Study guide
9.3 Muscular Dystrophies & Neuromuscular Disease in Children
ABPMR Physical Medicine and Rehabilitation Board Exam Study Guide Exam
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1 Chapter 1: Introduction & Exam Overview
1.1 ABPMR Part I Exam Overview & Format 1.2 Board Review Strategy & Content Blueprint Breakdown
2 Chapter 2: Musculoskeletal Medicine
2.1 Shoulder & Upper Extremity Diagnostics 2.2 Knee, Hip & Lower Extremity Conditions 2.3 Spine Pathology, Radiculopathy & Myelopathy 2.4 Inflammatory Arthritis, Tendinopathies & Concussion 2.5 Bone Health, Osteoporosis & Metabolic Bone Disease 2.6 Thoracic Outlet Syndrome & Peripheral Nerve Entrapment 2.7 Fracture Rehabilitation & Immobilization Complications 2.8 Overuse Tendinopathies & Regenerative Medicine 2.9 Gait Analysis & Abnormal Gait Patterns
3 Chapter 3: Spinal Cord Injury Rehabilitation
3.1 ASIA Examination & Impairment Scale 3.2 Spinal Cord Injury Syndromes 3.3 Autonomic Dysreflexia & Acute Complications 3.4 Neurogenic Bladder, Bowel & Secondary Complications
4 Chapter 4: Stroke & Neuromuscular Rehabilitation
4.1 Stroke Vascular Syndromes & Acute Assessment 4.2 Stroke Motor Recovery & Hemiplegic Complications 4.3 Spasticity Assessment & Multimodal Management 4.4 ALS, GBS, CIDP & Neuromuscular Disease Rehab 4.5 Multiple Sclerosis & Demyelinating Disease Rehabilitation 4.6 Parkinsonism, Movement Disorders & Functional Neurologic Disorder
5 Chapter 5: Electrodiagnostics (EMG/NCS)
5.1 Nerve Conduction Studies Principles & Physics 5.2 Needle Electromyography & MUAP Analysis 5.3 Mononeuropathies & Radiculopathy Patterns 5.4 Neuromuscular Junction Disorders & Myopathies
6 Chapter 6: Traumatic Brain Injury
6.1 TBI Severity Classification, Glasgow Coma Scale, and Pathophysiology 6.2 Rancho Los Amigos Levels of Cognitive Functioning and Disability Rating Scale 6.3 Neurobehavioral Sequelae and Agitation Management 6.4 Concussion Evaluation, Vestibular-Ocular Dysfunction, and Return-to-Play/Work Protocols
7 Chapter 7: Chronic Pain & Interventional Spine
7.1 Fluoroscopic Spine Interventions 7.2 Opioid Stewardship & Controlled Substance Compliance 7.3 Complex Regional Pain Syndrome, Fibromyalgia & Myofascial Pain 7.4 Spinal Cord Stimulation & Intrathecal Therapy
8 Chapter 8: Amputee Rehabilitation & Prosthetics
8.1 Amputation Surgical Levels & Medicare K-Levels 8.2 Socket Designs & Suspension Systems 8.3 Prosthetic Feet & Microprocessor Knee Mechanisms 8.4 Post-Amputation Care, Residual Limb & Phantom Pain
9 Chapter 9: Pediatric Rehabilitation
9.1 Cerebral Palsy Classification & GMFCS 9.2 Spina Bifida & Myelomeningocele Management 9.3 Muscular Dystrophies & Neuromuscular Disease in Children 9.4 Pediatric Brachial Plexopathy & Congenital Disorders
10 Chapter 10: Prosthetics, Orthotics & Assistive Technology
10.1 Lower Limb Orthoses — AFOs & KAFOs 10.2 Spinal & Upper Extremity Orthoses 10.3 Upper Limb Prosthetics — Body-Powered vs Myoelectric 10.4 Wheelchairs, Seating Systems & Pressure Mapping
11 Chapter 11: Medical Rehabilitation & Outcomes
11.1 Dysphagia Assessment & Management 11.2 Neurogenic Bladder & Bowel Management 11.3 Pressure Injuries & Wound Management 11.4 Outcome Measures & Medical Complications 11.5 Cancer Rehabilitation & Oncologic Rehabilitation 11.6 Burns, Endocrine/Metabolic & Medical Complications Rehabilitation
12 Chapter 12: Cardiopulmonary Rehabilitation
12.1 Cardiac Rehabilitation Phases & Metabolic Equivalents 12.2 Pulmonary Rehabilitation & COPD Management
13 Chapter 13: Foundational Rehabilitation Concepts
13.1 Kinesiology, Biomechanics & Functional Anatomy 13.2 Rehabilitation Pharmacology & Medication Management 13.3 Research Methodology, Biostatistics & Evidence-Based Practice 13.4 Nutrition, Metabolism & Energy Expenditure in Rehabilitation 13.5 Diagnostic Procedures: Imaging, MSK Ultrasound, Urodynamics & Neuropsych Assessment
9.3 Muscular Dystrophies & Neuromuscular Disease in Children
Key Takeaways Explain This Topic AI
Duchenne Muscular Dystrophy (DMD) is an X-linked recessive disorder caused by frame-shift mutations in the DMD gene on Xp21, leading to complete deficiency of the protein dystrophin.
