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Physiology of Hearing

  1. Role of outer ear : Sound localization
  2. Role of the middle ear
  • To match the impedance of the air to the much higher impedance of the cochlear fluids
  • Serves to apply sound preferentially to only one window of the cochlea [Round window baffle effect]
  • P.S : This efficient impedance transformer changes the low-pressure, high-displacement vibration into high- pressure, low displacement vibration. (Total transformer ratio = 18:1) (Areal ratio x lever ratio)
Area of the tympanic membrane
Areal ratio   =  Area of the foot plate
Effective vibratory area of TM – 45 mm2  =  14:1 Foot plate area = 3.2 mm2
Force on malleus  =  1.3 /1                      
Lever ratio  =  Force on the stapes

MCQ. Axis of ossicular rotation passes between anterior process of malleus to short process of incus.
MCQ. Lever ratio between the handle of malleus and the long process of incus is 1.3: 1.
MCQ. Malleus and incus are derived from the first arch. Stapes develops from second arch except its footplate and annular ligament which are derived from the otic capsule.

Mechanisms of bone conduction
  1. Compressional / Distortional bone conduction: (high frequency sounds)
    Vibratory energy of the sound ⇒ Causes alternate compression and expansion of the cochlear shell (due to flexibility of the round window membrane cochlear aqueduct)
    Cause movement of the cochlear fluids
  2. Inertial bone conduction
    Vibratory energy of the sound ⇒ Strikes the skull and makes it Vibrate: ⇒ ossicles because of their inertia
    Cochlear fluid sets in motion ⇒ Relative movement at the stapes Footplate   Ü do not vibrate
  3. Osteotympanic bone conduction

Hearing Loss (WHO Classification)
Extra Edge. Noise-
induced hearing loss shows a dip at 4000 Hz in air conduction curve of audiogram.
Normal 0-25 dB
Mild loss 26-40
Moderate 41-55
Moderate to severe 56-70
Severe 71-91
Profound >91

A. Complete obstruction of EAC 30dB
B. TM perforation   10-40 dB
C. Ossicular Interruption (with intact drum —54dB; with perforation —38dB) 

Table: Congenital causes of conductive hearing loss                                    
  1. Meatal atresia                           
  2. Fixation of stapes footplate
  3. Fixation of malleus head         
  4. Ossicular discontinuity  
  5. Congenital cholesteatoma
Table :  Acquired causes of conductive hearing loss
External ear: Any obstruction in the ear canal, e.g. wax, foreign body, furuncle, acute inflammatory swelling, benign or malignant tumour or atresia of canal.

Middle ear     
  1. Perforation of tympanic membrane, traumatic or infective
  2. Fluid in the middle ear, e.g. acute otitis media, serous otitis media or    hemotympanum
  3. Mass in middle ear, e.g. benign or malignant tumour
  4. Disruption of ossicles, e.g. trauma to ossicular chain, chronic suppurative otitis media,
  5. cholesteatoma
  6. Fixation of ossicles, e.g. otosclerosis, tympanosclerosis, adhesive otitis media
  7. Eustachian tube blockage, e.g. retracted tympanic membrane, serous otitis mediA.

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