Sensory Evoked Potentials (SEP)
The system that detects a stimulus coming from our environment such as touch, heat, pain, position, vibration and transmits it to the relevant centers in the brain via the spinal cord is called the “Somatosensory System”. This system also works in conjunction with other systems, such as the motion system, helping us to respond appropriately to our environment.
The arrival of an electrical stimulus, which is given to the nerves in the wrist and ankle so lightly that it does not cause pain, passes through the spinal cord and brain stem and reaches the relevant area in the brain, is recorded using electrodes placed on specific points on the scalp. During the transmission of the stimulus, whether there is damage to the somatosensory system, and if there is, its location is determined by looking at the characteristics of the waves representing the anatomical regions. It is very valuable in terms of revealing the loss of function that cannot be shown by MRI or not noticed by the patient.
It is a very valuable method in central nervous system diseases, especially in multiple sclerosis (MS) , and in cases such as neck-lumbar hernia, spinal canal enlargement (syringomyelia), spinal cord curvature (scoliosis), spinal cord injury, and tumors.
It can be applied in all age groups, including pregnant women. As in every test, it should be performed and interpreted by experienced neurologists.
What is the SEP Test?
SEP test; is performed for controlling and examining nerve conduction occurring in the afferent peripheral nervous system of patients.
Thanks to this test, it has become more possible to detect the problems of the thick myelinated fibers in the peripheral nervous system and column-medial lemniscus pathway in the central nervous system.
It is a frequently preferred method since it is easier to record and read than EMG. For this reason, the SEP test is preferred more than EMG, especially in the examination of mixed nerves in the head.
How is the SEP test performed?
The SEP test mostly consists of SEP examinations performed with stimulation of the median and ulnar nerves, the ankle part of the tibial nerve, and the knee part of the peroneal nerve.
In this complicated nerve stimulation test, even a stimulation intensity that is enough to create a slight finger movement is sufficient. This amount of stimulus can be tolerated by the patient.
The electrodes that perform the recording process are connected to the Erb point of the median and ulnar nerves of the peripheral nerve structures, the posterior tibial nerve at the knee, the cervical part of the spine for the median and the ulnar, the midline of the lumbar area of the tibial nerve, and the different channels of the recording device placed on the parietal region and vertex part of the scalp.
The remaining electrodes, called reference electrodes, are placed outside the head at a noncephalic site.
The finding that helps detect the presence of an abnormal condition when performing the SEP test examination is the absence of potentials originating from the lesion level or the presence of latency prolongation.
The waves recorded during the SEP test may vary from individual to individual, depending on factors such as the patient’s height, body temperature at the time of measurement, nerve conduction velocity, and age.
If the waves obtained after the right and left side stimulation measurements present a significant difference between these two sides, this finding is considered an indicator for unilateral lesions.
In the SEP test, findings of pathology conditions affecting the peripheral and central nervous systems can be easily gathered.
In lesions of the proximal parts of peripheral nerves, the findings of the SEP test, in addition to the EMG findings, are considered supportive for the detection of the presence of diseases.
In Which Diseases Is SEP Test Required
The SEP test can be performed in case of a lumbar hernia. In order to detect damage to the nerves, the SEP test is considered necessary for some patients in addition to radiological examinations.
The SEP test is also considered necessary in patients with spinal cord injury. In addition to the tests performed to understand the amount of damage and how the patient’s condition will develop, the SEP test is also performed.
For the diagnosis and course of the disease in patients with spinal cord paralysis, the SEP test may be requested along with other radiological examinations. Again, as in spinal cord injury, this examination is also frequently applied to make the right decision about the course of the disease, its progress, and the treatment strategy, or to determine whether there is permanent damage.
