Becker muscular dystrophy is an X-linked recessive muscular dystrophy caused by mutations in the dystrophin gene that produce a partially functional but reduced-quantity dystrophin protein, resulting in progressive proximal muscle weakness with later onset and slower progression than Duchenne muscular dystrophy.

Becker Muscular Dystrophy (BMD)

2. Epidemiology

Parameter	Value
Incidence	~1 in 18,000–30,000 live male births (approximately 1/3 to 1/5 as common as DMD)
DMD incidence (for comparison)	~1 in 3,500 male births ^[2]
Prevalence	~2.4–7.3 per 100,000 males (varies by population)
Male-to-female ratio	Almost exclusively males (X-linked recessive); female carriers rarely symptomatic (see below)

3. Anatomy and Function of Dystrophin

Understanding BMD requires understanding what dystrophin does. This is foundational.

Dystrophin is a large cytoskeletal protein (427 kDa) located on the inner surface of the sarcolemma (muscle cell membrane). It is the product of the DMD gene on Xp21.2 — one of the largest genes in the human genome (~2.4 Mb, 79 exons) ^[2].

Function: Dystrophin acts as a molecular shock absorber — it links the internal cytoskeleton (actin filaments inside the muscle cell) to the extracellular matrix (via the dystrophin-associated glycoprotein complex, DAGC) through the sarcolemma.

[Extracellular Matrix (Laminin-α2)]
        ↕
[DAGC: Dystroglycans + Sarcoglycans] ← embedded in sarcolemma
        ↕
[Dystrophin] ← subsarcolemmal scaffold
        ↕
[F-actin cytoskeleton]

Structural Domains of Dystrophin

Domain	Location	Function
N-terminal (actin-binding domain)	Intracellular	Binds F-actin cytoskeleton
Central rod domain	Middle (24 spectrin-like repeats + 4 hinges)	Flexible spacer; absorbs mechanical stress
Cysteine-rich domain	Near C-terminal	Binds β-dystroglycan (transmembrane link)
C-terminal domain	Intracellular, near membrane	Binds syntrophins and dystrobrevins; signalling

Why this matters for BMD: In BMD, mutations (usually in-frame deletions) most commonly affect the central rod domain. Because the rod domain is a flexible spacer with repeating units, deleting some repeats still allows the protein to fold and connect the N-terminal (actin-binding) and C-terminal (membrane-binding) domains → the protein is shorter but retains partial function. This is why BMD is milder than DMD.

4. Etiology

Feature	Detail
Gene	DMD gene, Xp21.2 (same gene as Duchenne) ^[2]
Inheritance	X-linked recessive ^[1]^[2]
Gene size	~2.4 Mb (largest known human gene), 79 exons
Protein product	Dystrophin (427 kDa, 3,685 amino acids)
BMD vs DMD	Allelic disorders — different mutations in the same gene ^[2]^[3]

The type of mutation determines whether the patient develops DMD or BMD. This is governed by the reading-frame rule (Monaco's rule), which holds true in ~90% of cases:

	DMD	BMD
Mutation effect on reading frame	Out-of-frame (frameshift)	In-frame
Dystrophin protein	Absent / non-functional (truncated, rapidly degraded)	Partially functional (shortened but folded) ^[2]^[3]
Phenotype	Severe	Milder

Common Mutation Types in BMD

Mutation Type	Frequency in BMD	Mechanism
In-frame deletions	~60–70%	Deletion of exons that maintains the reading frame → shortened but partially functional dystrophin
In-frame duplications	~5–10%	Duplication of exons maintaining reading frame
Missense mutations	Rare	Single amino acid change affecting protein function but not ablating it
Splice-site mutations	~5%	Altered splicing producing some functional transcript

Hotspot regions: Deletions cluster in two hotspot regions:

Exons 45–55 (most common, ~80% of deletions) — central rod domain
Exons 2–20 (~20% of deletions) — N-terminal/proximal rod domain

Why in-frame deletions in the rod domain are tolerated: The rod domain consists of 24 spectrin-like repeats. Deleting several repeats (if in-frame) simply shortens the "shock absorber" — it's less effective but still bridges actin to the DAGC. Think of it like removing some links from a chain — the chain is shorter but still connects both ends.

5. Pathophysiology

Understanding the pathophysiology of BMD requires tracing from the genetic defect all the way to the clinical phenotype.

Feature	DMD	BMD
Dystrophin	Absent	Partially functional
Sarcolemma stability	Severely compromised	Moderately compromised
Rate of necrosis-regeneration	Very rapid → early satellite cell exhaustion	Slower → delayed exhaustion
Onset of weakness	~2–5 years	Late childhood to adulthood
Loss of ambulation	By ~12 years	Beyond age 15 (often into 30s–40s) ^[2]^[3]
Cardiac involvement	Universal by teens	Variable; may be severe
Life expectancy	20s–30s	40–60 years ^[2]^[3]

6. Classification

BMD sits within the broader classification of muscular dystrophies. This classification has been tested heavily in exams.

Clinical classification of muscular dystrophies: Duchenne (DMD), Becker (BMD), Facioscapulohumeral (FSHD), Limb-girdle (LGMD), Myotonic dystrophy (MyoD) ^[1]^[5]

Classification System	Categories
Clinical classification ^[1]^[5]	DMD, BMD, FSHD, LGMD, Myotonic dystrophy
Immunohistochemical classification ^[2]	Dystrophinopathy (DMD, BMD), Sarcoglycanopathy (LGMD), Others (laminin, caveolin, calpain, dysferin)
Genetic classification ^[2]	By specific gene mutation

Dystrophinopathies exist on a continuum:

Phenotype	Dystrophin Level	Clinical Severity
DMD	< 3% of normal	Most severe
Intermediate DMD/BMD	3–10%	Intermediate
BMD (classic)	10–40%	Moderate
Mild BMD	> 40%	Mild; may present with only cramps or cardiomyopathy
Subclinical/carrier	Variable (in females)	Asymptomatic to mild
X-linked DCM	Cardiac-specific deficiency	Cardiomyopathy without skeletal weakness

Important Concept: The Dystrophinopathy Spectrum

BMD is NOT a single disease entity — it's a spectrum. Some patients are almost as severe as DMD (loss of ambulation in teens), while others remain ambulant into their 50s. The amount and functionality of residual dystrophin determines where on the spectrum a patient falls. X-linked dilated cardiomyopathy (isolated DCM in young males, no skeletal weakness) is the mildest end of the spectrum and is caused by DMD gene mutations that selectively affect cardiac dystrophin expression.

7. Clinical Features

BMD: usually later onset with milder symptoms and better prognosis ^[2]^[3]

Continue to ambulate beyond 15 ^[2]^[3]

Death between 40–60 ^[2]^[3]

Feature	Typical Timing
Symptom onset	Late childhood to adolescence (typically 5–15 years; some as late as 20s–30s)
Loss of ambulation	After age 15 (often 30s–40s; some never lose ambulation)
Cardiac involvement	Teens to 20s (may precede or be disproportionate to skeletal weakness)
Respiratory involvement	Late (30s–50s), milder than DMD
Death	40–60 years (primarily from cardiac or respiratory failure)

7.2 Symptoms (with Pathophysiological Basis)

Symptom	Pathophysiological Basis
Progressive proximal limb weakness	Dystrophin deficiency → sarcolemma instability → ongoing myofibre necrosis → fibrosis → weakness. Proximal muscles (hip girdle, shoulder girdle) are affected first because they are larger, more frequently used under load, and undergo more eccentric contractions
Difficulty climbing stairs, rising from floor, running	Hip girdle (glutei, iliopsoas, quadriceps) weakness — these are the first muscle groups affected. The patient cannot generate sufficient force for antigravity movements
Gower's sign — the patient "climbs up" their own body when rising from the floor	Pelvic girdle muscle weakness ^[2] — the hip extensors and knee extensors are too weak to extend the trunk against gravity from a squatting position, so the patient uses their arms to "walk" hands up the thighs to compensate
Waddling (Trendelenburg) gait	Hip abductor (gluteus medius/minimus) weakness — during single-leg stance phase, the pelvis drops on the unsupported side because the abductors cannot stabilise it
Toe-walking / tight Achilles tendon ^[2]	Progressive ankle plantar flexor contracture due to fibrosis of the gastrocnemius/soleus. The ankle dorsiflexors weaken first (relatively), leading to imbalanced pull → equinus deformity
Exercise intolerance, easy fatigability	Reduced functional muscle mass + inefficient contraction in dystrophic fibres + cardiac limitation
Muscle cramps and myalgia	May be the presenting symptom in milder BMD. Due to abnormal calcium handling in dystrophic fibres → sustained involuntary contraction
Upper limb weakness (later)	Shoulder girdle involvement typically follows hip girdle by years. Difficulty with overhead tasks, carrying heavy objects

Symptom	Pathophysiological Basis
Exertional dyspnoea, orthopnoea, PND	Dilated cardiomyopathy ^[4] → LV systolic dysfunction → ↑LVEDP → pulmonary congestion → dyspnoea
Palpitations	Ventricular or atrial arrhythmias due to myocardial fibrosis creating re-entrant circuits
Syncope / presyncope	Arrhythmia (VT) or ↓cardiac output
Peripheral oedema	Right heart failure (in advanced DCM) → ↑venous hydrostatic pressure

Clinical Pearl: Cardiac Symptoms in BMD

A young man presenting with unexplained dilated cardiomyopathy should ALWAYS prompt consideration of dystrophinopathy — even if he has minimal or no skeletal muscle complaints. BMD cardiomyopathy can be disproportionately severe compared to skeletal muscle involvement. This is a commonly missed diagnosis.

Symptom	Pathophysiological Basis
Nocturnal hypoventilation → morning headaches, daytime somnolence	Diaphragm and intercostal muscle weakness → restrictive ventilatory defect → CO₂ retention during sleep (when respiratory drive is lowest)
Recurrent chest infections	Weak cough → poor airway clearance → aspiration/atelectasis → infection
Progressive respiratory failure	End-stage diaphragm and accessory muscle weakness; may require non-invasive ventilation

Symptom	Pathophysiological Basis
Cognitive/learning difficulties (milder than DMD; ~10–20% of BMD vs ~1/3 of DMD) ^[3]	Dystrophin is expressed in brain (particularly the cerebellar Purkinje neurons and hippocampus). Brain-specific dystrophin isoforms (Dp71, Dp140) may be affected depending on mutation location. Mutations distal in the gene (affecting Dp71/Dp140 promoters) → more cognitive impact
Contractures (ankles, knees, hips, elbows)	Fibrosis of muscles and tendons → loss of elasticity → fixed joint deformity
Scoliosis (less common than DMD)	Paraspinal muscle weakness → loss of spinal support → progressive curvature

7.3 Signs (with Pathophysiological Basis)

Sign	Pathophysiological Basis
Pseudohypertrophy of calf muscles ^[2]^[3]	Necrotic muscle fibres are replaced by fat and fibrous tissue ^[3]. The calves appear enlarged (hypertrophied) but are actually non-functional tissue — hence "pseudo" (false) hypertrophy. This is virtually pathognomonic of dystrophinopathy
Muscle wasting (proximal > distal)	Progressive loss of functional muscle fibres. Proximal muscles waste faster due to greater mechanical stress and earlier involvement
Lordotic posture ^[2]	Trunk and hip extensor weakness → the pelvis tilts anteriorly → compensatory lumbar hyperlordosis to maintain the centre of gravity over the feet
Waddling gait ^[2]	Hip abductor weakness → Trendelenburg sign positive → the trunk lurches side to side during walking to shift the centre of gravity over the stance leg
Toe-walking	Achilles tendon contracture (see above)
Scapular winging	Periscapular muscle weakness (serratus anterior, lower trapezius) → the scapula protrudes posteriorly, especially during forward flexion of the arm

Sign	Pathophysiological Basis
Calves feel rubbery/firm	Fat and fibrotic tissue replacing muscle
Muscle tenderness (variable)	Active necrosis/inflammation in dystrophic fibres

