Somatosensory Evoked Potential testing (SSEP) is a non-invasive neurodiagnostic procedure. It measures the speed and strength of electrical signals traveling through the sensory nerves to the brain. SSEP assesses the functional integrity of the somatosensory pathway, the system responsible for relaying sensations like touch, vibration, and joint position from the body’s periphery to the central nervous system. By evaluating these signals, medical professionals can determine if there is damage or disruption along the nerve routes, including the limbs, spinal cord, and brain.
How SSEP Measures Nerve Signals
The SSEP procedure begins with applying a small electrical pulse to a peripheral nerve, usually the median nerve at the wrist or the posterior tibial nerve at the ankle. This stimulation generates an impulse that travels along the sensory nerve fibers, through the spinal cord’s dorsal columns, and up to the somatosensory cortex in the brain. The pulse intensity is adjusted to be just strong enough to cause a slight, visible twitch in the fingers or toes.
As the signal progresses, recording electrodes placed over various points, such as the limb, neck, and scalp, capture the resulting electrical activity. These electrodes record characteristic wave peaks, each corresponding to the signal reaching a specific point along the pathway, including the brachial plexus, cervical spine, or cerebral cortex. A specialized computer averages many responses to filter out background electrical noise, isolating the tiny nerve signal.
Analysis focuses on two primary measurements: latency and amplitude. Latency is the time the signal takes to travel from the stimulation point to the recording electrode, indicating the speed of nerve conduction. Prolonged latency suggests the signal is traveling too slowly, often due to demyelination or damage to the nerve’s protective coating. Amplitude measures the signal’s strength; reduced amplitude can indicate a loss of nerve fibers or a block in transmission along the sensory pathway.
Conditions Identified by SSEP Testing
SSEP testing provides functional insight into sensory pathways, complementing structural imaging tests like MRI or CT scans by showing how well the nerves are working. This assessment is useful for diagnosing conditions affecting the central nervous system, including the brain and spinal cord. SSEP helps evaluate Multiple Sclerosis (MS), a demyelinating disease where the myelin sheath is damaged. The test can reveal slowed nerve conduction (prolonged latency) in sensory pathways, even if the patient has not reported overt sensory symptoms.
SSEP is also applied to assess the impact of spinal cord injuries and diseases that compress or damage the spinal cord. By stimulating nerves in the legs and measuring the response at the scalp, clinicians determine the functional integrity remaining in the sensory tracts following trauma or a tumor. SSEP can identify silent lesions, which are areas of nerve damage not apparent during a standard neurological examination. This ability to detect subclinical damage makes it a valuable tool in predicting the neurological outcome after a stroke or severe trauma.
SSEP is also used to evaluate issues affecting the peripheral nervous system and nerve plexuses. It can help confirm severe nerve entrapment syndromes, such as Carpal Tunnel Syndrome, when standard nerve conduction studies are inconclusive. SSEP is useful for assessing injuries to the brachial or lumbar plexus. In these cases, SSEP helps localize the precise segment of the nerve pathway where the conduction problem is occurring.
SSEP Use in Surgical Monitoring
A distinct application of SSEP testing is within the operating room as part of Intraoperative Neurophysiological Monitoring (IONM). During complex surgical procedures, especially those involving the spine, brainstem, or major blood vessels, SSEP continuously monitors the integrity of the sensory nerve pathways in real-time. This monitoring function is preventative, aiming to protect the patient from neurological injury during the operation.
Electrodes stimulate peripheral nerves, and the resulting signals are recorded at the scalp and spine throughout the procedure. The surgical team watches for any significant change in the SSEP signal, defined by a decrease in amplitude or an increase in latency. A sudden, sustained drop in signal amplitude, typically 50% or more, alerts the surgeon that the sensory pathway is potentially being compressed, stretched, or experiencing a lack of blood flow.
This real-time feedback allows the surgeon to immediately adjust their technique or reposition the patient to relieve pressure on the neural structures. SSEP monitoring is standard practice in surgeries like spinal fusion, scoliosis correction, and certain brain tumor removals. While SSEP specifically monitors the sensory tracts, it serves as an early warning system for the entire spinal cord and is often used with tools that assess motor function.
What to Expect During the Test
The SSEP procedure is non-invasive and generally takes 30 to 60 minutes, though a full-body assessment may last longer. Patients should avoid applying lotions, oils, or heavy hair products beforehand, as these interfere with electrode connection. Small metal discs (electrodes) are temporarily attached to the skin with paste or tape on the scalp, neck, and the limbs being tested.
Once the electrodes are in place, the technician applies a mild electrical stimulus to the nerve, usually at the wrist or ankle. Patients report this sensation as a quick tapping, tingling, or a muscle twitch, which is rarely painful. The test involves repeating this mild stimulation multiple times, allowing the recording equipment to average the responses and obtain a clear signal. The patient remains relaxed, often lying down, while the computer records the signals traveling to the brain.