brainstem auditory evoked potentials

Brainstem Auditory Evoked Responses (BAER, BAEP, BEP) are a type of sensory evoked response  generated by the brain  when transmitting and processing responses to heard (auditory) stimuli. Similar to other evoked responses these electrical signals are very small and normally obscured by random electrical activity in the environment. In order to measure them dozens of responses are collected sequentially and then averaged together. The random electrical signals tend to cancel each other out, leaving the evoked response to be seen and measured. Delays or reductions in these responses help define or locate any problem in the auditory pathway beginning at the ear and extending to the midbrain.

IOM Use of BAERs:

Cochlear recordings preceded BAER recordings in the OR due to their larger size and ease of recording. Intraoperative use of BAERs extends back to 1978 (Levine RA et al) and have become the more frequently used recording of the two for posterior fossa surgeries due to their broader coverage of the auditory pathway. Previous reviews suggest that at least for microvascular decompression surgery complication rates drop with increased surgical experience, with the exception of retrocochlear hearing loss, making BAER monitoring of ongoing utility.  Currently BAERs are used (in conjunction with other modalities) for  monitoring posterior fossa tumor removal (vestibular neuromas and extra-axial tumors) and in microvascular decompression for hemifacial spasm, tic douloureux along with some aneurysms and arteriovenous malformations.

Recording is done from scalp electrodes in two channels that can be in various arrangements (montages) but usually include Cz to Ai and Ac. The stimulus is delivered as 'clicks' to the ear via small tubes with surrounding foam collars at their ends that fit into the ear canal.

BAER Waveforms:

Traditional BAER responses consist of 10 waveforms recorded in the first 10-15 milliseconds after stimulation. Latency depends upon the presence of underlying pathology and can not be relied upon for waveform identification. Waveform V (five) is the most important in IONM as it is the most easily identified and most consistently present.

Upside:

• minimally effected by anesthetics and temperature change
• gives relatively broad coverage of the auditory pathway

Downside:

• may be affected by operating room environmental noise such as drills
• may be of little use in patients with severe hearing loss

Last Word on BAERs:

BAERs are a useful tool for intraoperative monitoring for surgeries involving the posterior fossa, especially those close to the structures of the auditory pathway, and have been shown to help in reduction of hearing loss in surgeries such as microvascular decompressive cases.

R. O'Brien MD

Selected References:

RA Radtke, CW Erwin and RH Wilkins, Intraoperative brainstem auditory evoked potentials: significant decrease in postoperative morbidity, Neurology, Vol 39, Issue 2 187-191

Chiappa KH. Brainstem auditory evoked potentials: methodology. In: Chiappa KH, editor. Evoked potentials in clinical medicine. 3rd ed. Philadelphia: Lippincott-Raven Publishers, 1997:157-97.

Levine RA et al. Evoked potential detection of hearing loss during acoustic neuroma surgery. Neurology 1978;28:339