Abnormal Findings and Clinical Uses of the BAEP

The BAEP is an important means of evaluating function of the eighth cranial nerve and the central auditory pathways in the brain stem.

In infants, young children, and adults who are unable to cooperate for behavioral testing, BAEPs can be used to evaluate hearing. y The wave V component of the response is generated by auditory stimuli that are too weak to generate other components. As a screening test of hearing, then, the electrophysiological threshold for eliciting wave V is determined. An impairment of auditory acuity is suggested if a stimulus intensity greater than 40 dB is required. BAEPs provide a sensitive screening test for acoustic neuromas or other lesions in the cerebellopontine angle involving the eighth cranial nerve. Indeed, the BAEP findings may be abnormal at a time when imaging studies show no definite abnormality.

The BAEP is also useful in assessing the integrity of the brain stem (see Fig,,..24-13, ). Although the auditory pathways have a bilateral course, a unilaterally abnormal BAEP nevertheless suggests an ipsilateral, structural brain stem lesion. The presence of normal BAEPs in comatose patients suggests either that the coma is due to bihemispheric disease or that it relates to metabolic or toxic factors; abnormal BAEPs in this context imply brain stem pathology and a poorer prognosis than otherwise. When coma is due to brain stem pathology, the BAEP findings help in localizing the lesion: alteration of wave V suggests midbrain dysfunction, and alteration of waves I to III indicates a lesion in the lower brain stem. y In patients with brain death, the BAEP should be absent apart from wave I and the early part of wave II, which are generated peripherally. In many patients with suspected brain death, however, all BAEP components, including wave I, are absent, and it is then not possible to exclude other causes (such as technical factors of deafness) for the absent response. y

BAEPs have been used to detect subclinical brain stem pathology in patients with suspected multiple sclerosis. y , y However, the yield in this circumstance is less than with the visual or somatosensory evoked potentials, possibly because the auditory pathway is relatively short or is more likely to be spared.

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