Describe the vestibular apparatus and its role in the maintenance of equilibrium. Add a note on nystagmus and the caloric test.

Components

• The three canals lie at right angles to one another, so movement in any plane can be resolved
• The two labyrinths work as a push–pull pair — rotation excites one side and inhibits the other

Structure of the Receptors

• Hair cells with one kinocilium and many stereocilia
• Bending toward the kinocilium → depolarisation → ↑ firing
• Bending away → hyperpolarisation → ↓ firing
• There is a resting discharge, so movement can be signalled in both directions
• Crista — hair cells embedded in the gelatinous cupula, which moves with endolymph
• Macula — hair cells in the otolith membrane, weighted with otoconia (calcium carbonate crystals) → responds to gravity and linear acceleration

Mechanism During Rotation

Head begins to rotate → Endolymph lags behind (inertia) → Relative movement deflects the cupula → Hair cells depolarise on one side, hyperpolarise on the other → Vestibular nerve → vestibular nuclei → Reflex eye and postural adjustment

• The canals respond only to acceleration, not to constant velocity — at steady speed the endolymph catches up and the sensation ceases
• On sudden stopping, the endolymph continues to move → opposite deflection → sensation of turning the other way, with post-rotatory nystagmus and past-pointing

Connections and Functions

Nystagmus

• During rotation the slow phase is opposite to the direction of rotation, so the fast phase (and hence the name) is in the same direction
• Post-rotatory nystagmus is in the opposite direction to the rotation

Caloric test

• Irrigating the external auditory meatus sets up convection currents in the endolymph and tests each labyrinth separately
• COWS — Cold Opposite, Warm Same: cold water gives nystagmus to the opposite side, warm water to the same side
• Absence of response indicates a dead labyrinth; it is also used to assess brainstem function in coma

Maintenance of Equilibrium — the Three Inputs

• Any two of the three can compensate for loss of the third — hence a patient with vestibular failure manages in daylight but is unsteady in the dark
• Romberg test — positive (sways markedly on closing the eyes) in posterior column and vestibular disease, but negative in cerebellar disease, where the patient sways with eyes open as well

Postural Reflexes

Applied Aspects

• Ménière disease — endolymphatic hydrops; the triad of vertigo, fluctuating sensorineural deafness and tinnitus
• Benign paroxysmal positional vertigo — displaced otoconia in the posterior semicircular canal; brief vertigo on head movement; treated by the Epley repositioning manoeuvre
• Motion sickness — conflict between vestibular, visual and proprioceptive input; treated with hyoscine or antihistamines
• Vestibular schwannoma (acoustic neuroma) — at the cerebellopontine angle; progressive unilateral deafness with tinnitus, later involving the fifth and seventh nerves
• Space motion sickness — conflict between otolith and visual input in weightlessness; adaptation takes 2–3 days
• Vestibular rehabilitation exercises promote central compensation after unilateral vestibular loss