Special Hearing Tests

Electrodes can be kept over  the promontory,EAC or outside,the signals are better as much it is kept close to source.

Different waves  are cochlear microphonic:produced  from outer hair cells action potential is produced by all the nerve fibers Used in diagnosis and monitoring the meiners disease and endolymphatic hydrops

Otoacoustic Emissions (OAEs)

They are low intensity sounds produced by outer hair cells of a normal cochlea and can be elicited by a very sensitive microphone placed in the external ear canal and an analysis by a computer. Sound produced by outer hair cells travels in a reverse direction:

Outer hair cells→basilar membrane→ perilymph→ oval window→ ossicles→ tympanic membrane→ ear canal.

Otoacoustic emissions (OAEs) are objective, noninvasive, and rapid measures (typically less than 2 minutes) used to determine cochlear outer hair cell function. 

Brainstem evoked response audiometry is the investigation used to detect acoustic neuromas or determine hearing thresholds in children.

In Brainstem Evoked Response Audiometry electrodes are placed over various areas of the skull and sounds of various frequencies are projected to the ear with a headphone. The potentials generated by the brain in response to the sound are recorded and studied. 

Electrocochleography is a near-field technique that monitors the most peripheral components of the auditory system. It measures electrical potentials arising in the cochlea and auditory nerve in response to auditory stimuli within first 5 milliseconds.

Auditory stimuli are delivered to the cochlea commonly in the form of a wideband click, composed of many frequencies that stimulate the entire cochlea. Cochlear potentials that can be recorded from humans include the cochlear microphonic potentials, the summating potential(SP) and the compound action potential(AP) of the auditory nerve. The SP and AP are the most important components of electrocochleography.

Otoacoustic emissions are low intensity sounds produced by outer hair cells of a normal cochlea and can be elicited by a very sensitive microphone placed in the external ear canal and an analysis by a computer.

Most screening programmes for newborn hearing ability uses otoacoustic emission as the initial test. It this test fails then auditory brainstem response is used for screening.

Otoacoustic emissions are low level sound emitted spontaneously by the cochlea on presentation of an auditory stimulus. Persons with normal hearing produce oatoacoustic emissions whereas those who have hearing loss of 30-40db HL or greater do not produce OAE.

Auditory brainstem response testing is used with babies between the ages of birth and 5 months. It is the electrophysiological response to an acoustic stimulus and originates from the eight cranial nerve and auditory brain stem. It can be used to determine the degree of hearing loss at different audiometric frequencies. 

Ans. is ‘a’ i.e., Before doing exchange transfusion for hyperbilirubinemia, ‘b’ In premature babies,`c’ i.e., Before starting aminoglycoside therapy

o All neonates with risk factor for hearing loss must be screened with an auditory brainsteam response (ABR) or otoacoustic emissions (OAE) test to exclude hearing impairment.

Indications for Auditory screening ‑

Family history or hereditary childhood SNHL.

1. In utero infections – Toxoplasmosis, Rubella, CMV, herpes, syphilis.
2. Craniofacial anomaly-morphological abnormalities of the pinna and ear canal.
3. Birth weight less than 1500 gm.
4. Hyperbilirubinemia at a serum level requiring exchange transfusion.
5. Ototoxic drugs aminoglycosides, loop diuretics
6. Bacterial meningitis
7. Apgar score 0 to 4 at 1 min or 0 to 6 at 5 min.
8. Mechanical ventilation lasting more than 3 days.

Stigmata associated with a syndrome known to include hearing loss.

BERA is very useful in distinguishing between cochlear pathology and retrocochlear pathology for SNHL

Impedence audiometry and PTA tests the middle ear pathology.

Electrocochleography is diagnostic in Meniere’s disease. Ratio of Summating Potential (SP) to Action Potential (AP) is greater then 30%.

The screening investigation of choice of high-risk neonates in ICU for suspected hearing loss is otoacoustic emissions. 

Ans. is ‘a’ i.e., 1st month of life

The American Academy of Pediatrics (AAP), Joint Committee on Infant hearing (2007), has recommended that all newborn infants be screened for hearing impairment either as neonate or before 1 month of age and that those infants who fail newborn screening have an audiologic examination to varify hearing loss before age of 3 months.