ABEM Evoked Potentials & Late Response Studies 1 — Questions and Answers
Question 1: The H-reflex is most commonly elicited from which nerve-muscle combination?
- Median nerve - flexor carpi radialis
- Tibial nerve - soleus (Correct answer)
- Peroneal nerve - tibialis anterior
- Ulnar nerve - abductor digiti minimi
Correct answer: Tibial nerve - soleus
The H-reflex is most reliably elicited by stimulating the tibial nerve and recording from the soleus, representing the S1 monosynaptic reflex arc.
Question 2: Which of the following best describes the physiological basis of F-waves?
- Sensory nerve action potentials recorded from proximal muscles
- Late motor responses due to antidromic activation of motor neurons followed by orthodromic discharge (Correct answer)
- Direct muscle responses to submaximal nerve stimulation
- Reflexes mediated through the dorsal root ganglion synapse
Correct answer: Late motor responses due to antidromic activation of motor neurons followed by orthodromic discharge
F-waves result from antidromic activation of alpha motor neurons, causing a small percentage to 'backfire' orthodromically and produce a late muscle response.
Question 3: What does prolonged F-wave latency most commonly indicate in electrodiagnostic studies?
- Distal sensory nerve pathology
- Proximal motor nerve demyelination or radiculopathy (Correct answer)
- Neuromuscular junction dysfunction
- Anterior horn cell disease exclusively
Correct answer: Proximal motor nerve demyelination or radiculopathy
Prolonged F-wave latency indicates slowing in the proximal segment of the motor nerve, suggesting proximal demyelination, polyradiculopathy, or Guillain-Barré syndrome.
Question 4: H-reflex latency in adults is most significantly influenced by which factor?
- Age and body temperature
- Limb length and patient height (Correct answer)
- Muscle mass and body weight
- Stimulus frequency and repetition rate
Correct answer: Limb length and patient height
H-reflex latency is primarily determined by limb length (and patient height), since longer limbs require greater conduction distance for the same reflex arc.
Question 5: Bilateral absent H-reflexes at rest in an adult most strongly suggests which diagnosis?
- Bilateral carpal tunnel syndrome
- Unilateral S1 radiculopathy
- Diffuse peripheral neuropathy (Correct answer)
- Bilateral cubital tunnel syndrome
Correct answer: Diffuse peripheral neuropathy
Bilateral absent H-reflexes suggest diffuse peripheral neuropathy affecting the S1 arc bilaterally, whereas radiculopathy typically causes unilateral H-reflex changes.
Question 6: What stimulus rate is most appropriate for eliciting F-waves during a nerve conduction study?
- 0.5-1 Hz (Correct answer)
- 3-5 Hz
- 10-20 Hz
- 30-50 Hz
Correct answer: 0.5-1 Hz
F-waves are elicited at slow rates (0.5-1 Hz) to prevent suppression of the F-wave response by preceding motor neuron discharges.
Question 7: In F-wave analysis, 'F-wave persistence' refers to which measurement?
- The amplitude of F-waves relative to the M-wave amplitude
- The percentage of stimuli that successfully elicit an F-wave response (Correct answer)
- The minimum F-wave latency recorded across 10-20 trials
- The duration of the F-wave waveform in milliseconds
Correct answer: The percentage of stimuli that successfully elicit an F-wave response
F-wave persistence is the percentage of delivered stimuli that produce an F-wave, with normal persistence typically ≥80-90% for most commonly tested nerves.
ABEM Evoked Potentials & Late Response Studies 1
The H-reflex is most commonly elicited from which nerve-muscle combination?