EVOKED POTENTIAL STUDY

 

Visual Evoked Potential Study (VEP)
Brainstem Auditory Evoked Potential Study (BAESP)
Somatosensory Evoked Potential Study (SSEP)

Electronic averaging has permitted the recording of low amplitude potentials evoked by different types of sensory stimulus. These responses are commonly used in the diagnosis of Multiple Sclerosis (M.S), a relapsing and remitting condition which is characterized by patchy inflammation affecting the myelin sheath of the central nervous system. The object is to demonstrate abnormalities in regions of the nervous system not known from clinical manifestations to be involved -silent lesions. For example, the finding of abnormal VEPs or BAEPs in a patient with paraparesis would demonstrate abnormalities in at least 2 sites of the central nervous system characteristic of M.S.

Visual Evoked Potentials (VEPs)

The visual evoked potential is a gross electrical signal recorded from the occipital cortex in response a systematic change in some visual event such as a flashing light or an alternating chequered pattern. The recording electrode is placed over the occipital cortex and the amplitude and latency of the waveform generated can be measured. This method currently provides the most sensitive means of detecting subclinical lesions of the optic nerve and may enable a diagnosis of M.S. to be made at an earlier time. Abnormality in the VEP is also encountered with compressive lesions of the anterior visual pathways. In the paediatric age group, the flash VEP may be used as a screening test for the visual pathway. Examples of normal VEPs in adults with 20/20 vision.

Brainstem Auditory Evoked Potentials (BAEPs)

If one could record directly from several different levels of the subcortical auditory pathway, one would see during the first 10 milliseconds following an appropriate acoustic stimulus a series of potentials corresponding to the sequential activation of peripheral, pontomedullary, pontine and midbrain portions of the pathway. When these acoustic nerve and brainstem potentials are volume conducted to surface recording electrodes at the vertex and earlobe, they form a composite series of waves known as brainstem auditory evoked potentials. Since the amplitude of these responses are very small (about 1/100 of spontaneous EEG activity), special computer averaging technique is required.

The neurologic applications of BAEPs have been proven useful in the diagnosis of M.S.. It may be used as a screening test for early detection of acoustic neuromas and in the assessment of comatose patients. Like the VEPs, it may also be a useful screening test for hearing in the paediatric age group such as neonatal screening and those who do not cooperate sufficiently with behavioural testing.

Somatosensory Evoked Potentials (SEPs)

SEPs can be recorded from electrodes placed over the somatosensory cortex of the cerebral hemisphere contralateral to the applied stimulus. Automatic averaging techniques are used to facilitate recording of these small potentials. In the upper limb, the stimulus is usually applied to the median or ulnar nerve whereas in the lower limb the tibial nerve is most frequently used. Stimulation of the median nerve at the wrist elicits a response at the brachial plexus at about 9 msecs (N9), the cuneate nucleus 12 msec (N12) and somatosensory cortex at 20msec (N20). Normal SEPs elicited by stimulation of the median nerve and recording at Erb’s point (EP), the 7th and 2nd cervical spine (CV7 and CV2) and somatosensory cortex (C4).

Clinical uses of SEPs

The SEPs findings may help in the detection and localization of lesions of the central somatosensory pathways but are not pathognomonic of specific diseases. In Multiple Sclerosis (MS) the presence of SEP abnormalities may reveal subclinical lesions involving the central somatosensory pathways thus aiding in early diagnosis. In patients with definite MS, the incidence of SEP abnormality is about 80% whereas in the category of possible MS, the yield is only about 30%. The interpretation of electrophysiologic results must therefore always be taken in a clinical context.