Sign	Pathophysiological Basis
Proximal weakness > distal	Proximal muscles are preferentially affected in dystrophinopathies (larger muscles, more mechanical stress)
Gower's sign positive ^[2]	Pelvic girdle weakness (see above)
Normal or reduced reflexes (NOT brisk)	This is a myopathic pattern — the reflex arc (LMN) is intact, but the effector (muscle) is weak. Reflexes are reduced late when muscle is severely wasted. No upper motor neuron signs
No sensory loss ^[2]	Dystrophin is a muscle protein — sensory nerves are completely unaffected. Sensory loss would point you away from myopathy
No fatigability ^[2]	Unlike myasthenia gravis (NMJ disorder), repeated testing does NOT worsen the weakness because the problem is structural (muscle), not synaptic transmission
Symmetrical weakness ^[2]	Genetic disorder affecting all muscle fibres → bilateral and symmetrical. Asymmetry would suggest an alternative diagnosis

Sign	Pathophysiological Basis
Displaced, diffuse apex beat	LV dilatation in DCM → the apex shifts laterally and inferiorly
S3 gallop	Rapid ventricular filling into a volume-overloaded, dilated LV
Pansystolic murmur (mitral regurgitation)	Functional MR due to LV dilatation → mitral annular dilatation → valve leaflets cannot coapt ^[4]
Elevated JVP, peripheral oedema, hepatomegaly	Right heart failure secondary to biventricular DCM

Sign	Pathophysiological Basis
Reduced chest expansion	Intercostal muscle weakness → restrictive defect
Paradoxical abdominal breathing	Diaphragm weakness → reliance on accessory muscles; the abdomen may move inward during inspiration (instead of outward) when the diaphragm is severely weak
Weak cough	Expiratory muscle weakness → inability to generate adequate intrathoracic pressure

Feature	DMD	BMD
Dystrophin	Absent	Partial
Mutation	Out-of-frame	In-frame
Onset	2–5 years	5–15 years (or later)
Gower's sign	Early	Present but later
Pseudohypertrophy	Prominent	Present
Loss of ambulation	< 12–13 years	> 15 years
Cognitive impairment	~1/3	~10–20% (milder)
Cardiomyopathy	Universal	Variable; can be severe/predominant
CK elevation	Massively elevated ( > 10,000)	Elevated (typically 200–10,000) ^[2]
Life expectancy	20s–30s	40–60 years

8. Relevant Associations and Syndromic Features

BMD patients have an increased risk of rhabdomyolysis under general anaesthesia (particularly with succinylcholine and volatile agents)
Not classic malignant hyperthermia (which is RYR1-related), but a "malignant hyperthermia-like" reaction due to sarcolemma fragility → massive Ca²⁺ release → rhabdomyolysis
Anaesthetic precaution: avoid succinylcholine and volatile agents; use total intravenous anaesthesia (TIVA) ^[7]

Anaesthetic Safety in BMD

ALL patients with dystrophinopathies (DMD or BMD) should be flagged for anaesthetic risk. Succinylcholine can cause fatal hyperkalemia and cardiac arrest due to massive rhabdomyolysis from fragile muscle membranes. This is a life-threatening complication and must be communicated to the anaesthetist before any procedure.

High Yield Summary

Definition: BMD is an X-linked recessive dystrophinopathy caused by in-frame mutations in the DMD gene (Xp21.2), producing partially functional dystrophin — allelic with DMD but milder.

Epidemiology: ~1/18,000–30,000 male births. 1/3 cases are de novo. Female carriers may develop cardiomyopathy.

Pathophysiology: Partial dystrophin → reduced sarcolemma stability → contraction-induced membrane tears → Ca²⁺ influx → myofibre necrosis → fibrosis + fatty replacement → progressive weakness. Same process in cardiac myocytes → dilated cardiomyopathy.

Key Clinical Features:

Later onset (5–15 years or later), milder symptoms, better prognosis than DMD
Continue to ambulate beyond age 15; death 40–60 years
Proximal weakness, Gower's sign, pseudohypertrophy of calves, waddling gait
Dilated cardiomyopathy — can be disproportionately severe; leading cause of death
No sensory loss, no fatigability, normal/reduced reflexes
Elevated CK (typically 200–10,000 U/L)

Must-Know Classifications:

Clinical: DMD, BMD, FSHD, LGMD, Myotonic dystrophy
Immunohistochemical: Dystrophinopathy, Sarcoglycanopathy, Others
Old definition: inherited, all symptoms due to weakness, progressive, no histopathological abnormalities other than degeneration and regeneration

Cardiac Association: BMD is a cause of familial dilated cardiomyopathy — consider dystrophinopathy in any young male with unexplained DCM.

Active Recall - Becker Muscular Dystrophy (Overview, Etiology, Pathophysiology, Clinical Features)

Differential Diagnosis of Becker Muscular Dystrophy

When a patient — typically a young male — presents with progressive, symmetrical, proximal muscle weakness, the differential diagnosis is broad. The clinical challenge is to localise the lesion first (is it muscle, neuromuscular junction, nerve, anterior horn cell, or central?) and then narrow within the myopathy category. BMD sits within the hereditary/congenital myopathy group, but many acquired conditions mimic it.

Before diving into specific differentials, you must first determine whether the weakness is truly myopathic. This is because proximal weakness can arise from lesions at multiple levels of the neuraxis.

Anatomical differentials for weakness include: cerebrum, brainstem, spinal cord, anterior horn cells, nerve root, brachial plexus, peripheral nerve, neuromuscular junction, muscle, bone and joints, metabolic, functional ^[8]

The key clinical features that localise to muscle (myopathy) are ^[5]^[9]:

Feature	Myopathy	Neuropathy	NMJ (e.g. MG)	Anterior Horn Cell (e.g. SMA/MND)
Distribution	Proximal > distal	Distal > proximal (usually)	Ocular, bulbar, proximal	Variable (proximal or distal)
Symmetry	Symmetrical	May be asymmetric	Symmetrical	May be asymmetric
Reflexes	Normal or reduced (late)	Absent early	Normal	May be brisk (if UMN) or absent (if LMN)
Sensory loss	Absent	Present	Absent	Absent
Fatigability	No	No	Yes	No
Fasciculations	No	Rare	No	Yes (LMN component)
Muscle bulk	Wasting ± pseudohypertrophy	Wasting	Normal early	Wasting
CK	Elevated	Normal/mildly elevated	Normal	Normal/mildly elevated

High Yield Localisation Principle

Clinical features of myopathy: symmetrical proximal muscle weakness, normal reflex, no fatigability, no sensory loss ^[5]. If any of these are violated, think of an alternative localisation. Sensory loss → neuropathy. Fatigability → NMJ. Fasciculations + hyperreflexia → motor neuron disease. This is fundamental to your exam approach.

3. Differential Diagnosis by Category

This is organised using the comprehensive framework from lecture slides and senior notes ^[6]^[7]^[9]^[10].

Differential diagnosis of myopathy categories: Infection, Inflammation, Congenital, Endocrine, Drug-induced, Channelopathy, Others ^[7]^[9]

Pathological differentials for lower limb weakness — Congenital: muscular dystrophy ^[6]^[8]

These are the conditions most commonly confused with BMD because they share a genetic, progressive, proximal weakness pattern.

Condition	Inheritance	Key Distinguishing Features	Why it differs from BMD
Duchenne muscular dystrophy (DMD) ^[2]^[3]	X-linked recessive	Earlier onset (~2–5y), more severe, loss of ambulation by 9–12y, death by late teens–20s, CK massively elevated ( > 10,000)	Same gene but out-of-frame mutation → absent dystrophin → much more rapid progression. BMD patients continue to ambulate beyond 15 ^[2]^[3]
Limb-girdle muscular dystrophy (LGMD) ^[2]^[3]^[5]	AD or AR	Predominantly shoulder and hip muscle weakness, no pseudohypertrophy (usually), variable CK	Different genes (sarcoglycans, calpain, dysferlin, etc.). Immunohistochemically: sarcoglycanopathy vs dystrophinopathy ^[2]. Can closely mimic BMD clinically — genetic testing is essential to distinguish
Facioscapulohumeral muscular dystrophy (FSHD) ^[2]^[3]^[5]	AD	Facial weakness (can't whistle, can't close eyes tightly), periscapular and humeral weakness, scapular winging, asymmetric involvement	BMD does NOT typically involve facial muscles. FSHD has a characteristic facial-first, descending pattern. Caused by D4Z4 repeat contraction on chromosome 4q35, NOT the DMD gene
Emery-Dreifuss muscular dystrophy ^[5]	X-linked or AD	Early contractures (elbows, Achilles tendons, posterior cervical muscles) BEFORE significant weakness, cardiac conduction defects (heart block > cardiomyopathy)	The contractures-first pattern is distinctive. Cardiac involvement is conduction disease (AV block), not primarily DCM as in BMD. Caused by emerin (X-linked) or lamin A/C (AD) mutations
Myotonic dystrophy (MyD) ^[3]^[5]	AD	Distal weakness (not proximal!), myotonia (delayed muscle relaxation), myopathic facies, frontal baldness, cataracts, CTG trinucleotide repeat in DMPK gene	BMD has proximal weakness and NO myotonia. MyD is the only "dystrophy" with predominantly distal weakness — a classic distinguishing point ^[5]
Oculopharyngeal muscular dystrophy ^[5]	AD	Late onset (50s–60s), ptosis, dysphagia, proximal limb weakness	Much later onset than BMD. The ocular and pharyngeal involvement is distinctive. Caused by GCN trinucleotide repeat in PABPN1 gene

High Yield: DMD vs BMD — The Critical DDx

The most important differential for BMD is DMD — they are allelic disorders (same gene, Xp21.2) ^[2]^[3]. The reading-frame rule predicts ~90% of cases: in-frame mutations → partially functional dystrophin → BMD; out-of-frame mutations → absent dystrophin → DMD. Clinically: BMD patients continue to ambulate beyond age 15; DMD patients lose ambulation by 9–12 years ^[2]^[3]. When in doubt, genetic testing and muscle biopsy with dystrophin immunostaining resolve the question.

These are critical differentials because they are treatable — missing them has major consequences.

Condition	Key Distinguishing Features	Why it differs from BMD
Polymyositis (PM) ^[7]^[9]^[10]	Subacute onset (weeks–months), proximal weakness, very high CK ( > 10× ULN), EMG shows spontaneous fibrillation potentials, muscle biopsy shows endomysial lymphocytic infiltrates	BMD is chronic (years–decades), with childhood/adolescent onset and family history. PM has no pseudohypertrophy. PM responds to immunosuppression; BMD does not
Dermatomyositis (DM) ^[7]^[9]^[10]	As for PM plus characteristic skin findings: heliotrope rash (periorbital), Gottron's papules (knuckles), V-sign, shawl sign. Associated with malignancy in adults	Skin findings are absent in BMD. DM has a subacute course. Adult DM: 5× risk of malignancy ^[3]
Inclusion body myositis (IBM) ^[9]^[10]	Insidious onset, distal > proximal weakness (finger flexors, quadriceps), asymmetric, poor response to immunosuppression, elderly males	BMD is proximal and symmetrical. IBM's characteristic pattern of wrist/finger flexor + quadriceps weakness is distinctive ^[10]

Hypothyroidism can mimic inflammatory myopathy with subacute onset of proximal muscle weakness and elevated CK ^[10]. Always check TFT in the workup of any proximal myopathy.

