcation, can self-amputate. Wet = infected, no demarcation, surgical emergency.
PAD prognosis: IC patients more likely to die of MI/stroke than lose limb. Critical CLI: 25% dead, 25% amputated at 1 year.
Post-thrombolysis: stroke and major haemorrhage risk.
Amputation complications: phantom limb pain (Mx: gabapentin/amitriptyline), stump neuroma, DVT, fixed flexion deformity.
CRPS: persistent burning pain disproportionate to injury. Think of it for any limb pain not resolving after trivial trauma.
Volkmann's contracture: late sequela of untreated compartment syndrome, classically in supracondylar fracture.
Diabetic foot: previous ulceration is the most important RF. Neuropathy causes 80% of ulcers.
Immobility complications: DVT/PE, pressure sores, contractures, muscle atrophy, depression.

Active Recall - Complications of Limb Pain

References

^[1] Senior notes: Ryan Ho Cardiology.pdf (pages 212, 226) ^[2] Senior notes: felixlai.md (Complications of acute arterial ischaemia section) ^[3] Senior notes: Ryan Ho Endocrine.pdf (pages 94–95, 98–99, Diabetic complications) ^[5] Senior notes: Ryan Ho Rheumatology.pdf (pages 38, 40, Tophaceous gout complications) ^[9] Senior notes: Ryan Ho Neurology.pdf (pages 80, 82, 174, 180, Stroke complications, Peripheral neuropathy) ^[11] Senior notes: maxim.md (Acute limb ischaemia complications, Amputation, Fracture complications) ^[12] Senior notes: Ryan Ho Rheumatology.pdf (page 67, Septic arthritis) ^[13] Senior notes: maxim.md (Complications of trauma, Volkmann's contracture, Open fracture management) ^[15] Lecture slides: murtagh merge.pdf (Arm and hand pain p21 — CRPS)

High Yield Summary

Key Concepts for Limb Pain:

Acute vs Chronic arterial ischaemia: 2-week cutoff. Acute = limb-threatening emergency (6-hour window). Chronic = progressive atherosclerosis with collateral formation.
Fontaine classification: I (asymptomatic) → IIa/b (claudication) → III (rest pain) → IV (tissue loss). Stages III–IV = critical limb ischaemia.
6 Ps of acute limb ischaemia: Pain, Pallor, Perishingly cold, Pulseless, Paraesthesia, Paralysis. Pain = early (nerves first), Paralysis = late (muscle infarction).
Embolism vs Thrombosis: Different history, different examination, different management. Emboli = acute/complete/cardiac source. Thrombus = subacute/incomplete/background PVD.
Vascular vs Neurogenic claudication: Constant distance + relief by standing = vascular. Variable distance + relief by bending = neurogenic.
Rest pain vs Neuropathic pain: Rest pain = unilateral, toes/forefoot, relieved by dependency. Neuropathic = bilateral, glove-and-stocking, position-independent.
ABI: 0.9–1.3 = normal, < 0.9 = PAD, < 0.5 = critical ischaemia. Falsely elevated in diabetics (calcification).
Buerger's test: Elevation pallor (Buerger's angle) + dependent reactive hyperaemia.
Compartment syndrome: Pain out of proportion + pain with passive stretch. Don't wait for absent pulses!
Risk factors: Smoking (strongest for PAD), DM (26% risk per 1% HbA1c), plus standard CV risk factors.
LeRiche's syndrome triad: Buttock claudication + absent femoral pulses + erectile dysfunction (aortoiliac occlusion).
Hot, swollen joint = septic arthritis until proven otherwise — rheumatological emergency.

High Yield Summary — Differential Diagnosis of Limb Pain

System-based approach: Vascular → Neurological → Musculoskeletal → Referred/Systemic
Acute limb ischaemia DDx: compartment syndrome and phlegmasia cerulea dolens
DVT DDx: cellulitis, lymphoedema, haematoma, ruptured Baker's cyst
Claudication DDx: vascular (constant distance, standing relief) vs neurogenic (variable distance, flexion relief) vs venous (leg elevation relief)
Hot joint DDx: septic arthritis, gout, pseudogout — always aspirate the joint!
Joint aspirate crystals: MSU = needle, strong negative birefringence; CPP = rhomboid, weak positive birefringence
Polyarthritis pattern: RA = symmetrical small joints (spares DIP); OA = DIP/PIP/weight-bearing; SpA = asymmetrical oligoarthritis LL > UL + enthesitis/dactylitis
Neuropathy pattern: polyneuropathy = glove-and-stocking; mononeuritis multiplex = vasculitis; radiculopathy = dermatomal
Red flags in back/limb pain: continuous day-and-night pain (cancer/infection); cauda equina signs; fever + immunosuppression; chronic steroids (fracture)
Masquerades: diabetes, drugs (statins, fluoroquinolones), spinal dysfunction, malignancy
Referred pain traps: hip → knee (obturator nerve); cardiac → left arm (T1-T4); spine → limb (radiculopathy)
Murtagh's rules: Continuous pain = cancer/infection; Rest pain relieved by activity = inflammatory; Activity pain relieved by rest = mechanical

