We interpret positive HIT to be due to peripheral vestibular deficit. But it can be seen in patients with cerebellar ataxia also.
False-Positive Head-Impulse Test in Cerebellar Ataxia
Abnormal Head Impulse Test in a Unilateral Cerebellar Lesion
It’s explained by deficit in floccular function.
Isolated floccular infarction: impaired vestibular responses to horizontal head impulse
Here’s a femail patient 59yr; her instability had started 2yr ago very progressively so she can’t walk, even sit unsupported; appendicular ataxia and dysarthria are also obvious signs of cerebellar dysfunction. MRI document marked cerebellar atrophy.
- gaze evoked Ny
- smooth pursuit: saccadic
- very reduced OKN
- bilaterally positive HIT
- positive VVOR test
Very reduced VOR gain
BUT caloric test is normal
Obviously, peripheral vestibular function is preserved. SPV is not elevated, meaning that nodular control of vestibular nuclei is preserved. Markedly reduced VOR gain could be explained by floccular dysfunction.
That means that in a regard of peripheral vestibular assessment in patients with cerebellar ataxia isn’t enough to perform just vhit, but also a caloric test.
Positive VVOR test was explained by three non-functioning compensatory mechanisams: smooth pursuit, OKR, VOR (first two as a sign of central vestibular dysfunction and third as peripheral vestibular dysfunction) and was considered as pathognomonic sign of CANVAS.