Describe the physiology of equilibrium.

The Vestibular End Organs

• Balance is maintained by three inputs integrated centrally — the vestibular apparatus, vision, and proprioception from the neck and limbs. Any two can compensate for the loss of the third.
• The semicircular canals detect angular (rotational) acceleration. When the head rotates, endolymph lags behind by inertia, deflecting the cupula and the stereocilia within it
• The cupula has the same specific gravity as endolymph, so it is not deflected by gravity — which is why the canals respond to rotation but not to head position. When otoconia become attached to it (cupulolithiasis), it becomes gravity-sensitive, and that is one mechanism of positional vertigo

Reflexes and Clinical Application

• The vestibulo-ocular reflex (VOR) is the most important vestibular reflex: head movement produces an equal and opposite eye movement, stabilising the image on the retina. It is the fastest reflex in the body (latency 10 ms) — far faster than visual tracking.
• Its failure causes oscillopsia — the world appearing to bounce with each step — which is the disabling symptom of bilateral vestibular loss, as after gentamicin
• The head impulse (Halmagyi) test tests it directly: a rapid, small, unpredictable head turn toward the affected side produces a corrective saccade because the reflex fails. It is central to the hints examination distinguishing peripheral from central vertigo
• The vestibulospinal reflexes maintain posture and tone, and are what is tested by romberg, sharpened Romberg, unterberger stepping and gait testing
• Nystagmus has a slow phase generated by the vestibular system and a fast corrective phase generated centrally; by convention it is named by the direction of the fast phase, although the slow phase is the pathological one
• Caloric testing applies this physiology — with the head raised 30 degrees to bring the lateral canal vertical, warm water induces nystagmus toward the same side and cold toward the opposite side, remembered as COWS: Cold Opposite, Warm Same. It is the only test that examines each labyrinth separately
• Central compensation is the reason most vestibular disease recovers: the cerebellum and brainstem re-balance the resting tone over days to weeks. It is promoted by early mobilisation and vestibular exercises and delayed by prolonged bed rest and by vestibular sedatives — which is why prochlorperazine should be stopped within a few days and is one of the commonest errors in managing vertigo
• Compensation is also impaired in the elderly, in visual impairment, in cerebellar disease and by anxiety
• Motion sickness arises from sensory conflict between vestibular, visual and proprioceptive inputs — classically reading in a moving car, where the vestibular system signals motion and vision does not
• The cupula has the same specific gravity as endolymph, so gravity does not deflect it and the canals sense rotation only
• Otoconia attached to the cupula make it gravity-sensitive, which is the cupulolithiasis mechanism of positional vertigo
• Ampullopetal flow excites the lateral canal and ampullofugal flow excites the vertical canals.
• Nystagmus occurs in the plane of the canal stimulated, which is how the offending canal is identified in positional testing
• The canals work in push-pull pairs, so the brain reads a difference and gains both sensitivity and noise rejection
• Acute loss makes the intact side appear maximally excited, giving violent vertigo with nystagmus beating away from the affected ear