Condition	Key Distinguishing Features	Why it differs from BMD
Hypothyroid myopathy ^[7]^[9]^[10]	Proximal weakness, elevated CK, associated with other hypothyroid features (fatigue, weight gain, bradycardia, delayed relaxation of reflexes)	Acquired, reversible with thyroid replacement. No pseudohypertrophy. TFT diagnostic
Cushing's syndrome / steroid myopathy ^[7]^[9]	Proximal weakness (especially hip girdle), normal CK (distinctive!), cushingoid features	CK is normal in steroid myopathy — this is a key differentiator. BMD always has elevated CK
Hyperthyroid myopathy ^[7]^[9]	Proximal weakness, weight loss, tremor, tachycardia, hyperreflexia	Acquired and reversible. Hypermetabolic features are absent in BMD
Electrolyte disturbance (hypoK, hyperCa, hypoNa, hypoPO₄) ^[7]^[9]	Acute/subacute weakness, often generalised, correctable with electrolyte correction	Acute onset distinguishes from the chronic progressive course of BMD. Check electrolytes in any new-onset weakness
Metabolic myopathies (glycogen storage diseases, lipid metabolism disorders) ^[5]	Exercise intolerance, cramps, second-wind phenomenon (McArdle's), myoglobinuria	These present with episodic rather than progressive weakness. Forearm exercise test and specific enzyme assays help distinguish

Drug-induced myopathy: glucocorticoids, statins, colchicine ^[6]^[9]

Drug	Mechanism	Key Features
Statins (HMG-CoA reductase inhibitors) ^[7]^[9]	Interfere with cholesterol synthesis in muscle membranes (coenzyme Q10 depletion, membrane instability)	Myalgia, proximal weakness, elevated CK. Usually temporal relationship with drug initiation. Resolves on discontinuation
Corticosteroids ^[7]^[9]	Type II fibre atrophy via protein catabolism	Proximal weakness, normal CK (important!), cushingoid features. Dose-dependent
Colchicine ^[9]	Disrupts microtubules → impaired autophagy in muscle	Often combined myopathy + neuropathy. Check drug history

Clinical Pearl: CK Normal in Steroid Myopathy

If a patient with proximal weakness has a normal CK, think steroid myopathy or endocrine myopathy. In BMD, CK is always elevated (typically 200–10,000 U/L) ^[5]. A normal CK in the context of proximal weakness essentially rules out dystrophinopathy.

Condition	Key Distinguishing Features
Viral myositis (influenza, Coxsackie, HIV, CMV, EBV) ^[6]^[7]^[9]	Acute onset, myalgia, elevated CK, temporal relationship with viral illness. Self-limiting in most cases
Pyomyositis ^[7]^[9]	Bacterial infection of muscle (usually Staph aureus). Focal, painful, swollen muscle ± abscess. Fever, leucocytosis
Parasitic (toxoplasmosis, trichinosis, cysticercosis) ^[7]^[9]	Travel history, eosinophilia, specific serologies

These are easily distinguished from BMD by their acute onset, systemic infection features, and temporal relationship with illness.

Condition	Key Distinguishing Features
Myasthenia gravis (MG) ^[10]	Fatigability (weakness worsens with repeated use), fluctuating course, ptosis and diplopia (ocular involvement), normal CK, positive anti-AChR or anti-MuSK antibodies, decremental response on repetitive nerve stimulation

Myasthenia gravis is distinguished from myositis by presence of facial muscle weakness, normal muscle enzymes, characteristic EMG changes, and AChR antibodies ^[10]

BMD does NOT have fatigability or ocular involvement. This is a fundamental distinction.

Condition	Key Distinguishing Features
Spinal muscular atrophy (SMA) ^[7]	AR inheritance, degeneration of anterior horn cells, hypotonia, fasciculations, areflexia, tongue fasciculations. 2nd most common neuromuscular disease after DMD ^[7]. Onset depends on type (Type 1: infancy; Type 3: childhood — may overlap with BMD presentation age)
Motor neuron disease (ALS)	Mixed UMN + LMN signs (fasciculations + hyperreflexia), progressive, typically adult onset. No pseudohypertrophy

SMA Type 3 (Kugelberg-Welander) can present with proximal weakness in childhood/adolescence and may mimic BMD. Key distinguishers: fasciculations, areflexia (LMN pattern), and EMG showing denervation (not myopathic pattern). Genetic testing for SMN1 confirms SMA.

Condition	Key Distinguishing Features
Periodic paralysis (hypokalemic, hyperkalemic, thyrotoxic) ^[7]^[9]	Episodic weakness (not progressive), often triggered by meals/exercise/rest after exercise, serum K+ abnormalities during attacks. Between attacks, patient is normal
Myotonia congenita	Myotonia WITHOUT progressive weakness. Distinguished by warm-up phenomenon. No dystrophic changes on biopsy

Periodic paralysis is episodic, not progressive — this is the key distinction from BMD.

Condition	Why it may be confused	How to distinguish
Cardiac deconditioning / heart failure	Exercise intolerance, fatigue	No true muscle weakness on formal testing. Cardiac investigations reveal the cause
Chronic fatigue syndrome / functional weakness	Subjective weakness, fatigue	No objective weakness on examination, normal CK, normal EMG. Positive Hoover's sign

4. Differential Diagnosis for Specific BMD Presentations

BMD can present in several ways. The differential changes depending on the presentation:

Differential diagnosis of floppy infant syndrome ^[7]:

Category	Differential
Non-paralytic (central)	Cerebral palsy, Down syndrome, Prader-Willi syndrome
Paralytic — anterior horn cells	SMA, poliomyelitis
Paralytic — peripheral nerve	Charcot-Marie-Tooth disease
Paralytic — NMJ	Congenital myasthenia gravis, transient neonatal myasthenia
Paralytic — muscle	DMD, BMD

Note: BMD rarely presents as a floppy infant (it is typically later-onset), but severe BMD can present with early hypotonia and delayed motor milestones, entering this differential.

Feature	BMD	DMD	LGMD	FSHD	MyD	PM/DM	SMA
Inheritance	XLR	XLR	AD/AR	AD	AD	None	AR
Onset	Late child–adult	2–5y	Variable	Teens–20s	15–40y	Any age	Infancy–adult
Distribution	Proximal	Proximal	Proximal	Face/scapula	Distal	Proximal	Proximal
Pseudohypertrophy	Yes	Yes	Rare	No	No	No	No
Myotonia	No	No	No	No	Yes	No	No
CK	↑↑ (200–10k)	↑↑↑ ( > 10k)	↑↑	N/↑	N/↑	↑↑↑	N/↑
Cardiac	DCM	DCM	Variable	Rare	DCM + conduction	Rare	Rare
Sensory loss	No	No	No	No	No	No	No
Fatigability	No	No	No	No	No	No	No
Fasciculations	No	No	No	No	No	No	Yes

References

^[2] Senior notes: Ryan Ho Neurology.pdf (p192 — Muscular Dystrophies, X-linked Dystrophinopathies, BMD vs DMD classification) ^[3] Senior notes: Adrian Lui Pediatrics Notes.pdf (p144–145 — DMD vs BMD clinical features, myotonic dystrophy, inflammatory myopathies) ^[4] Senior notes: Block A - Inherited Cardiac conditions.pdf (p5 — Familial DCM, neuromuscular causes including BMD, TTN and LMNA genes) ^[5] Senior notes: Maksim Medicine Notes.pdf (p276 — Myopathy general approach, causes, clinical features, CK levels, muscular dystrophy key features) ^[6] Lecture slides: Neurology- Two cases of lower limb weakness.pdf (p38 — Differential diagnosis of myopathy categories) ^[7] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p535 — Floppy infant DDx; p706 — Differential diagnosis of myopathies table) ^[8] Lecture slides: CFB_Neuro clinical skills demonstration_01.08.22_file to students.pdf (p7–8 — Anatomical and pathological differentials for weakness) ^[9] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p1116 — Differential diagnosis of myopathy comprehensive table) ^[10] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p1757 — DDx of inflammatory myopathy: hypothyroidism, IBM, drug-induced, MG, muscular dystrophy, myotonic dystrophy)

Diagnostic Criteria, Algorithm, and Investigations for Becker Muscular Dystrophy

1. Diagnostic Criteria

Unlike many medical conditions (e.g. rheumatoid arthritis with ACR/EULAR criteria, or SLE with SLICC criteria), there is no single universally accepted "checklist" diagnostic criteria for BMD. Instead, the diagnosis rests on a convergence of clinical, biochemical, histopathological, and genetic evidence. The definitive diagnosis is genetic ^[2]^[3].

Dx: usually based on genetics ^[2]^[3]

A clinical diagnosis of BMD should be strongly suspected when ALL of the following are present:

Criterion	Rationale
Male sex	X-linked recessive inheritance — almost exclusively males affected
Progressive proximal muscle weakness	The hallmark of all dystrophinopathies — proximal > distal
Onset later than typical DMD (usually > 5 years, often 5–15y or even later)	Partial dystrophin → slower rate of muscle destruction → later presentation
Ambulant beyond age 15 (or age 16 in some classification systems)	This is the traditional clinical cut-off used to distinguish BMD from DMD operationally ^[2]^[3]
Calf pseudohypertrophy ^[5]	Virtually pathognomonic of dystrophinopathy (fat + fibrosis replacing muscle)
Gower's sign ^[5]	Indicates pelvic girdle weakness — classic for dystrophinopathy
Elevated serum CK (typically 1,000–10,000 U/L)	Sarcolemma instability → CK leaks into blood
X-linked family history (if present; but 1/3 are de novo)	Compatible inheritance pattern
Absence of sensory loss, fatigability, UMN signs, myotonia	Rules out alternative localisations and diagnoses

The definitive diagnosis requires one of the following:

Level	Method	Findings for BMD
Gold standard	Genetic testing ^[2]^[3]^[5]	Identification of an in-frame mutation in the DMD gene (Xp21.2) — deletions, duplications, or point mutations that preserve the reading frame
Supportive	Muscle biopsy with dystrophin immunostaining	Reduced but present dystrophin (patchy, variable intensity) — in contrast to DMD where dystrophin is absent
Supportive	Western blot of dystrophin	Dystrophin protein of reduced quantity and/or abnormal molecular weight (truncated but present)

High Yield: The Clinical BMD-vs-DMD Rule

The traditional clinical distinction between BMD and DMD is based on ambulation status: BMD patients continue to ambulate beyond age 15; DMD patients lose ambulation by 9–12 years ^[2]^[3]. However, this is a retrospective clinical criterion — genetic testing resolves the question prospectively. The reading-frame rule (in-frame = BMD, out-of-frame = DMD) is ~90% accurate.

3. Investigation Modalities

The investigations for BMD serve three purposes:

Confirm the diagnosis (CK, genetic testing, muscle biopsy)
Exclude differentials (NCS/EMG, TFT, autoantibodies)
Characterise the phenotype and monitor complications (ECG, echo, PFTs)

CK: released from damaged muscle membrane ^[2]^[3] CK 200–1,000 IU/L: most myopathies, SMA/MND, rigorous exercise, IM injection, after EMG/muscle biopsy, tonic-clonic seizures, statin/neuroleptic-induced ^[2]^[3] CK 1,000–10,000 IU/L: inflammatory myopathies, acute rhabdomyolysis, DMD/BMD ^[2]^[3]

Parameter	Detail
What is CK?	Creatine kinase is an intracellular enzyme in skeletal and cardiac muscle. When the sarcolemma is damaged, CK leaks into the bloodstream. It is the most sensitive blood marker of muscle damage
Normal range	~22–198 U/L (varies by lab; slightly higher in males and with exercise)
BMD typical range	1,000–10,000 U/L ^[2]^[5] — significantly elevated, reflecting ongoing sarcolemma instability
DMD typical range	Often > 10,000–50,000 U/L (much higher because dystrophin is completely absent → more severe membrane damage)
Temporal pattern	CK is highest in early/active disease when necrosis rate is high. As muscle mass is replaced by fat and fibrosis in late disease, CK may paradoxically fall (less muscle left to leak CK from)
Key point	CK is elevated even in presymptomatic patients and female carriers — this can be a screening tool

Exam Pearl: CK Interpretation

A normal CK essentially excludes dystrophinopathy. If CK is normal in a patient with proximal weakness, think steroid myopathy (CK always normal), endocrine myopathy, or a non-myopathic cause. In BMD, CK is always elevated, even before symptoms appear.

