Somatosensory evoked potentials

Somatosensory Evoked Potentials (SSEPs) are used in neuromonitoring to asses the function of a patient's spinal cord during surgery. They are recorded by stimulating peripheral nerves, most commonly the posterior tibial nerve, median nerve or ulnar nerve, typically with an electrical stimulus. The stimulus is then recorded from the patient's scalp.

Averaging
Because of the low amplitude of the signal once it reaches the patient's scalp and the relatively high amount of electrical noise caused by background EEG, scalp muscle EMG or electrical devices in the room, the signal must be averaged. The use of averaging improves the signal-to-noise ratio. Typically, in the operating room, over 100 and up to 1,000 averages must be used to adequately resolve the evoked potential.

Interpretation
The two most looked at aspects of an SSEP are the amplitude and latency of the peaks. The most predominant peaks have been studied and named in labs. Each peak is given a letter and a number in its name. For example, N20 refers to a negative peak (N) at 20ms. This peak is recorded from the cortex when the median nerve is stimulated. It most likely corresponds to the signal reaching the somatosensory cortex. When used in intraoperative monitoring, the latency and amplitude of the peak relative to the patient's post-intubation baseline is a crucial piece of information. Dramatic increases in latency or decreases in amplitude are indicators of neurological dysfuncion.

Anesthetic
During surgery, the large amounts of anesthetic gases used can affect the amplitude and latencies of SSEPs. Any of the halogenated agents or nitrous oxide will increase latencies and decrease amplitudes of responses, sometimes to the point where a response can no longer be detected. To resolve this issue, less than a half MAC of volatile gas and less than 50% N2O is recommended.