The classical application of VEP measurements for clinical purposes is its contribution to the diagnosis of multiple sclerosis (MS). The main characteristic of this disease is the patchy demyelination of afferent and efferent nerve fibers distributed all over the nervous system, and the patients have neurological symptoms that cannot be explained by a single lesion. Myelin is an insulating sheath, found on most axons, that increases conduction velocity (cf. Kandel et al., 2000), thus, electrophysiologically, latency prolongations are expected when demyelination occurs. In many patients the visual system is affected at an early state of the disease, and one finds patients with pathological VEP results who, however, do not display any subjective visual symptoms (i.e., they have normal visual acuity and visual fields). An optic neuritis occurs frequently at a very early stage of the disease, which in general is followed by a recovery after several weeks, whereas other symptoms— hemiparesis, ataxia, and sensory disturbances—may be seen only after some years. The diagnosis of MS is warranted only if several independent lesions can be quantified, or if several repeated attacks of similar neurological symptoms occur over time. Thus, with pathological VEP measurements clinically silent lesions can be detected in patients with no visual symptoms, and this may contribute to a final diagnosis of MS. For more information on pathophysiological and clinical details of MS the reader is referred to Bauer et al., (1980) and McKhann (1982).
Other applications of VEP measurements in ophthalmology and neurology comprise the documentation of visual development in infants (as discussed in the section on stereoscopic vision) as well as the topologic localization of disturbances in the visual system (Heckenlively and Arden, 1991). The combined recordings of VEP and ERG activity allow localization of lesions in the afferent visual pathway in patients with visual field defects. Skrandies and Leipert (1988) could demonstrate a significant relationship between pathological electrical acivity and the site of the lesion in a group of neuro-ophthalmologi-cal patients. After lesions of the optic nerve or optic tract, ERG changes appear in parallel to a retrograde degeneration of the axons of the retinal ganglion cells. On the other hand, in adult patients with cortical lesions, no electrophysiological sign of subsequent retinal alterations can be found. This has also been demonstrated in controlled lesion experiments performed on adult cats (see Skrandies and Leipert, 1988). In summary, such data illustrate the topodiagnostic possibilities of the combination of various electrophysiological recordings in patients with defective vision. Due to their noninvasive nature and their sensitivity to functional (and not only structural and anatomical) changes, these methods are commonly applied for a wide variety of diagnostic questions.
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