ABEM Nerve Conduction Studies & Electromyography 4 — Questions and Answers
Question 1: The blink reflex R1 response primarily assesses which pathway?
- Trigeminal sensory — facial motor via the pons (oligosynaptic) (Correct answer)
- Trigeminal sensory — facial motor via the medulla (polysynaptic)
- Facial sensory — trigeminal motor
- Acoustic — facial motor
Correct answer: Trigeminal sensory — facial motor via the pons (oligosynaptic)
The R1 response is an oligosynaptic reflex traversing the trigeminal afferent and facial efferent within the pons.
Question 2: In a patient with suspected thoracic outlet syndrome, which NCS finding is most diagnostically helpful?
- Prolonged median distal motor latency
- Reduced ulnar SNAP amplitude with normal median SNAP
- Reduced medial antebrachial cutaneous SNAP (Correct answer)
- Prolonged peroneal F-wave latency
Correct answer: Reduced medial antebrachial cutaneous SNAP
Reduced medial antebrachial cutaneous SNAP amplitude reflects lower trunk/medial cord involvement characteristic of neurogenic thoracic outlet syndrome.
Question 3: Which feature distinguishes a fasciculation potential from a fibrillation potential on EMG?
- Fasciculations arise from single muscle fibers; fibrillations arise from entire motor units
- Fasciculations have the morphology of MUAPs and fire irregularly; fibrillations are small and fire regularly (Correct answer)
- Fasciculations are always pathological; fibrillations can be benign
- Fasciculations occur only at rest; fibrillations occur during voluntary contraction
Correct answer: Fasciculations have the morphology of MUAPs and fire irregularly; fibrillations are small and fire regularly
Fasciculations represent spontaneous discharge of entire motor units and have MUAP morphology with irregular firing; fibrillations are single muscle fiber potentials with regular firing.
Question 4: In Lambert-Eaton myasthenic syndrome (LEMS), high-frequency repetitive stimulation (50 Hz) or brief maximal voluntary contraction characteristically produces:
- Decrement >10% in CMAP amplitude
- Increment >100% in CMAP amplitude (Correct answer)
- No change in CMAP amplitude
- Loss of the CMAP after the third stimulus
Correct answer: Increment >100% in CMAP amplitude
LEMS is a presynaptic disorder where high-frequency stimulation causes calcium accumulation and acetylcholine release, producing >100% CMAP increment.
Question 5: A patient has weakness and numbness in the lateral forearm after shoulder dislocation. Which nerve is most likely injured?
- Musculocutaneous nerve (Correct answer)
- Axillary nerve
- Radial nerve
- Median nerve
Correct answer: Musculocutaneous nerve
The musculocutaneous nerve innervates the biceps, coracobrachialis, and provides the lateral antebrachial cutaneous sensory nerve, commonly injured with shoulder dislocation.
Question 6: Electrodiagnostic criteria for Guillain-Barré syndrome (AIDP) classically include:
- Reduced SNAP amplitudes with normal motor NCS
- Absent or prolonged F-waves, prolonged distal latencies, and reduced conduction velocities (Correct answer)
- Widespread fibrillations with neurogenic MUAPs
- Decrement on repetitive nerve stimulation
Correct answer: Absent or prolonged F-waves, prolonged distal latencies, and reduced conduction velocities
AIDP is a demyelinating polyradiculoneuropathy characterized by prolonged or absent F-waves, prolonged distal latencies, and reduced conduction velocities.
Question 7: On needle EMG, 'early recruitment' of motor units with small amplitude, short duration MUAPs is most consistent with:
- Neurogenic disorder
- Myopathy (Correct answer)
- Upper motor neuron lesion
- Normal variant
Correct answer: Myopathy
Myopathies reduce the force output per motor unit, requiring early recruitment of many small MUAPs to generate adequate force.
ABEM Nerve Conduction Studies & Electromyography 4
The blink reflex R1 response primarily assesses which pathway?