Ocular vestibular evoked myogenic potentials: skull taps can cause a stimulus direction dependent double-peak

To explore the mechanisms for skull tap induced ocular vestibular evoked myogenic potentials (oVEMP).

An electro-mechanical "skull tapper" was used to test oVEMP in response to four different stimulus sites (forehead, occiput and above each ear) in healthy subjects (n=20) and in patients with unilateral loss of vestibular function (n=10).

In normals, the oVEMP in response to forehead taps and the contra-lateral oVEMP to taps above the ears were similar. These responses had typical oVEMP features, i.e. a short-latency negative peak (n10) followed by a positive peak (p15). In contrast, the ipsi-lateral oVEMP to the laterally directed skull taps, as well as the oVEMP to occiput taps, had an initial double negative peak (n10+n10b). In patients with unilateral loss of vestibular function, the crossed responses from the functioning labyrinth were very similar to the corresponding oVEMP in normals.

The present data support a theory that skull tapping may cause both a response that is more stimulus direction dependent and one that is less so.

Whereas the stimulus direction dependent occurrence of the negative double-peak might reveal the functional status of one part of the labyrinth, the rather stimulus direction-independent response might reveal the functional status of other parts.