Recording evoked responses (your body’s reaction to an external stimulus) makes it possible to confirm delays or breaks in the transmission of information in the nervous system. In cases of MS, such breaks are attributable to damage to the myelin sheath, which provides nerve insulation. The illustration shows a typical evoked response test, in this case using a stimulus coil and magnetic field around the head.
Evoked responses help diagnose and assess MS development
Evoked responses (evoked potential) are responses that do not occur of themselves but in response to external stimuli. The speed is measured at which different sensitive or sensory stimuli result in a measurable response in the brain.
Evoked responses play an important role when diagnosing MS and in assessing the development of the disease. They make it possible to detect changes in nerve pathways that do not lead to obvious symptoms and impairments. Changes in nerve pathways can also be detected by means of evoked responses when the symptoms have been in remission for a long time or occurred several years previously.
Electrical responses (electrical potential) are differences in tension in human nerve and muscle cells which are detected through the skin, e.g. during electroencephalographic investigation (EEG), which is adiagnostic procedure that records electrical activity generated by brain cells.
A distinction can be made between the various forms of evoked response:
Visual evoked responses (VER/VEP) make it possible to measure the conduction of impulses through the optical nerve pathways.
The electrodes convey every cerebral response to an optical stimulus. The results of several repetitions are then added up or computer averaged. With an healthy eye, a response to stimulus occurs after 100 to 120 milliseconds. A characteristic delay of this response can indicate an injury to the optic nerve or its pathways. View related illustration.
Acoustic or auditory evoked responses (AER/AEP) help a specialist to detect disturbances of the sensory pathway in the auditory or acoustic nerve, as well as disturbances of the cerebellum and of the centripetal pathways.
A click stimulus is presented to each ear by earphone and the electrical potential is measured over the corresponding occipital area. This method of investigation may be helpful if no disturbances of cerebral function have been previously clinically identified. View related illustration
Somatosensory evoked responses (SSER/SSEP) measure sensitivity to touch (tactile sensitivity) on specific parts of the body, usually of the hands and feet, and their conduction of impulses to the brain.
The stimulus is applied with light electrical impulses either on the anklebone of the foot above the course of the tibial nerve (a tibial SSER/SSEP) or on the inner side of the wrist joint above the median nerve or ulnar nerve. The measurement is related to the corresponding area of the brain.
Sometimes it is also necessary to measure responses during impulse conduction, over the spinal cord, which is sometimes referred to as fractionated SSER/SSEP. View related illustration.
Magnetic evoked responses differ from the previously discussed forms of evoked response in that the nerve cells themselves are directly stimulated.
Superficial electrodes are used to assess muscular responses on the muscles of the arm or leg, depending on the arm or leg function for which the brain area under stimulus is responsible. The time between the stimulation of the brain cells and the muscle response is measured. Stimuli are also applied over the region of neck vertebrae or of the lumbar vertebrae in order to distinguish between conduction in the brain and conduction in the spinal cord. View related illustration.
