【 摘 要 】
A brief historyThe auditory system neurophysiology and its means of assessment were restrict to audiology research labs until thirty years ago. With the progress in electronic equipment it became possible to develop systems for stimulation, capture, amplification, computation e recording the neurophysiologic activities of our hearing system that made it possible its clinical application. As a consequence, such equipment started to be produced in large scale, making auditory neurophysiology and its methods of assessment shift from highly sophisticated laboratories to the clinical settings. Thus, a new and fascinating chapter started in audiology: electrophysiology of hearing (audiometry of electrical response). Electrical response audiometry is the generic denomination of the methods that allow us to study the bioelectrical phenomenon that occurs in the auditory system as a response to sound stimulus (evoked auditory potentials), from the inner ear all the way to the cerebral cortex. All these electrophysiological tests aim at detecting a bioelectrical activity related to an event (presentation of a sound signal and the mental activity triggered by changes in the sound signal characteristics). This bioelectrical activity is produced when the sound stimulus is transformed into electricity within the inner ear and in all the synapses that occur successively in the auditory pathway.The development of computers able to individualize and mediate different responses made feasible the exclusive recording of neural potentials emitted by the auditory pathways. It is likely that the ultimate step towards the clinical use of evoked response audiometry was the work of Jewett, who in 1970, based on experimental studies in cats, described the presence of 4 waves that were related to specific sites or origins located in the brainstem, which follow a first potential, the action potential (wave I)1. Since then, evoked response audiometry has been increasingly used in the clinical setting, broadening the potentials of its application. Currently, hearing electrophysiological tests are indispensable tools for the assessment, diagnosis and monitoring of numerous neurotologic diseases. Among evoked potentials, it is possible that the brainstem evoked response audiometry (BERA) be the one most used in the clinical practice. It allows us to acquire the hearing system electrophysiological activity in the brainstem, mapping auditory synapses of auditory pathways from the cochlear nerve, cochlear nuclei, superior olivary complex (pons) all the way to the inferior colliculus (mesencephalus).A series of seven waves (the components of the BERA traces are called waves or potentials) may be recorded from the forehead-lobule or mastoid leads during the first 12ms after a moderate sound stimulus. These waves, sequentially labeled with Roman numerals seem to represent successive tracts and/or auditory path synapses. Of these seven waves, the first five of them are the ones that interest us, and among them, waves I, III and V are the ones that offer us the most important parameters for BERA interpretation. Waves I and II are generated in the auditory nerve, wave III in the neurons that leave the cochlear nuclei complex2-4, waves IV and V in the lateral superior lemniscus and the major negative potential that follows wave V in the inferior colliculus depolarization.5At present, ABR applications are increasingly more used, in investigating childhood hearing loss, the screening of cochleo-vestibular syndromes, the search for retro-cochlear lesions, in monitoring coma status (brain death), in monitoring the brainstem in skull base surgeries etc.One of ABRÂ’s quality is its very capacity of assessing the neurophysiologic integrity of brainstem hearing paths. We are able to compare stimulus progression speed (latencies) in both ears. Thus, sometimes, during the study of childrenÂ’s hearing thresholds, we see ABRs that suggest the presence of retro-cochlear lesions in the auditory pathways (asymmetry of traces, increase in inter-peak intervals), often times proved by an image exam. It is an occasional finding of some neurologic disease when in search of childrenÂ’s hearing thresholds.In this present investigation we report 2 cases of neurologic diseases as occasional findings during the investigation of childhood hearing loss through the ABR.
【 授权许可】
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