Vestibular Asymmetry: Some Theoretical and Practical Considerations

Pathological vestibular asymmetry can be divided into static and dynamic types. Static asymmetry results from a unilateral change of the resting neural input. Acute, chronic, and recovery stages can be recognized if one interprets the direction and intensity of the resultant spontaneous nystagmus relative to the clinical picture. Static asymmetry is additive with induced asymmetry and manifests itself as directional preponderance or as the direction-fixed or direction-changing feature of positional nystagmus. Dynamic asymmetry refers to abnormal asymmetry induced by normal head movements. For example, with unilateral hypofunction, a greater gain is observed with head movement toward the unaffected side, suggesting nonlinearity as specified by Eswald's second law. Visually induced vestibular asymmetry is a form of dynamic asymmetry generated by convergence of visual-vestibular information, and causing symptoms in certain "motion-active" visual environments.