Timothy C. Hain, MD • Page last modified: March 2, 2021
There are several methods of activating the SCM muscle when doing a VEMP - -head lifting off the table, head turning to one side being the main two.
This material is to comment on how one might run into technical problems during VEMP testing related to head position when one is lying on a flat table.
Our goal is to have our subjects lift their head slightly off the table, as shown below.
There are several factors that interact in determining how much muscle activation a person needs to exert when asked to lift their head off the table and that need to be considered when doing a VEMP
Clinicians all realize that necks can vary - -some people cannot lie flat -- their neck or trunk is bent foward and just doesn't go back that far. Other people are very flexible, and their necks are easily manipulated. For the person with the neck bent forward, perhaps due to bony changes through life, lifting the head off the table will be a very simple thing for them -- they will need to exert no force at all ! This will make their VEMP useless.
A person with a flexible neck has to exert maximum force to get the head just barely off the table. The force on the head from gravity directed downward is proportional to the sine of the angle that the head makes with the table. It is not all that sensitivie -- sine of 90 degrees is 1. sine of 60 is 0.866, sine of 45 0.5, and sine of 0 degrees is 0. Thus you are pretty safe up to 30 degrees, and things go downhill rapidly for head angles more than 30 degrees.
Of course, if the head is not even off the table (i.e. 90+), no force at all. More off the table means less force.
Someone who lifts their head up for 30 seconds, and then puts it back on the table for another 30 seconds will have less of a VEMP than someone who has their head up for the entire time. This would result in a VEMP that is half of what it would be if their head had been up for a full minute.
This pattern can be adjusted for if the operator of the VEMP apparatus keeps the interpreter informed of the situation, or alternatively just stops the test.
When the test is stopped, the amplitude is normalized to the # of samples. So amplitude might be roughly correct (perhaps a noisy trace). When the test is continued without muscle contraction, the amplitude goes down.