Needle Electromyography: Spontaneous Activity, MUAPs, and Recruitment
Needle EMG: spontaneous activity, MUAPs, recruitment
Needle EMG samples individual motor units. Assess at rest for spontaneous activity, then with mild voluntary activation for MUAP morphology, then with maximal effort for recruitment and interference pattern. Spontaneous activity includes fibrillations and positive sharp waves, which indicate active denervation or muscle disease; fasciculations, which indicate motor unit instability; myotonic discharges, which wax and wane in amplitude and frequency; and complex repetitive discharges, which have abrupt onset and offset. Endplate noise and endplate spikes are normal at the motor endplate.
MUAP morphology includes duration, amplitude, phases, and turns. Neuropathic MUAPs are large, long, and polyphasic because of collateral reinnervation. Myopathic MUAPs are small, short, and polyphasic. Recruitment is reduced in neuropathy, where units fire fast; in myopathy, units recruit early with many small units at low effort. The interference pattern is reduced in neuropathy and full but low amplitude in myopathy.
Traps: fibrillations are not specific for neuropathy and occur in inflammatory myopathy, ALS, and after nerve injury; fasciculations can be benign; paraspinal sampling is needed for radiculopathy; avoid needling through infected skin; recognize that endplate activity is normal; do not overcall myopathy from a single muscle; temperature and contraction effort affect MUAPs.
Worked example: a 45-year-old with proximal weakness and CK 900 has EMG showing short-duration, low-amplitude, polyphasic MUAPs with early recruitment and fibrillations. This is a myopathic pattern with active muscle fiber necrosis, consistent with inflammatory myopathy rather than a neuropathic process.
Rule of thumb: 'Large, long, late equals neuropathy; small, short, early equals myopathy.'