Anatomy of the Inner Ear

Timothy C. Hain, MD

While there are several other surveys of inner ear anatomy, we have diffidently here set out to put the content in our lecture on this subject on the web.

Overview of peripheral anatomy

• Mechanical sensors (canals and otoliths) respond to angular and linear movement
• Cochleo-Vestibular nerve
• Spiral and Scarpa’s ganglion
• Brainstem nuclei

Purposes of the auditory-vestibular system

VOR (vestibulospinal Reflex)

• Stabilizes eye in space
• Necessary to see while head is in motion
• Disorders include
  • Bilateral loss
    • Test for bilateral loss with the DIE (Dynamic Illegible 'E' test)
      • Distance vision with head still
      • Distance vision with head moving (horizontal or vertical, 1-2 hz)
      • Normal: 0-2 lines change.
      • Abnormal: 4-7 lines drop with movement
    • Other tests of lesser utility
      • OCR
      • Head-thrust
      • Ophthalmoscope test
      • Head-shaking test
  • Unilateral loss or imbalance
    • Test for unilateral loss at bedside with head-shaking nystagmus or vibration
      • Head-shaking is about 75% sensitive to unilateral loss
      • Vibration is about 95% sensitive to unilateral loss
  • Test for imbalance by assessing spontaneous nystagmus
    • This test works best acutely (i.e. in the first few days)
  • VEMP test for unilateral saccule loss (research test)
  • Cross-coupling
  • Manifests as positional nystagmus
  • Test with positional testing such as Dix-Hallpike test

VSR (Vestibulo-spinal reflex)

• Stabilizes body
• Helps maintain desired orientation to environment
• Test for VSR using Romberg test
  • Eyes Closed Tandem Romberg (6 seconds)
  • Eyes Open Tandem Romberg
  • Eyes Closed Regular Romberg Eyes
  • Open Regular Romberg
• Other tests of uncertain utility
  • Quix test
  • Fukuda tests

 

Going from through ear structures from the periphery to the central structures

External Ear

• EAC
• Auricle

Middle Ear

• Tympanic Membrane
  • TM perforation
  • Tympanosclerosis
• Ossicles
  • Ossicular chain discontinuity
  • Otosclerosis
• Stapes footplate and round window
  • Otosclerosis
• The mastoid sinus
  • Mastoiditis
• The Eustachian Tube
  • ETD

Inner Ear

• Cochlea
  • Disorders
    • Sensorineural hearing loss
      • Hair cell loss or damage
        • Noise trauma
        • Age -- Presbyacusis
        • Toxins -- Vancomycin toxicity
        • Genetic -- many
        • Viruses -- labyrinthitis
        • Vascular: AICA syndrome
• Vestibular labyrinth
  • Three semicircular canals
  • Disorders
    • Trauma
    • Age -- vestibular atelectasis
    • Toxins -- gentamicin toxicity
    • Genetic -- Mondini malformation
    • Viruses -- Vestibular neuritis
    • Vascular: AICA syndrome
• Vestibule
  • Otolith organs
    • Utricle
    • Saccule
    • Disorders -- little is known
      • Age ?
      • Genetic ?
      • Trauma ?
  • Cochlear aqueduct
  • Vestibular aqueduct

The 8th nerve and related structures

• Vestibular portion
• Cochlear portion
• Nervus Intermedius
• Labyrinthine artery
• Internal auditory Canal
• Common Disorders
  • Acoustic neuroma
  • Microvascular compression

Brainstem structures

• Cochlear nucleus
• Vestibular nuclei
• Commisures
• Cerebellar connections
• Disorders
  • Multiple sclerosis
  • Stroke
    • AICA syndrome
    • PICA syndrome (Wallenberg's syndrome)

Ascending connections

• Trapezoid body
  • Because of cross-connections, central hearing loss is unusual and subtle
• Inferior colliculus
  • Little is known about effects of lesions
• Medial Geniculate
  • Little is known about lesions

Auditory and vestibular cortex