A Somatosensory Evoked Potential (SEP) test is a neurophysiological procedure that assesses the function of sensory nerve pathways from the body to the brain. It measures the electrical activity generated along these pathways in response to specific sensory stimulation. This non-invasive test evaluates how efficiently sensory signals are transmitted through the spinal cord and into the brain, providing information about the integrity of the somatosensory system.
Understanding Somatosensory Evoked Potentials
Somatosensory evoked potentials are electrical signals produced by the nervous system when sensory receptors are stimulated. These responses are measured at the skin’s surface following controlled stimulation of peripheral nerves. The process involves stimulating a sensory pathway and recording the resulting electrical activity from the scalp or other locations. This tracks how sensory information travels from the body to the brain.
The sensory information travels along specific neural pathways, primarily the dorsal column-medial lemniscus pathway. This pathway conveys sensations such as fine touch, vibration, pressure, two-point discrimination, and proprioception (body position) from the skin and joints to the brain. It begins with first-order neurons in the dorsal root ganglia, which send axons up the ipsilateral dorsal columns of the spinal cord.
These axons then synapse with second-order neurons in the medulla’s nucleus gracilis and nucleus cuneatus. From there, the fibers cross over to the opposite side of the brain (decussate) and ascend as the medial lemniscus to the thalamus. Finally, third-order neurons project from the thalamus to the primary somatosensory cortex, where the sensation is interpreted. SEP measures electrical activity generated along this route, including the peripheral nerves, spinal cord, brainstem, and cerebral cortex.
Reasons for Performing a Somatosensory Evoked Potential Test
The SEP test assesses the integrity of sensory pathways and helps identify various neurological issues. It is useful for detecting and localizing disorders within the somatosensory pathways, including problems affecting peripheral nerves, spinal roots, the spinal cord, brainstem, or primary somatosensory cortex.
Healthcare providers may recommend an SEP test if a person experiences symptoms like numbness, tingling, or weakness in their arms or legs. It can help diagnose conditions such as multiple sclerosis (MS), which involves damage to the protective coating of nerve fibers (demyelination), leading to slower and weaker signals. The test also aids in evaluating spinal cord injuries, detecting compression of the spinal cord or nerves, and identifying spinal cord tumors.
SEP tests are used for monitoring neurological function during complex surgical procedures, especially those involving the spine, central nervous system, or vascular structures. They are employed during scoliosis surgery or other interventions where the spinal cord is at risk of damage. This real-time monitoring allows surgeons to detect potential nerve injury early, enabling adjustments to surgical techniques to prevent permanent neurological deficits.
The Somatosensory Evoked Potential Procedure
Before an SEP test, patients are advised to avoid lotions or oils on their skin and wear comfortable clothing. The procedure usually takes about 1.5 to 2 hours, though it can extend up to 3 hours depending on the areas being tested. No dietary restrictions, such as fasting, are required for this non-invasive test.
During the test, a healthcare professional attaches small metal disc electrodes to specific areas of the body and scalp. For upper limb testing, electrodes might be placed on the wrist (over the median or ulnar nerve), shoulder, neck, and scalp. For lower limb testing, electrodes are commonly placed on the ankle (over the posterior tibial nerve), knee, lower back, and scalp. The skin underneath the electrodes may be abraded with gel to ensure good electrical contact.
Once the electrodes are in place, small electrical pulses are delivered through stimulating electrodes, usually at the wrist or ankle. These pulses are described as a mild tingling sensation or a slight muscle twitch, but they are not typically painful. The electrical signals travel along the sensory nerves, and recording electrodes pick up the activity as it progresses towards the brain. The signals are then amplified and displayed on a monitor for interpretation.
What Somatosensory Evoked Potential Results Indicate
The electrical signals recorded during an SEP test appear as waveforms, which are plots of voltage against time. Analysis of these waveforms focuses on two key parameters: latency and amplitude. Latency refers to the time it takes for the electrical signal to travel from the stimulation point to the recording electrode, measured in milliseconds (ms). Amplitude indicates the strength of the electrical response, measured in microvolts (μV).
Abnormalities in these parameters can indicate dysfunction or damage within the somatosensory pathways. For instance, a prolonged latency suggests the signal is taking longer than expected to travel, which can point to issues like nerve demyelination or compression. Reduced amplitude indicates a weaker signal, potentially due to fewer nerve fibers or a disruption in their function.
Specific waveform components are identified by their polarity (positive or negative, labeled P or N) and their latency in milliseconds (e.g., N20, P37). The N20 peak, typically occurring around 20 ms after median nerve stimulation, reflects activity in the primary somatosensory cortex. Changes in the N9, N13-P14, or N20-P23 components can help localize the problem to the peripheral nerve, cervical spinal cord, or thalamocortical radiations. These changes help healthcare professionals understand the nature and location of a neurological issue.