EMG EMG Clinical Applications & Pathology 2 — Questions and Answers
Question 1: Which electrodiagnostic feature is most helpful in distinguishing axonal Charcot-Marie-Tooth (CMT2) from demyelinating CMT (CMT1)?
- Markedly reduced CMAP amplitudes with relatively preserved conduction velocities in CMT2 versus uniformly slowed velocities in CMT1 (Correct answer)
- Absent sensory responses only in CMT1
- Fibrillation potentials present in CMT1 but not CMT2
- H-reflex absent only in CMT2
Correct answer: Markedly reduced CMAP amplitudes with relatively preserved conduction velocities in CMT2 versus uniformly slowed velocities in CMT1
CMT2 is axonal, producing reduced amplitudes with near-normal velocities, while CMT1 is demyelinating with uniformly slowed conduction velocities (often below 38 m/s in the upper limbs).
Question 2: In myasthenia gravis, which electrodiagnostic test is most sensitive for confirming neuromuscular junction dysfunction?
- Single-fiber EMG (SFEMG) measuring jitter (Correct answer)
- Repetitive nerve stimulation at 3 Hz showing >10% CMAP decrement
- Needle EMG showing fibrillation potentials
- Sensory nerve conduction studies
Correct answer: Single-fiber EMG (SFEMG) measuring jitter
Single-fiber EMG measuring jitter is the most sensitive electrodiagnostic test for neuromuscular junction dysfunction, detecting abnormalities even when RNS is normal.
Question 3: Which finding on needle EMG is most characteristic of Lambert-Eaton Myasthenic Syndrome (LEMS)?
- Small baseline CMAP that markedly increases (>100%) after brief exercise or high-frequency stimulation (Correct answer)
- Decremental response at 3 Hz with no post-exercise facilitation
- Widespread fibrillation potentials with polyphasic MUPs
- Normal EMG with prolonged sensory latencies
Correct answer: Small baseline CMAP that markedly increases (>100%) after brief exercise or high-frequency stimulation
LEMS shows a small baseline CMAP with dramatic post-exercise or high-frequency facilitation (>100% increment) due to calcium-mediated acetylcholine vesicle release.
Question 4: A patient with fatigable weakness, ptosis, and diplopia undergoes RNS testing showing a 12% CMAP decrement at 3 Hz. What is the most likely diagnosis?
- Myasthenia gravis (Correct answer)
- Lambert-Eaton myasthenic syndrome
- Myotonic dystrophy
- Polymyositis
Correct answer: Myasthenia gravis
A >10% CMAP decrement at low-frequency (3 Hz) repetitive nerve stimulation in the clinical context of fatigable ptosis and diplopia is consistent with myasthenia gravis.
Question 5: Which EMG pattern is most consistent with inclusion body myositis (IBM)?
- Mixed myopathic and neurogenic MUPs with fibrillations, affecting distal muscles (finger flexors, foot extensors) early (Correct answer)
- Pure myopathic pattern limited to proximal muscles with no fibrillations
- Pure neurogenic pattern with fasciculations and large MUPs
- Normal needle EMG with abnormal sensory NCS
Correct answer: Mixed myopathic and neurogenic MUPs with fibrillations, affecting distal muscles (finger flexors, foot extensors) early
IBM characteristically shows a mixed myopathic-neurogenic pattern with both short and long MUPs, fibrillation potentials, and early distal involvement of finger flexors and foot extensors.
Question 6: During EMG evaluation of suspected diabetic polyneuropathy, which nerve is typically studied first as a sensitive indicator of early disease?
- Sural sensory nerve (Correct answer)
- Median motor nerve
- Ulnar motor nerve
- Common peroneal motor nerve
Correct answer: Sural sensory nerve
The sural sensory nerve is among the longest and most vulnerable in length-dependent neuropathy; its reduced amplitude or absent response is often the earliest electrodiagnostic finding in diabetic polyneuropathy.
EMG EMG Clinical Applications & Pathology 2
Which electrodiagnostic feature is most helpful in distinguishing axonal Charcot-Marie-Tooth (CMT2) from demyelinating CMT (CMT1)?