Nerve Conduction Studies and Electromyography
Domain 6 - Nerve Conduction Studies and Electromyography
The exam tests stimulation technique, measurement of latency and amplitude, waveform interpretation, and needle EMG findings that separate neuropathy from myopathy.
Core rules
- Motor NCS. Stimulate supramaximally; record with G1 over the motor point and G2 over the tendon. Measure distal latency, baseline-to-negative-peak amplitude, and conduction velocity between stimulation sites. Typical settings: 20 Hz–10 kHz, 5 ms/div.
- Sensory NCS. Antidromic or orthodromic; measure peak latency and SNAP amplitude; average if needed. Use 20 Hz–2 kHz and 1–2 ms/div.
- Late responses. F-waves assess proximal conduction (minimal latency, persistence); H-reflex assesses S1 radiculopathy.
- EMG. Four phases: insertional activity, spontaneous activity, voluntary MUAP analysis, and recruitment. Fibrillations and positive sharp waves indicate active denervation; fasciculations are less specific; myotonic discharges wax and wane. Neuropathy gives long-duration, large, polyphasic MUAPs with reduced recruitment; myopathy gives short, small, polyphasic MUAPs with early recruitment.
- Temperature. Keep limbs at least 32 °C, ideally near 34 °C; cooling slows conduction.
Worked example
Median motor study: distal latency 3.8 ms, proximal latency 8.2 ms, elbow-to-wrist distance 250 mm. Conduction velocity = 250 mm ÷ (8.2 − 3.8 ms) = 250 ÷ 4.4 = 56.8 m/s. If the amplitude is normal and the velocity is above 50 m/s, the segment is normal. A latency difference of 4.4 ms across 250 mm is what produces that value; forgetting to subtract the distal latency is the common error.
Traps
Submaximal stimulation; measuring distance along the skin rather than the nerve; anode drift; not subtracting distal latency; cold limbs; misreading voluntary contraction as spontaneous activity.