Excerpt from Somatosensory Evoked Potentials: Clinical Applications

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Somatosensory evoked potentials (SEPs) are generated by stimulation of afferent peripheral nerve fibers by either physiological or electrical means. SEPs were first recorded more than 40 years ago. For an explanation of their physiologic and anatomic basis and description of techniques, see Somatosensory Evoked Potentials: General Principles. In this article, the focus is on the clinical applications of SEPs.

SEPs can be recorded after physiological stimuli (eg, muscle stretch). However, electrical stimulation usually is administered to elicit the potential. Typically, a square wave of 0.2- to 2-millisecond duration is delivered to a peripheral nerve by electrodes, usually surface electrodes. For intraoperative monitoring, needle electrodes are used for stimulation since they require smaller currents, which reduce the stimulus artifact. The usual sites for SEP stimulation are the median nerve at the wrist, the common peroneal nerve at the knee, and the posterior tibial nerve. A SEP also may be recorded by stimulating the skin in various dermatomal areas, but the response is much weaker.

In mixed peripheral nerves, the threshold for sensory perception is lower than the threshold to elicit movement. For stimulation of mixed peripheral nerves, the stimulating current is adjusted to produce a minimal movement of the joint involved. This stimulation intensity typically is well tolerated by patients. Recording electrodes are placed on the scalp and over the cervical spine. For recording upper extremity SEPs, electrodes are placed over the Erb point. For recording lower extremity SEPs, electrodes are placed over the lumbosacral spine.

Waveforms are described in terms of morphology, amplitude, and dispersion. Each laboratory should establish reference values for latencies and interpeak latencies that are based on a patient's age and height. Because limb cooling affects peripheral nerve conduction velocity, minimum skin temperature norms should be established for each laboratory.

Responses recorded are classified according to specific latencies. "Short-latency" SEPs refer to the portion of the SEP waveform that occurs within 25 milliseconds after stimulation of the upper extremity nerves, 40 milliseconds after stimulation of the peroneal nerve, or 50 milliseconds after stimulation of the tibial nerve. "Long-latency" refers to the waveforms recorded more than 100 milliseconds following stimulation of these nerves. "Middle-latency" SEP refers to waveforms that occur between these 2 periods.

Mixed nerve stimulation has become the standard for clinical use. Other methods include cutaneous nerve stimulation, dermatomal stimulation (which is more specific than cutaneous nerve stimulation), motor point stimulation, and paraspinal stimulation.

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