Special Tests of Hearing
- The ear which does not hear low intensity sound begins to hear greater intensity sounds louder than normal hearing ear
- Patients with recruitment are poor candidates for hearing aids
- Typically seen in lesions of cochlea – Meniere's disease and Presbycusis
- Patients with cochlear lesions distinguish smaller changes in intensity of pure tone better than normal persons and those with conductive or retro-cochlear pathology Thus used to differentiate cochlear and retro-cochlear lesions
Threshold tone decay
- Measure of nerve fatigue and used to detect retrocochlear lesions
- Electrocochleography (EcoG)
- Auditory brain stem response (ABR) or Brainstem evoked response audiometry (BERA)
- Useful to find fine threshold of hearing in young infants and children within 5-10 dB
- To differentiate cochlear lesions from VIII nerve lesions
AUDITORY BRAINSTEM RESPONSE
- The ABR is a surface-recorded averaged response representing the activity of the distal portion of the auditory pathway.
- As a rule, five to seven peaks occurring within a time frame of less than 10 ms make up the ABR. For neurodiagnostic purposes, the first five positive polarity peaks (waves I through V) are typically considered.
- The ABR may be recorded with standard or disposable surface electrodes placed high on the forehead below the hairline or at the vertex (noninverting electrode); on the medial surface of the ipsilateral earlobe; or on the medial surface of the contralateral earlobe (ipsilateral and contralateral inverting electrodes) and on the center of the forehead (ground electrode).
- These electrodes may be used for a typical two-channel montage with the ipsilaterally referenced channel emphasizing wave I (synonymous to the N1 of the electrocochleogram) and the contralaterally referenced channel emphasizing the separation between waves IV and V.
- Waves I and II of the ABR reflect the activation of the distal and proximal segments of the cochlear nerve, respectively.
- Waves III and IV reflect the activation of the cochlear nucleus complex and the superior olivary complex.
- Wave V is associated primarily with the activation of the lateral lemniscus and not the inferior colliculus as was previously considered.
- In normal persons 7 waves are produced in the first 10 ms
- The first, third and fifth waves are most stable and used in measurements
- Anatomical site from where the waves arise
? Wave II proximal part of 8th nerve
? Wave III cochlear nucleus
? Wave IV superior olivary complex
? Wave V lateral lemniscus
? Waves VI & VII inferior colliculus
Uses of BERA
- Screening of infants
- To determine threshold of hearing in infants, children; adults and malingerers who do not cooperate
- To diagnose retrocochlear & brainstem pathology
- To monitor VIII cranial nerve intraoperatively during surgery of acoustic neuroma
- ABR cannot test hearing losses above 80 dB and in infants with severe to profound deafness
- ASSR is useful in such situations
- OAEs are low intensity sounds produced by outer hair cells of a normal cochlea.
- Do not disappear in 8th nerve damage as cochlear cells are normal
- screening test for hearing in neonates and uncooperative, mentally challenged after sedation
- distinguish cochlear from retrocochlear lesions
- used to diagnose retrocochlear lesions especially auditory neuropathy
- 50% normal individuals
- Cochlear lesions
- Middle ear disorder
- Hearing loss >30 dB
- PTA and speech audiometry - inconsistent results on repeat. Normally the results are within ±5DB. Variation greater than +5dB is diagnostic of NOHL
- Absence of shadow curve while testing bone conduction when the healthy ear is not masked
- Inconsistence in PTA and SRT
- Stenger's test
- Acoustic reflex threshold
- Electric response audiometry
- In Electrocochleography,action potentional is produced by outer hair cells.
- Electrocochleography is mostly used in monitoring the Meniers disease.
- Electrocochleography is the gold standard investigation for diagnosis of Meniers disease.
- In Electrocochleograpgy,the electrode must be kept close to source of amplitude.
- Otoacoustic emissions arise from Outer hair cells.
- Recruitment phenomenon is seen in Presbyacusis.
- Otoacoustic Emissions is the best screening test to evaluate hearing in a neonate.
- Brainstem Evoked Response Audiometry(BERA)
- Electrocochleography is a method of recording auditory evoked response taken from cochlea and auditory nerve is the investigation of choice in assessing hearing loss in neonates.
- Auditory screening is required in children under following conditions:Before doing exchage transfusion for hyperbilirubinemia, In premature babies , Before starting
- aminoglycoside therapy
- To distinguish between cochlear and post cochlear damage, test done is BERA.
- In Electrocochleography,Outer hair cells are mainly responsible for cochlear microphonics and summation potential.
- Recruitment test is positive in Meniere's disease.
- Best time for hearing assessment in an infant is 1st month of life
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