Auditory brainstem evoked response (ABR) also known as Brainstem auditory evoked responses (BAER), test both the inner ear system and the the VIIIth nerve pathway up to the brainstem. This test measures the timing of different electrical waves in response to small clicks of noise or tone bursts in the ear. These tone bursts and clicks are delivered through ear phones placed into the inner ear that are then recorded through electrodes typically placed on the neck and forehead to measure the latencies of the evoked responses.
This data is then averaged over a period of time, and filters are applied to the data to eliminate any background noise during the test. The computer software then generates an averaged response of the auditory pathway on a graph (waveform) for the technician to review. There are three waves (1, 3, 5) plotted on the graph for each ear. This graph represents specific anatomical points along the auditory neural pathway, they are: the cochlear nerve and nuclei (waves I and II), superior olivary nucleus (wave III), lateral lemniscus (wave IV), and inferior colliculi (wave V). Delays of one side relative to the other suggests a lesion in the VIIIth nerve pathway between the vestibular system and brainstem, or sometimes even the brainstem itself.
An ABR test will allow you to see if there has been a "block" along the VIIIth nerve pathway that lead to the brainstem. Common "blocks" include tumors, infections, and degeneration to the nerve itself. This test can be especially useful if acoustic neuroma is suspected. A positive indication for acoustic neuroma is the interaural latency differences in wave V. The latency in the ear with the suspected neuroma is a prolonged wave form that differs from the normal ear.
ABR studies are also useful for detecting brainstem disorders like multiple sclerosis, parkinson's disease, brainstem strokes, or brainstem degenerative disorders. These conditions are less common then vestibular disorders, but discovering one of these conditions early can lead to more options, and better outcomes for the patient, so ABR can be quite useful when used as a screening study. An ABR can also be more cost effective for this purpose then a MRI. While an MRI provides additional details, if the physician only requires the condition of the auditory pathway, an ABR is quite cost effective.