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 (SEPs) consist of a series of waves that reflect sequential activation of neural structures along the somatosensory pathways. While SEPs can be elicited by mechanical stimulation, clinical studies use electrical stimulation of peripheral nerves, which gives larger and more robust responses.
The stimulation sites typically used for clinical diagnostic SEP studies are the median nerve at the wrist, the common peroneal nerve at the knee, and the posterior tibial nerve at the ankle. SEPs can also be recorded following stimulation of branches of the trigeminal nerve, but this is technically challenging because of the proximity of the stimulating and recording electrodes and the short latencies of the responses; the electrical stimulus artifact often overlaps with, and obscures, the trigeminal nerve SEPs. Recordings of SEPs to stimulation of the ulnar nerves at the wrists are useful for intraoperative monitoring when the mid-cervical spinal cord or parts of the brachial plexus are at risk. Recording electrodes are placed on the scalp, over the spine, and over peripheral nerves proximal to the stimulation site.
SEPs are used for clinical diagnosis in patients with neurologic diseases, to evaluate patients with sensory sympotoms that might be psychogenic, for prognostication in comatose patients, and for intraoperative monitoring during surgeries that place parts of the somatosensory pathways at risk. Abnormal SEPs 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 (e.g., the anterior spinothalamic tract and the dorsal column tracts within the spinal cord), SEPs can be normal in patients with significant sensory deficits. However, an abnormal SEP result demonstrates that there is dysfunction within the somatosensory pathways; subjects cannot volitionally make their SEPs abnormal.
The SEP components generated in the brainstem and in the cerebral cortex are mediated entirely by the dorsal columns (posterior columns) of the spinal cord, the fasciculus cuneatus for upper limb SEPs and the fasciculus gracilis for lower limb SEPs. Lesioning of the dorsal columns of the spinal cord rostral to the root levels where the afferent somatosensory activity enters the spinal cord abolishes the SEPs generated in the brain. SEPs can persist following lesions of the anterolateral spinal cord, however. SEPs are abnormal in diseases of the dorsal columns in which joint position sense and proprioception are impaired.
The development of, and easy access to, sophisticated neuroradiologic imaging has had a great impact on the usage of SEPs in clinical settings; fewer diagnostic SEP studies are being performed now than in the pre-MRI era. Nevertheless, SEPs are valuable as a diagnostic test in several clinical situations, and as a prognostic tool in patients who are comatose as a result of anoxic brain injury. Their role in the operating room has expanded, and interest remains high in SEPs as research tools for unraveling of fundamental aspects of sensory physiology.
An overview of the general principles of recording and interpretation of SEPs is presented in this article.