We have completed skull vibration testing as part of our clinical protocol for several years and have found it to be a helpful cross check measure for detecting peripheral (inner ear) vestibular abnormalities.
It is important to note that we use skull vibration as a part of our test battery and not as an isolated measure.
In general, abnormal skull vibration testing is best at detecting larger peripheral vestibular deficits and a milder hypofunction (inner ear weakness) could be missed if this measure was used in alone. We have discussed skull vibration testing previously on the blog including an overview of the test procedure as well as our experiences implementing this test clinically. To date though this is not a widely utilized test and there is a scarcity of research on how skull vibration testing fits into a vestibular testing protocol.
Skull Vibration Test and Vestibular Function
I recently stumbled on an interesting article that investigated the agreement between skull vibration and tests of semicircular canal function and otolith function. This article examined the agreement between skull vibration and other commonly performed vestibular tests including: caloric, ocular and cervical VEMPs.
The authors found that 80% of the time the caloric test and skull vibration were in agreement on the finding of normal or abnormal. The oVEMP was in agreement with skull vibration 63% of the time and the cVEMP was in agreement 76% of the time. The sensitivity for caloric asymmetry was only 47%, oVEMP was 26%, and cVEMP was 33%.
To summarize, the authors found that skull vibration was best at detecting abnormalities of the horizontal semicircular canal through caloric testing and was not very effective at detecting otolith abnormalities as measured with VEMP testing. The authors felt the disagreement between VEMPs and skull vibration could be related to the different frequencies that they are assessed with. Typically skull vibration testing is completed with a 100 Hz stimulus and the most widely utilized stimulus frequency for VEMP testing is 500 Hz.
This study is a step in the right direction in helping better determine how skull vibration fits into a vestibular test battery. There are some limitations to this study and further research in this area is still needed. At present, we know that skull vibration is best at detecting larger degrees of vestibular hypofunction and it appears to agree best with measures of semicircular canal function as measured with caloric irrigation.
Additional investigation into the agreement between rotational chair and vHIT testing would be helpful. These measures assess the vestibular ocular reflex (VOR) at head speeds encountered during daily life while the caloric irrigation is an extremely low frequency stimulus. We have previously discussed the flaws in making broad judgments on one’s overall vestibular function solely based on caloric testing.
One can speculate that because we know that skull vibration induced nystagmus is most likely to occur with larger degrees of caloric hypofunction (>50%) and abnormal vHIT is also more likely to occur with a larger caloric hypofunction, then vHIT and skull vibration should be in agreement the majority of the time.
If we had a better understanding of the agreement between vHIT, rotary chair, and skull vibration testing, we may be able to better identify vestibular abnormalities that are causing patients a functional deficit.
We maintain that skull vibration is a useful measure as part of a vestibular assessment protocol because it is quick, easy to perform, comfortable for patients, and can potentially shorten the test protocol.
There is a need for additional investigation in this area but this study marks a step in the right direction to better determining how to best utilize skull vibration as part of a vestibular testing protocol.