Written by Travis M. Moore

Last edited 7-Nov-2019

When calculating the percent of unilateral weakness (UW), you are comparing all the right-ear responses to all the left-ear responses. That sounds pretty straightforward, but let's be really specific. What we're comparing is all of the tests done using the right ear (i.e., right warm and right cool), to all the tests done using the left ear (i.e., left warm and left cool). It is important to have this straight so you don't mix up a UW with a directional preponderance (described next).

To compare the two ears' responses, we first need to add together
the warm and cool responses for each ear. This number is referred
to as the *total right ear (TotRE)* and *total left
ear (TotLE)* response, respectively. The calculation is
simple:

Once the total response from each ear is calculated, we use the following equation to quantify the relationship of the responses between the ears. If the total response of one ear is less than the other, it will show up in the result as a percent of weakness to one side.

Use the calculator below to get a feel for the kind of results that lead to a weakness on a specific side. Can you come up with values that result in a right-sided UW? How about a left-sided UW?

The UW% formula must be modified for the monothermal test because there won't be any values for one temperature. If you're following best practice, you will always be starting with warm, so there won't be any data for cool. The formula below has been changed to be able to compare the right warm total SPV to the left warm total SPV.

A directional preponderance is similar in concept to a UW, but we are comparing the total right-beating nystagmus to the total left-beating nystagmus. Again, let's be very specific: What we're comparing is all of the tests that produce right-beating nystagmus (i.e., right warm and left cool), to all the tests that produce left-beating nystagmus (i.e., left warm and right cool). It is important to have this straight so you don't mix up DP with a unilateral weakness (described above).

To compare the total left/right nystagmus, we first need to add
together the peak nystagmus of all right-beating tests and all
left-beating tests. These numbers are referred to as the
*total right-beating (TotRB)* and *total left-beating
(TotLB)* nystagmus, respectively. The calculation is simple:

Once the response for each direction is calculated, we use the following formula to quantify the relationship between right- and left-beating nystagmus. If the nystagmus beats more in one direction than the other, it will show up as a percent of increased SPV in one direction.

Use the calculator below to get a feel for the kind of results that lead to stronger nystagmus in a particular direction. Can you come up with values that result in overall stronger nystagmus to the right? How about a left-sided DP?

A patient with an intact vestibular system will be able to
suppress caloric nystagmus. The degree of suppression is
quantified as the *fixation index*, and is simply the
ratio of nystagmus SPV before and after fixation. Note that
the fixation index is calculated based on the direction of
nystagmus. That means you need to calculate two fixation indexes:
one for right-beating nystagmus (i.e., right warm or left cool),
and one for left-beating nystagmus (i.e., right cool or left warm).
This calculation is simple enough that no calculator is provided.

Next Topic: Caloric Interpretation

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