Short-latency scalp somatosensory evoked potentials and central spine to scalp propagation characteristics during peroneal and median nerve stimulation in multiple sclerosis

Peripheral (cauda-lumbar, wrist-Erb, Erb-cervical) and central (cauda-vertex, cervical-scalp) nervous impulse propagation velocities and times to peroneal and median nerve stimulation were investigated in 34 patients suffering from definite (17 cases), probable (6 cases) and possible (11 cases) forms of multiple sclerosis (MS). In 6 cases short- and intermediate-latency scalp somatosensory evoked potentials to peroneal nerve stimulation were recorded with 'open' (1-5000 Hz, -6 dB) bandpass filters and subsequently digitally filtered through a 'narrow' bandpass (200-5000 Hz, -6 dB). The lumbar response was abnormal in 2.95% of legs, while the Erb response was always within normal limits. The cauda-vertex conduction was altered in 75% of the examined limbs (86.2% definite, 58.3% probable, 63.6% possible MS). Absent scalp responses to peroneal stimulation were often encountered during narrow bandpass recording (54.9%), while a slowed central conduction was less frequent (33.3%). Scalp responses when recorded with open bandpass were always identifiable, being delayed in 3 out of 6 cases. In 5 of these the short-latency wavelets were either absent or showed a prolonged interpeak time even when open filter records were normal. Median nerve SEPs were altered in 60.3% of cases, more frequently because of a delayed scalp response or of a prolonged cervical-scalp conduction time than because of an absent cervical or scalp response. When peroneal and median nerve data were considered together, the rate of abnormality rose to 88.2% of patients. Due to their length, afferent pathways from the lower limb might suffer from a loss of high frequency impulse coding as an early sign of defective impulse propagation.