Excerpt from Somatosensory Evoked Potentials: General Principles

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Evoked potentials are the electrical signals generated by the nervous system in response to sensory stimuli. Auditory, visual, and somatosensory stimuli are used commonly for clinical evoked potential studies. Somatosensory evoked potentials (SSEPs) consist of a series of waves that reflect sequential activation of neural structures along the somatosensory pathways. While SSEPs can be elicited by mechanical stimulation, clinical studies utilize electrical stimulation of peripheral nerves, which gives larger and more robust responses. The stimulation sites typically used for clinical diagnostic SSEP studies are the median nerve at the wrist, the common peroneal nerve at the knee, and/or the posterior tibial nerve at the ankle. Recording electrodes are placed over the scalp, the spine, and peripheral nerves proximal to the stimulation site. The dorsal column-lemniscal system is the major anatomical substrate of the SSEPs within the CNS.

SSEPs are used for clinical diagnosis in patients with neurologic disease for prognostication in comatose patients and for intraoperative monitoring during surgeries that place parts of the somatosensory pathways at risk. Abnormal SSEPs can result from dysfunction at the level of the peripheral nerve, plexus, spinal root, spinal cord, brain stem, thalamocortical projections, or primary somatosensory cortex. Since individuals have multiple parallel afferent somatosensory pathways (eg, anterior spinothalamic tract and dorsal column tracts within the spinal cord), recordings of SSEPs can be normal even in patients with significant sensory deficits.

SSEPs depend on the functional integrity of the rapidly conducting, large-diameter group IA muscle afferent fibers and group II cutaneous afferent fibers, which travel in the posterior column of the spinal cord. When a mixed peripheral nerve (with both sensory and motor components) is stimulated, both group IA muscle afferents and group II cutaneous afferents contribute to the resulting SSEP. Selective ablation of the dorsal column of the spinal cord abolishes the SSEPs generated rostral to the lesion. Diseases of the dorsal columns in which joint position sense and proprioception are impaired invariably are associated with abnormal SSEPs.

The development of and easy access to sophisticated neuroradiologic imaging have had a great impact on the usage of SSEPs in clinical settings; fewer diagnostic SSEP studies are being performed now than in the pre-MRI era. Nevertheless, SSEPs are valuable as a diagnostic test in several clinical situations. Their role in the operating room has expanded, and interest remains high in SSEPs as research tools for unraveling of fundamental aspects of sensory physiology.

An overview of the general principles of recording and interpretation of SSEPs is presented in this article.

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