Other muscle enzymes to check: ALT, AST, LDH — these are also released from damaged muscle. A common pitfall: elevated ALT/AST in a BMD patient is from muscle, NOT liver. Always check CK alongside "liver enzymes" in any patient with myopathy to avoid unnecessary hepatological workup.

This is the first-line and definitive diagnostic test for BMD ^[2]^[3]^[5].

Key investigations for muscular dystrophy: genetic testing (dystrophin – Xp21.2) ^[5]

Test	Method	What It Detects	Sensitivity
MLPA (Multiplex Ligation-dependent Probe Amplification)	Hybridisation-based technique quantifying copy number of each of the 79 exons of the DMD gene	Deletions (~65% of BMD mutations) and duplications (~5–10%)	~70–75% for all dystrophinopathy mutations
NGS panel / Sanger sequencing	DNA sequencing of all 79 exons + splice sites	Point mutations, small insertions/deletions, splice-site mutations NOT detected by MLPA	Picks up the remaining ~25–30%
RNA analysis (from muscle biopsy)	RT-PCR of dystrophin mRNA	Deep intronic mutations, complex rearrangements	Reserved for cases negative on MLPA + NGS

Stepwise approach:

MLPA first — cheap, fast, detects the most common mutations (large deletions/duplications)
If MLPA negative → NGS / Sanger sequencing — detects point mutations
If both negative but clinical suspicion remains high → muscle biopsy (see below) and/or RNA studies

Interpreting the result — the reading-frame rule:

In-frame mutation → dystrophin mRNA is translated into a shorter but partially functional protein → BMD
Out-of-frame (frameshift) mutation → premature stop codon → truncated, non-functional protein rapidly degraded → DMD
This rule holds in ~90% of cases. The ~10% exceptions include unusual splice effects, internal translation re-initiation, and exon-skipping events

Genetic studies are listed as a key investigation for lower limb weakness workup ^[6]

NCS: to rule out neuropathy (can also cause reduced motor response) ^[2]^[5] EMG: polyphasic, low-amplitude motor unit potential ^[5]

These are neurophysiological tests used to confirm the localisation is myopathic and exclude neuropathic mimics.

Test	What It Measures	Expected Findings in BMD	Why
NCS (motor)	Speed and amplitude of nerve conduction	Normal conduction velocities and distal latencies; normal or mildly reduced compound muscle action potential (CMAP) amplitudes	The nerves are completely intact in BMD. CMAP may be slightly reduced because the muscle (effector) is weak, but the nerve conducts normally
NCS (sensory)	Sensory nerve action potentials (SNAPs)	Completely normal	No sensory nerve involvement in myopathy
EMG	Electrical activity of muscle fibres	Myopathic pattern: polyphasic, short-duration, low-amplitude motor unit potentials (MUPs) ^[5]; early recruitment	Individual motor units recruit fewer and smaller muscle fibres (because many fibres are necrotic/fibrotic) → each MUP is smaller and shorter. To generate force, more motor units are recruited simultaneously (early/rapid recruitment)
EMG at rest	Spontaneous activity	May show fibrillation potentials and positive sharp waves (indicating active denervation of individual fibres from necrosis, not neurogenic denervation per se)	Active myofibre necrosis disrupts the motor endplate on individual fibres → those fibres become functionally denervated even though the nerve is intact

Neurophysiological tests: nerve conduction studies, electromyography — used to confirm lesion location and cause of presentation ^[6]

Important: EMG Myopathic vs Neurogenic Pattern

The key EMG distinction:

Myopathic (BMD): short duration, low amplitude, polyphasic MUPs; early recruitment
Neurogenic (e.g. SMA, MND): long duration, high amplitude MUPs; reduced recruitment (fewer motor units available, so each fires faster)

This distinction is fundamental and frequently tested. If EMG shows a neurogenic pattern in a patient you suspected had BMD, reconsider SMA or motor neuron disease.

Muscle biopsy: at muscle that is most affected ^[2] Site: done on weak but not atrophied muscle, guided by P/E, EMG ± MRI ^[12]

Muscle biopsy is no longer the first-line diagnostic test for suspected dystrophinopathy (genetic testing has replaced it), but it remains critical when:

Genetic testing is inconclusive (no mutation found on MLPA + NGS)
Clinical phenotype is atypical and you need to differentiate from LGMD or inflammatory myopathy
You need to quantify dystrophin for prognostication or trial eligibility

Technique	Findings in BMD
H&E staining	Dystrophic changes: variation in fibre size (both atrophic and hypertrophic fibres), central nuclei (regenerating fibres), necrotic fibres, fibrosis, fatty infiltration. These are non-specific "cycles of necrosis, regeneration, eventual fibrosis and fatty tissue replacement" ^[2]^[3]
Dystrophin immunohistochemistry	Reduced, patchy staining — some fibres show partial dystrophin, others have absent or faint staining. In DMD, staining is completely absent. This is the key histological discriminator
Western blot (immunoblot)	Dystrophin band of reduced intensity and/or abnormal molecular weight (truncated protein runs at a lower kDa than normal 427 kDa). In DMD, the band is absent
Other immunostains	Sarcoglycan complex may be secondarily reduced (because the DAGC partially disassembles without full dystrophin) → helps distinguish from primary sarcoglycanopathy (LGMD) where sarcoglycans are primarily absent but dystrophin is normal

Biopsy site selection:

Choose a muscle that is clinically weak but NOT severely atrophied — a completely wasted muscle will show only end-stage fibrosis and fat with no diagnostic information
Common sites: vastus lateralis (quadriceps), biceps brachii, deltoid
Avoid muscles recently tested with EMG needle (causes artefactual changes)

Cardiac assessment is mandatory in ALL BMD patients because cardiomyopathy is the leading cause of death and can be disproportionately severe relative to skeletal muscle involvement.

ECG / echo: detect cardiac muscle involvement ^[2]^[5] Key investigations for muscular dystrophy: ECG / echo: dilated cardiomyopathy ^[5]

Investigation	Expected Findings in BMD	Interpretation
ECG	May show: tall R waves in V1, deep Q waves in lateral leads (inferolateral fibrosis pattern), RBBB or LBBB, ST-T changes, atrial/ventricular arrhythmias	Myocardial fibrosis → abnormal conduction pathways; LV dilatation → axis shifts. No specific ECG features for DCM, but usually abnormal: AF, AV block, LBBB ^[4]
Echocardiography	LV dilatation, reduced LVEF ( < 50%) ^[4], ± functional MR, ± RV dilatation	Dystrophin-deficient cardiac myocytes undergo progressive necrosis → fibrosis → LV dilatation → systolic dysfunction. The LV end-diastolic volume/diameter > 2SD from age/BSA-corrected nomograms and LVEF < 50% define DCM ^[4]
Cardiac MRI (CMR)	Late gadolinium enhancement (LGE) in the inferolateral/subepicardial wall; oedema on T2-weighted sequences ^[4]	LGE indicates myocardial fibrosis — this is often present before echo shows reduced EF and can guide early ACE-inhibitor therapy. Oedema suggests active "hot phase" of cardiomyopathy ^[4]. CMR is more sensitive than echo for early disease
24-hour Holter monitor	Ventricular ectopics, non-sustained VT, supraventricular arrhythmias	Fibrotic myocardium creates re-entrant circuits → arrhythmia risk. Holter screening helps risk-stratify for sudden cardiac death

Frequency of cardiac screening:

At diagnosis, then annually (or more frequently if abnormalities detected)
Echocardiography ± cardiac MRI from age 10 onwards (some guidelines recommend from diagnosis regardless of age)
Earlier and more frequent if symptoms develop

High Yield: Cardiac MRI in BMD

Cardiac MRI with late gadolinium enhancement (LGE) can detect myocardial fibrosis BEFORE echocardiographic changes become apparent ^[4]. This is why many centres now perform CMR routinely in BMD patients — it allows for early initiation of cardioprotective therapy (ACE inhibitors / beta-blockers) even before LVEF drops. The typical fibrosis pattern is subepicardial, starting in the inferolateral wall — this is distinct from the subendocardial pattern of ischaemic cardiomyopathy.

Test	Expected Findings	Interpretation
Forced vital capacity (FVC)	Reduced in advanced disease (restrictive pattern)	Respiratory muscle (diaphragm + intercostal) weakness → unable to fully expand the chest → ↓FVC
FEV1/FVC ratio	Normal or increased (restrictive, NOT obstructive)	The airways are normal; the problem is weak respiratory muscles, not airway obstruction
Peak cough flow (PCF)	Reduced	Weak expiratory muscles → ineffective cough → ↑risk of aspiration/atelectasis/infection
Supine vs upright FVC	> 20% drop in supine FVC suggests diaphragm weakness	When supine, abdominal contents push up against the weak diaphragm, further reducing lung volumes
Nocturnal oximetry / sleep study	Desaturations during REM sleep	Diaphragm is the main respiratory muscle during REM sleep; weakness → nocturnal hypoventilation

In BMD, respiratory involvement is typically later and milder than in DMD, but should still be monitored from diagnosis onwards with annual PFTs.

Basic blood tests: CBC, inflammatory markers, basic biochemistry, other metabolic markers — vitamin B12, thyroid function tests, lipid profile, glucose etc., creatine kinase ^[6]

Further blood tests: autoimmune markers (ANA, anti-dsDNA, anti-ENA, anti-AChR Ab, anti-HMG-CoA reductase Ab…), genetic studies ^[6]

Investigation	Purpose	Interpretation
TFT	Rule out thyroid myopathy ^[2]^[3]	Hypothyroidism causes proximal weakness + elevated CK → can mimic BMD. If TSH is elevated, treat and reassess
Electrolytes (K⁺, Ca²⁺, PO₄³⁻, Na⁺)	Rule out electrolyte-related myopathy	HypoK, hypoCa, hypoPO₄ → acute/subacute weakness. Should be normal in BMD
ESR/CRP	Rule out inflammatory myopathy	Elevated in PM/DM, normal in BMD (no systemic inflammation)
ANA, anti-ENA, anti-Jo-1	Rule out autoimmune/inflammatory myopathy ^[2]	Positive in PM/DM/overlap syndromes, negative in BMD
CBC	General screen	Usually normal in BMD. Eosinophilia might suggest parasitic myositis
LFT	Assess "liver enzymes"	ALT/AST may be elevated in BMD — this is from muscle, not liver. Always correlate with CK to avoid misinterpretation

Feature	Detail
Role	Non-invasive assessment of muscle composition; increasingly used for monitoring disease progression and selecting biopsy sites
Findings in BMD	T1W: fatty infiltration (bright signal replacing muscle in affected compartments). Preferential involvement of posterior compartment of thigh (hamstrings, adductors) and medial gastrocnemius. T2W/STIR: oedema in actively inflamed/degenerating muscles
Utility	Guides muscle biopsy site selection (identifies a muscle that is affected but not completely replaced by fat). Monitors disease progression over time. Research tool for clinical trials

Action	Detail
Carrier testing for female relatives	MLPA/NGS of the DMD gene in mother, sisters, and other at-risk females. CK can be a screening tool (elevated in ~60% of carriers) but is NOT definitive
Prenatal diagnosis	Chorionic villus sampling (CVS) at 10–12 weeks or amniocentesis at 15–18 weeks with genetic testing of fetal DNA if mother is a known carrier
Preimplantation genetic testing (PGT)	Available for families undergoing IVF; selects embryos without the DMD mutation
Cascade screening	Screen first-degree male relatives of the proband with CK ± genetic testing
3-generation family tree ^[4]	Essential component of genetic counselling for any suspected familial condition
Germline mosaicism counselling	Even if the mother tests negative for the mutation, there is a ~7–10% recurrence risk due to germline mosaicism → must be communicated

For idiopathic DCM: genetic counselling → 3-generation family tree, genetic screening ^[4]

