Abstract
In 1967, Sohmer and Feinmesser recorded four negative peaks from extracochlear surface electrodes and they stated that the later components may be due to repetitive firing of auditory nerve fibers or may be due to the discharge of neurons in brain stem auditory nuclei. In 1970, Jewett, Romano and Williston presented the first full description of the scalp responses, postulated to be generated by brainstem nuclei. Since then, the auditory brainstem response (ABR) technique has attracted attention because it satisfied two previously unattainable needs; 1) a means of quantitative measurement of changes in auditory function in newborn babies or uncooperative patients, and 2) the detection of brainstem neurological abnormalities. Recording during wakefulness and in sleep or under drug intoxication has shown that ABRs are independent of the level of arousal or attention, and this feature is particularly useful for people unable to cooperate appropriately. Because of the close relationship to the anatomy of brainstem pathways, ABRs offer a neurophysiological index of neuronal function at the level of the brainstem. At present, these responses have become widely recognized as an important tool for the diagnosis of neurological diseases.
One specific feature of the ABR is its dependence on maturation. The ABR appears at around 26 weeks of gestation and thereafter undergoes systematic changes in latency, amplitude and threshold. By using a supramaximal intensity of more than 60 dBHL, it is possible to evoke in all newborn babies with sufficient hearing ability, waves I, III and V. The principal pathophysiological findings in neurologic diseases concern the absolute and interpeak latencies of each wave, the amplitude ratio (wave V /wave I), the response threshold, the response to changes in the rate of stimulation, and the elimination of specific components. The amplitude ratio for which there are sufficient data is wave V /wave I, which usually exceeds 1 in normal subjects. The younger the patient, the longer are all the component latencies, the smaller the amplitude ratios, and the larger the effect of changes in the rate of stimulation. Therefore, age specific norms must be determined. Besides absolute latencies, interwave latencies are clinically significant since they allow evaluation of the brainstem function even in the presence of a hearing disturbance. The application of the ABR technique to various auditory and neurological diseases was discussed. It is noteworthy that none of these abnormalities is specific for any disease, and normal ABRs do not guarantee that hearing is normal in the perceptual sense, because further transmission may be disturbed. MLPs may be useful in such a situation. Conversely, the absence of several brainstem components dose not necessarily mean structural damage to the brainstem neurons or hearing impairment. The diagnosis of an audiological or a neurological disorder must be made on the basis of the clinical status and other available examination.
