Somatosensory Evoked Potentials
Jacob R. Berger and Andrew S. Blum
Summary
Somatosensory evoked potentials (SSEPs) are electrical potentials generated by various portions of the ascending sensory pathways in response to stimulation of peripheral sensory nerves. SSEPs can be easily elicited and recorded and can be used to examine the functional integrity of somatosensory path- ways. This chapter summarizes the methodology for the recording of SSEPs, as well as their role in the evaluation of processes that may affect ascending sensory pathways (e.g., demyelination), and highlights their particular usefulness as an intraoperative tool during spinal cord surgery.
Key Words: Demyelination; dorsal columns; intraoperative monitoring; latency; multiple sclerosis;
signal averaging.
1. INTRODUCTION
Somatosensory evoked potentials (SSEPs) are electrical potentials generated by various portions of the ascending sensory pathways in response to stimulation of peripheral sensory nerves. SSEPs can be easily elicited and recorded and can be used to examine the functional integrity of somatosensory pathways. SSEPs conceptually resemble brainstem auditory evoked potentials (BAEPs) and visual evoked potentials (VEPs); both of these are produced after stimulation of auditory and visual pathways, respectively, and have been discussed in previous chapters (Chapters 25 and 26). Motor evoked potentials (MEPs), recorded from peripheral muscles in response to cortical stimulation, also have important and emerging clinical uses, but, historically, have been used with less frequency than SSEPs. MEPs will not be discussed in this chapter.
2. PHYSIOLOGY AND ANATOMY
As with electroencephalography, evoked potential surface electrodes record changes in extracellular voltage at the skin or scalp surface. SSEP waveforms reflect the activity of sum- mated dendritic potentials of underlying gray matter as well as the propagating electrical activity of nerve fiber conduction along white matter pathways. Most elicited SSEP wave- forms probably have components of both gray and white matter activity, but generally one or the other predominates. When gray matter activity in the cortex or spinal cord is responsible for the main electrical generators of the recorded SSEP waveform, the evoked potential is termed a “near-field” potential. “Far-field” potentials reflect electrical activity distant to the recording electrode, typically operant in white matter tracts or subcortical structures. Unlike
From: The Clinical Neurophysiology Primer
Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