Priority	Investigation	Purpose
1st	Serum CK	Screening; massively elevated in dystrophinopathy
2nd	Genetic testing (MLPA → NGS)	Definitive diagnosis; identifies specific mutation and reading frame
3rd	EMG/NCS	Confirms myopathic pattern; excludes neuropathy
4th	ECG + Echocardiography	Cardiac phenotyping; screens for DCM
5th	Muscle biopsy (if genetics inconclusive)	Dystrophin immunostaining and Western blot
6th	Cardiac MRI	Early fibrosis detection (LGE), prognostication
7th	PFTs	Respiratory monitoring
8th	Bloods (TFT, electrolytes, autoAb)	Exclude treatable differentials
9th	Genetic counselling + family screening	Carrier detection, reproductive planning
10th	Neuropsychological testing	Cognitive assessment

High Yield Summary: Diagnosis of BMD

Diagnosis is usually based on genetics ^[2]^[3] — MLPA is first-line; NGS if MLPA negative
CK is always elevated in BMD (typically 1,000–10,000 U/L) — a normal CK essentially excludes the diagnosis
The reading-frame rule predicts ~90% of cases: in-frame = BMD, out-of-frame = DMD
Muscle biopsy shows reduced/patchy dystrophin on immunostaining (only needed if genetic testing is inconclusive)
EMG shows myopathic pattern: polyphasic, short-duration, low-amplitude MUPs ^[5]
Cardiac screening is mandatory: ECG + echo ± cardiac MRI ^[4]^[5] — cardiomyopathy is the leading cause of death
Always exclude treatable differentials: TFT, electrolytes, autoantibodies, drug history ^[2]^[3]
Genetic counselling with 3-generation family tree for the entire family ^[4]

Active Recall - Diagnosis and Investigations of Becker Muscular Dystrophy

References

^[2] Senior notes: Ryan Ho Neurology.pdf (p191–192 — Myopathy investigations: CK ranges, NCS, EMG, muscle biopsy, genetic testing; BMD diagnosis) ^[3] Senior notes: Adrian Lui Pediatrics Notes.pdf (p134, p143–144 — Investigation of floppy infant, myopathy investigations: CK, EMG, muscle biopsy, genetic testing; BMD diagnosis usually based on genetics) ^[4] Senior notes: Block A - Inherited Cardiac conditions.pdf (p5, p8 — Familial DCM diagnostic criteria, echocardiographic criteria, cardiac MRI LGE, genetic counselling) ^[5] Senior notes: Maksim Medicine Notes.pdf (p276 — Myopathy investigations: CK ranges, EMG findings, key investigations for muscular dystrophy) ^[6] Lecture slides: Neurology- Two cases of lower limb weakness.pdf (p22–25 — Investigations for lower limb weakness: imaging, NCS, EMG, blood tests, autoimmune markers, genetic studies) ^[11] Senior notes: Ryan Ho Cardiology.pdf (p169 — DCM aetiology, association with X-linked muscular dystrophy, clinical presentation) ^[12] Senior notes: Ryan Ho Rheumatology.pdf (p92 — Muscle biopsy site selection, MRI for guiding biopsy, EMG triad in inflammatory myopathy)

Management of Becker Muscular Dystrophy

3. Treatment Modalities in Detail

3.1 Corticosteroid Therapy

Drug	Dose	Notes
Prednisone / Prednisolone	0.75 mg/kg/day (continuous) or 0.75 mg/kg/day 10 days on/10 days off	Standard regimen from DMD data
Deflazacort	0.9 mg/kg/day	Oxazoline derivative of prednisolone; may have slightly less weight gain than prednisone but equal or superior efficacy; FDA-approved for DMD (not specifically BMD)

Side Effect	Mechanism	Monitoring / Prevention
Weight gain / cushingoid habitus	Glucocorticoid excess → central fat redistribution, appetite stimulation	Dietary counselling, caloric restriction
Osteoporosis / fractures	Osteoblast suppression + osteoclast activation → bone loss	DEXA scan, calcium + vitamin D supplementation, bisphosphonates if needed
Growth retardation (in children)	Suppression of growth hormone axis	Monitor height velocity
Behavioural changes	CNS effects of steroids	Behavioural monitoring, dose adjustment
Cataracts	Posterior subcapsular type; mechanism unclear	Annual ophthalmology review
Glucose intolerance / diabetes	Counter-regulatory effects on insulin signalling	Fasting glucose monitoring
Adrenal suppression	Chronic exogenous steroids suppress the HPA axis	Stress-dose steroids for illness/surgery; never abruptly discontinue
Immunosuppression	Lymphocyte suppression	Infection vigilance; ensure vaccinations are up to date before starting

Clinical Decision: Steroids in BMD

The decision to use corticosteroids in BMD is not automatic (unlike in DMD where it is standard of care). It must be an individualised risk-benefit discussion. Consider steroids if the patient is showing measurable decline in motor function on serial assessments (e.g., timed functional tests worsening, losing ability to climb stairs). The side-effect burden is significant, especially in an adult who may be on treatment for decades.

3.2 Cardiac Management

Cardiac disease (dilated cardiomyopathy, arrhythmias) is the leading cause of death in BMD. Aggressive cardiac management extends survival significantly.

Timing	Investigation
At diagnosis (regardless of age)	ECG, echocardiography ± cardiac MRI
Annually thereafter	ECG, echocardiography
Every 3–5 years or if abnormalities detected	Cardiac MRI with LGE
If symptomatic or arrhythmia suspected	24-hour Holter monitor

Stage	Treatment	Rationale
Prophylactic (even with normal LVEF)	ACE inhibitor (e.g., perindopril, ramipril) or ARB (e.g., losartan)	Evidence from DMD studies (e.g., landmark study by Duboc et al.) shows that early ACEi treatment delays onset and progression of cardiomyopathy even before LVEF drops. Mechanism: reduces afterload, attenuates neurohormonal activation, may have direct anti-fibrotic effects. Current guidelines recommend starting ACEi/ARB by age 10 or at diagnosis in older patients
Established LV systolic dysfunction (LVEF < 50%)	ACEi/ARB + beta-blocker (e.g., bisoprolol, carvedilol)	Standard HF therapy: ACEi reduces afterload and remodelling; beta-blocker reduces heart rate, myocardial oxygen demand, and arrhythmia risk. Together they improve survival in all forms of DCM
Symptomatic HF or progressive decline	Add MRA (e.g., spironolactone/eplerenone)	Blocks aldosterone → reduces fibrosis, fluid retention, and potassium loss. Part of guideline-directed medical therapy for HFrEF
Further optimisation (2024/2026 guidelines)	SGLT2 inhibitor (e.g., dapagliflozin, empagliflozin)	Now recommended in all HFrEF regardless of diabetes status. Reduces HF hospitalisation and cardiovascular death. Mechanism: osmotic diuresis, natriuresis, reduced preload, direct cardioprotective effects
Diuretics (for congestion)	Loop diuretics (furosemide)	Symptomatic relief of fluid overload; does not improve survival
Arrhythmia	ICD (implantable cardioverter-defibrillator) if LVEF ≤ 35% or significant ventricular arrhythmia	Sudden cardiac death risk from VT/VF in fibrotic myocardium. ICD delivers a shock to terminate lethal arrhythmia
End-stage HF	Heart transplantation or LVAD (left ventricular assist device)	When medical therapy fails. BMD patients are candidates for transplant; however, assessment must consider respiratory muscle function and ability to comply with post-transplant immunosuppression

High Yield: Cardiac Management is the Key to Survival in BMD

Unlike DMD (where respiratory failure is the traditional cause of death), in BMD cardiac failure and sudden arrhythmic death are the leading causes of mortality. The reason is that BMD patients' skeletal muscle function is preserved long enough that cardiac disease "catches up" and becomes the dominant problem. Early, aggressive cardiac therapy (ACEi from diagnosis, add beta-blocker when LVEF drops) is the single most impactful intervention for prolonging survival in BMD.

Drug	Key Contraindications	Monitoring
ACEi/ARB	Bilateral renal artery stenosis, pregnancy, angioedema history, hyperkalemia	RFT + K⁺ at 1–2 weeks after initiation, then periodically
Beta-blocker	Decompensated HF (start only when euvolaemic), severe bradycardia, 2nd/3rd degree AV block	HR, BP; start low and titrate up slowly
MRA	Severe renal impairment (eGFR < 30), K⁺ > 5.0	K⁺ and RFT closely; risk of hyperkalemia
SGLT2i	Type 1 DM, recurrent UTI/genital infections, eGFR < 20	Glucose, ketones, RFT

Respiratory failure develops later and more gradually in BMD than DMD, but still requires monitoring.

Intervention	Indication	Mechanism / Rationale
Annual PFTs (FVC upright and supine, PCF)	All BMD patients from diagnosis	Tracks decline; a > 20% drop in supine FVC indicates significant diaphragm weakness
Cough-assist techniques	When peak cough flow < 270 L/min	Weak expiratory muscles → ineffective cough → mucus retention → pneumonia. Manual assisted cough or mechanical insufflation-exsufflation (CoughAssist device)
Non-invasive ventilation (NIV / BiPAP)	FVC < 50% predicted, or nocturnal hypoventilation (SpO₂ < 88% or pCO₂ > 45 mmHg during sleep)	Provides positive pressure ventilation to offload weakened respiratory muscles, particularly during sleep when respiratory drive is lowest
Pneumococcal and influenza vaccination	All BMD patients	Preventive — respiratory infections are dangerous in patients with weak cough and restrictive lung disease
Tracheostomy and invasive ventilation	Very late stage; rarely needed in BMD compared to DMD	Only if NIV fails or patient cannot protect airway

3.4 Orthopaedic and Rehabilitation Management

Supportive care with multidisciplinary approach: PT, OT, genetic counselling ^[5]

Goal	Modality	Rationale
Maintain range of motion	Daily stretching of Achilles tendons, hip flexors, iliotibial bands, hamstrings	Fibrosis causes progressive contracture if muscles are not regularly stretched through full ROM
Maintain strength	Submaximal aerobic exercise (swimming, cycling); avoid eccentric/high-resistance exercise	Why avoid eccentric exercise? In BMD, the sarcolemma is fragile — eccentric contractions (lengthening under load) generate the most mechanical stress and cause the most membrane tears → accelerated damage. Moderate aerobic exercise is beneficial and does NOT accelerate disease
Prevent deconditioning	Regular activity	Inactivity → rapid loss of whatever muscle function remains → disuse atrophy superimposed on dystrophic atrophy

Exercise in BMD: What to Tell Patients

Patients often ask: "Should I exercise or rest?" The answer is yes, exercise — but the right type. Submaximal aerobic exercise (swimming is ideal — it is non-weight-bearing and works all muscle groups) is safe and beneficial. High-resistance and eccentric exercise (heavy weight lifting, downhill running) should be avoided because it damages already-fragile sarcolemma. Think of it this way: you want to keep the engine running, but not rev it so hard that the fragile engine block cracks.

