What the study looks at
An SSEP is a sensory pathway study, not a test of muscle and not a test of pain or temperature sensation. It follows large sensory fibers. For the arms, the median nerve at the wrist is the usual starting point. For the legs, the tibial nerve at the ankle is the usual starting point. Other nerves can be used when the clinical question calls for them.
Responses are recorded at several places along the route, including over the spine and the scalp. Each response is named for about when it is expected to appear, in milliseconds, and for where it was recorded. A delay between two sites draws attention to that segment of the pathway. An absent response raises the same kind of question. The physician decides what the pattern means.
How the visit is performed
The patient lies quietly. Surface electrodes stimulate the nerve with small, repeated pulses, and recording electrodes sit on the skin over the pathway. Many responses are averaged so a small signal can be seen above background activity. Movement, muscle tension, and a cold limb make the average harder to read, so comfort and warmth matter.
The stimulus is noticeable but brief. The visit takes longer than a single nerve conduction recording because the signal has to be averaged. Atlas brings the equipment and a technician to the office.
Why a practice orders it
Physicians request SSEPs when they want information about the central continuation of a sensory nerve: through the plexus and roots, along the spinal cord, and up to the cortex. The study can add pathway information when the question is myelopathy, a cord-level process, or the difference between a peripheral and a central localization.
SSEPs do not diagnose a disease by themselves. They are read with the history, the exam, imaging, and any EMG or nerve conduction study done around the same time.