Direct Recording of Spinal Evoked Potentials to Peripheral Nerve Stimulation by a Specially Modified Electrode

Abstract

Intraoperative spinal cord monitoring of somatosensory evoked potentials (SEP) has become widely accepted in the past decade, and is becoming a standard of care in major spinal centers. Various techniques now exist for continuous monitoring of spinal cord function by cortical SEP (16, 18, 21), vertebral bone recording (17), epidural recording (1, 11, 13, 23), and intraspinal ligament placement of recording needle electrode (9). Additional studies have also examined SEP pickup at cervical, thoracic, and lumbosacral levels of the spine (20). Stimulation sites vary, but are either unilateral or bilateral peripheral nerve and/or centrally in the cauda equina (12) or thoracic spine (23). Of these techniques, the simplest and least invasive is cortical somatosensory evoked potentials. The origin of cortical somatosensory evoked potentials is thought to be the pyramidal cells of the cerebral cortex (26). It is affected by various factors, such as inhalation of halogenated anesthetic agents (21), the stimulation rate of peripheral nerve (19), drugs (diazepam, haloperidol) (8), and hypotension (1, 4). The technique of recording spinal evoked potentials by placement of a Kirschner wirein the spinous process (17) or placement of a needle in the intraspinal ligament (9) usually will permit reliable monitoring of spinal cord function in the low thoracic and lumbar region, but in the high thoracic and cervical spine, signals are too small to permit reliable interpretation of changes in responses. Placing a needle (11) or a pair of wire (13) electrodes in the epidural space causes concern about dural laceration.