Goal	Modality
Maintain independence	Adaptive devices (long-handled shoehorns, dressing aids, adapted utensils), home modifications (grab rails, ramps, stair lifts)
Mobility	Ankle-foot orthoses (AFOs) for foot drop, powered wheelchairs when ambulation becomes too difficult
Upper limb function	Mobile arm supports when shoulder girdle weakness progresses

Procedure	Indication	Rationale
Achilles tendon release / lengthening	Progressive equinus contracture despite stretching	Fibrotic gastrocnemius/soleus causes fixed plantar flexion → toe-walking → falls. Surgical release + post-operative splinting restores dorsiflexion
Spinal fusion	Progressive scoliosis (Cobb angle > 20–30° and progressing; usually in non-ambulant patients)	Paraspinal weakness → progressive curvature → compromises respiratory function (restricts chest wall expansion) and sitting balance. Less common in BMD than DMD
Fracture management	Osteoporotic fractures (compounded by steroid use and reduced weight-bearing)	Standard fracture care, but consider underlying bone fragility

Domain	Interventions
Psychological support	Counselling for adjustment to chronic progressive illness, depression screening, peer support groups
Educational support	Special educational needs assessment for the ~10–20% with cognitive involvement; classroom accommodations
Vocational planning	Career counselling acknowledging physical limitations; most BMD patients can work in sedentary occupations for decades
Genetic counselling ^[5]	Carrier testing for at-risk female relatives, prenatal diagnosis (CVS/amniocentesis), preimplantation genetic testing, 3-generation family tree construction. Discuss germline mosaicism risk (~7–10% recurrence even if mother tests negative)
Transition planning	Structured transition from paediatric to adult neuromuscular services (typically at 16–18 years)

This is a life-saving management point that must be communicated to every BMD patient and documented prominently in their medical record:

Precaution	Rationale
Avoid succinylcholine	Depolarising agent causes massive sustained depolarisation of already-fragile dystrophic muscle membranes → rhabdomyolysis → hyperkalemia → cardiac arrest. This is potentially fatal
Avoid volatile anaesthetic agents (sevoflurane, desflurane, isoflurane)	Risk of a "malignant hyperthermia-like" reaction: massive rhabdomyolysis, metabolic acidosis, hyperkalemia. Not true MH (RYR1-mediated) but clinically similar
Use total intravenous anaesthesia (TIVA)	Propofol + remifentanil/fentanyl avoids the above risks. Non-depolarising muscle relaxants (e.g., rocuronium, atracurium) are safe
MedicAlert bracelet	Recommended so that in emergency situations, anaesthetists are immediately alerted

Anaesthetic Safety: A Must-Know Point

Succinylcholine + dystrophinopathy = potentially fatal hyperkalemic cardiac arrest. This is examined repeatedly. EVERY BMD patient should have their anaesthetic risk documented, and a MedicAlert bracelet is recommended. If you are the admitting doctor for a BMD patient undergoing any procedure, ensure the anaesthetist is aware.

These represent the future of BMD management and are increasingly examined:

Therapy	Mechanism	Current Status for BMD
Exon-skipping (antisense oligonucleotides)	Small synthetic RNA molecules that mask specific exons during pre-mRNA splicing → can convert an out-of-frame transcript into an in-frame one → ↑partially functional dystrophin. Examples: eteplirsen (exon 51 skip), golodirsen (exon 53 skip), viltolarsen (exon 53 skip), casimersen (exon 45 skip)	Primarily developed and FDA-approved (accelerated) for DMD — not for BMD (BMD already produces some dystrophin). However, the concept is relevant: exon-skipping essentially aims to convert a DMD phenotype into a BMD-like phenotype
Gene replacement therapy (micro/mini-dystrophin)	AAV (adeno-associated virus) vectors deliver a truncated but functional "micro-dystrophin" gene into muscle cells → cell produces a BMD-like shorter dystrophin. Delandistrogene moxeparvovec (Elevidys) FDA-approved 2023 for DMD (4–5y ambulant boys)	Under investigation for BMD; in BMD the patient already produces some dystrophin, so the added micro-dystrophin may supplement it. Trials ongoing as of 2026
Utrophin upregulation	Utrophin is a dystrophin homologue (autosomal, expressed at NMJs in normal muscle). Upregulating utrophin could compensate for dystrophin deficiency without genetic correction. Ezutromid was an early candidate	Preclinical / early clinical. The advantage is that it is mutation-agnostic — would work for any DMD gene mutation
CRISPR/Cas9 gene editing	Directly corrects the mutation in the DMD gene in patient cells. Can remove exons to restore reading frame, or correct point mutations	Preclinical / early-phase trials. Enormous potential but challenges include delivery to all muscle cells (including cardiac), off-target effects, immune response to Cas9
Myostatin inhibitors	Myostatin is a negative regulator of muscle growth. Blocking it → increased muscle mass. Agents: domagruzumab, RO7239361	Phase 2/3 trials in DMD have been disappointing so far; not yet proven beneficial
Givinostat (HDAC inhibitor)	Histone deacetylase inhibitor → reduces fibrosis and fat replacement in dystrophic muscle	FDA-approved for DMD (2024) — the first non-steroidal oral drug approved for DMD. May be applicable to BMD but trials are ongoing
Cardiac-specific gene therapy	AAV-delivered micro-dystrophin targeted to cardiac tissue	Important for BMD because cardiomyopathy can be the dominant/lethal feature even when skeletal muscle is relatively preserved

Understanding Exon-Skipping: A Clever Molecular Trick

Exon-skipping uses antisense oligonucleotides to make the cell's splicing machinery "skip" a particular exon during mRNA processing. If a patient has an out-of-frame deletion (say, exons 49–50 are deleted), the downstream exon 51 no longer matches the reading frame → frameshift → truncated protein → DMD. By masking exon 51 with an ASO, the splicing machinery skips exon 51 too → exons 48–52 are now all removed → the remaining exons re-align in-frame → a shortened but partially functional dystrophin is produced → the phenotype converts from DMD to BMD-like. The goal is to turn a DMD patient into a BMD patient. This beautifully illustrates the reading-frame rule in clinical application.

Assessment	Frequency	Test
Motor function	Every 6–12 months	Timed functional tests (6-minute walk test, 10-metre walk time, time to rise from floor, North Star Ambulatory Assessment)
Cardiac	Annual	ECG + echocardiography (cardiac MRI every 3–5y or if new abnormalities)
Respiratory	Annual	PFTs (FVC upright and supine, PCF); nocturnal oximetry if symptomatic
Orthopaedic	Annual	Joint ROM assessment; spine XR if non-ambulant (scoliosis screening)
Bone health	Every 2–3 years (more often if on steroids)	DEXA scan; calcium, vitamin D, PTH levels
Endocrine (if on steroids)	Annual	Fasting glucose, HbA1c, growth velocity (children), cortisol
Ophthalmology (if on steroids)	Annual	Slit lamp exam for posterior subcapsular cataract
Psychological	As needed; formal reassessment every 1–2 years	Mood screening, neuropsychological reassessment if cognitive concerns
Genetic counselling	At diagnosis and before reproductive decisions	Update family as new therapies become available

Pillar	Key Interventions
Skeletal muscle	PT (stretching + submaximal aerobic exercise), OT, assistive devices; consider corticosteroids if declining function
Cardiac	Early ACEi/ARB (prophylactic from age 10); add beta-blocker, MRA, SGLT2i, ICD as needed
Respiratory	Annual PFTs; cough-assist; NIV when FVC < 50% or nocturnal hypoventilation
Orthopaedic	Contracture prevention; surgical release if needed; scoliosis monitoring
Anaesthesia	Avoid succinylcholine and volatile agents; use TIVA
Psychosocial	Psychological support, educational accommodations, vocational planning
Genetic	Genetic counselling, carrier testing, prenatal/preimplantation diagnosis ^[5]
Emerging	Exon-skipping (mainly for DMD → BMD conversion), gene therapy, CRISPR, givinostat

High Yield Summary: Management of BMD

No specific treatment; management is supportive with a multidisciplinary approach ^[2]^[3]^[5]
Corticosteroids: standard in DMD (↑motor function, strength, pulmonary function, ↓risk of scoliosis ^[2]^[3]); considered in BMD only if actively declining, with individualised risk-benefit analysis
Management of complications: heart failure, arrhythmia, respiratory failure ^[5] — cardiac management is the single most impactful intervention for survival
Cardiac: prophylactic ACEi/ARB from early stage; add beta-blocker → MRA → SGLT2i → ICD → transplant as disease progresses
Respiratory: annual PFTs, cough-assist, NIV when indicated
Anaesthesia: NEVER use succinylcholine or volatile agents — use TIVA
Physiotherapy: submaximal aerobic exercise is safe; avoid eccentric/high-resistance exercise
Genetic counselling is essential for the whole family
Emerging therapies: exon-skipping, micro-dystrophin gene therapy, CRISPR gene editing are transforming the landscape — most are currently for DMD, with BMD trials following

Active Recall - Management of Becker Muscular Dystrophy

References

^[2] Senior notes: Ryan Ho Neurology.pdf (p192 — BMD management: supportive; DMD management: supportive + steroid therapy) ^[3] Senior notes: Adrian Lui Pediatrics Notes.pdf (p144 — DMD Mx: supportive + steroid therapy; BMD Mx: supportive) ^[4] Senior notes: Block A - Inherited Cardiac conditions.pdf (p5, p8 — Familial DCM workup, cardiac MRI LGE significance, genetic counselling) ^[5] Senior notes: Maksim Medicine Notes.pdf (p276 — Myopathy management: no specific treatment, supportive care with multidisciplinary approach, management of complications: heart failure, arrhythmia, respiratory failure; genetic counselling)

Complications of Becker Muscular Dystrophy

BMD is a chronic progressive multisystem disease. Although it is "milder" than DMD, the complications are still serious and ultimately life-limiting. Every complication traces back to the same root cause: partial dystrophin deficiency → ongoing sarcolemma instability → cumulative organ damage in any tissue that expresses dystrophin (skeletal muscle, cardiac muscle, smooth muscle, brain).

Management of complications: heart failure, arrhythmia, respiratory failure ^[5]

The complications can be organised by organ system, and understanding the pathophysiology of each explains "why" it happens and "when" to expect it.

1. Cardiac Complications (Leading Cause of Death)

This is the most important section for BMD because cardiac disease — not respiratory failure — is the dominant killer in this condition, distinguishing it from DMD where respiratory failure historically predominated.

Cardiomyopathy is listed as a cardinal feature of dystrophinopathies ^[2]^[3] BMD is a cause of familial dilated cardiomyopathy ^[4] Complications of DCMP: arrhythmia, thromboembolism, sudden cardiac death ^[11]

Feature	Detail
Prevalence	~60–75% of BMD patients develop clinically significant cardiomyopathy; subclinical cardiac fibrosis (on MRI) is present in an even higher proportion
Pathophysiology	Dystrophin is expressed in cardiac myocytes. Partial dystrophin → membrane instability during each cardiac contraction (the heart beats ~100,000 times/day) → micro-tears → Ca²⁺ influx → cardiomyocyte necrosis → fibrosis (subepicardial, starting in the inferolateral LV wall) → progressive chamber dilatation → ↓contractility → DCMP: dilatation and impaired contraction of LV (± RV), ↑LV mass but with normal/↓ thickness ^[11]
Why it can be disproportionate	In BMD, the partially functional dystrophin may be "enough" for skeletal muscle (which contracts intermittently) but insufficient for cardiac muscle (which contracts ceaselessly). The cardiac workload is relentless, so the cumulative sarcolemma damage accrues faster in the heart than in resting skeletal muscles
Clinical features	Exertional dyspnoea, orthopnoea, PND, peripheral oedema, fatigue. Signs: displaced apex, S3 gallop, functional MR/TR. Presentation: unexplained premature HF, incidental cardiomegaly ^[11]
Onset	Typically teens–20s (subclinical); symptomatic HF in 20s–40s

High Yield: Cardiac vs Skeletal Muscle — The Paradox of BMD

A key exam concept: BMD patients can have near-normal skeletal muscle function but severe, life-threatening cardiomyopathy. This is because the heart never rests — it undergoes ~100,000 contraction cycles per day, each one stressing the dystrophin-deficient sarcolemma. Skeletal muscles can be rested between activities, but the heart cannot. This is why cardiac screening is mandatory from diagnosis, even in asymptomatic patients.

Feature	Detail
Mechanism	DCM → ↓systolic function → ↓cardiac output → neurohormonal activation (RAAS, sympathetic) → fluid retention, vasoconstriction → pulmonary congestion (left HF) ± systemic congestion (right HF)
Clinical significance	Heart failure is the most common cause of death in BMD (accounting for ~50% of deaths)
Progression	Insidious; patients may not report dyspnoea because their physical activity is already limited by skeletal weakness — this masks cardiac symptoms. By the time HF is symptomatic, LV function may already be severely impaired

Clinical Trap: Masked Cardiac Symptoms

BMD patients may NOT report exertional dyspnoea because they cannot exert themselves enough (due to skeletal weakness) to provoke cardiac symptoms. This means HF can be "silent" until advanced. Never rely on symptom absence to rule out cardiac involvement — echocardiographic screening is essential regardless of symptoms.

