WAVE-FORM ANALYSIS OF SPINAL SOMATOSENSORY-EVOKED POTENTIAL - PARADOXICALLY ENHANCED NEGATIVE PEAKS IMMEDIATELY CAUDAL TO THE SITE OF CONDUCTION BLOCK

Objectives: We studied waveform changes associated with a focal conduc tion block in compression myelopathies. Design and Methods: A total of 26 patients underwent serial intervertebral recording of spinal somat osensory evoked potentials (SSEPs) after epidural stimulation. The sit e of compression identified by abrupt reduction in size of the negativ e peak was designated as '0' level with the other levels numbered in o rder of distance assigning a minus sign caudally. Results: Considering the response recorded at '-4' as baseline (100%), SSEPs showed a prog ressive increase rostrally, reaching an average of 154% in amplitude a nd 216% in area at '-1' followed by an abrupt decline to 32% and 31% a t '0'. The incremental change of the negative peak was accompanied by a small reduction in area of the initial positive component to 90% at '-1' considering the value at '-4' as baseline (100%). Conclusions: Th e theory of solid angle approximation and the concept of phase cancell ation best explain the apparently paradoxical enhancement of the negat ive peak which characterize typical waveform changes at the site of co nduction block. (C) 1998 Elsevier Science Ireland Ltd. All rights rese rved.