What is the clinical interpretation of somatosensory evoked potentials (SEPs)?

Updated: Aug 20, 2019
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: Selim R Benbadis, MD  more...
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Answer

Answer

The SEP stimulus preferentially excites only the largest myelinated fibers in the peripheral nerve. These include cutaneous and subcutaneous fibers, somesthetic and proprioceptive fibers, and motor axons of equivalent diameter. The stimulus activates predominantly the large-diameter, fast-conducting group in muscles and group II cutaneous fibers. Stimulation produces an action potential that travels up the axon toward the spinal cord and past the cell bodies of the sensory axons of the large-fiber sensory system in the dorsal root ganglia to the ipsilateral posterior columns of the spinal cord. These signals then traverse a synapse in the dorsal column nuclei at the cervicomedullary junction.

The signals travel in second-order neurons to the ventroposterolateral nucleus of the thalamus (VPL) via the medial lemniscus. In the VPL, a synapse is made with a third-order neuron that travels to area 3b of the parietal sensory cortex. SEPs provide information concerning the integrity of the pathway through the brain, brain stem, spinal cord, dorsal roots, and peripheral nerves.

Although SEPs may reveal or localize a lesion involving the somatosensory pathways, they should be interpreted as extended neurologic examination. They do not indicate the underlying disease process and normal findings do not exclude an organic basis for symptoms. Despite these limitations, SEP studies may be helpful diagnostically as well as prognostically to determine the extent of pathologic involvement in different disorders.


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