EMG Nerve Conduction Studies 1 — Questions and Answers
Question 1: Which nerve conduction parameter directly reflects the speed of the fastest-conducting motor nerve fibers between two stimulation points?
- Distal motor latency
- CMAP amplitude
- Nerve conduction velocity (Correct answer)
- CMAP duration
Correct 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.
Question 2: What does a reduced compound muscle action potential (CMAP) amplitude on motor nerve conduction studies primarily indicate?
- Demyelination of motor fibers
- Loss of functioning motor axons (Correct answer)
- Slowing of the fastest-conducting fibers
- Increased temporal dispersion
Correct 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.
Question 3: In adults, the H-reflex is most reliably and consistently obtained from which nerve-muscle pair?
- Ulnar nerve / abductor digiti minimi
- Median nerve / abductor pollicis brevis
- Tibial nerve / soleus (Correct answer)
- Peroneal nerve / extensor digitorum brevis
Correct 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.
Question 4: A prolonged F-wave latency with a normal distal motor latency and normal nerve conduction velocity most likely suggests:
- Focal entrapment at a distal site
- Proximal conduction slowing or nerve root pathology (Correct answer)
- Neuromuscular junction dysfunction
- Pure sensory neuropathy
Correct 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.
Question 5: In carpal tunnel syndrome, which NCS finding is typically the earliest and most sensitive indicator?
- Reduced CMAP amplitude at the wrist
- Prolonged median sensory peak latency across the wrist (Correct answer)
- Slowed median motor NCV in the forearm
- Absent median F-wave
Correct 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.
Question 6: 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:
- 20%
- 35%
- 50% (Correct answer)
- 75%
Correct 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.
Question 7: Temporal dispersion in nerve conduction studies refers to:
- Increased CMAP amplitude with proximal stimulation
- Widening of CMAP duration with proximal versus distal stimulation (Correct answer)
- Decreased sensory latency at distal stimulation sites
- Uniform slowing of conduction velocity across the entire nerve
Correct 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.
EMG Nerve Conduction Studies 1
Which nerve conduction parameter directly reflects the speed of the fastest-conducting motor nerve fibers between two stimulation points?