Duchenne and Becker Muscular Dystrophy

• Loss of Dystrophin Function: The DMD gene mutation results in frameshift or nonsense mutations (DMD) versus in-frame deletions (BMD). In DMD, truncated dystrophin cannot properly localize to the sarcolemmal membrane, leading to complete loss of function. BMD mutations preserve the reading frame, producing a shorter but partially functional protein capable of maintaining some structural integrity. The dystrophin protein normally anchors the dystrophin-associated protein complex (DAPC) to the muscle cell membrane, linking the cytoskeletal actin filaments to the extracellular matrix.

• Calcium Influx and Myonecrosis: Loss of dystrophin disrupts the DAPC, destabilizing the sarcolemma and causing uncontrolled calcium (Ca²⁺) influx into muscle fibers during contraction and eccentric movement. Excess intracellular Ca²⁺ activates calcium-dependent proteases (calpains) and phospholipases, triggering myofibrillar necrosis. Oxidative stress is amplified through increased mitochondrial respiration and reactive oxygen species (ROS) production, further compromising cellular integrity.

• Inflammatory Cascade and Fibrosis: Myonecrosis triggers infiltration of macrophages and lymphocytes, establishing a chronic inflammatory environment. Initially, inflammatory cytokines (TNF-α, IL-1β, IL-6) attempt reparative processes but ultimately promote muscle fiber apoptosis and replacement with fat and connective tissue. Progressive fibrosis and fatty infiltration reduce contractile protein density, accelerating functional decline. Additionally, reduced expression of associated proteins (sarcoglycans, syntrophins) amplifies the degenerative cascade.

• DMD Gene Mutations (Xp21.2): Approximately 65% are deletions (usually large, multi-exon), 7% are duplications, and 26% are point mutations (nonsense, frameshift, or splice site mutations). Deletions in the central "hot spot" region (exons 45–55) account for most cases. Two-thirds of mutations are inherited from carrier mothers; one-third represent de novo mutations with equal likelihood of paternal or maternal origin.

• Inheritance Pattern: X-linked recessive inheritance means affected males have the condition and carrier females (heterozygotes) typically remain asymptomatic due to random X-inactivation (lyonization). However, symptomatic female carriers (approximately 5–10% of carrier women) may develop muscular dystrophy, cardiac involvement, or cognitive issues due to skewed X-inactivation patterns favoring the mutant allele. Homozygous females are rare but present with severe phenotypes.

• Risk Modifiers: Severity correlates with the position and type of mutation. Mutations resulting in complete dystrophin absence are universally severe (DMD phenotype). In-frame deletions generally produce the milder BMD phenotype, though rare in-frame mutations can cause severe disease depending on disruption of critical protein domains.

Early Presentation (DMD—Infancy to Early Childhood)

• Delayed motor milestones: Sitting delayed until 6–8 months; walking delayed until 12–18 months; affected boys rarely walk before age 2
• Waddling gait and difficulty climbing stairs visible by ages 2–3
• Frequent falls and clumsiness; difficulty rising from the floor
• Toe-walking (equinus stance) and apparent calf hypertrophy (pseudohypertrophy from fatty infiltration)

Progressive Phase (Ages 4–12)

• Proximal muscle weakness (hip and shoulder girdle) far exceeding distal weakness
• Gower's maneuver (pathognomonic): Child uses hands to push body upright from floor by progressively "walking" hands up the legs due to severe hip extensor weakness
• Winging of scapulae during arm elevation
• Loss of independent ambulation by age 8–12 in untreated DMD (now often delayed to early teens with modern therapy)
• Lumbar lordosis and rigid spine from hip extensor weakness and flexor contractures
• Reduced vital capacity and respiratory effort dyspnea; sleep-disordered breathing

Cardiac Manifestations (Ages 10–20)

• Dilated cardiomyopathy (80% of DMD patients by late teens); often clinically silent until advanced
• Arrhythmias (premature ventricular contractions, atrial fibrillation); sudden cardiac death risk
• Dystonic features (unusual postures) from severe weakness and pain

