ABEM Neuromuscular Disorders & Diagnostic Criteria 5 — Questions and Answers
Question 1: A patient presents with pure sensory ataxia, absent deep tendon reflexes, and normal motor strength. NCS shows absent SNAPs with normal CMAPs. This pattern is most consistent with:
- Length-dependent sensorimotor polyneuropathy
- Sensory neuronopathy (dorsal root ganglionopathy) (Correct answer)
- Mononeuritis multiplex
- Anterior horn cell disease
Correct answer: Sensory neuronopathy (dorsal root ganglionopathy)
Sensory neuronopathy affects the dorsal root ganglion, producing diffuse, non-length-dependent loss of all SNAPs with preserved CMAPs, causing sensory ataxia without motor involvement.
Question 2: Which of the following best characterizes the electrodiagnostic findings in botulism?
- Decremental response at 3 Hz repetitive stimulation similar to MG
- Low CMAP amplitudes with facilitation at high-frequency stimulation (50 Hz) similar to LEMS (Correct answer)
- Normal NCS with only needle EMG abnormalities
- Normal repetitive stimulation with pure sensory NCS abnormalities
Correct answer: Low CMAP amplitudes with facilitation at high-frequency stimulation (50 Hz) similar to LEMS
Botulism, like LEMS, is a presynaptic NMJ disorder causing low baseline CMAPs with incremental response at high-frequency stimulation, though the facilitation is typically less dramatic than in LEMS.
Question 3: In a patient with suspected polymyositis, which finding on needle EMG would suggest an alternative diagnosis of IBM?
- Fibrillation potentials in proximal muscles
- Mixed myopathic and neuropathic MUAP changes with finger flexor weakness (Correct answer)
- Myotonic discharges throughout
- Absent fibrillation potentials
Correct answer: Mixed myopathic and neuropathic MUAP changes with finger flexor weakness
IBM classically involves early and severe finger flexor and quadriceps weakness with a mixed EMG pattern of myopathic and neuropathic features, distinguishing it from polymyositis which shows pure myopathic changes.
Question 4: On NCS, which finding most reliably indicates axonal degeneration rather than demyelination as the primary pathology?
- Prolonged distal motor latency
- Reduced CMAP and SNAP amplitudes with relatively preserved conduction velocity (Correct answer)
- Increased temporal dispersion on proximal stimulation
- Absent F-waves with normal distal latency
Correct answer: Reduced CMAP and SNAP amplitudes with relatively preserved conduction velocity
Axonal loss reduces the number of conducting fibers, decreasing CMAP and SNAP amplitudes while conduction velocity (reflecting the fastest surviving fibers) is relatively preserved unless axonal loss is very severe.
Question 5: In thoracic outlet syndrome (TOS) with true neurogenic form, which electrodiagnostic pattern is most expected?
- Absent median SNAP with normal ulnar SNAP
- Low or absent medial antebrachial cutaneous SNAP and low ulnar CMAP amplitude with relatively preserved median motor studies (Correct answer)
- Conduction block across the clavicle on ulnar nerve stimulation
- Normal NCS with only abnormal needle EMG in cervical paraspinals
Correct answer: Low or absent medial antebrachial cutaneous SNAP and low ulnar CMAP amplitude with relatively preserved median motor studies
True neurogenic TOS predominantly affects the lower trunk (C8-T1), causing low medial antebrachial cutaneous SNAP amplitude and reduced ulnar CMAP, with the median motor studies relatively spared early.
Question 6: Which statement correctly describes the electrodiagnostic approach to suspected neuromuscular junction disease when repetitive nerve stimulation is normal?
- A normal RNS definitively excludes NMJ disease
- Single-fiber EMG should be performed as it is more sensitive than RNS for detecting NMJ dysfunction (Correct answer)
- Nerve conduction studies should be repeated on a different day
- High-dose pyridostigmine should be given and RNS repeated immediately
Correct answer: Single-fiber EMG should be performed as it is more sensitive than RNS for detecting NMJ dysfunction
Single-fiber EMG measuring jitter and blocking is significantly more sensitive than repetitive nerve stimulation for NMJ disorders, detecting abnormalities in ~95% of generalized MG cases where RNS may be normal.
Question 7: A patient with suspected vasculitic neuropathy is most likely to show which pattern on electrodiagnostic studies?
- Symmetric length-dependent sensorimotor axonal polyneuropathy
- Asymmetric, multifocal axonal sensorimotor neuropathy (mononeuritis multiplex pattern) (Correct answer)
- Uniform demyelinating sensorimotor polyneuropathy
- Pure motor neuropathy with conduction block at multiple sites
Correct answer: Asymmetric, multifocal axonal sensorimotor neuropathy (mononeuritis multiplex pattern)
Vasculitic neuropathy causes ischemia in individual nerve trunks due to vasculitis of the vasa nervorum, producing a stepwise, asymmetric, multifocal axonal pattern (mononeuritis multiplex) rather than symmetric polyneuropathy.
ABEM Neuromuscular Disorders & Diagnostic Criteria 5
A patient presents with pure sensory ataxia, absent deep tendon reflexes, and normal motor strength.
NCS shows absent SNAPs with normal CMAPs.
This pattern is most consistent with: