A somatosensory evoked potentials test (SSEP) measures the speed with which the nervous system sends and receives signals that communicate sensations, such as touch, pain, and vibration. If the spinal cord or one of its nerve roots is significantly pinched, these signals will travel slower than usual.

How SSEP Works

A somatosensory evoked potentials test is done in a doctor’s office or hospital.

Electrodes are attached to the skin over the scalp or spine. These electrodes sense when signals are sent and received from the central nervous system.
A second set of electrodes are placed on the skin over the wrist, the back of the knee, or another location. These electrodes measure when signals are sent and received from peripheral nerves.
A mild electrical current is transmitted to the electrodes on the wrist, knee or other peripheral area.
How long it takes for the current to be sensed by the peripheral nerves and communicated to the central nervous system is recorded.

The test is considered safe and can take anywhere from 20 to 90 minutes.

In some cases, needles are used instead of over-the-skin sensors. When needles are used, there is a small risk of bleeding and infection. The possibility of nerve damage also exists, though it is rare.

When SSEP Is Considered

For patients with neck or back pain, an SSEP test may be ordered if the pain is accompanied by numbness or weakness in the arms and/or legs. This test can identify a problem with nervous system function but cannot necessarily pinpoint the underlying cause.

A somatosensory evoked potentials test may also be used to monitor spinal cord function during surgical procedures. However, the spinal cord ends before the lumbar spine, so SSEP is of limited value during lumbar spinal surgery. It is used more often in cervical (neck) or thoracic (chest) spine surgery and particularly during surgery for correcting scoliosis.

SSEP may be conducted along with electromyography (EMG), which assesses the electrical activity of muscle and nerve function.