72: Somatosensory-Evoked Potentials and Spinal Surgery

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CHAPTER 72 Somatosensory-Evoked Potentials and Spinal Surgery

Daniel J. Janik, MD [Colonel (Retired), USAF, MC]

1 What are somatosensory-evoked potentials?

Somatosensory-evoked potentials (SSEPs) are the electrophysiologic responses of the nervous system to the application of a discrete stimulus at a peripheral nerve anywhere in the body. They reflect the ability of a specific neural pathway to conduct an electrical signal from the periphery to the cerebral cortex.

2 How are somatosensory-evoked potentials generated?

Using a skin surface disc electrode or subcutaneous fine-needle electrode placed near a major peripheral mixed function (motor and sensory) nerve such as the median, a square-wave electrical stimulus of 0.2 to 2 ms is applied to the nerve at a rate of 1 to 2 Hz. The stimulus intensity is adjusted to produce minimal muscle contraction (usually 10 to 60 mA). The resulting electrical potential is recorded at various points along the neural pathway from the peripheral nerve to the cerebral cortex.

3 What major peripheral nerves are most commonly stimulated?

In the upper extremity the common sites of stimulation are the median and ulnar nerves at the wrist. In the lower extremity the common peroneal nerve at the popliteal fossa and the posterior tibial nerve at the ankle are used. Less commonly the tongue, trigeminal nerve, and pudendal nerve have been studied.

4 Trace the neurosensory pathway from the peripheral nerves to the cerebral cortex

The axons of the peripheral sensory nerves enter the spinal cord via the dorsal spinal roots. These first-order neurons continue rostrally in the ipsilateral posterior column of the spinal cord until they synapse with nuclei at the cervicomedullary junction. Second-order neurons from these nuclei immediately decussate to the contralateral side of the brainstem, where they continue their ascent via the medial lemniscus through the midbrain, synapsing in the thalamus. Third-order neurons then travel via the internal capsule to synapse in the postcentral gyrus, the primary somatosensory cortex.

5 At what points along the neurosensory pathway are somatosensory-evoked potentials most commonly recorded?

After upper limb stimulation, potentials are recorded at the brachial plexus (Erb’s point, 2 cm superior to the clavicular head of the sternocleidomastoid muscle), the cervicomedullary junction (posterior midline of the neck at the second cervical vertebra), and the scalp overlying the somatosensory cortex on the contralateral side.

After stimulation of the lower extremity, potentials are recorded at the popliteal fossa, lumbar and cervical spinal cord, and somatosensory cortex. It is important to record nerve and subcortical potentials to verify adequate stimulation and delineate anesthetic effects.

6 Describe the characteristics of the somatosensory-evoked potential waveform

The SSEP is plotted as a waveform of voltage vs. time. It is characterized by:

image Amplitude (A), which is measured in microvolts from baseline to peak or peak to peak
image Latency (L), which is the time, measured in milliseconds, from onset of stimulus to occurrence of a peak or the time from one peak to another
image Morphology, which is the overall shape of the waveform, described as positive (P, below the baseline) or negative (N, above the baseline)

A waveform is identified by the letter describing its deflection above or below the baseline followed by a number indicating its latency (e.g., N20) (Figure 72-1).

image

Figure 72-1 Characteristics of the somatosensory-evoked potential waveform.

7 Name several characteristic peaks important for the evaluation of somatosensory-evoked potentials

See Tables 72-1 and 72-2.

TABLE 72-1 Characteristic Peaks for Evaluation of Median Nerve Stimulation

Peak Generator
N9 Brachial plexus (Erb’s point)
N11 Dorsal root entry zone (cervical spine)
N13, 14 Posterior column (nucleus cuneatus)
P14 Medial lemniscus
N20 Somatosensory cortex

TABLE 72-2 Characteristic Peaks for Evaluation of Posterior Tibial Nerve Stimulation

Peak Generator
N20 Dorsal root entry zone (lumbar spine)
N40 Somatosensory cortex

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