Electrophysiologic evaluation of lumbosacral radiculopathies: electromyography, late responses, and somatosensory evoked potentials.

We compared the diagnostic utility of EMG, F wave and H-reflex studies, and peroneal and dermatomal SEPs in evaluating 28 patients with clinically unequivocal L-5 or S-1 compressive root lesions. The single most useful electrophysiologic technique was EMG, which often provided evidence of denervation in a myotomal pattern when other electrophysiologic findings were normal. We found abnormal late responses in 14 patients, but always in association with EMG abnormalities. Peroneal-derived SEPs were always normal. Dermatomal SEPs confirmed the diagnosis in seven patients, including two in whom other electrophysiologic studies were normal.