How to Interpret SVV Overestimations

Regarding which side is considered the cause of a pathological SVV measurement. I've watched the webinar you've uploaded online and it really clarifies a lot of things in regards to how the equipment works and how to interpret the results. However I’m a bit in doubt about how you interpret overestimations?

Answer: In discussing the interpretation we decided to offer a starting point as the webinar is aimed at an introductory level. It offers some description of the underestimations that might occur following common peripheral vestibular disorders i.e. the discussion focussed on the “classic” unilateral weakness in both acute and various chronic phases but stops short of considering interpretation of overestimations.

When considering overestimation results, one point we would strongly advise is that SVV is not only assessing otolith (utricular) function, but as a behavioural test will also be influenced by subjective factors, and of course any central neurological disorders involving the neural connections from each ear to the brainstem (e.g. vestibular nuclei) and also the motor control of the eyes. From this you might quickly see how a variety of central lesions and disorders might also influence the SVV results, and some of these might be associated with overestimations. For example traumatic brain injuries that cause dizziness and postural instability, plus demyelinating diseases affecting the central vestibular circuits and cerebellar abnormalities, and ischemia / infarcts. On this note, unilateral cerebellar lesions have been found to produce contraversive ocular tilt results, and one might speculate that the SVV findings in a case like this might appear as an overestimation in head tilt conditions to one side;  contraversive tilt occurs with a unilateral cerebellar lesion and ipsiversive with cerebellar stimulation. It would be interesting to learn whether certain irritative disorders (e.g. Vestibular Paroxysmia) might also lead to an overestimation of SVV.

References and caveats
Mossman S, Halmagyi GM. Partial ocular tilt reaction due to unilateral cerebellar lesion. Neurology 1997;49:491–493