Brainstem auditory evoked potentials (BAEPs) are also known as brainstem auditory evoked responses (BAERs) or auditory brainstem responses (ABRs). These physiological measures can be used to evaluate the auditory pathways from the ear to the upper brainstem (Picton 1990). In addition, ABR threshold testing may be used to determine behavioral threshold sensitivity in infants or uncooperative patients. The most consistent and reproducible potentials are a series of five submicrovolt waves that are seen within 10 msec of an auditory stimulus. These potentials are recorded by averaging 1,000 to 2,000 responses from click stimuli by use of a computer system and amplifying the response (Figure 16).

Figure 16. Brainstem auditory evoked potential (BAEP) in a normal adult. Responses were recorded between electrodes on the vertex and the ipsilateral mastoid. Waves I, III and V are labeled. ms/div = milliseconds per division, V/div = microvolts per division.

The anatomical correlates of the five reliable potentials have only been roughly approximated. Wave I of the BAEP is a manifestation of the action potentials of the VIII nerve and is generated in the distal portion of the nerve adjacent to the cochlea. Wave II may be generated by the VIII nerve or cochlear nuclei. Wave III is thought to be generated at the level of the superior olive, and waves IV and V are generated in the rostral pons or in the midbrain near the inferior colliculus. The complex anatomy of the central auditory pathway (Benjamin and Troost 1988), with multiple crossing of fibers from the level of the cochlear nuclei to the inferior colliculus, makes interpretation of central disturbances in the evoked responses difficult. Excellent reviews of the generation of the potential, and interpretation of abnormality, are found in recent contributions.

The brainstem auditory evoked potential (BAEP) is a sensitive, noninvasive diagnostic test for the diagnosis of cerebellopontine angle tumors (Picton 1990). This test is used to differentiate cochlear from VIII nerve hearing defects and, on some occasions, demonstrates an auditory abnormality when behavioral audiometric testing is still normal. The majority of patients with acoustic tumors had abnormal responses (Baloh and Honrubia 1990).

The absence of waves III and V has been seen in some patients with vestibular schwannoma and in cerebellopontine angle meningiomas. Such patients often have marked hearing deficits with poor discrimination on behavioral testing, suggesting retrocochlear disease. The absence of all waves should not occur unless a severe hearing loss exists. The most specific evoked potential abnormality is the presence of an increase in interwave intervals. Abnormal interwave latencies (I-III or I-V) are the most specific and sensitive abnormalities seen with cerebellopontine angle tumors. The abnormal prolongation or absence of wave V at increased click rates is also characteristic of retrocochlear pathology. Increased absolute latencies of all waves, when compared to responses from the other ear or to clinical normative data may signify a conductive deficit.

Electrocochleography

Electrocochleography (ECochG) is a method of recording the stimulus-related electrical potentials associated with the inner ear and auditory nerve, including the cochlear microphonic, summating potential (SP) and the compound action potential (AP) of the auditory nerve. This measure is beneficial in the differential diagnosis of certain types of sensory disorders, such as Ménière's disease or cochlear hydrops. The amplitude of the SP and AP is measured and is of primary interest when evaluating an ear for increased endolymphatic pressure.