Cognitive and Behavioral

• Intellectual disability (30–50% of DMD patients); average IQ approximately 85, but spectrum ranges from normal to severe
• Attention deficit, autism spectrum features, and learning disabilities in subset of patients

Becker Muscular Dystrophy (Milder Phenotype)

• Onset typically ages 5–25 years (later than DMD)
• Proximal weakness progresses slowly; many retain ambulation into adulthood (age 40s–60s)
• Pseudohypertrophy of calf muscles often prominent
• Cardiac involvement less frequent than DMD but still present in significant minority; risk increases with disease duration

Late-Stage DMD (Teens and Adulthood)

• Wheelchair dependence and immobility-related complications
• Severe flexor contractures (hips, knees, ankles) limiting positioning
• Scoliosis (60–90% of DMD patients); often progressive and painful
• Respiratory failure (primary cause of death); vital capacity <20% predicted

Clinical Suspicion Triggers

• Male child with proximal weakness, Gower's maneuver, family history of DMD/BMD, or elevated CK
• Delayed motor milestones, frequent falls, or difficulty climbing stairs in young boys

Serum Creatine Kinase (CK)

• Markedly elevated CK (10–100× upper limit of normal, often 10,000–100,000 IU/L) in early-stage DMD; may be extremely high (>100,000) in infants before clinical symptoms manifest
• Mild-to-moderate elevation in BMD (1–20× normal) or in asymptomatic carrier females
• CK levels decrease over time as muscle mass is lost and replaced by fat/fibrosis; normalcy does NOT exclude advanced disease
• CK elevation precedes clinical weakness, making it useful for screening

Electromyography (EMG)

• Myopathic pattern: Short-duration, low-amplitude, polyphasic motor unit action potentials with early recruitment
• Distinguishes myopathy from neurogenic weakness; fibrillations and positive sharp waves indicate active myonecrosis
• Less commonly performed now given improved genetic testing availability

Muscle Imaging (MRI/Ultrasound)

• MRI with fat-suppression sequences reveals selective muscle involvement: thigh adductors and vastus lateralis affected early; rectus femoris relatively spared (creates characteristic "splaring pattern")
• Progressive fatty infiltration replaces functional muscle, visible as hyperintensity on T1-weighted and short-tau inversion recovery (STIR) sequences
• Calf muscle hypertrophy with fatty infiltration visible on imaging despite weakness
• Ultrasound may show increased echogenicity reflecting fibrosis and fat

Genetic Testing (Gold Standard)

• DNA sequencing and deletion/duplication analysis of DMD gene identifies ~99% of mutations
• Next-generation sequencing (NGS) detects point mutations, indels, and complex rearrangements
• Multiplex ligation-dependent probe amplification (MLPA) efficiently detects large deletions and duplications
• Confirmatory muscle biopsy with Western blot (less commonly used now): Demonstrates absent or severely reduced dystrophin (DMD) versus internally deleted but present dystrophin (BMD) using anti-dystrophin antibodies
• Immunofluorescence shows markedly reduced or absent sarcolemmal dystrophin staining in DMD; partial or normal pattern in BMD

Additional Diagnostic Tests

• Electrocardiogram (ECG): Abnormalities in ~50% of DMD patients; tall R waves in V1–V2 and deep Q waves in lateral/inferior leads; arrhythmias
• Echocardiography: Assesses left ventricular ejection fraction (LVEF), wall motion abnormalities, and mitral regurgitation; baseline at diagnosis and serial monitoring recommended
• Pulmonary function tests: Vital capacity, FEV₁, FVC, maximum inspiratory/expiratory pressure; essential for monitoring respiratory decline

Diagnostic Criteria (Clinical-Genetic)

• Confirmed DMD gene mutation on sequencing/MLPA in affected male, OR
• Characteristic clinical phenotype PLUS markedly elevated CK PLUS absent dystrophin on muscle biopsy/immunofluorescence, OR
• Confirmed carrier status in mother/female relative with similar clinical presentation (symptomatic carrier)

Corticosteroids (First-Line, Disease-Modifying)

