Normal SEP Findings

SEP components are defined by polarity and latency. With stimulation of a nerve in the arm, recording between the contralateral "hand area" of scalp and contralateral Erb's point, that is, in a referential derivation, demonstrates a P9 potential that reflects activity at or just beyond the brachial plexus. A P13-P14 potential reflects postsynaptic activity in the medial lemniscus, and an N20 component is generated in the primary somatosensory cortex (...Fig 24-14. ). A variety of other potentials may also be detected but are clinically less helpful and are not discussed further. With stimulation of the tibial nerve at the ankle, a P38 response is recorded over the scalp in the midline and represents activation of the primary somatosensory cortex ( ..Fig 24-15 ). Recordings over the cervical spine after stimulation of the nerve in the arm reveals an N13/P13 whereas stimulation of a leg nerve elicits a negative peak over the cauda equina and lumbar spine.

The presence or absence of these obligate components of the response and their latency and interpeak latency are recorded. Amplitude measurements may also be made, but abnormalities should not be based solely on amplitude criteria, except when an obligate response is absent. The absolute latency of individual components, but not the interpeak latency, varies with limb length.

As with the BAEP, it has generally been assumed that different components of the SEP reflect sequential activation of neural generators. However, potentials can be generated by a change in the physical characteristic of the volume conductor as, for example, when an impulse passes from the arm to the axilla. y , y

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