EDT EDT Clinical Correlations & Pathology 2 — Questions and Answers
Question 1: In suspected myasthenia gravis, what is the most sensitive EDX test?
- Routine motor NCS
- Single-fiber EMG (SFEMG) (Correct answer)
- Repetitive nerve stimulation at 50 Hz
- Standard needle EMG
Correct answer: Single-fiber EMG (SFEMG)
Single-fiber EMG, which measures jitter (variability in fiber discharge timing within a motor unit), is the most sensitive EDX test for neuromuscular junction disease including myasthenia gravis.
Question 2: Chronic inflammatory demyelinating polyneuropathy (CIDP) is differentiated from Charcot-Marie-Tooth (CMT) type 1 on EDX by:
- Presence of slowed conduction velocity in CIDP only
- Acquired features in CIDP: conduction block, temporal dispersion, and asymmetric slowing versus uniform slowing in CMT (Correct answer)
- Absent F-waves in CMT only
- Normal SNAP amplitudes in CMT
Correct answer: Acquired features in CIDP: conduction block, temporal dispersion, and asymmetric slowing versus uniform slowing in CMT
CIDP shows acquired, often asymmetric demyelination with conduction block and temporal dispersion, while CMT1 shows uniform, symmetric slowing without conduction block.
Question 3: A patient with wrist drop after humeral fracture has EDX showing preserved radial sensory SNAP amplitude. What does this suggest?
- Complete axonal interruption of the radial nerve
- The lesion may be proximal to where the superficial radial sensory branch originates, sparing it (Correct answer)
- No nerve injury is present
- The lesion involves the posterior cord of the brachial plexus
Correct answer: The lesion may be proximal to where the superficial radial sensory branch originates, sparing it
A preserved SNAP despite motor weakness suggests the sensory branch may be anatomically spared, or the timing of testing is too early for Wallerian degeneration to cause amplitude loss.
Question 4: In ALS, sensory nerve conduction studies are typically:
- Markedly abnormal in all extremities
- Normal or minimally abnormal, as ALS primarily affects motor neurons (Correct answer)
- Absent in lower extremities only
- Slowed without amplitude reduction
Correct answer: Normal or minimally abnormal, as ALS primarily affects motor neurons
ALS is primarily a motor neuron disease; sensory NCS are typically normal because sensory neurons and their axons are largely spared.
Question 5: EDX findings in critical illness myopathy (CIM) include:
- Normal needle EMG with reduced SNAP amplitudes
- Reduced CMAP amplitudes, short-duration MUPs with early recruitment, and reduced muscle membrane excitability (Correct answer)
- Absent F-waves with normal CMAP amplitudes
- Fibrillations confined to paraspinal muscles
Correct answer: Reduced CMAP amplitudes, short-duration MUPs with early recruitment, and reduced muscle membrane excitability
CIM shows reduced CMAP amplitudes (reflecting muscle membrane inexcitability), short-duration small MUPs (myopathic), and early recruitment on EMG.
Question 6: In evaluating hereditary neuropathy with liability to pressure palsies (HNPP), EDX characteristically shows:
- Diffuse uniform slowing consistent with CMT type 1
- Mild baseline slowing with markedly prolonged latencies and conduction block at typical compression sites (Correct answer)
- Pure axonal polyneuropathy
- Normal NCS with myopathic EMG
Correct answer: Mild baseline slowing with markedly prolonged latencies and conduction block at typical compression sites
HNPP shows mild background slowing with exaggerated, disproportionate slowing and conduction block at typical entrapment sites (carpal tunnel, fibular head, cubital tunnel).
EDT EDT Clinical Correlations & Pathology 2
In suspected myasthenia gravis, what is the most sensitive EDX test?