Auditory Brainstem Response Audiometry- Method and Indications

Introduction:

The ear consists of the inner, middle, and outer ear. The auditory brainstem response test helps to determine how the cochlea (a snail-shaped organ in the inner ear), brain, and the auditory nerve (hearing nerve) are responding to the sounds. It is also known as brainstem auditory. Usually, the sound heard in the ears is converted from mechanical to electrical signals in the cochlea (a snail-shaped organ in the inner ear), which is further transmitted to the brain by the auditory nerves (the eighth cranial nerves). An auditory brainstem response audiometry test can detect any defects in the hearing pathway. Auditory brainstem response tests are very safe and painless procedures used to measure the thresholds of the nerve fibers in the hearing pathway.

How Is the Auditory Brainstem Response Test Done?

Auditory brainstem response (ABR) tests are safe and painless procedures done by audiologists (specialists in hearing and balance disorders). Audiologists attach an adhesive electrode to the scalp and ears.

Procedure to Attach Electrodes During the Test:

• Even-numbered electrodes are placed on the right side of the scalp, and odd-number electrodes are placed on the left side of the scalp.

• Positive electrodes are placed on the vertex (highest point on the head).

• Two electrodes are placed on the right and the left ear lobes.

• One ground electrode is placed on the lower side of the forehead.

The Procedure of Sound Induction in the Ears:

• A sound is produced by the earphones at 80 to 90 decibels normal hearing levels (dBnHL) for adult patients, and if the patient has hearing loss, it is reduced to 60 to 70 decibels sensational hearing.

• Sound stimulation is increased gradually by 10 decibels until the waveform is achieved.

• The normal ear is masked by white noise (soothing sound) at 30 to 40 decibels.

• The waveforms formed are then assessed for their morphology and latency (delay).

• The wave “V” latency should not be more than 0.4 milliseconds difference between the ears.

Interpretation of the Test Results:

Interpretation of the auditory brainstem response depends on the latency (delay), amplitude, and interwave intervals between waves one to five.

Wave Latency:

• The time gap between the initial auditory (hearing) response and the peak of the wave. It is altered depending on the age (longer in newborns and decreases with age) and stimulation intensity. Different anatomical points and latency (delay) periods for the generation of waves one to five are as follows:

Wave I: The wave is generated at the distal part (away from the center) of the auditory nerve (the eighth cranial nerve) and the latency period measures around 1.5 milliseconds.

Wave II: This wave is generated in the proximal part (towards the center) of the auditory nerve with a latency period of 2.5 milliseconds.

Wave III: Cochlear (part of the inner ear) nucleus and superior olivary complex (brainstem nucleus) is related to this wave having a latency period of 3.5 milliseconds.

Wave IV: It is related to the upper olivary complex (brainstem nucleus) and auditory nerve fibers. The latency period is 4.5 milliseconds.

Wave V: It is associated with the lateral lemniscus (nerve tract of the brainstem that transfers the sound from the cochlea nucleus to other parts of the brain). The latency period is around 5.5 milliseconds.

• Wave IV and wave V are affected due to lesions present on the brain stem.

Wave Amplitude: It measures the height between the wave peak and depression points of the wave (trough).

How is Peripheral and Central Hearing Loss Assessed?

Assessment of Peripheral Hearing Loss:

• Due to delayed latency of wave I.

• Absence of all waveforms.

• Indicates cochlear defects.

Assessment of Central Hearing Loss:

• The long duration between wave I and wave V.

• Indicates brainstem defects.

• Increased latency between wave I to wave III indicates hearing nerve defects.

• Increased latency between wave III to wave V indicates cochlear nerve (the eighth cranial nerve) defects like neuromas.

• The absence of waves IV and V indicates lesions affecting the pons (part of the brain).

What Are the Indications for the Auditory Brainstem Response Test?

Indications for auditory brainstem response test are as follows:

• For diagnosing hearing loss in newborns, children, and adults.

• To diagnose the effectiveness of artificial hearing aids, cochlear implants (an artificial device placed in the inner ear bones surgically), and bone-anchored hearing aids in patients in vestibular rehabilitation centers (rehabilitation centers for hearing and speech therapies for the patients).

• Early screening of hearing, speech, and language development in newborns and children.

• For early diagnosis of the auditory nerve (the eighth cranial nerve) lesions like acoustic neuromas (non-cancerous tumors compressing the nerve connecting the inner ear and the brain).

• For diagnosing and treating brainstem tumors while doing surgical procedures like intracranial cerebral-pontine angle tumor removal to preserve the auditory nerve (the eighth cranial nerve).

• For diagnosis of neurological (nerve) disorders like multiple sclerosis (a disease damaging the outer covering of the nerves).

What Is the Preparation Done Before Performing the Auditory Brainstem Response Test?

• Auditory brainstem auditory tests are done while sleeping for children younger than six months.

• This test is usually done only when the child is in a still position or sleeping with their eyes closed.

• Anesthesia or sedation is used to perform the test on children between six months to seven years old.

• For children above seven years of age, tests are performed in the hospital (audiology department).

• The patients undergoing auditory brainstem tests under anesthesia or sedation are advised not to drink and eat six hours before the test.

Conclusion:

Auditory brainstem response audiometry tests are one of the most important diagnostic tools used for detecting cancerous or non-cancerous tumors of the brain or hearing nerve tumors, severity or type of hearing loss, and defects related to the inner ear. It helps to detect any defect in the hearing pathway, which is the brainstem's response to the sound transferred from the cochlea (inner ear organ) to the brain through the hearing nerve. ABR (auditory brainstem response test) has served as a boon for the medical field for early diagnosis and screening of newborn babies with suspected hearing loss so that early intervention and management can be done to prevent any delay in speech and language development in the child.