Audiology

 Hearing loss : 2 types : i) conductive hearing loss (pinna to footplate)

                                         ii) sensory hearing loss 

  Conductive hearing loss causes : 

                                                    Wax 

                                                    CSOM 

                                                    Glue Ear 

                                                    Otosclerosis 

Sensory Neural Hearing Loss : 2 types : i) Cochlear hearing loss Ex. Meniere's disease 

                                                               ii) Retrocochlear hearing loss Ex. Acoustic neuroma 

 We can hear sound in 2 ways : Air conduction and Bone conduction 

 Bone conduction : directly stimulate cochlea without using conductive pathway , so bone conduction checks cochlea only . ( Bone conduction is poor in SNHL only) 

  Tuning Fork Test : 512 Hz is used because of good sound and less vibration . 

False Negative Rinne : U/L severe SNHL d/t transcranial transmission of sound to other side of cochlea when BC is being checked on the diseased side . It is corrected by masking the other ear . 

 AUDIOMETRY (=  AUDIOGRAM= Pure Tone Audiometry ) :

Normal Hearing : All frequencies within 0-10 dB can be hearing .

 Conductive Hearing Loss : BC Normal AC Poor 

 SNHL : Both BC and AC are poor 

 NOTE : In audiogram upper line is always of bone conduction . 

             500, 1000,2000 Hz are most important human speech frequencies 

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  How to differentiate between cochlear and retrocochlear hearing loss ? 

 1) Tone decay test : Normal tone decay time is 1 min . In retrocochlear hearing loss Tone Decay is within 10 sec . 

 2) Recruitment   : Abnormal perception of loudness . Seen in cochlear hearing loss . 

 3) SISI : Short Increment Sensory Index : Normal SISI score 50 % . In cochlear hearing loss SISI score is better than normal (70%)

 4) Speech Audiometry : This test checks the understanding capability for the heard words. Score is out of 10 words and it is called Discrimination Score . 

      In retrocochlear hearing loss the discrimination score is very poor i.e 40% and roll over phenomenon is positive . On increasing the intensity of sound the discrimination even falls further . 

    BERA  ( Brainstem Evoked Response Audiometry ) 

      #  Objective test of hearing 

      # Principle : we give sound to the ear and we record the electrical activity from auditory pathway . 

     Audiometry Pathway :   ECOLI : E - Eighth Nerve C- Cochlear nucleus O-Olivary complex superior L- Lateral meniscus I - Inferior colliculus

Note : Wave 5 is the most important wave in BERA . BERA is the best diagnostic test for hearing loss . 

  Otoacoustic emission (OAE) : These are cochlear echoes . Healthy cochlea generate echoes . It is produced by outer hair cells of cochlea . OAE is the ideal hearing screening investigation . BERA is the diagnostic test .