Auditory Brainstem Response Audiometry (ABR)

With what populations is ABR used?

is primarily used to evaluate hearing in "difficult-to-test" populations where a voluntary or reliable behavioral response cannot be obtained.

These include:

• Infants who lack the motor control or cognitive ability to perform tasks like raising a hand or dropping a block in response to sound.

• Individuals with severe developmental delays or cognitive limitations.

• Malingerers—individuals suspected of faking a hearing loss for monetary or legal gain.

• Patients suspected of having a vestibular schwannoma (tumor on the VIIIth nerve) or other site-of-lesion pathologies.

What is an ABR?

is a test of neural response to sound that requires nerve fibers and brainstem cells to fire synchronously. It is part of a larger family of Auditory Evoked Potentials (AEPs), which represent electrical activity stimulated by acoustic signals from the auditory nerve to the cortex

What is the timing of evoked potentials?

AEPs are categorized by when they occur after a stimulus: early (0–10ms), middle (10–50ms), and late (50–400+ms)

What is ABR specifically concerned with the early responses?

occurring within the first 10ms following the onset of the stimulus

What is a common electrode montage?

Electrodes are secured to the head in an arrangement to pick up electrical signals

What are the typical set up of a common electrode montage?

• Top of the head (Vertex/Cz): Used in conjunction with the ear electrodes to measure responses to sound.

• Earlobes or Mastoid (A1/A2): Placed on the stimulus ear.

• Forehead (Fpz): Serves as a ground electrode to ensure environmental electrical signals do not interfere with the recording.

• Preparation: Electrodes must be attached using conduction paste to reduce skin resistance (impedance) and guarantee a clear signal

Top of the head (Vertex/Cz)

Used in conjunction with the ear electrodes to measure responses to sound

Earlobes or Mastoid (A1/A2)

Placed on the stimulus ear

Forehead (Fpz)

Serves as a ground electrode to ensure environmental electrical signals do not interfere with the recording

Preparation of the common electrode montage

Electrodes must be attached using conduction paste to reduce skin resistance (impedance) and guarantee a clear signal

What are the typical stimuli used in ABRs?

Sounds are delivered via earphones, insert phones, or bone oscillators. Primary types of stimuli include: CLICKS & FREQUENCY SPECIFIC STIMULI

Clicks

Historically the standard; they are very fast, high-frequency signals that trigger a strong response from many neurons.

Frequency Specific Stimuli

Known as tone bursts, tone pips, or chirps, these consist of a few cycles of a pure tone and allow for ABR testing at specific individual frequencies

What is the relationship between an EEG and an ABR?

The ABR was developed following the discovery of Electroencephalography (EEG), which is a technique for recording ongoing electrical activity from different parts of the brain.

• The ABR is actually an extraction of stimulus-related events from the ongoing background EEG

• Because these tiny events are buried within background noise (such as muscle movement or building electricity), the recording equipment must filter out the noise and amplify the signal so it can be measured

Be familiar with ABR waveforms.

An ABR consists of seven waves labeled with Roman numerals (I–VII) that occur within the first 10ms. The first five waves are the most diagnostically significant

Waves I and II

Arise from the VIIIth (auditory) nerve

Wave III

Generated in the area of the cochlear nucleus and Superior Olivary Complex (SOC)

Wave IV

Arises from multiple brainstem sites

Wave V

Generally the most prominent peak, arising from the lateral lemniscus and inferior colliculus

What are some characteristics of ABRs for CONDUCTIVE HEARING LOSS?

Wave I latency is markedly delayed, but interwave latencies (the time between waves) remain normal

What are some characteristics of ABRs for SENSORINEURAL HEARING LOSS?

Often presents with poor morphology; Wave I may be small or absent, and Wave V is typically delayed .

What are some characteristics of ABRs for NEURAL/BRAINSTEM PATHOLOGY?

characterized by delayed interwave latencies (e.g., I–III or I–V) or, in some cases, no response present after Wave I .

What is a Latency/Intensity Function and what does it tell us?

The Latency/Intensity Function involves recording the ABR at progressively lower decibel levels .

• As the intensity of the sound decreases, the latency of Wave V increases (the peak occurs later in time) .

• This function is used to determine the hearing threshold, which is defined as the lowest intensity level where Wave V can still be identified.

As the intensity of the sound decreases…

the latency of Wave V increases (the peak occurs later in time) .