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What is it?
Somatosensory Evoked Potential (SSEP) is a test showing the electrical signals of sensation going from the body to the brain. The signals show whether the nerves that connect to the spinal cord are able to send and receive sensory information like pain, temperature, and touch. When ordering electrical tests to diagnose spine problems, SSEP is combined with an electromyogram (EMG), a test of how well the nerve roots leaving the spine are working.
Why is it done?
An SSEP indicates whether the spinal cord or nerves are being pinched. It is helpful in determining how much the nerve is being damaged and if there is a bone spur, herniated disc, or other source of pressure on the spinal cord or nerve roots. EMG is used to show if a nerve is being irritated or pinched as it leaves the spine on its way down the arm or leg. During spine surgery, the EMG is used to monitor nerve output to the muscles in procedures where screws are placed in the middle or lower part of the spine. SSEP is used to double check whether the sensory part of the nerve is working correctly.
How is it done?
Either an electrode is placed over the skin or a needle is inserted into the nerve or sensory center of the brain. Measurements of how long it takes an electrical signal to travel through the nerve pathway are recorded. The function of the nerve is determined by the speed of these electrical signals. When the nerve pathway is pinched, the signals are slower than expected.
What are the limitations?
An SSEP does not show why the problem occurred or what is causing it. The test looks primarily at whether sensation is affected by a problem in the spinal cord or nerves. There could be a problem in the nerve somewhere between the central nervous system and the area tested. The problem may not necessarily be in the spine itself.
What are the risks?
There are few risks associated with SSEP. There is a small chance of infection anytime a needle is inserted into the body. However, the risk is almost absent in this test.
source -- www.allaboutbackpain.com/html/spine_diagnostics/spine_diagnostics_ssep.html