Evoked potentials are the electrical signals generated by the nervous system in response to sensory stimuli.

Somatosensory evoked potentials consist of a series of responses [presented in wave form] that reflect sequential activation of neural structures along the somatosensory pathways.

Following stimulation sites typically used

• Median nerve at the wrist
• Common peroneal nerve at the knee
• Posterior tibial nerve at the ankle.

The Test

Eectrical stimulation,  a square wave of 0.2- to 2-millisecond duration is delivered to a peripheral nerve by  surface electrodes or needle elctrodest at the usual sites.

Electrodes are placed over the Erb point [site at the lateral root of the brachial plexus located 2–3 cm above the clavicle] for recording upper extremities and  over the lumbosacral spine for lower extremity.

Wave morphology, amplitude, and dispersion is recorded. Depending upon latency of response, the potentials are described as short, middle and long latency somatosensory electric potentials.

Surgical monitoring

SEP monitoring is used during spinal surgeries especially scoliosis correction to recognize any harm caused to the cord. Ischemia of ascending somatosensory pathways produces a drop in amplitude or loss of waveforms, thus warning the surgeon in time to take corrective action.

Use

• Diagnostic aid in neurological disorders
• Intraoperative monitoring of surgeries that place parts of the somatosensory pathways at risk.

An abnormal somatosensory evoked potential can be due to problem [dysfunction] of

• Peripheral nerve
• Neural plexus
• Spinal root
• Spinal cord
• Brain stem
• Thalamocortical projections
• Primary somatosensory cortex.

Diseases of the dorsal columns in which joint position sense and proprioception are impaired invariably are associated with abnormal somatosensory evoked potentials.

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