Clinical hallmarks of DMD include early motor delays, calf pseudohypertrophy, Gower sign, serum CK >10,000 U/L, and historical loss of independent ambulation at age 10-12 years.
Corticosteroid therapy (prednisone 0.75 mg/kg/day or deflazacort 0.9 mg/kg/day) is the mainstay medical treatment, delaying loss of ambulation by 2-5 years and reducing scoliosis incidence.
Targeted therapies for DMD include exon-skipping antisense oligonucleotides (e.g., Eteplirsen for exon 51) and AAV gene replacement therapy delivering micro-dystrophin.
Becker Muscular Dystrophy (BMD) stems from in-frame Xp21 mutations producing partially functional dystrophin, resulting in a milder phenotype with ambulation preserved past age 16.
Last updated: July 2026
Pathophysiology and Genetics of Duchenne Muscular Dystrophy
Duchenne Muscular Dystrophy (DMD) is an X-linked recessive disorder affecting 1 in 3,500-5,000 live male births. It is caused by mutations in the DMD gene on the short arm of the X chromosome ( Xp21 )—the largest human gene, making it susceptible to spontaneous mutations (~33% of cases).
Genetic Diagnostics & Carrier Manifestations
Genetic confirmation utilizes Multiplex Ligation-dependent Probe Amplification (MLPA) to detect exon deletions or duplications (accounting for ~70% of mutations), followed by gene sequencing for point mutations. Female carriers (heterozygous) are typically asymptomatic but carry a 2-5% risk of carrier cardiomyopathy or mild muscle weakness, warranting baseline cardiac screening with echocardiography or cardiac MRI starting in early adulthood (every 5 years).
The Role of Dystrophin
Dystrophin anchors the actin cytoskeleton to the extracellular matrix via the dystrophin-glycoprotein complex (DGC) . In DMD, frame-shift mutations (deletions/duplications disrupting the reading frame) cause complete absence of dystrophin (<1% of normal) . Sarcolemmal instability results in micro-tearing during contraction, calcium influx, chronic muscle necrosis, and replacement of muscle with fibroadipose tissue .
Clinical Presentation and Early Functional Signs
Symptoms manifest between ages 2 and 5 years:
Motor Delays : Walking delayed past 18 months, frequent falls, stair difficulty.
Gower Sign : Proximal hip extensor (gluteus maximus) weakness. To stand from prone, the child turns, pushes onto hands/knees, and "walks" hands up thighs.
Calf Pseudohypertrophy : Gastrocnemius enlargement due to fibroadipose infiltration replacing lost muscle fibers.
Serum Creatine Kinase (CK) : Markedly elevated ( >10,000 to 30,000 U/L ), reflecting continuous sarcolemmal leakage.
- Clinical Feature Pathophysiology / Mechanism Diagnostic Significance
- Gower Sign Proximal hip extensor & quadriceps weakness Classic physical hallmark of DMD
Calf Pseudohypertrophy Fibroadipose tissue replacement of muscle Differentiates dystrophy from neurogenic atrophy
- Toe Walking Achilles tendon tightness & contracture Compensatory posture for quadriceps weakness
- Serum CK >10,000 U/L Massive sarcolemmal leakage of enzyme Essential screening test for early diagnosis
- Natural History and Multisystem Complications
Loss of Ambulation : Occurs historically at age 10 to 12 years without steroids.
Spinal Deformity : Rapidly progressive scoliosis develops after loss of ambulation in >90% of untreated boys.
Pulmonary Complications : Progressive diaphragm and intercostal weakness causes restrictive lung disease and nocturnal hypoventilation. BiPAP is typically required by late adolescence.
Cardiac Complications : Dilated cardiomyopathy and arrhythmias occur in virtually all patients by age 18. Baseline cardiac imaging (ECHO/MRI) is recommended at diagnosis or age 6 , repeating every 2 years until age 10, then annually.
- Pharmacological and Disease-Modifying Therapies
- Corticosteroids (Standard of Care)
Daily corticosteroids are recommended for ambulatory boys aged ≥4 years:
Regimens : Prednisone (0.75 mg/kg/day) or Deflazacort (0.9 mg/kg/day).
Benefits : Delays loss of ambulation by 2-5 years, preserves upper limb and respiratory function, reduces scoliosis progression by >70%, and delays cardiomyopathy onset.
Side Effects : Weight gain, cushingoid appearance, growth suppression, osteopenia, vertebral fractures, cataracts.
Targeted Genetic Therapies
Exon-Skipping Antisense Oligonucleotides (ASOs) : Bind pre-mRNA to skip target exons and restore the open reading frame, generating a shortened, partially functional dystrophin:
Eteplirsen : Skips exon 51 (~13% of patients).
Golodirsen & Viltolarsen : Skip exon 53 (~8% of patients).
Casimersen : Skips exon 45 (~8% of patients).
Gene Therapy : AAV vector delivery ( Delandistrogene moxeparvovec ) of micro-dystrophin .
Multidisciplinary Care & Transition of Care
As life expectancy expands into the 30s with advanced cardiopulmonary management, structured transition of care from pediatric to adult multidisciplinary clinics is vital. Key components include continuous cardiac monitoring (ACE inhibitors/beta-blockers for early left ventricular dysfunction), bone health management (bisphosphonates for osteoporotic compression fractures), and adaptive equipment (power tilt-in-space wheelchairs, environmental control units, and customized seating).
Becker Muscular Dystrophy (BMD)
Becker Muscular Dystrophy (BMD) results from in-frame mutations in the DMD gene (Xp21).
Distinction between DMD and BMD:
Dystrophin Expression : Dystrophin is truncated but retains partial activity (10-40% of normal) .
Ambulation : Preserved past age 16 years (often into adulthood).
Onset : Later onset (mean age 11-12 years).
Cardiac Risk : Dilated cardiomyopathy can occur early and be disproportionately severe relative to skeletal weakness.
Diagnostic Category Gene Mutation Type Dystrophin Protein Age at Ambulation Loss Cardiac Involvement
Duchenne (DMD) Out-of-frame (frame-shift) Absent (<1% of normal) 10 - 12 years (untreated) Universal by age 18; dilated cardiomyopathy
Becker (BMD) In-frame mutation Truncated/Reduced (10-40%) Preserved past age 16 Can be severe; out of proportion to muscle weakness
Differential Diagnosis: Spinal Muscular Atrophy (SMA)
Autosomal recessive degeneration of anterior horn cells caused by SMN1 gene (5q13) mutations. Disease-modifying therapies have revolutionized outcomes:
Nusirensern (Spinraza): Intrathecal antisense oligonucleotide that modifies SMN2 pre-mRNA splicing to increase full-length SMN protein production.
Risdiplam (Evrysdi): Daily oral small-molecule SMN2 splicing modifier.
Onasemnogene abeparvovec (Zolgensma): One-time intravenous AAV9 gene replacement therapy delivering a functional copy of the SMN1 gene to motor neurons.
Clinical Subtypes of SMA:
SMA Type 1 (Werdnig-Hoffmann) : Onset <6 months; severe hypotonia ("floppy infant"), paradoxical breathing, tongue fasciculations; non-sitters.
SMA Type 2 : Onset 6-18 months; achieve sitting, non-ambulators.
SMA Type 3 (Kugelberg-Welander) : Onset >18 months; achieve independent ambulation.