Feature	Detail
Pathophysiology	Myocardial fibrosis creates electrically heterogeneous tissue → slow conduction zones interspersed with normal tissue → re-entrant circuits → ventricular tachycardia (VT) or ventricular fibrillation (VF) → sudden cardiac death (SCD)
Types	Ventricular ectopics, non-sustained VT, sustained VT, VF. Also atrial fibrillation/flutter (from atrial dilatation)
Risk	SCD accounts for ~25–30% of deaths in BMD. Risk is highest in patients with LVEF ≤ 35% and/or significant LGE on cardiac MRI
Prevention	ICD implantation when LVEF ≤ 35% or documented significant ventricular arrhythmia

Feature	Detail
Pathophysiology	Dilated, poorly contracting ventricles → blood stasis → thrombus formation (especially in LV apex or left atrial appendage in AF) → systemic embolisation → stroke, peripheral arterial embolism
Additional risk	Reduced mobility (wheelchair-bound patients) → venous stasis → DVT → pulmonary embolism
Prevention	Anticoagulation if AF develops; DVT prophylaxis during immobilisation/surgery

Respiratory involvement in BMD is later and milder than in DMD, but it still contributes to morbidity and mortality, particularly in the non-ambulant phase.

Complication	Pathophysiology	Clinical Manifestation	Typical Onset
Restrictive ventilatory defect	Diaphragm + intercostal muscle weakness → inability to fully expand the chest → ↓lung volumes (↓FVC, ↓TLC) with preserved FEV1/FVC ratio	Reduced exercise tolerance, dyspnoea on exertion	30s–50s (later than DMD)
Nocturnal hypoventilation	Diaphragm weakness is worst during REM sleep (accessory muscles are atonic in REM; diaphragm is the sole pump) → CO₂ retention during sleep → morning headaches, daytime somnolence, poor concentration	Morning headache, unrefreshing sleep, daytime hypersomnia	Precedes daytime respiratory failure by months–years
Ineffective cough	Expiratory muscle (abdominals, intercostals) weakness → ↓peak cough flow → inability to clear mucus	Mucus retention → atelectasis → recurrent lower respiratory tract infections / pneumonia	Gradual; worsens as weakness progresses
Respiratory failure	End-stage diaphragm and accessory muscle weakness → inability to maintain adequate ventilation even at rest → hypercapnic (Type II) respiratory failure	Chronic respiratory failure requiring NIV, eventually invasive ventilation in some	40s–60s; less common than in DMD as a primary cause of death
Aspiration pneumonia	Bulbar weakness (rare in BMD, more common in later stages) + weak cough → aspiration of oropharyngeal contents → chemical/bacterial pneumonitis	Recurrent pneumonia, fever, productive cough	Late disease

Why Respiratory Failure is Less Dominant in BMD Than DMD

In DMD, dystrophin is completely absent → respiratory muscles fail rapidly → respiratory failure is the traditional cause of death in the late teens/20s. In BMD, partial dystrophin provides enough sarcolemma stability to sustain respiratory muscle function for decades. This means BMD patients live long enough for cardiac disease to "catch up" and become the leading killer rather than respiratory failure. However, respiratory complications still occur and contribute to mortality, especially in the 5th–6th decades.

3. Musculoskeletal Complications

Feature	Detail
Pathophysiology	Progressive fibrosis within muscles → shortening and loss of elasticity → the muscle can no longer be stretched to its full length → the joint is held in a fixed position. Imbalanced muscle forces across a joint (e.g., stronger plantar flexors vs weaker dorsiflexors at the ankle) accelerate contracture
Common sites	Achilles tendons (equinus contracture → toe-walking ^[5]), hip flexors (hip flexion contracture → difficulty standing upright), iliotibial bands (abduction contracture), hamstrings, elbows
Clinical significance	Contractures impair gait, reduce functional independence, and accelerate loss of ambulation. They are painful and contribute to falls
Prevention	Daily stretching, night splints, standing frames. Surgical release if conservative measures fail

Feature	Detail
Pathophysiology	Paraspinal muscle weakness → loss of spinal support → progressive lateral curvature of the spine. Develops primarily after loss of ambulation (gravity acts on the unsupported spine during sitting)
Clinical significance	Scoliosis compresses the thorax → reduces lung volumes → worsens restrictive ventilatory defect. Also causes pain and difficulty with seating/positioning
Prevalence in BMD	Less common and less severe than in DMD because BMD patients remain ambulant longer (weight-bearing helps maintain spinal alignment)
Management	Bracing (limited evidence of efficacy in neuromuscular scoliosis), spinal fusion surgery if Cobb angle > 20–30° and progressing

In DMD, corticosteroids ↓risk of scoliosis ^[3] — partly by prolonging ambulation. This benefit is less well-studied in BMD.

Feature	Detail
Pathophysiology	Reduced weight-bearing (progressive weakness → reduced physical activity) → decreased mechanical loading on bone → osteoporosis. Compounded by corticosteroid use (if prescribed), which directly suppresses osteoblast activity
Clinical significance	Increased fracture risk, especially of long bones (femur) and vertebral compression fractures. A long bone fracture can precipitate loss of ambulation if it occurs during the ambulant phase
Prevention	Weight-bearing exercise (as tolerated), calcium + vitamin D supplementation, bisphosphonates if T-score ≤ −2.5 or vertebral fracture. DEXA screening every 2–3 years (more frequently if on steroids)

Complication	Pathophysiology	Prevalence in BMD
Mild mental impairment ^[3]	Dystrophin is expressed in the brain — particularly in cerebellar Purkinje neurons, hippocampus, and cerebral cortex. Brain-specific dystrophin isoforms (Dp140, Dp71) may be disrupted depending on the location of the mutation within the DMD gene. Mutations in the distal portion of the gene (affecting Dp71 or Dp140 promoters) have a greater impact on cognition	~10–20% of BMD patients (milder than DMD, where ~1/3 are affected ^[3])
Learning disabilities	Impaired working memory, processing speed, and executive function. Verbal IQ tends to be lower than performance IQ	Most common cognitive phenotype; may present as academic underperformance in school-age children
Attention deficit	Frontal lobe dystrophin isoform involvement → impaired sustained attention	Higher prevalence than general population; may overlap with ADHD
Depression and anxiety	Combination of organic (brain dystrophin deficiency) and reactive (chronic progressive disability, loss of function, social isolation) factors	Common, underdiagnosed. Formal screening recommended
Autism spectrum features	Dystrophin's role in synaptic plasticity; more common when Dp71/Dp140 isoforms are affected	Modest increase in prevalence compared to general population

Why Some BMD Patients Have Cognitive Issues and Others Don't

The DMD gene is enormous (79 exons) and has multiple internal promoters that produce tissue-specific dystrophin isoforms. The brain-specific isoforms Dp140 (promoter upstream of exon 45) and Dp71 (promoter upstream of exon 63) are relevant for cognition. If the mutation is proximal in the gene (e.g., exons 1–44), only full-length dystrophin is disrupted and brain isoforms are preserved → normal cognition. If the mutation is distal (e.g., exons 45–79), it may also disrupt Dp140 and/or Dp71 promoters → cognitive impairment. This genotype-phenotype correlation is increasingly recognised and tested.

Complication	Pathophysiology	Prevention
Rhabdomyolysis from succinylcholine	Depolarising agent causes sustained depolarisation of fragile dystrophic sarcolemma → massive, uncontrolled Ca²⁺ influx → widespread myofibre necrosis → release of intracellular K⁺ and myoglobin → fatal hyperkalemia + cardiac arrest; myoglobinuria → acute kidney injury	Absolute avoidance of succinylcholine. Use non-depolarising agents (rocuronium, atracurium)
Malignant hyperthermia-like reaction from volatile agents	Volatile anaesthetics (sevoflurane, isoflurane, desflurane) in dystrophinopathy patients → massive rhabdomyolysis, metabolic acidosis, hyperthermia, hyperkalemia. Not true MH (which is RYR1-mediated) but clinically indistinguishable	Avoidance of volatile anaesthetic agents. Use total intravenous anaesthesia (TIVA)
Respiratory complications under anaesthesia	Weakened respiratory muscles → difficulty weaning from mechanical ventilation post-operatively; atelectasis, aspiration	Pre-operative PFT assessment, careful ventilatory management, early post-op NIV support
Cardiac complications under anaesthesia	Underlying cardiomyopathy → haemodynamic instability during anaesthesia (hypotension, arrhythmia)	Pre-operative echocardiography, invasive monitoring, avoidance of myocardial depressants

Complication	Pathophysiology
Constipation	Smooth muscle of the gut also expresses dystrophin. Partial deficiency → reduced gut motility → constipation (a frequently underappreciated symptom). Compounded by reduced physical activity and poor fluid intake
Gastroparesis / delayed gastric emptying	Gastric smooth muscle dysfunction → bloating, nausea, early satiety
Dysphagia (rare in BMD)	Pharyngeal muscle weakness → difficulty swallowing → risk of aspiration. Much less common in BMD than in oculopharyngeal or myotonic dystrophy
Acute gastric dilatation	Rare but serious. Can occur post-operatively in non-ambulant patients → abdominal distension, vomiting, and if unrecognised → gastric necrosis
Nutritional issues	Obesity (reduced activity + steroid use) or malnutrition (dysphagia, poor oral intake in advanced disease). Weight management is important — obesity increases the biomechanical load on already-weak muscles

Complication	Pathophysiology
Deep vein thrombosis (DVT) / Pulmonary embolism (PE)	Reduced lower limb mobility → venous stasis (Virchow's triad) → thrombus formation. PE can be fatal, especially in patients with underlying cardiomyopathy
Pressure ulcers	Prolonged sitting in wheelchair or bed → sustained pressure over bony prominences → tissue ischaemia → ulceration
Social isolation and reduced quality of life	Loss of mobility → reduced participation in social activities, education, employment → psychological morbidity

Treatment	Complication	Mechanism
Corticosteroids (if used)	Osteoporosis, weight gain, growth retardation, cushingoid habitus, cataracts, glucose intolerance, adrenal suppression, immunosuppression	Chronic glucocorticoid excess → multiple metabolic and endocrine effects
ACEi/ARB	Hypotension, renal impairment, hyperkalemia, ACEi cough, angioedema	Pharmacological side effects; monitor RFT and K⁺
Surgery (tendon release, spinal fusion)	Anaesthetic risks (see above), wound healing complications, post-operative respiratory deterioration	Dystrophic muscle heals poorly; respiratory muscles may decompensate under anaesthesia

Age Range	Dominant Complications
5–15 years	Progressive proximal weakness, contractures begin, Gower's sign, toe-walking. Learning difficulties may manifest
15–30 years	Loss of ambulation in more severe cases. Subclinical cardiomyopathy developing (LGE on CMR). Contractures progress. Psychological adjustment challenges
30–50 years	Symptomatic DCM and heart failure. Arrhythmias. Respiratory function declining. Scoliosis (if non-ambulant). Osteoporosis
40–60 years	Death between 40–60 ^[2]^[3] — primarily from cardiac failure or sudden arrhythmic death; respiratory failure contributory

BMD: usually later onset with milder symptoms and better prognosis; continue to ambulate beyond 15; death between 40–60 ^[2]^[3] Becker muscular dystrophy: dystrophin stain — partial deficiency ^[1]

Although BMD is X-linked recessive and primarily affects males, female carriers can develop complications:

Complication	Prevalence	Mechanism
Dilated cardiomyopathy	~8–10% of carriers develop clinically significant DCM	Skewed X-inactivation → the normal X chromosome is preferentially inactivated in cardiac myocytes → the mutant allele is predominantly expressed → partial dystrophin deficiency in the heart
Mild skeletal muscle weakness	~5–10% (manifesting carriers)	Same mechanism of skewed lyonization in skeletal muscle
Elevated CK	~60% of carriers	Even without symptoms, carrier status causes low-level sarcolemma instability in some muscle fibres → mild CK leak

Don't Forget the Carriers

Female carriers of DMD gene mutations can develop dilated cardiomyopathy even without skeletal muscle symptoms. All identified female carriers should undergo cardiac screening with echocardiography at least every 5 years (or more frequently if abnormalities are found). This is a commonly examined point and a frequently missed diagnosis in clinical practice.

