5.4 Neuromuscular Junction Disorders & Myopathies
Key Takeaways
- 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.
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 |
| Steroid Myopathy | Proximal weakness, normal CK, chronic steroids | Normal EMG or mild short MUAPs; NO fibs/PSWs |
A 38-year-old female presents with fluctuating ptosis, diplopia, and proximal arm fatigue that worsens toward the end of the day. Repetitive nerve stimulation (RNS) of the spinal accessory nerve recording the trapezius muscle is performed at 3 Hz. Which of the following findings is diagnostic of Myasthenia Gravis?
A 65-year-old male smoker with a 40 pack-year history presents with progressive leg weakness and severe dry mouth. On physical examination, his lower extremity deep tendon reflexes are initially absent, but elicitable immediately following 10 seconds of isometric quadriceps contraction. Resting CMAP amplitudes are severely reduced. High-frequency (50 Hz) repetitive nerve stimulation produces a 250% increase in CMAP amplitude. What is the molecular target of the autoantibodies in this condition?
A 61-year-old male presents with asymmetric weakness and atrophy involving his wrist flexors and quadriceps muscles. Creatine kinase is mildly elevated. Needle EMG of the flexor digitorum profundus demonstrates fibrillation potentials alongside a mixture of short-duration, low-amplitude MUAPs and long-duration, high-amplitude MUAPs with complex repetitive discharges. Which of the following diagnosis is most consistent with this presentation?