High Yield Summary — Diagnostics for Limb Pain

ABI ≤ 0.9 = diagnostic for PAD. Use exercise testing if ABI normal but symptomatic. Use TBI if ABI > 1.3 (calcified/diabetic).
Duplex USG = first-line imaging for all PAD. Normal flow is triphasic; single-level disease → biphasic; multi-level → monophasic.
DSA = gold standard for arterial imaging but reserved for planned intervention only (invasive + risks).
Acute limb ischaemia is a clinical diagnosis graded by Rutherford classification using clinical exam + bedside Doppler.
Joint aspiration = MOST IMPORTANT TEST for acute monoarthritis. Always send crystals + Gram stain + culture.
Gout crystals: needle-shaped, strong negative birefringence (yellow parallel). CPPD crystals: rhomboid, weak positive birefringence (blue parallel).
DVT diagnosis: Wells score → D-dimer (low probability) or Duplex USG (high probability). D-dimer: sensitive but not specific.
NCS/EMG: differentiates axonal (reduced amplitude) from demyelinating (reduced velocity) neuropathy — only demyelinating is typically treatable.
MRI spine: investigation of choice for cord compression, radiculopathy, cauda equina — order urgently for cauda equina!
Murtagh's screening bloods: FBE, ESR/CRP, urinalysis, ALP, PSA (males 50–75). XR if red flags. CT/MRI reserved for serious disease.
Compartment syndrome: clinical diagnosis. Pressure measurement if in doubt (delta P ≤ 30 mmHg of diastolic BP). Do NOT delay fasciotomy for measurement if clinically obvious.

High Yield Summary — Management of Limb Pain

Acute limb ischaemia: ABC → IV heparin (ALL patients) → assess Rutherford grade → revascularise (IIa/IIb) or amputate (III). Differentiate embolism (embolectomy) vs thrombosis (thrombolysis/bypass).
Chronic limb ischaemia: Conservative FIRST for intermittent claudication (smoking cessation, supervised exercise, aspirin, statin, cilostazol). Surgery only if failed 6 months conservative or critical ischaemia.
TASC II: A/B → endovascular preferred; C/D → surgery preferred.
Thrombolysis C/I: recent stroke, ICH, intracranial malignancy, active bleeding, major surgery/trauma, uncontrolled HTN.
Cilostazol: PDE3 inhibitor. C/I in CHF.
DVT: proximal DVT → anticoagulate (DOAC or LMWH → warfarin). Cancer → LMWH/DOAC > warfarin. Pregnancy → LMWH. HIT → non-heparin anticoagulant.
Compartment syndrome: emergency fasciotomy. Do not delay for pressure measurement if clinical picture is obvious.
Septic arthritis: aspirate → IV antibiotics → joint washout. Emergency!
Neuropathic pain: gabapentinoids (pregabalin/gabapentin) or amitriptyline first-line. Treat underlying cause.
Gout: NSAIDs/colchicine for acute flare; allopurinol for long-term ULT (check HLA-B*5801 in Chinese patients). Do NOT start allopurinol during acute flare.
RA: early DMARDs (methotrexate first-line); biologics if inadequate response. Surgery: LL before UL; forefoot → knee → hip.
SpA: NSAIDs first-line (continuous use); biologics if BASDAI ≥ 4 despite 2–3 NSAIDs tried.
Surgery rationale for IC: failed surgery is worse than no surgery; second surgery less likely to succeed. Conservative Mx addresses the bigger killer (cardiovascular mortality).

