Special Hearing Tests

Special Tests of Hearing
Recruitment
  • 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
Short Increment Sensitivity Index (SISI)
  • 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
Evoked response audiometry
  • Electrocochleography (EcoG)
  • Auditory brain stem response (ABR) or Brainstem evoked response audiometry (BERA)
EcoG
  • Useful to find fine threshold of hearing in young infants and children within 5-10 dB
  • To differentiate cochlear lesions from VIII nerve lesions
BERA/ABR
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 I distal part of 8th nerve
? 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
Auditory Steady State Response (ASSR)
  •  ABR cannot test hearing losses above 80 dB and in infants with severe to profound deafness
  •  ASSR is useful in such situations
OTOACOUSTIC EMISSIONS
  •  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
  •  Uses: 
  1. screening test for hearing in neonates and uncooperative, mentally challenged after sedation
  2. distinguish cochlear from retrocochlear lesions
  3. used to diagnose retrocochlear lesions especially auditory neuropathy
OAEs absent in
  •  50% normal individuals
  •  Cochlear lesions
  •  Middle ear disorder
  •  Hearing loss >30 dB
Tests to detect psychogenic HL or malingering or Non organic hearing loss:
  •  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
Exam Question
  • 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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