As indicated above, PSVER is especially valuable in proving the existence of active or residual disease of an optic nerve. Patients with previous optic neuritis almost invariably have prolonged latencies. Furthermore, prolongations of PSVER are found in about one-third of multiple sclerosis patients who have had no history or clinical evidence of optic nerve involvement. This acquires significance in that the finding of abnormal PSVER in a patient with a clinically apparent demyelinating lesion elsewhere in the CNS may usually be taken as evidence of multiple sclerosis, as discussed in Chap. 37.
A compressive lesion of an optic nerve will have the same effect as a primarily demyelinating one. Many other diseases of the optic nerves—including toxic and nutritional amblyopias, ischemic optic neuropathy, and the Leber type of hereditary optic neuropathy—show abnormalities of the PSVER. Glaucoma and other diseases of the eye, if severe enough to affect the optic nerve, may also produce increased latencies. Impaired visual acuity has little effect on the latency but does correlate well with the amplitude of the PSVER (a property that is exploited in some computerized testing for visual acuity).
The effects of auditory stimuli can be studied in the same way as visual ones by a procedure called brainstem auditory evoked responses, or potentials (BAERs, or BAEPs). Between 1,000 and 2,000 clicks, delivered first to one ear and then to the other, are recorded through scalp electrodes and superimposed on each other by computer and thereby maximized. A series of seven waves appears at the scalp within 10 ms after each stimulus. On the basis of depth recordings and the study of lesions produced in cats as well as pathologic studies of the brainstem in disease, it has been established that each of the first five waves is generated by a specific brainstem structure, as indicated in Fig. 2-9. The generators of waves VI and VII in particular are uncertain. The presence of wave I and its absolute latency test the integrity of the auditory nerve.
Clinical interpretations of BAERs are based mainly on latency measurements from the stimulus and interwave latencies. The most important are the interwave latencies between I and III, and III and V (see Table 2-4). A lesion that affects one of the auditory nuclear relay stations or its immediate connections manifests itself by a delay in the appearance or an absence of all subsequent waves; in other words, the nuclei behave as if they are connected in series. These effects are more pronounced on the side of the stimulated ear than contralaterally. This is difficult to understand, as a majority of the cochlear-superior olivary-lateral lemniscal-medial geniculate fibers cross to the opposite side. It is also surprising that a lesion of one relay station would allow impulses, even though delayed, to continue their ascent and be recordable in the cerebral cortex.
BAERs are a particularly sensitive means of detecting lesions of the eighth cranial nerve (vestibular schwannoma and other tumors of the cerebellopontine angle) and of the auditory pathways of the brainstem. Almost one-half of patients with definite multiple sclerosis and a lesser number with a possible or probable diagnosis of this disease will show abnormalities of the BAERs, (usually a prolongation of interwave latencies I to III or III to V), even in the absence of clinical symptoms and signs of brainstem disease. The BAERs are also useful in assessing hearing in infants who have been exposed to ototoxic drugs, in young children who cannot cooperate with audiometry, and in those with psychogenic or feigned deafness.