Electrodiagnostic Medicine and Clinical Neuromuscular Assessment
Electrodiagnostic medicine and clinical assessment
Nerve conduction studies must use supramaximal stimulation, fixed distances, and adequate limb temperature (at least 32°C for the upper limb and 30°C for the lower limb); cooling slows conduction and increases amplitudes, a common source of error. Sensory studies are antidromic or orthodromic; amplitude reflects axonal loss, while latency reflects demyelination or distance. Motor studies measure distal latency, CMAP amplitude, conduction velocity, and F-wave latency. Conduction block is a fall in CMAP amplitude of more than 50% across a segment with preserved distal response; temporal dispersion broadens and lowers the proximal CMAP.
Needle EMG assesses spontaneous activity, motor unit potential morphology, and recruitment. Fibrillations and positive sharp waves indicate active denervation and appear two to three weeks after injury. Fasciculations are less specific. Large, long-duration, polyphasic MUAPs indicate chronic reinnervation; small, short MUAPs indicate myopathy. Recruitment is reduced in neurogenic disorders and early in myopathy.
Late responses: F waves assess proximal segments, H reflexes assess S1 and tibial afferents, and blink reflexes assess the trigeminal-facial arc. The H reflex is the electrophysiologic equivalent of the ankle jerk.
Traps include anode-break excitation, volume conduction, submaximal stimulation, and misidentifying a normal variant such as a Martin-Gruber anastomosis. Always correlate with clinical findings; a normal study does not exclude small-fiber neuropathy, which requires quantitative sensory testing or skin biopsy.
Worked example: a patient with foot drop has a low peroneal CMAP, normal peroneal SNAP, and active denervation in the tibialis anterior; if conduction block is present at the fibular head, the lesion is compressive, whereas a length-dependent pattern suggests a neuropathy.
Rule of thumb: perform NCS before needle EMG, and interpret amplitude with temperature and distance in mind.