What are Somatosensory Evoked Potentials?

Somatosensory Evoked Potentials (SSEP) are the electrical responses of the nervous system to a specific stimulus. Although this procedure can be performed for any sensory nerve, better responses are obtained from larger nerves of the upper extremity, such as the median, ulnar and radial, or of the lower extremity such as the peroneal and tibial. It gives information about the proprioceptive pathways, which includes the above-mentioned nerves, the spinal cord, and the spinocerebellar pathways.

Indications for performing Somatosensory Evoked Potentials include:

• Detection of clinically silent second lesions in multiple sclerosis
• Compressive lesions of spinal cord
• Intrinsic cord lesions such as tumors, AV malformations, and syringomyelia
• Spinocerebellar degeneration
• Subacute combined degeneration due to b12 deficiency
• Leukodystrophies
• Neuropathies and focal traumatic lesions of peripheral nerves
• Brachial plexopathy
• Radiculopathy
• Motor neuron disease
• Coma

How is Somatosensory Evoked Potentials procedure performed:

• SSEP can be recorded from the extremities, spine or scalp
• In the upper extremity, stimulation is done at the wrist, and the potentials can be recorded at different points from the elbow to the scalp
• For lower extremity recordings, stimulation is done at the ankle for the tibial or peroneal nerve, and potentials are recorded over the knee, the spinal cord and scalp
• The patterns of the waveforms at the various recording sites helps localize the lesion
• The intensity of the stimulus should be high enough to obtain the desired response, while ensuring it does not induce pain, which will make relaxation difficult and not necessarily enhance the amplitude of the response
• The highest amplitudes are generated by stimulation of the median and tibial nerves

Factors interfering with an accurate response include background noise, electrical activity from the muscles, or nearby devices. Analog filters can be used to minimize this interference. Results of this test should be interpreted in the light of clinical findings, and results of other diagnostic tests. Intraoperative SSEP is a very helpful tool in monitoring the activity of the brain and spinal cord during surgery. It detects and localizes any injury during neurosurgery and helps prevent potential complications including any neurologic trauma from surgery.