High Yield Summary — Complications of Limb Pain

Compartment syndrome: mechanism = ischaemia → membrane damage → oedema in non-distensible compartment → secondary ischaemia. Earliest symptom = pain out of proportion. Most sensitive sign = pain with passive stretch. Pulses may be PRESENT. Anterior tibial compartment most common. Tx = emergency fasciotomy.
Rhabdomyolysis: K+ → arrhythmia; myoglobin → AKI (ATN); lactic acid → metabolic acidosis. Tx = aggressive hydration + IV bicarbonate + mannitol ± dialysis.
Reperfusion injury: oxygen free radicals → capillary damage → oedema → no-reflow. Thrombolysis has less reperfusion injury than open embolectomy (more gradual reperfusion).
Gangrene: Dry = non-infected, clear demarcation, can self-amputate. Wet = infected, no demarcation, surgical emergency.
PAD prognosis: IC patients more likely to die of MI/stroke than lose limb. Critical CLI: 25% dead, 25% amputated at 1 year.
Post-thrombolysis: stroke and major haemorrhage risk.
Amputation complications: phantom limb pain (Mx: gabapentin/amitriptyline), stump neuroma, DVT, fixed flexion deformity.
CRPS: persistent burning pain disproportionate to injury. Think of it for any limb pain not resolving after trivial trauma.
Volkmann's contracture: late sequela of untreated compartment syndrome, classically in supracondylar fracture.
Diabetic foot: previous ulceration is the most important RF. Neuropathy causes 80% of ulcers.
Immobility complications: DVT/PE, pressure sores, contractures, muscle atrophy, depression.

Limb Pain

Limb Pain

Definition

Epidemiology

Peripheral Arterial Disease (PAD) — the major vascular cause of limb pain

Musculoskeletal causes

Neurological causes

Venous causes

Risk Factors

For Arterial Limb Pain (PAD/Chronic Limb Ischaemia)

For Venous Limb Pain (DVT/VTE)

For Neurogenic Limb Pain

For Musculoskeletal Limb Pain

Anatomy and Function

Arterial Anatomy of the Lower Limb

Vessel Wall Anatomy [2]

Peripheral Nerve Anatomy

Venous Anatomy of the Lower Limb

Compartments of the Lower Leg

Etiology (Focus on Hong Kong)

A. Vascular Causes

1. Arterial — Acute Limb Ischaemia

2. Arterial — Chronic Limb Ischaemia

3. Buerger's Disease (Thromboangiitis Obliterans) [1]

4. Venous — Deep Vein Thrombosis (DVT)

5. Venous — Chronic Venous Insufficiency (CVI) [10]

B. Neurological Causes

1. Peripheral Neuropathy [3][9]

2. Diabetic Peripheral Neuropathy [3]

3. Entrapment Neuropathies [4][9]

4. Neurogenic (Spinal) Claudication [1][2]

C. Musculoskeletal Causes

1. Osteoarthritis (OA)

2. Crystal-Induced Arthritis [5]

3. Rheumatoid Arthritis [6]

4. Septic Arthritis [12]

5. Soft Tissue Injuries

6. Bone Pathology

D. Referred Pain

E. Other Systemic Causes

Classification

A. Chronic Limb Ischaemia — Fontaine Classification [2]

B. Chronic Limb Ischaemia — Rutherford Classification [11]

C. Acute Limb Ischaemia — Rutherford (SVS/ISCVS) Classification [1]

D. Limb Pain by Temporal Pattern

Clinical Features

A. Symptoms

1. Intermittent Claudication (Non-Critical Chronic Limb Ischaemia)

2. Rest Pain (Critical Limb Ischaemia)

3. Acute Limb Ischaemia — The 6 Ps [11]

4. Venous Pain (DVT)

5. Venous Claudication [1]

6. Neurogenic Pain

7. Musculoskeletal Pain

8. Compartment Syndrome [1]

B. Signs

1. Signs of Chronic Limb Ischaemia

2. Signs of Acute Limb Ischaemia

3. Signs of DVT [7]

4. Signs of Chronic Venous Insufficiency

5. Signs of Peripheral Neuropathy [3][9]

6. Signs of Entrapment Neuropathy

7. Signs of Septic Arthritis [12]

8. Signs of Compartment Syndrome [1]

Summary of Pathophysiological Mechanisms Linking to Clinical Features

Active Recall - Limb Pain: Definition to Clinical Features

References

Differential Diagnosis of Limb Pain

A. Region-Specific Differential Diagnoses

1. Lower Limb Pain — Vascular Differential

2. Lower Limb Pain — Claudication Differential

3. Knee Pain [15]

4. Foot and Ankle Pain [15]

5. Arm and Hand Pain [15]

6. Shoulder Pain [13]

7. Neck Pain [15]

8. Lower Back Pain [15]

B. System-Based Unifying Framework

C. Acute Monoarthritis — A Critical Differential

D. Polyarthritis — Differential