Amplitude of Somatosensory Evoked Potentials (SEPs) Recorded in Short-Latency SEP Condition Is 80% of That in Giant SEP Condition.

ABSTRACT

PURPOSE: Giant somatosensory evoked potentials (SEPs) with enhanced long-loop reflex (C-reflex) are useful to detect cortical motor hyperexcitability in patients with myoclonic epilepsy. The recording conditions of giant SEPs are different from those of short-latency SEPs (SSEPs). We investigated the waveform characteristics obtained for each condition. METHODS: Forty-eight upper limbs of 24 adult normal subjects (12 men, age 35.5 ± 9.7 years [mean ± SD]) were investigated. Somatosensory evoked potentials of each subject were recorded in both conditions on the same day. The main differences in recording conditions were reference electrodes (SSEP Fz vs. giant SEP the earlobe electrode ipsilateral to the stimulated limb), stimulus rate (5 vs. 1 Hz), and bandpass filter (20 Hz-3 kHz vs. 1 Hz-1 kHz). Somatosensory evoked potentials were elicited by unilateral percutaneous electrical stimulation of the median nerve at the wrist with intensity of 110% of the movement threshold and recoded at C3'/C4'. RESULTS: The amplitudes of N20 onset-N20 and N20-P25 were significantly larger in giant SEP condition than in SSEP condition (p < 0.001). The mean + 3SD of N20-P25 amplitude was 10.0 µV in giant SEP condition and 7.8 µV in SSEP condition. The N20-P25 amplitude was significantly correlated between giant SEP condition and SSEP condition (R = 0.64, p < 0.001). C-reflex was not elicited. CONCLUSIONS: The amplitude of SEPs in SSEP condition is equivalent to 80% of that in giant SEP condition. The information is useful for detecting cortical hyperexcitability in various neurological disorders including myoclonic epilepsy.