• Glucocorticoids (prednisone or deflazacort) slow disease progression and delay loss of ambulation by 1–3 years when initiated early (optimal starting age 4–7 years)
• Mechanism: Anti-inflammatory; reduce macrophage infiltration, inflammatory cytokine production, and NF-κB activation; improve muscle protein synthesis
• Dosing: Prednisone 0.75 mg/kg/day (max 40 mg/day) daily, or deflazacort 0.9 mg/kg/day (potentially fewer side effects and metabolic complications)
• Intermittent dosing (10 days on/10 days off weekly) achieves similar efficacy with reduced adverse effects compared to daily dosing
• Monitoring required: Bone density (DEXA scans annually), glucose tolerance, growth charts, behavioral/mood changes, osteoporosis prevention (calcium, vitamin D, bisphosphonates if indicated)
• Side effects: Weight gain, mood changes, immunosuppression, hypertension, hyperglycemia, osteoporosis; must be weighed against disease-modifying benefit

Newer Disease-Modifying Therapies

• Ataluren: Nonsense mutation read-through drug for ~10–15% of DMD patients with premature stop codons (nonsense mutations); allows limited full-length dystrophin production; modest functional benefit in some trials but not universally recommended
• Eteplirsen and other antisense oligonucleotides (ASOs): Skip exons to restore the reading frame and allow internally deleted but functional dystrophin production; FDA-approved for DMD patients with mutations amenable to exon 51 skipping (eteplirsen) or exon 53 skipping (golodirsen); intravenous infusions; real-world efficacy variable
• Gene therapy (dmd-based): Under investigation; viral vectors deliver microdystrophin or engineered small dystrophin; early trials show promise but accessibility currently limited to specialized centers
• Dystrophin modulation (e.g., myostatin inhibitors): Experimental agents under development

Cardiac Management

• ACE inhibitors (e.g., lisinopril) or angiotensin II receptor blockers (ARBs, e.g., losartan): Initiated when LV dysfunction detected (LVEF <55%) or as early preventive therapy; reduce disease progression and sudden death risk
• Beta-blockers (e.g., carvedilol): Added for cardiomyopathy; improves outcomes in DMD-associated dilated cardiomyopathy
• Aldosterone antagonists (e.g., spironolactone): May provide additional cardioprotection
• Diuretics for clinical heart failure
• Implantable cardioverter-defibrillator (ICD) placement considered for sustained arrhythmias, syncope, or markedly reduced EF
• Cardiac monitoring: Baseline echocardiography, repeat annually or with clinical changes; ECG surveillance

Respiratory Support

• Noninvasive positive-pressure ventilation (NIPPV) using BiPAP or CPAP at night when vital capacity drops below 40–50% predicted or sleep-disordered breathing develops
• Mechanical in-exsufflation devices (cough-assist devices) help clear secretions and reduce pneumonia risk
• Assisted ventilation during sleep extends lifespan and improves quality of life substantially
• Pulmonary function monitoring guides timing of ventilator initiation (typically when VC <25% or rapid decline)

Orthopedic Management

• Physical therapy and stretching to minimize contractures; passive range-of-motion exercises critical
• Ankle-foot orthoses (AFOs) and knee-ankle-foot orthoses (KAFOs) prolong ambulation phase
• Surgical correction of contractures when limiting function (e.g., Achilles tendon lengthening)
• Scoliosis spinal fusion surgery: Considered when Cobb angle >40–50° or rapidly progressive; typically performed in teenage years; extends survival by reducing respiratory compromise from severe spinal deformity
• Wheelchair optimization and seating systems for comfort and pressure relief

Metabolic and Nutritional Support

• Vitamin D and calcium supplementation to support bone health, especially critical with corticosteroid use
• Bisphosphonates (e.g., alendronate) if osteoporosis develops or accelerated bone loss on steroids
• Nutritional counseling to avoid excessive weight gain (exacerbates mobility loss) while maintaining adequate protein intake

Pain and Symptom Management

• Nonsteroidal anti-inflammatory drugs (NSAIDs) for muscle pain; caution with renal/cardiac complications
• Antispasticity agents (baclofen, tizanidine) for muscle tone management
• Psychosocial support: Counseling, school accommodations, adaptive equipment, transition planning to adulthood

