ABEM Nerve Conduction Studies & Electromyography 2 — Questions and Answers
Question 1: Which parameter is most affected by limb temperature cooling during nerve conduction studies?
- Amplitude of CMAP
- Conduction velocity (Correct answer)
- Duration of CMAP
- Latency difference between stimulation sites
Correct answer: Conduction velocity
Cooling slows sodium channel kinetics, prolonging action potential duration and reducing conduction velocity by approximately 2 m/s per degree Celsius.
Question 2: A patient presents with median nerve mononeuropathy. On NCS, the distal motor latency is prolonged but conduction velocity across the forearm segment is normal. This pattern is most consistent with:
- Axonal loss at the wrist
- Focal demyelination at the carpal tunnel (Correct answer)
- Proximal median nerve entrapment
- Martin-Gruber anastomosis
Correct answer: Focal demyelination at the carpal tunnel
Focal demyelination at the carpal tunnel produces prolonged distal latency while leaving proximal conduction velocity intact.
Question 3: On EMG, which finding most specifically indicates active denervation rather than chronic reinnervation?
- Large polyphasic motor unit action potentials
- Fibrillation potentials and positive sharp waves (Correct answer)
- Reduced recruitment
- Increased motor unit duration
Correct answer: Fibrillation potentials and positive sharp waves
Fibrillation potentials and positive sharp waves reflect spontaneous activity of denervated muscle fibers and indicate active denervation.
Question 4: In ulnar neuropathy at the elbow, inching technique is used to:
- Measure distal latency accurately
- Localize the exact site of focal conduction slowing (Correct answer)
- Differentiate axonal from demyelinating lesions
- Confirm Martin-Gruber anastomosis
Correct answer: Localize the exact site of focal conduction slowing
Inching stimulates at 1–2 cm intervals across the elbow to pinpoint the site of maximal conduction slowing or conduction block.
Question 5: Which EMG finding is characteristic of myotonic dystrophy type 1?
- Fasciculation potentials
- Complex repetitive discharges
- Myotonic discharges waxing and waning in amplitude and frequency (Correct answer)
- Early recruitment of small MUAPs
Correct answer: Myotonic discharges waxing and waning in amplitude and frequency
Myotonic discharges classically wax and wane in both amplitude and frequency, producing the 'dive bomber' sound on EMG.
Question 6: The H-reflex predominantly tests which nerve root when recorded from the soleus muscle?
- L4
- L5
- S1 (Correct answer)
- S2
Correct answer: S1
The soleus H-reflex traverses the tibial nerve, travels through S1 afferents, and returns via S1 motor fibers to the soleus.
Question 7: Which of the following best describes a complex repetitive discharge (CRD) on EMG?
- A discharge that waxes and wanes and begins and ends abruptly
- A high-frequency uniform discharge beginning and ending abruptly without waxing or waning (Correct answer)
- A single large polyphasic potential firing repetitively
- A group discharge of fasciculation potentials
Correct answer: A high-frequency uniform discharge beginning and ending abruptly without waxing or waning
CRDs are high-frequency, uniform discharges with abrupt onset and cessation, created by ephaptic activation between adjacent muscle fibers.
ABEM Nerve Conduction Studies & Electromyography 2
Which parameter is most affected by limb temperature cooling during nerve conduction studies?