Vestibular compensation and substitution.

This is a very brief update on the major papers since August 1998. Unilateral vestibular loss causes oculomotor, postural and sensory symptoms, all of which would be appropriate responses in a healthy person to a strong maintained angular and linear acceleration stimulus directed towards the healthy side. Within hours or days these static symptoms (so called because they are present without any externally imposed vestibular stimulation) reduce, and their progressive disappearance is called 'vestibular compensation'. However, careful testing with natural vestibular stimuli shows that the dynamic vestibular response after unilateral vestibular loss to passively imposed vestibular stimuli does not recover; it is usually asymmetric and functionally ineffective. Major recent developments are: (1) the permanent asymmetrical and functionally ineffective dynamic rotational vestibulo-ocular reflex responses to passive natural vestibular stimulation after unilateral vestibular loss and canal blocks in human patients; (2) evidence for the substitution of other sensory input and responses during vestibular compensation; (3) perceptual testing using visual perception of a horizontal line to confirm permanent otolith dysfunction; (4) the clear and substantial differences in post-unilateral vestibular loss vestibulo-ocular reflex responses between passive and active head turning; and (5) new results in brainstem physiology explaining the disappearance of static symptoms.