Nerve Conduction Studies Flashcards

Which nerve conduction parameter directly reflects the speed of the fastest-conducting motor nerve fibers between two stimulation points?

Answer: Nerve conduction velocity

Nerve conduction velocity is calculated by dividing the distance between two stimulation sites by the difference in their latencies, reflecting the fastest-conducting myelinated motor fibers.

What does a reduced compound muscle action potential (CMAP) amplitude on motor nerve conduction studies primarily indicate?

Answer: Loss of functioning motor axons

CMAP amplitude reflects the number of functional motor axons contributing to the compound response; a reduced amplitude indicates axonal loss rather than demyelination.

In adults, the H-reflex is most reliably and consistently obtained from which nerve-muscle pair?

Answer: Tibial nerve / soleus

The H-reflex is most consistently elicited by stimulating the tibial nerve and recording from the soleus, serving as the electrophysiologic equivalent of the Achilles tendon reflex via the S1 reflex arc.

A prolonged F-wave latency with a normal distal motor latency and normal nerve conduction velocity most likely suggests:

Answer: Proximal conduction slowing or nerve root pathology

F-waves travel the full length of the motor nerve proximally and back distally; a prolonged latency with normal distal NCS indicates pathology in the proximal segment, such as radiculopathy or proximal plexopathy.

In carpal tunnel syndrome, which NCS finding is typically the earliest and most sensitive indicator?

Answer: Prolonged median sensory peak latency across the wrist

Prolonged median sensory latency across the wrist is the most sensitive NCS indicator of carpal tunnel syndrome, as sensory fibers are more vulnerable to compressive ischemia than motor fibers.

The widely accepted AANEM criterion for definite conduction block is a reduction in CMAP amplitude or area with proximal versus distal stimulation of more than:

Answer: 50%

A greater than 50% reduction in CMAP amplitude or area between proximal and distal stimulation sites is the established criterion for definite conduction block, indicating focal failure of impulse propagation.

Temporal dispersion in nerve conduction studies refers to:

Answer: Widening of CMAP duration with proximal versus distal stimulation

Temporal dispersion occurs when demyelination causes different motor fibers to conduct at varying speeds, producing phase cancellation and a widened CMAP duration when stimulating proximally.