ABEM Neuromuscular Disorders & Diagnostic Criteria 3 — Questions and Answers
Question 1: On needle EMG, which MUAP characteristic most strongly suggests a myopathic process?
- Increased duration with high amplitude
- Early recruitment of short-duration, low-amplitude, polyphasic MUAPs (Correct answer)
- Reduced recruitment with large polyphasic MUAPs
- Normal MUAPs with fibrillation potentials only
Correct answer: Early recruitment of short-duration, low-amplitude, polyphasic MUAPs
Myopathic MUAPs are short in duration and low in amplitude due to loss of muscle fibers per motor unit, and early recruitment occurs because more units fire to compensate for reduced force per unit.
Question 2: In critical illness polyneuropathy (CIP), which electrodiagnostic pattern is expected?
- Primarily demyelinating sensorimotor neuropathy with conduction block
- Axonal sensorimotor neuropathy with low CMAP and SNAP amplitudes and active denervation (Correct answer)
- Pure motor axonal neuropathy sparing sensory nerves
- Length-independent sensory neuropathy with normal motor studies
Correct answer: Axonal sensorimotor neuropathy with low CMAP and SNAP amplitudes and active denervation
CIP is an axonal sensorimotor polyneuropathy occurring in critically ill patients, characterized by reduced CMAP and SNAP amplitudes with fibrillations on needle EMG, without significant conduction velocity slowing.
Question 3: A patient with diabetic lumbosacral radiculoplexus neuropathy (diabetic amyotrophy) would most likely show which finding on needle EMG of the affected limb?
- Purely sensory abnormalities with normal paraspinal muscles
- Active denervation in multiple lumbar myotomes including paraspinal muscles (Correct answer)
- Myotonic discharges restricted to proximal muscles
- Normal EMG with abnormal repetitive stimulation only
Correct answer: Active denervation in multiple lumbar myotomes including paraspinal muscles
Diabetic amyotrophy (Bruns-Garland syndrome) affects lumbosacral plexus and roots, causing active denervation across multiple myotomes including paraspinal muscles, confirming the proximal and radicular involvement.
Question 4: Which electrodiagnostic feature differentiates multifocal motor neuropathy (MMN) from ALS?
- Fasciculation potentials are present only in ALS
- MMN shows conduction block in motor nerves without sensory involvement; ALS does not (Correct answer)
- MMN has abnormal SNAPs while ALS has normal SNAPs
- Both show identical motor NCS findings but differ only on EMG
Correct answer: MMN shows conduction block in motor nerves without sensory involvement; ALS does not
MMN is defined by persistent, multifocal motor conduction block with preserved sensory conduction, while ALS shows pure lower motor neuron and upper motor neuron signs without conduction block.
Question 5: In hereditary neuropathy with liability to pressure palsies (HNPP), NCS most commonly reveals:
- Uniform demyelination throughout all nerve segments
- Focal conduction slowing or conduction block at typical entrapment sites with mild diffuse slowing (Correct answer)
- Axonal loss without any slowing at entrapment sites
- Normal NCS between episodes of pressure palsy
Correct answer: Focal conduction slowing or conduction block at typical entrapment sites with mild diffuse slowing
HNPP (PMP22 deletion) causes disproportionate focal slowing or block at entrapment sites (carpal tunnel, fibular head) superimposed on mild diffuse sensorimotor demyelinating changes.
Question 6: Which pattern on NCS best supports a diagnosis of chronic inflammatory demyelinating polyneuropathy (CIDP) rather than CMT1A?
- MNCV uniformly below 38 m/s in all nerves
- Asymmetric, multifocal slowing with conduction block and temporal dispersion (Correct answer)
- Reduced CMAP amplitudes with normal conduction velocities
- Absent sural SNAP with normal motor studies
Correct answer: Asymmetric, multifocal slowing with conduction block and temporal dispersion
CIDP characteristically shows asymmetric or multifocal demyelination with conduction block and temporal dispersion, while CMT1A shows uniform, symmetric slowing without block.
Question 7: In inclusion body myositis (IBM), which combination of needle EMG findings is most characteristic?
- Pure myopathic changes with no denervation potentials
- Mixed pattern with both myopathic and neuropathic MUAP features plus active denervation (Correct answer)
- Only fasciculation potentials without fibrillations
- Normal EMG with normal NCS in early disease
Correct answer: Mixed pattern with both myopathic and neuropathic MUAP features plus active denervation
IBM uniquely shows a mixed pattern on EMG with both short-duration myopathic MUAPs and long-duration neuropathic MUAPs, along with fibrillation potentials, reflecting the combined inflammatory and degenerative pathology.
ABEM Neuromuscular Disorders & Diagnostic Criteria 3
On needle EMG, which MUAP characteristic most strongly suggests a myopathic process?