Study guide
5.4 Neuromuscular Junction Disorders & Myopathies
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
5.4 Neuromuscular Junction Disorders & Myopathies
Key Takeaways Explain This Topic AI
Low-frequency repetitive nerve stimulation (2-3 Hz) depletes presynaptic ACh stores, producing a >10% CMAP decrement in Myasthenia Gravis (postsynaptic AChR disorder).
High-frequency RNS (20-50 Hz) or 10-second exercise causes presynaptic calcium accumulation, producing a >100% CMAP increment in Lambert-Eaton Myasthenic Syndrome (presynaptic VGCC disorder).
Single-fiber EMG is the most sensitive diagnostic test (>95-99%) for Myasthenia Gravis, measuring microsecond transmission variability (jitter) and impulse blocking.
Inclusion Body Myositis affects finger flexors (FDP) and quadriceps in males >50, demonstrating a unique mixed EMG pattern of short myopathic and long neurogenic MUAPs with CRDs.
Last updated: July 2026
- 5.4 Neuromuscular Junction Disorders & Myopathies
- Neuromuscular Junction (NMJ) Transmission Physiology
Neuromuscular junction transmission converts nerve action potentials into skeletal muscle contraction through sequential electrical and biochemical events:
Depolarization of presynaptic nerve terminals opens P/Q-type Voltage-Gated Calcium Channels (VGCC) , driving calcium influx into the terminal axon.
Calcium entry triggers exocytosis of synaptic vesicles containing acetylcholine (ACh) into the synaptic cleft.
ACh molecules diffuse across the cleft and bind to postsynaptic nicotinic Acetylcholine Receptors (AChR) on the muscle endplate fold.
Channel opening generates an Endplate Potential (EPP) . In healthy junctional tissue, EPP amplitude exceeds threshold—the safety factor of neuromuscular transmission. PRESYNAPTIC TERMINAL (LEMS Target)
- [ VGCC Channels ] --> Ca2+ Influx
- |
- ( ACh Vesicles )
- | Exocytosis
- ================= Synaptic Cleft
- |
- [ ACh Receptors ] --> Postsynaptic EPP
- POSTSYNAPTIC MEMBRANE (MG Target)
- Repetitive Nerve Stimulation (RNS) Principles
Repetitive Nerve Stimulation (RNS) tests NMJ safety factor under physiological stress:
Low-Frequency RNS (2–3 Hz) : Depletes presynaptic ACh stores. In postsynaptic disorders (Myasthenia Gravis), compromised safety factor causes EPPs to fall below threshold, yielding a >10% drop in CMAP amplitude—a decremental response .
High-Frequency RNS (20–50 Hz) / Post-Exercise Facilitation : Rapid stimulation or 10–15s maximal voluntary contraction causes calcium accumulation in the presynaptic terminal. In presynaptic disorders (LEMS), this surge in ACh release produces a >100% CMAP increase—an incremental response .
- Postsynaptic NMJ Disorders: Myasthenia Gravis (MG)
- Pathophysiology & Electrodiagnostics
Myasthenia Gravis is an autoimmune disease with autoantibodies against postsynaptic AChR (85%), MuSK , or LRP4 , causing receptor destruction and endplate fold simplification.
Low-Frequency RNS (2–3 Hz) : Classic decremental response >10% in CMAP amplitude/area (1st to 4th/5th wave).
Post-Exercise Protocols : Post-exercise facilitation (brief improvement) followed by post-exercise exhaustion (worsening decrement at 2–5 min).
Muscle Selection : Proximal muscles ( Trapezius via Spinal Accessory, Nasalis / Orbicularis Oculi via Facial) yield higher sensitivity (75–85%) than distal hand muscles (50%).
Single-Fiber EMG (SFEMG)
SFEMG is the most sensitive diagnostic test for MG (>95–99% sensitivity).
Technique : Uses a specialized single-fiber needle electrode with a small recording surface (25 µm diameter) to record individual muscle fiber action potentials from the same motor unit.
Key Diagnostic Parameters : Measures jitter (microsecond variability in transmission time between two muscle fibers of the same motor unit) and blocking (impulse failure). In MG, jitter is markedly prolonged and impulse blocking occurs frequently. REPETITIVE NERVE STIMULATION PATTERNS
- Low-Frequency RNS (2-3 Hz) in MG Post-Exercise Facilitation in LEMS
- Decremental Response (>10% drop) Incremental Response (>100% surge)
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- 1st 4th Wave Baseline Post-10s Exercise
- Presynaptic NMJ Disorders: LEMS & Botulism
- Lambert-Eaton Myasthenic Syndrome (LEMS)
Presynaptic autoimmune disorder with antibodies against P/Q-type Voltage-Gated Calcium Channels (VGCC) . Over 50% are paraneoplastic (Small Cell Lung Cancer).
Clinical Triad : Proximal weakness (legs > arms), autonomic dysfunction (dry mouth), and reduced reflexes that facilitate post-exercise.
EMG Triad :
Low baseline CMAP amplitudes (<20% normal).
Low-frequency RNS: Decremental response.
High-frequency RNS (20–50 Hz) or 10–15s exercise: Massive incremental response (>100% increase , often 200–400% surge).
Botulism
Neurotoxin cleaves SNARE proteins (SNAP-25, synaptobrevin), blocking presynaptic ACh exocytosis. Shows low baseline CMAPs, low-frequency decrement, and mild-to-moderate post-exercise increment (20–100%).
- Feature Myasthenia Gravis (MG) Lambert-Eaton Syndrome (LEMS) Botulism
- Primary Target Postsynaptic AChR / MuSK Presynaptic P/Q-type VGCC Presynaptic SNARE proteins
- Baseline CMAP Normal Severely Low (<20% normal) Low
- Low-Freq RNS (2-3 Hz) Decrement >10% Decrement Decrement
- High-Freq RNS / Exercise Minimal change Increment >100% (Surge) Increment 20–100%
- Clinical Autonomic Signs Absent Present (dry mouth) Present (pupils fixed/dilated)
- Single-Fiber EMG Increased jitter & blocking Increased jitter & blocking Increased jitter & blocking
- Myopathies & Inflammatory Muscle Diseases
- Electromyographic Features of Myopathies
Primary diseases of muscle (myopathies) damage muscle fibers while leaving alpha motor axons intact.
EMG Triad : Increased insertional activity (fibs/PSWs), short-duration, low-amplitude, polyphasic MUAPs , and early/excessive recruitment .
Inflammatory Myopathies
Polymyositis & Dermatomyositis : Autoimmune proximal muscle weakness with high CK. EMG demonstrates abundant fibs/PSWs, short/low MUAPs, and early recruitment.
Inclusion Body Myositis (IBM) : Asymmetric weakness in males >50 targeting Flexor Digitorum Profundus and Quadriceps . EMG shows a mixed pattern of short myopathic MUAPs alongside long-duration high-amplitude neurogenic MUAPs and complex repetitive discharges (CRDs).
Steroid-Induced Myopathy : Type 2 muscle fiber atrophy presenting with proximal weakness and normal CK. Characterized by normal needle EMG (absence of fibs/PSWs) because muscle membrane stability remains intact.
Myopathy Type Key Clinical Features Characteristic EMG Patterns
Polymyositis / Dermatomyositis Symmetric proximal weakness, high CK, heliotrope rash Abundant fibs/PSWs, short/low MUAPs, early recruitment
Inclusion Body Myositis (IBM) Asymmetric, finger flexors & quads, >50 yrs Mixed pattern: short & long MUAPs, CRDs, fibs