Neuromuscular Disorders & Diagnostic Criteria Flashcards

According to the Awaji criteria for ALS diagnosis, which of the following is true?

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.

What is the diagnostic significance of a 'giant' motor unit action potential (amplitude >5 mV, duration >15 ms) on needle EMG?

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.

Which diagnostic criterion is used to define Guillain-Barré syndrome electrodiagnostically as the AIDP subtype versus AMAN subtype?

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.

In a patient with suspected myasthenia gravis, single-fiber EMG (SFEMG) is most useful because:

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.

Which clinical and electrodiagnostic feature helps distinguish spinal muscular atrophy (SMA) from ALS?

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.

On NCS, a Martin-Gruber anastomosis (MGA) is suspected when:

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.

Which electrodiagnostic finding is characteristic of Eaton-Lambert syndrome involving autonomic dysfunction?

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.