Vestibular Asymmetry: Some Theoretical and Practical Considerations

Abstract

\s=b\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\x=req-\ changing feature of positional nystagmus. Dynamic asymmetry refers to abnormal asymmetry induced by normal head movements. For example, with unilateral hypo\x=req-\ function, a greater gain is observed with head movement toward the unaffected side, suggesting nonlinearity as specified by Ewald'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. (Arch Otolaryngol 1983;109:682-687) The vestibular system is a sensory system whose function is to mon¬ itor how the head moves or is posi¬ tioned relative to inertial space.

Vestibular asymmetry in caloric and video head impulse testing: Do we interpret it correctly?

“…Although the consequences of Ewald's second law make the vHIT appear to be unilateral, it is still assumed that the contralateral side contributes to some extent because when the head is moved, both ears move together and the two vestibular organs interact as a push-pull pair with a gradual contralateral inhibition saturation, which is more evident in unilateral vestibular loss. [15][16][17][18][19] This is especially true for low-velocity ipsilesionally directed head movements, as a residual contribution from the healthy contralateral side cannot be excluded. 20,21 In contrast, "rapid […], unlike slow, […] head movements" are required to silence the contralateral input and ultimately "demonstrate […] asymmetry" 22 .…”

Vestibular asymmetry in caloric and video head impulse testing: Do we interpret it correctly?

“…Although the consequences of Ewald's second law make the vHIT appear to be unilateral, it is still assumed that the contralateral side contributes to some extent because when the head is moved, both ears move together and the two vestibular organs interact as a push-pull pair with a gradual contralateral inhibition saturation, which is more evident in unilateral vestibular loss. [15][16][17][18][19] This is especially true for low-velocity ipsilesionally directed head movements, as a residual contribution from the healthy contralateral side cannot be excluded. 20,21 In contrast, "rapid […], unlike slow, […] head movements" are required to silence the contralateral input and ultimately "demonstrate […] asymmetry" 22 .…”

Vestibular asymmetry in caloric and video head impulse testing: Do we interpret it correctly?

“…Although the consequences of Ewald's second law make the vHIT appear to be unilateral, it is still assumed that the contralateral side contributes to some extent because when the head is moved, both ears move together and the two vestibular organs interact as a push-pull pair with a gradual contralateral inhibition saturation, which is more evident in unilateral vestibular loss. [15][16][17][18][19] This is especially true for low-velocity ipsilesionally directed head movements, as a residual contribution from the healthy contralateral side cannot be excluded. 20,21 In contrast, "rapid […], unlike slow, […] head movements" are required to silence the contralateral input and ultimately "demonstrate […] asymmetry" 22 .…”

Vestibular asymmetry in caloric and video head impulse testing: Do we interpret it correctly?

“…Although the consequences of Ewald's second law make the vHIT appear to be unilateral, it is still assumed that the contralateral side contributes to some extent because when the head is moved, both ears move together and the two vestibular organs interact as a push-pull pair with a gradual contralateral inhibition saturation, which is more evident in unilateral vestibular loss. [15][16][17][18][19] This is especially true for low-velocity ipsilesionally directed head movements, as a residual contribution from the healthy contralateral side cannot be excluded. 20,21 In contrast, "rapid […], unlike slow, […] head movements" are required to silence the contralateral input and ultimately "demonstrate […] asymmetry" 22 .…”