High Yield Summary: Complications of BMD

Cardiac (Leading cause of death):

Dilated cardiomyopathy ^[3]^[4] — progressive, can be disproportionate to skeletal weakness
Heart failure — may be "silent" due to limited exertion
Arrhythmia and sudden cardiac death ^[5]^[11] — VT/VF from myocardial fibrosis
Thromboembolism ^[11] — from LV stasis or AF

Respiratory (Later and milder than DMD):

Restrictive ventilatory defect → nocturnal hypoventilation → respiratory failure
Ineffective cough → recurrent pneumonia

Musculoskeletal:

Contractures (Achilles, hip flexors)
Scoliosis (less common than DMD)
Osteoporosis and fractures

Cognitive: Mild mental impairment ^[3] in ~10–20%; depends on mutation location affecting brain dystrophin isoforms (Dp140, Dp71)

Anaesthetic: Fatal hyperkalemia from succinylcholine; MH-like reaction from volatile agents — use TIVA

Gastrointestinal: Constipation, gastroparesis (smooth muscle dystrophin deficiency)

Immobility: DVT/PE, pressure ulcers, social isolation

Female carriers: DCM (~8–10%), mild weakness, elevated CK — cardiac screening mandatory

Death between 40–60 years ^[2]^[3], primarily from cardiac failure or SCD

Active Recall - Complications of Becker Muscular Dystrophy

References

^[1] Lecture slides: GC 056. Generalized muscle weakness.pdf (p25 — BMD: continue to ambulate beyond 15, death between 40–60, dystrophin stain partial deficiency) ^[2] Senior notes: Ryan Ho Neurology.pdf (p192 — BMD clinical features: later onset, milder symptoms, better prognosis, continue to ambulate beyond 15, death between 40–60) ^[3] Senior notes: Adrian Lui Pediatrics Notes.pdf (p144 — DMD/BMD cardiomyopathy, mild mental impairment, pseudohypertrophy, BMD management supportive) ^[4] Senior notes: Block A - Inherited Cardiac conditions.pdf (p5 — BMD as cause of familial DCM, neuromuscular disorders causing DCM) ^[5] Senior notes: Maksim Medicine Notes.pdf (p276 — Management of complications: heart failure, arrhythmia, respiratory failure; key investigations for muscular dystrophy) ^[11] Senior notes: Ryan Ho Cardiology.pdf (p169 — DCM: dilatation and impaired contraction of LV, complications: arrhythmia, thromboembolism, sudden cardiac death; clinical presentation)

High Yield Summary

Definition: BMD is an X-linked recessive dystrophinopathy caused by in-frame mutations in the DMD gene (Xp21.2), producing partially functional dystrophin — allelic with DMD but milder.

Epidemiology: ~1/18,000–30,000 male births. 1/3 cases are de novo. Female carriers may develop cardiomyopathy.

Key Clinical Features:

Later onset (5–15 years or later), milder symptoms, better prognosis than DMD
Continue to ambulate beyond age 15; death 40–60 years
Proximal weakness, Gower's sign, pseudohypertrophy of calves, waddling gait
Dilated cardiomyopathy — can be disproportionately severe; leading cause of death
No sensory loss, no fatigability, normal/reduced reflexes
Elevated CK (typically 200–10,000 U/L)

Must-Know Classifications:

Clinical: DMD, BMD, FSHD, LGMD, Myotonic dystrophy
Immunohistochemical: Dystrophinopathy, Sarcoglycanopathy, Others
Old definition: inherited, all symptoms due to weakness, progressive, no histopathological abnormalities other than degeneration and regeneration

Cardiac Association: BMD is a cause of familial dilated cardiomyopathy — consider dystrophinopathy in any young male with unexplained DCM.

High Yield Summary: Diagnosis of BMD

Diagnosis is usually based on genetics ^[2]^[3] — MLPA is first-line; NGS if MLPA negative
CK is always elevated in BMD (typically 1,000–10,000 U/L) — a normal CK essentially excludes the diagnosis
The reading-frame rule predicts ~90% of cases: in-frame = BMD, out-of-frame = DMD
Muscle biopsy shows reduced/patchy dystrophin on immunostaining (only needed if genetic testing is inconclusive)
EMG shows myopathic pattern: polyphasic, short-duration, low-amplitude MUPs ^[5]
Cardiac screening is mandatory: ECG + echo ± cardiac MRI ^[4]^[5] — cardiomyopathy is the leading cause of death
Always exclude treatable differentials: TFT, electrolytes, autoantibodies, drug history ^[2]^[3]
Genetic counselling with 3-generation family tree for the entire family ^[4]

High Yield Summary: Management of BMD

No specific treatment; management is supportive with a multidisciplinary approach ^[2]^[3]^[5]
Corticosteroids: standard in DMD (↑motor function, strength, pulmonary function, ↓risk of scoliosis ^[2]^[3]); considered in BMD only if actively declining, with individualised risk-benefit analysis
Management of complications: heart failure, arrhythmia, respiratory failure ^[5] — cardiac management is the single most impactful intervention for survival
Cardiac: prophylactic ACEi/ARB from early stage; add beta-blocker → MRA → SGLT2i → ICD → transplant as disease progresses
Respiratory: annual PFTs, cough-assist, NIV when indicated
Anaesthesia: NEVER use succinylcholine or volatile agents — use TIVA
Physiotherapy: submaximal aerobic exercise is safe; avoid eccentric/high-resistance exercise
Genetic counselling is essential for the whole family
Emerging therapies: exon-skipping, micro-dystrophin gene therapy, CRISPR gene editing are transforming the landscape — most are currently for DMD, with BMD trials following

High Yield Summary: Complications of BMD

Cardiac (Leading cause of death):

Dilated cardiomyopathy ^[3]^[4] — progressive, can be disproportionate to skeletal weakness
Heart failure — may be "silent" due to limited exertion
Arrhythmia and sudden cardiac death ^[5]^[11] — VT/VF from myocardial fibrosis
Thromboembolism ^[11] — from LV stasis or AF

Respiratory (Later and milder than DMD):

Restrictive ventilatory defect → nocturnal hypoventilation → respiratory failure
Ineffective cough → recurrent pneumonia

Musculoskeletal:

Contractures (Achilles, hip flexors)
Scoliosis (less common than DMD)
Osteoporosis and fractures

Cognitive: Mild mental impairment ^[3] in ~10–20%; depends on mutation location affecting brain dystrophin isoforms (Dp140, Dp71)

Anaesthetic: Fatal hyperkalemia from succinylcholine; MH-like reaction from volatile agents — use TIVA

Gastrointestinal: Constipation, gastroparesis (smooth muscle dystrophin deficiency)

Immobility: DVT/PE, pressure ulcers, social isolation

Female carriers: DCM (~8–10%), mild weakness, elevated CK — cardiac screening mandatory

Death between 40–60 years ^[2]^[3], primarily from cardiac failure or SCD

Becker Muscular Dystrophy

Becker Muscular Dystrophy (BMD)

2. Epidemiology

3. Anatomy and Function of Dystrophin

Structural Domains of Dystrophin

4. Etiology

Common Mutation Types in BMD

5. Pathophysiology

6. Classification

7. Clinical Features

7.2 Symptoms (with Pathophysiological Basis)

7.3 Signs (with Pathophysiological Basis)

8. Relevant Associations and Syndromic Features

Active Recall - Becker Muscular Dystrophy (Overview, Etiology, Pathophysiology, Clinical Features)

Differential Diagnosis of Becker Muscular Dystrophy

3. Differential Diagnosis by Category

4. Differential Diagnosis for Specific BMD Presentations

Active Recall - Differential Diagnosis of Becker Muscular Dystrophy

References

Diagnostic Criteria, Algorithm, and Investigations for Becker Muscular Dystrophy

1. Diagnostic Criteria

3. Investigation Modalities

Active Recall - Diagnosis and Investigations of Becker Muscular Dystrophy

References

Management of Becker Muscular Dystrophy

3. Treatment Modalities in Detail

3.1 Corticosteroid Therapy

3.2 Cardiac Management

3.4 Orthopaedic and Rehabilitation Management

Active Recall - Management of Becker Muscular Dystrophy

References

Complications of Becker Muscular Dystrophy

1. Cardiac Complications (Leading Cause of Death)

3. Musculoskeletal Complications

Active Recall - Complications of Becker Muscular Dystrophy

References

On this page

Becker Muscular Dystrophy

1. Definition

2. Epidemiology

2.1 Incidence and Prevalence

2.2 Key Epidemiological Points

2.3 Female Carriers

3. Anatomy and Function of Dystrophin

3.1 The Dystrophin-Associated Glycoprotein Complex (DAGC)

3.2 Role of Dystrophin in Muscle Physiology

3.3 Cardiac Dystrophin

4. Etiology

4.1 Genetics

4.2 Types of Mutations

4.3 De Novo Mutations

4.4 Hong Kong–Specific Considerations

5. Pathophysiology

5.1 Molecular Pathophysiology

5.2 Step-by-Step Pathophysiology

5.3 Why BMD is Milder Than DMD

5.4 Cardiac Pathophysiology (Important for Exams)

6. Classification

6.1 Classification of Muscular Dystrophies

6.2 Old vs Modern Definition

6.3 Phenotypic Spectrum of Dystrophinopathies

7. Clinical Features

7.1 Overview and Natural History

7.2 Symptoms (with Pathophysiological Basis)

A. Skeletal Muscle Symptoms

B. Cardiac Symptoms

C. Respiratory Symptoms (Late)

D. Other Symptoms

7.3 Signs (with Pathophysiological Basis)

A. Inspection

B. Palpation

C. Motor Examination

D. Cardiac Signs

E. Respiratory Signs (Late)

7.4 Summary Comparison: BMD vs DMD

7.5 Key Distinguishing Features of BMD from Other Myopathies

8. Relevant Associations and Syndromic Features

8.1 X-linked Dilated Cardiomyopathy

8.2 Asymptomatic Hyperckemia

8.3 Malignant Hyperthermia Risk

Active Recall - Becker Muscular Dystrophy (Overview, Etiology, Pathophysiology, Clinical Features)

References

1. Framework: Anatomical Localisation First

2. Differential Diagnosis Algorithm (Mermaid Diagram)

3. Differential Diagnosis by Category

3.1 Hereditary / Congenital Myopathies (The Primary DDx Group for BMD)

3.2 Acquired Inflammatory Myopathies

3.3 Endocrine and Metabolic Myopathies

3.4 Drug-Induced Myopathies

3.5 Infectious Myopathies

3.6 Neuromuscular Junction Disorders

3.7 Anterior Horn Cell Diseases

3.8 Channelopathies and Periodic Paralysis

3.9 Non-Neuromuscular Mimics

4. Differential Diagnosis for Specific BMD Presentations

4.1 Young Male with Unexplained Dilated Cardiomyopathy

4.2 Floppy Infant

4.3 Asymptomatic Hyper-CK-emia

5. Key Discriminating Features — Summary Table

6. Approach to Narrowing the Differential

Active Recall - Differential Diagnosis of Becker Muscular Dystrophy

1. Diagnostic Criteria

1.1 Clinical Criteria (Suspicion-Level)

1.2 Confirmatory Criteria (Definitive Diagnosis)

1.3 Diagnostic Criteria for Associated Dilated Cardiomyopathy

2. Diagnostic Algorithm

3. Investigation Modalities

3.1 Serum Creatine Kinase (CK)

3.2 Genetic Testing (Definitive Investigation)

3.3 Nerve Conduction Studies (NCS) and Electromyography (EMG)

3.4 Muscle Biopsy

3.5 Cardiac Investigations

3.6 Pulmonary Function Tests (PFTs)

3.7 Bloods to Exclude Differentials

3.8 Muscle MRI

3.9 Neuropsychological Assessment

3.10 Genetic Counselling and Family Screening