12. Fluid systems of the labyrinth, Meniere’s disease, toxic lesions of the inner ear

Fluid systems of the labyrinth
- Membranous labyrinth
  - Filled with K-rich endolymph
  - Contains hair cells
  - Divided into vestibule and cochlear duct
- Bony labyrinth
  - Contains membranous labyrinth in Na-rich perilymph
  - Three cavities
    - Scala media (cochlear duct)
      - Filled with endolymph
      - Basilar membrane forms the floor of it
    - Scala vestibuli
      - Filled with perilymph
      - Above and separated from scala media
      - Connected to scala tympani at helicotrema
    - Scala tympani
      - Filled with perilymph
      - Below and separated from scala media
      - Runs downward from helicotrema to the round window
- Oval window vibrates -> sound waves transmitted in perilymph in scala vestibuli
  - -> sound waves vibrate the vestibular membrane and basilar membrane, thus stimulating hair cells
  - -> sound waves travel through helicotrema -> down the scala tympani -> to the round window
- Low frequency waves act at apex of cochlea
- High frequency waves act at base of cochlea
Toxic lesions of the inner ear
- Etiology
  - Endogenous
    - Metabolic diseases
      - Diabetes mellitus
      - Uraemia
    - Bacterial toxins
  - Exogenous
    - Aminoglycosides
    - Loop diuretics
    - Cytostatic drugs
    - Salicylates
    - Industrial solvents
    - Heavy metals
    - Alcohol
    - Illegal drugs
- Serous labyrinthitis
  - Bacterial toxins from chronic otitis media

Meniere disease (idiopathic endolymphatic hydrops)
- Impaired resorption of endolymph causes accumulation
- Epidemiology
  - Females
  - Older adults
- Clinical features
  - Meniere triad
    - Episodes lasting from minutes to hours
    - 3 yes 1 no
    - Yes: Sensorineural HL
    - Yes: Repeated attacks of vertigo
    - Yes: Tinnitus
    - No: No neurological signs
- Diagnosis
  - Criteria
    - >1 episode that lasts 20 minutes to 12 hours
    - Low-mid frequency SN hearing loss
    - Tinnitus
- Treatment
  - Avoid triggers (stress, alcohol, caffeine)
  - Low sodium diet
  - Rehabilitation
  - Bed rest
  - Drugs
    - First generation antihistamines
    - Histamine analogues
    - Gentamycin in ear – destroy vestibule
  - Surgery
    - Labyrinthectomy
    - Sacculotomy
    - Vestibular neurectomy