Wave V latency on ABR is predominately influenced by high-frequency regions in the cochlea, and small acoustic tumors may not disrupt high-frequency fibers enough to result in abnormal ABR waveforms. ![]() ABR has a diagnostic accuracy of 92 to 98% for large acoustic tumors (> 1.5 cm), but smaller tumors (< 1.5 cm) are missed in 30% of cases. In addition to hearing loss, ABR can be used to diagnose vestibular schwannomas, brainstem lesions, de-myelination disease, and auditory neuropathy. In children who pass their newborn hearing screen but are exposed to ototoxicity agents such as chemotherapy, aminoglycoside antibiotics, or heavy metals, ABR can be used to monitor for developing hearing loss. ĪBR is also indicated for young children or even adults with inconclusive audiometric thresholds or who are unable to tolerate conventional behavioral audiometry testing. Testing takes place while the infant is asleep or sedated with thresholds highly correlative with those of conventional behavioral audiometry. Ĭlinical applications of the ABR include identification of suspected neurological abnormalities of the cranial nerve VIII as well as the associated auditory pathways and the estimation of hearing sensitivity for those who are not able to accurately provide behavioral hearing evaluation information.ĪBR has been the gold standard for assessing and confirming hearing loss in infants who fail their newborn hearing screen. The medial geniculate nucleus in the thalamus receives afferent input from the inferior colliculus and projects it to the tonotopically organized auditory cortex. The neural fibers from the superior olivary nuclei are then transmitted to the ipsilateral inferior colliculus via the lateral lemniscus tract. The cochlear nucleus is located at the posterolateral portion of the brainstem and is cochleo-topically divided with low-frequency fibers on its ventral region and high-frequency fibers on its dorsal region.Īfter the cochlear nucleus, the majority of neural fibers decussate across the midline into the contralateral superior olivary complex. The cochlear nucleus is the first relay point of the brainstem along the auditory pathway. Auditory nerve fibers from the low-frequency, apical portion of the cochlea and the high-frequency, basal portion travel along with the central and peripheral parts of the auditory nerve, respectively. The cochlea has a unique tonotopic organization with high frequency sounds stimulating the basal component and low frequency sounds stimulating the apical component. ![]() The cochlear hair cells transform physical sound input into bioelectrical action potentials that are then transmitted to the spiral ganglion which comprise the main component of the auditory nerve. Electrical information to the auditory cortex travels along an ascending path through a series of nuclei including the cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculi, and medial geniculate nuclei. Mechanical sound is transformed into electrical signals in the cochlea and transmitted to the brain through the auditory nerve or vestibulocochlear nerve (cranial nerve VIII). ĪBR measures the synchronous neural fiber activity along the auditory pathway to determine hearing thresholds. It is now essential in the diagnosis of hearing loss, acoustic tumors, and cerebellopontine angle (CPA) tumors. It was first introduced into audiology in the 1970s, and its importance has increased with the contemporary guidelines of newborn hearing screening for hearing loss within 3 months of age. ![]() The readings consist of a sequence of up to 7 positive wave peaks labeled I-VII with negative troughs in-between. Įlectrical activity from the eighth cranial nerve and neurons along the brainstem auditory pathway are recorded by surface electrodes placed on the scalp, forehead, and near the ears for 10 milliseconds after an acoustic stimulus. ABR tests synchronous neural function and can estimate hearing sensitivity thresholds in individuals who are unable to tolerate traditional behavioral audiometry. Auditory brainstem response (ABR), also known as brainstem auditory Evoked Potentials (BAEP), is an objective measurement of auditory pathway function from the auditory nerve to the mesencephalon.
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