Pathophysiology of Vestibular System Disorders

• The majority of people had dizziness due to cardiovascular disease including stroke which could involve the central vestibular system followed by peripheral vestibular disease.
• Differential diagnosis


  Key in identifying the causes of dysfunction in people who are dizzy or have vertigo
• As many as 35% of adults aged 40 years or older, have experienced some form of vestibular dysfunction with up to 65% of individuals older than 60 years of age experiencing dizziness or loss of balance, often on a daily basis.
• Elderly individuals with dizziness also report memory problems and anxiety more frequently than non-dizzy elderly, which negatively impacts their quality of life.
• Peripheral vestibular disorders


  Involve the inner ear vestibular structures and/or the vestibular nerve
• Central vestibular disorders


  Primarily result from damage to the vestibular nuclei, the cerebellum, and the brainstem, including vestibular pathways within the brainstem that mediate vestibular reflexes
• Possible causes of peripheral vestibular disease

  • Trauma
  • BPPV
  • Neuritis
  • Meniere's disease
  • Aminoglycoside antibiotic toxicity
  • Labyrinthitis
  • Vestibular neuropathy
  • Herpes zoster
  • Tumors
• Types of peripheral vestibular disorders

  • Unilateral vestibular hypofunction
  • Bilateral vestibular hypofunction
  • Recurrent pathological excitation or inhibition of the peripheral vestibular system
• Vestibular disorders that produce recurrent disruptions in vestibular function include:

  • benign paroxysmal positional vertigo
  • Méniere's disease
  • perilymphatic fistula
  • These disorders are characterized by intermittent periods of normal vestibular function with periods of abnormal function.
• Unilateral vestibular hypofunction (UVH)

  Characterized by a reduction or loss of peripheral vestibular function
• Causes of UVH


  • Viral or bacterial infections as in vestibular neuritis and labyrinthitis
  • Head trauma
  • Vascular occlusion
  • Unilateral vestibulopathy
  • Following surgical procedures such as a labyrinthectomy and acoustic neuroma resection
• Symptoms of UVH


  • Acute onset of severe rotational vertigo
  • Spontaneous horizontal-rotatory nystagmus, beating toward the unaffected ear
  • Slight oscillopsia, when turning the head quickly to the affected side
  • Postural instability with a tendency to fall toward the affected side
  • Nausea and vomiting
• Damage to the vestibular system that decreases function unilaterally creates an imbalance between the two sides that results in the brain's misperception that movement of the head has occurred which is in the direction of the more neutrally active healthy ear. This triggers the vestibulo-ocular reflex or VOR in a corrective response.
• Resolution of vertigo and spontaneous nystagmus usually occurs within 3–7 days in environments with light, because the patient is able to suppress the nystagmus with visual fixation. The spontaneous nystagmus may always be present in the dark. Resolution of the dynamic VOR and postural instability usually is slower to recover and may take up to a year.
• Vestibular neuritis


  The second most common peripheral cause of vertigo
• Infection of the vestibular nerve results in nerve degeneration that is usually unilateral but can be bilateral. Onset is often preceded by a viral infection of the upper respiratory or gastrointestinal tracts.
• Symptoms of vestibular neuritis

  • Sudden, prolonged, severe vertigo that is worsened by head movements
  • Associated with spontaneous horizontal-rotatory nystagmus, beating toward the good ear
  • Postural imbalance to the affected side
  • Nausea
  • Hearing usually remains intact

  
• Symptoms for vestibular neuritis improve over a period of 48–72 hours and gradually return to normal over 6 weeks

  Patients may continue to experience oscillopsia and impaired balance when rapidly turning the head to the affected side.
• Labyrinthitis


  An inflammatory disorder of the membranous labyrinth, typically caused by viral and bacterial infections
• The inflammation caused by labyrinthitis damages the vestibular and cochlear end organs, resulting in vestibular symptoms such as vertigo, nystagmus, postural imbalance, and nausea similar to vestibular neuritis along with tinnitus and/or hearing loss.
• Acoustic neuroma


  Slow-growing, benign tumors that originate from the Schwann cells, lining the vestibular portion of the eighth cranial nerve, often within the internal auditory canal

  
• Symptoms of acoustic neuroma

  • Early in the disease, when the tumor is small, patients may complain of vertigo, disequilibrium, tinnitus, and asymmetric hearing loss due to compression of the vestibulocochlear nerve
  • As the tumor enlarges, symptoms of hearing loss, tinnitus, and vestibular hypofunction worsen, resulting in the primary deficits
  • Eventually, the tumor grows to a size where it compresses the brainstem and cerebellum, causing difficulty in swallowing, impaired eye movements, gait ataxia, and possibly death

  
• Bilateral vestibular hypofunction (BVH)


  Most commonly caused by ototoxicity from taking certain classes of antibiotics specifically aminoglycosides gentamicin and streptomycin
• Symptoms of BVH


  • Disequilibrium
  • Severe postural instability with resultant gait ataxia
  • Oscillopsia with head movement due to bilaterally impaired or absent vestibulospinal and vestibulo-ocular reflexes
• BVH symptoms are likely permanent, but people can return to high functional levels.
• Benign paroxysmal positional vertigo (BPPV)

  The most common peripheral vestibular disorder and is the cause of approximately 50% of dizziness in older people
• Benign Paroxysmal Positional Vertigo

  The condition occurs as a result of otoconia detaching from the otolithic membrane in the utricle and migrating into one of the semicircular canals.
• Types of BPPV


  • Posterior canalithiasis
  • Horizontal canalithiasis
  • Cupulolithiasis
• Because the otoconia have more than twice the density of the endolymph within the semicircular canals, the involved canal becomes gravity-sensitive. The shifting of otoconia with head movements causes deflection of the cupula, thereby sending abnormal input to the brain, increasing firing of the hair cells creating a sensation of rotation of the head.

  
• Cupulolithiasis occurs when the otoconia is adhered to the cupula, which results in more persistent and more easily triggered dizziness with typical head movements of daily life.
• Horizontal semicircular canals


  Gravity-sensitive due to higher density of otoconia compared to endolymph
• Otoconia movement with head movements

  1. Deflection of cupula

  2. Abnormal input to brain

  3. Increased firing of hair cells

  4. Sensation of head rotation

  5. Vertigo and nystagmus

  
• Head stops moving


  Otoconia come to rest, pressure on hair cells relieved, nystagmus subsides
• Cupulolithiasis


  Otoconia adhered to cupula, causing direct pull on hair cells, not dependent on endolymph drag
• BPPV


  • Transient episodes of vertigo lasting 5-30 seconds
  • Caused by changes in head position
  • May include disequilibrium, mild postural instability, nausea
• BPPV is typically unilateral, bilateral involvement is rare
• Activities that provoke BPPV symptoms

  Rapidly changing head position with respect to gravity, e.g. looking up, rolling over, bending over
• Causes of BPPV


  • Head injury in <50 years old
  • Age-related degeneration of otolithic membrane in ≥50 years old
• Meniere's disease


  Disorder of inner ear, increased endolymphatic pressure, inappropriate nerve excitation
• Typical Meniere's attack


  1. Pressure, discomfort, fullness in ears

  2. Reduced hearing, tinnitus

  3. Rotational vertigo, postural imbalance, nystagmus, nausea and vomiting