What are 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

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 50 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 SEPs refer 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.

Normal baseline SEPs. Left median on upper left co Normal baseline SEPs. Left median on upper left corner, right median on upper right corner, left tibial on lower left corner and right tibial on lower right corner.
Effects of reduced temperatures from 36º to 34.5º, Effects of reduced temperatures from 36º to 34.5º, resulting in increase in latency throughout evoked potentials from N9-P20.

Animal studies

Madhok et al concluded that in the absence of brain injury in a rodent model, hypothermia induces significant increases to the SEP amplitude while increasing SEP latency. [2] Hypothermia also suppressed EEGs at different regions of the brain by different degrees. The changes to SEPs and EEG are both reversible with subsequent rewarming.


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