EDT EDT EMG Waveform Analysis 3 — Questions and Answers
Question 1: Which EMG pattern is most consistent with Lambert-Eaton Myasthenic Syndrome (LEMS) on repetitive nerve stimulation?
- Decremental response at 3 Hz stimulation
- Incremental response >100% at high-frequency (20–50 Hz) stimulation or post-exercise (Correct answer)
- No change in CMAP amplitude at any stimulation frequency
- Decremental response only at high frequencies
Correct answer: Incremental response >100% at high-frequency (20–50 Hz) stimulation or post-exercise
LEMS shows a markedly incremental CMAP response (>100%) at high-frequency stimulation or after brief exercise because calcium accumulation enhances ACh vesicle release.
Question 2: Neuromyotonia (Isaacs syndrome) is characterized on EMG by:
- Waxing and waning discharges that stop abruptly
- Continuous high-frequency spontaneous MUP discharges that persist after nerve block (Correct answer)
- Regular fibrillation potentials at rest
- Normal EMG with abnormal NCS
Correct answer: Continuous high-frequency spontaneous MUP discharges that persist after nerve block
Neuromyotonia shows continuous high-frequency spontaneous discharges that persist even after nerve block, indicating the abnormality is at the nerve terminal membrane level.
Question 3: When performing needle EMG of the first dorsal interosseous (FDI), the main clinical purpose is to evaluate:
- C6 nerve root
- Ulnar nerve and/or C8/T1 roots (Correct answer)
- Median nerve at the wrist
- Radial nerve in the forearm
Correct answer: Ulnar nerve and/or C8/T1 roots
The FDI is innervated by the ulnar nerve (deep branch) and reflects C8/T1 myotomal innervation, making it useful for evaluating ulnar nerve or C8/T1 pathology.
Question 4: The insertional activity on needle EMG is normally:
- Absent in all muscles
- A brief burst of electrical activity lasting only as long as needle movement (Correct answer)
- Prolonged trains of fibrillations lasting >1 second
- Large polyphasic potentials at rest
Correct answer: A brief burst of electrical activity lasting only as long as needle movement
Normal insertional activity is a brief burst of electrical potentials generated by mechanical irritation of muscle fibers that stops immediately when needle movement ceases.
Question 5: In evaluating a patient with suspected ALS, EMG evidence of active denervation must be found in how many body regions to meet El Escorial criteria?
- 1 region
- 2 regions
- 3 regions (Correct answer)
- 4 regions
Correct answer: 3 regions
Definite ALS by El Escorial criteria requires EMG evidence of lower motor neuron involvement (fibrillations, large MUPs, reduced recruitment) in at least 3 of 4 body regions.
Question 6: Satellite potentials seen on needle EMG are:
- Independent discharges of denervated single fibers
- Late components of MUPs linked to the main potential, indicating reinnervation (Correct answer)
- Normal findings in younger patients
- Artifacts from nearby muscle activity
Correct answer: Late components of MUPs linked to the main potential, indicating reinnervation
Satellite potentials are small late components time-locked to the main MUP, reflecting slow-conducting newly reinnervated muscle fibers linked to the same motor unit.
EDT EDT EMG Waveform Analysis 3
Which EMG pattern is most consistent with Lambert-Eaton Myasthenic Syndrome (LEMS) on repetitive nerve stimulation?