ABR

Overview of ABR

The Auditory Brainstem Response (ABR, also Brainstem Auditory Evoked Potential, BAER) is a series of electrical responses recorded from the auditory pathway between the ear and the upper brainstem following presentation of sound to the ear. For example, auditory click stimuli are delivered to the ears through expandable foam ear buds inserted into the external auditory canal, and responses are recorded from electrodes placed near the ear/mastoid.

Surface electrodes placed on the skull allow recording of 5 main waves of electrical activity through the brainstem.

Surface electrodes placed on the skull allow recording of 5 main waves of electrical activity through the brainstem.

The ABR is lateralized and consists of a waveform with five short-latency peaks, marked with Roman Numberals I-V, which reflect neuronal activity at various points along the ascending auditory pathway. The neural generators for the peaks are a little controversial, but generally considered to be I) distal auditory nerve, II) proximal auditory nerve, III) cochlear nucleus, IV) superior olivary complex, and V) lateral lemniscus or inferior colliculus.

Schematic of auditory neural pathway. The ABR is initiated by stimulation of the cochlea with a broadband click stimulus given via an ear insert in the external auditory canal. Neural generators of the ABR peaks are shown.

Schematic of auditory neural pathway. The ABR is initiated by stimulation of the cochlea with a broadband click stimulus given via an ear insert in the external auditory canal. Neural generators of the ABR peaks are shown.

Interpretation:

Changes in the ABR may be indicative of evolving injury somewhere along the auditory pathway between cochlea and the inferior colliculus. In order to detect changes in ABRs and localize the evolving injury, the responses must be quantified.  Thus, when we record ABRs, we look at the following variables:

Amplitude:

Each wave of the ABR is measured from peak to trough. Baseline ABR amplitude varies by stimulus presentation, recording location and patient (individual variability, pathology, etc). Once recorded, the response is rather stable. Neural transmission is altered by low temperatures, nerve compression/stretch injury, vascular changes, etc.  So, decreases in amplitude, can be indicative of evolving injury.

Absolute Latency:

Nerve conduction times are fairly well established for the ABR, but they differ by age. Given when we know about neural generators, we can expect to see waves/peaks at a given time after presentation of click stimulation to the ear. Conduction time is altered by low temperatures, nerve compression/stretch injury, vascular changes, etc.  So, increases in latency, can be indicative of evolving injury. Normative data for adults are presented in the table below.

Absolute Latency (msec)
Wave Mean SD
I 1.7 0.15
II 2.8 0.17
III 3.9 0.19
IV 5.1 0.24
V 5.7 0.25

Inter-peak Latency:

We also have a good idea of how long it takes a neural signal to get from point A to point B in the auditory pathway. Changes in inter-peak latency help to localize changes in neural conduction. Normative data for adults are presented in the table below.

Inter-Peak Latency (msec)
Wave Mean SD
I-III 2.1 0.15
I-V 4.0 0.23
III-V 1.2 0.16

ABR in IONM

The ABR is ultimately an average of all responses recorded after many successive stimulus presentations.  The number of presentations required to record a well-formed, reliable ABR ranges from hundreds to thousands. This is because the ABR is a relatively small response recorded in an electrically-noisy environment.  Because many trials are required to record the ABR, it can take seconds to minutes to record a reliable response. Thus, the response is delayed and not a real-time indicator of neural function.

How we view it:

Each peak is marked with a Roman Numeral, and each trough is marked with a prime (I’, III’, V’). Then we measure and label the amplitude (μV), absolute latency (msec) and inter-peak latency (msec).

Normal ABR. Taken from http://www.operativemonitoring.com/ep.htm

Normal ABR. Taken from http://www.operativemonitoring.com/ep.htm

Why we use it:

I can’t really talk about all of the different changes that you might see in the ABR, because it would consume a lot of space, and there are some really good articles already written on the topic (see references).

1.  Monitoring the Auditory Nerve (aka VIIIth Cranial Nerve, aka Vestibulocochlear Nerve):

Compression or stretch of the auditory nerve, and reduced blood flow to the cochlea, can result in changes in amplitude, absolute latency and interpeak latency.

2. Monitoring Global Brainstem Function/Perfusion:

Direct injury to the brainstem, or reduced blood flow to the brainstem, can result in changes in amplitude, absolute latency and interpeak latency.

Benefits of ABRs in Surgery:

  • Relatively noninvasive.
  • Consistent standards and protocols for use.
  • Fairly well-established criteria for alert.
  • Minimally effected by anesthetics.
  • Can be used with paralytics.
  • Broad coverage of the auditory pathway through the brainstem.

Limitations of ABRs in Surgery:

  • Delayed results due to averaging (seconds to minutes).
  • Affected by severe hearing loss, may not be recorded.
  • May remain unchanged in the face of evolving long-track motor/sensory deficit.
  • Cannot tell you about the function of other cranial nerves and their nuclei.
  • Susceptible to contamination from electrical and mechanical noise.

Surgical Procedures for which ABRs are Indicated:

Spine (extradural, decompression/fusion):

  • High Posterior Cervical (risk to vertebral artery).

Spinal Cord (intradural):

  • High Cervical Extramedullary tumor.
  • High Cervical Intramedullary tumor, syrinx.

Intracranial (infratentorial):

  • Brain stem (CPA, acoustic neuroma, cranial nerves, etc).

Intracranial Vascular:

  • Brainstem aneurysm clipping.
  • Brainstem repair of arteriovenous malformation (AVM).
  • Brainstem aneurysm coiling (interventional radiology; IR).

Diagnostics (non-surgical):

  • Clinical Audiology

References:

  1. Amano M, Kohno M, Nagata O, Taniguchi M, Sora S, Sato H. Intraoperative continuous monitoring of evoked facial nerve electromyograms in acoustic neuroma surgery. Acta Neurochir (Wien). 2011;153(5): 1059–67.
  2. Bess FH, Humes LE. Audiology: the fundamentals. 4th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2008.
  3. Legatt AD. Mechanisms of intraoperative brainstem auditory evoked potential changes. J Clin Neurophysiol. 2002;19(5):396–408.
  4. Legatt AD, Arezzo JC, Vaughan HG. Short-latency auditory evoked potentials in the monkey. II. Intracranial generators. Electroencephalogr Clin Neurophysiol. 1986;64(1):53–73.
  5. Legatt AD, Arezzo JC, Vaughan HG. The anatomic and physiologic bases of brain stem auditory evoked potentials. Neurol Clin. 1988;6(4):681–704.
  6. Legatt ADAJ, Vaughan HG. Short-latency auditory evoked potentials in the monkey. I Wave shape and surface topography. Electroencephalogr Clin Neurophysiol. 1986;64(1):41–52.

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