Normal hearing

• A toddler is brought to the GP with a 2 day history of earache (tugging on ear), fever and reduced hearing (less responsive to parents). What is the diagnosis? What type of hearing loss?

  Otitis media, conductive
• A 50 year old female presents with left ear pain, itchiness and discharge. She has noticed some left sided hearing loss. She is a regular swimmer. What is the diagnosis? What type of hearing loss?

  Otitis externa Conductive
• A 30 year old woman presents with bilateral hearing loss which has progressed. She sometimes has ringing in her ears. What is your diagnosis? What type of hearing loss?

  Otosclerosis, conductive
• Sound waves travel through the ear canal, are conducted through the middle ear and converted to an electrical signal in the inner ear.
• A 45 year old male presents with unilateral hearing loss, tinnitus and vertigo. What is the abnormality on his MRI? What type of hearing loss?

  Acoustic neuroma, sensorineural

  
• Muscles in middle ear

  • stapedius
  • tensor tympani
• Nerves found in middle ear

  • facial
  • chorda tympani
• Fluid spaces in cochlea

  • scala vestibuli
  • scala tympani
  • scala media (cochlear duct)
• The scala vestibuli and scala tympani contain perilymph (ECF)
• The scala media contains endolymph with very high K+
• The tips of hair cells are bathed in endolymph in scala media
• Ear bones

  A) malleus

  B) incus

  C) stapes

  
• Normal hearing mechanism

  • waves of vibration in fluid
  • membranes move
  • hair cells bend
  • movement of ions
  • action potential
• • apex of hair cells bathed in endolymph (high K+)
  • K+ passing through apical channels goes into hair cells, then leaves via basal membrane and is recycled to endolymph via stria vascularis
  • inner hair cells transduce mechanical signal (vibration) into electrical one via opening of mechanically-gated K+ channels
  • outer hair cells act to amplify the vibration by adjusting their length to modify the basilar membrane (positive feedback)
• Sound recognition

  • vibration of the oval window generates a fluid wave down the scala vestibuli then back through the scala tympani to the round window
  • the wave is conducted to the scala media, leading to a displacment of the basilar membrane
• Pitch is determined by the frequency of the signal - these are detected at different positions along the cochlea

  • higher frequencies closer to oval window
  • lower frequencies nearer the helicotrema
• Loudness is identified by the amplitude of deflection at a given part of the cochlea.
• Identification of pitch - tonotopy

  • width of basilar membrane increases towards apex
  • therefore location on membrane has a different resonant frequency
• volume depends on amount of pressure exerted by sound waves on the tympanic membrane
• Decibel = ratio between two sounds. The scale is logarithmic
• dB difference = 10 log10 (sound 1/sound 2)
• Mechanism of damage from noise

  • damage to hair cells/cilia
  • inflammation
  • excitotoxicity
• Conductive hearing loss - outer/middle ear e.g. infection, wax, fluid
• Sensorineural hearing loss - inner ear/brain e.g. damage to hair cells, nerves
• Loss of higher frequencies is a normal part of ageing
• A 50 year old male presents with a painful rash on the left ear and with a facial droop and hearing loss on the left side. What is the diagnosis? What type of hearing loss?

  Herpes zoster affecting facial and vestibulocochlear nerve, sensorineural