EMG Clinical Applications & Pathology Flashcards

Which electrodiagnostic finding is most specific for conduction block in a demyelinating neuropathy?

Answer: A >50% reduction in CMAP area or amplitude when stimulating proximally versus distally without significant temporal dispersion

Conduction block is defined as a >50% reduction in proximal versus distal CMAP area/amplitude (AANEM criteria) without significant temporal dispersion, indicating focal demyelination.

In ulnar neuropathy at the elbow, which NCS comparison is most diagnostic?

Answer: Comparing ulnar motor conduction velocity across the elbow segment to the forearm segment, with >10 m/s slowing across the elbow being significant

A conduction velocity drop of >10 m/s (or >10 m/s slower than the forearm segment) across the elbow localizes the ulnar lesion to that segment.

Which feature on needle EMG helps confirm that a peroneal neuropathy is at the fibular head rather than an L5 radiculopathy?

Answer: Normal EMG in the tibialis posterior and short head of the biceps femoris with abnormal EMG in the peroneal-innervated muscles

The tibialis posterior (tibial nerve, L5) and short head of biceps femoris (peroneal nerve above fibular head) are normal in peroneal neuropathy at the fibular head, helping exclude L5 radiculopathy.

Myotonic discharges on needle EMG are characterized by which sound and waveform pattern?

Answer: Waxing and waning frequency and amplitude producing a 'dive-bomber' sound

Myotonic discharges wax and wane in frequency and amplitude, producing the classic 'dive-bomber' sound that is pathognomonic for myotonic disorders.

A patient presents with acute onset wrist drop after sleeping with the arm compressed. Which electrodiagnostic finding best supports a radial nerve injury at the spiral groove with axonotmesis?

Answer: Normal distal radial motor conduction velocity with absent/reduced CMAP and fibrillations in radial-innervated forearm muscles 2–3 weeks after injury

Axonotmesis produces axon loss, so fibrillations appear 2–3 weeks post-injury and the CMAP amplitude is reduced, while conduction velocity and distal latency are relatively preserved.

Which pattern of H-reflex abnormality is most consistent with an S1 radiculopathy?

Answer: Absent or significantly prolonged H-reflex on the affected side compared to the contralateral side

A unilaterally absent or prolonged H-reflex (compared to the contralateral side) is the most sensitive NCS finding for S1 radiculopathy, as the reflex arc travels through the S1 root.