Monitoring and Multidisciplinary Care

• Neuromuscular specialist coordination of DMD care
• Annual or biannual multidisciplinary clinic visits (neurology, cardiology, pulmonology, orthopedics, rehabilitation medicine, genetics)
• Serial functional assessments: Timed 6-minute walk, North Star Ambulatory Assessment (NSAA), or modified functional assessment tools
• Family genetic counseling and carrier testing for female relatives

Musculoskeletal

• Contractures (hip, knee, ankle flexors): Severely limit positioning, increase pain, and can lead to immobility-related complications; managed by aggressive physical therapy, orthoses, and sometimes surgery
• Scoliosis: Develops in 60–90% of DMD patients; progressive and often rapidly accelerates after loss of ambulation; increases sitting imbalance, pain, and respiratory compromise; requires spinal fusion in many cases
• Fractures from falls during ambulatory phase; osteoporosis accelerated by corticosteroid use and immobility increases fracture risk in later stages

Cardiac

• Dilated cardiomyopathy: Present in 80% of older DMD patients; progressive LV dysfunction, arrhythmias, and sudden cardiac death are major late causes of death; requires aggressive preventive and symptomatic therapy
• Arrhythmias (atrial fibrillation, premature ventricular contractions, ventricular tachycardia): Risk of syncope and sudden death
• Conduction abnormalities: Manifested on ECG; may require pacemaker or ICD

Respiratory

• Progressive respiratory muscle weakness: Vital capacity declines progressively; when VC <20–25%, risk of hypoxemia and hypercap

• The reading-frame rule is the single most tested concept: out-of-frame (frameshift/nonsense) mutations abolish dystrophin → Duchenne; in-frame deletions yield a truncated but partly functional protein → Becker. A stem describing a large deletion that "preserves the reading frame" in a 20-year-old still walking is Becker, not DMD.
• Classic triad in a 3–5-year-old boy: Gowers sign (walking the hands up the thighs), calf pseudohypertrophy, and waddling gait with lumbar lordosis. Pseudohypertrophy is fat and fibrous replacement — a distractor stem calling it "true muscle hypertrophy" is wrong.
• Best next step after a markedly elevated CK: genetic testing of the DMD gene (copy-number analysis plus sequencing, commonly on a single NGS platform), not muscle biopsy. Biopsy with dystrophin immunostaining/Western blot is reserved for genetically unconfirmed cases — the DMD Care Considerations Working Group (CDC-sponsored) makes molecular diagnosis the standard entry point.
• Anesthesia association examiners love: succinylcholine and volatile inhaled anesthetics can precipitate rhabdomyolysis, hyperkalemic cardiac arrest, and a malignant-hyperthermia-like reaction in dystrophinopathy. Total intravenous anesthesia is preferred.
• Glucocorticoids (prednisone or deflazacort) are the only widely applicable disease-modifying therapy, supported by American Academy of Neurology guidance and the DMD Care Considerations; they delay loss of ambulation. Exon-skipping antisense oligonucleotides apply only to specific mutations.
• Cardiac disease is universal and silent early: surveillance echocardiography (or cardiac MRI) plus ECG begins at diagnosis. In DMD, cardioprotective therapy with an ACE inhibitor or ARB is commonly started by about age 10 or at the first sign of declining function, whichever comes first, though exact triggers differ between guidance documents. Classic ECG: tall R waves in V1–V2 with deep lateral Q waves.
• Death results from progressive respiratory failure and/or cardiomyopathy: with widespread nocturnal noninvasive ventilation and cough assist extending survival, cardiac deaths now account for a comparable or greater share than respiratory deaths — avoid ranking them rigidly.
• Carrier females are not always silent: skewed X-inactivation can produce elevated CK, weakness, or isolated dilated cardiomyopathy — offer genetic counseling and cardiac screening to mothers and sisters.
• Common distractors: floppy infant with tongue fasciculations = spinal muscular atrophy; grip myotonia with frontal balding and cataracts = myotonic dystrophy; normal CK with fatigable ptosis = myasthenia gravis.