ABEM Neuromuscular Disorders & Diagnostic Criteria 4 — Questions and Answers
Question 1: According to the Awaji criteria for ALS diagnosis, which of the following is true?
- Fibrillations on EMG do not count as evidence of lower motor neuron dysfunction
- Fasciculation potentials in a chronic neurogenic background are equivalent to fibrillations for LMN evidence (Correct answer)
- Clinical upper motor neuron signs must precede EMG abnormalities
- At least four body regions must be involved for definite ALS diagnosis
Correct answer: Fasciculation potentials in a chronic neurogenic background are equivalent to fibrillations for LMN evidence
The Awaji criteria allow fasciculation potentials in a background of chronic neurogenic change to serve as equivalent evidence of LMN dysfunction, potentially allowing earlier diagnosis than the original El Escorial criteria.
Question 2: What is the diagnostic significance of a 'giant' motor unit action potential (amplitude >5 mV, duration >15 ms) on needle EMG?
- It indicates acute denervation with early reinnervation
- It reflects chronic, long-standing reinnervation over months to years (Correct answer)
- It is a normal variant in large muscles
- It is pathognomonic of myasthenia gravis
Correct answer: It reflects chronic, long-standing reinnervation over months to years
Giant MUAPs develop from extensive collateral reinnervation over many months, incorporating large numbers of muscle fibers into single motor units, indicating chronic denervating disease such as old polio or slowly progressive ALS.
Question 3: Which diagnostic criterion is used to define Guillain-Barré syndrome electrodiagnostically as the AIDP subtype versus AMAN subtype?
- AIDP shows axonal loss with preserved F-waves; AMAN shows demyelination
- AIDP shows demyelination (slowed CV, conduction block, temporal dispersion); AMAN shows primary axonal pattern with low CMAPs and preserved velocities (Correct answer)
- AIDP affects only motor nerves; AMAN affects sensory nerves preferentially
- Both subtypes are indistinguishable on NCS within the first two weeks
Correct answer: AIDP shows demyelination (slowed CV, conduction block, temporal dispersion); AMAN shows primary axonal pattern with low CMAPs and preserved velocities
AIDP (Acute Inflammatory Demyelinating Polyneuropathy) shows demyelinating NCS features, while AMAN (Acute Motor Axonal Neuropathy) shows primary axonal loss with reduced CMAP amplitudes but relatively preserved conduction velocities.
Question 4: In a patient with suspected myasthenia gravis, single-fiber EMG (SFEMG) is most useful because:
- It provides the highest specificity of any MG test
- It provides the highest sensitivity (~95%) for detecting NMJ dysfunction, even when repetitive stimulation is normal (Correct answer)
- It can differentiate MG from LEMS with certainty
- It quantifies acetylcholine receptor antibody titers
Correct answer: It provides the highest sensitivity (~95%) for detecting NMJ dysfunction, even when repetitive stimulation is normal
SFEMG measuring jitter is the most sensitive electrodiagnostic test for neuromuscular junction dysfunction (~95% sensitivity in generalized MG), detecting abnormalities when standard RNS is still normal.
Question 5: Which clinical and electrodiagnostic feature helps distinguish spinal muscular atrophy (SMA) from ALS?
- SMA shows upper motor neuron signs while ALS does not
- SMA lacks upper motor neuron signs and typically presents in childhood with very chronic large MUAPs; ALS has UMN involvement and faster progression (Correct answer)
- SMA has normal EMG; ALS shows fibrillations
- Both are clinically and electrodiagnostically identical
Correct answer: SMA lacks upper motor neuron signs and typically presents in childhood with very chronic large MUAPs; ALS has UMN involvement and faster progression
SMA is a pure lower motor neuron disease without UMN involvement, often with very large chronic MUAPs reflecting long-standing reinnervation, while ALS involves both UMN and LMN with faster progression.
Question 6: On NCS, a Martin-Gruber anastomosis (MGA) is suspected when:
- Ulnar CMAP amplitude is higher than median at the wrist
- Median nerve stimulation at the elbow gives a larger CMAP than stimulation at the wrist for ulnar-innervated hand muscles (Correct answer)
- The first lumbrical shows an anomalously short distal latency
- F-wave latencies are shorter than expected for ulnar nerve
Correct answer: Median nerve stimulation at the elbow gives a larger CMAP than stimulation at the wrist for ulnar-innervated hand muscles
MGA is a forearm crossover of motor fibers from median to ulnar nerve; elbow median stimulation activates these crossing fibers, producing a larger CMAP in ulnar-innervated muscles than wrist stimulation, which can mimic or mask carpal tunnel syndrome.
Question 7: Which electrodiagnostic finding is characteristic of Eaton-Lambert syndrome involving autonomic dysfunction?
- Absent H-reflex bilaterally
- Low baseline CMAP amplitude that dramatically increases after brief maximum voluntary contraction (post-exercise facilitation) (Correct answer)
- Absent blink reflex R1 component
- Slowed motor conduction velocity in all limbs
Correct answer: Low baseline CMAP amplitude that dramatically increases after brief maximum voluntary contraction (post-exercise facilitation)
LEMS characteristically shows low baseline CMAP amplitudes that increase dramatically (>100%) after 10–15 seconds of maximum voluntary contraction due to calcium accumulation and enhanced ACh release at presynaptic terminals.
ABEM Neuromuscular Disorders & Diagnostic Criteria 4
According to the Awaji criteria for ALS diagnosis, which of the following is true?