Written by Travis M. Moore
Last edited 11-Nov-2019
At this point you have finished your caloric examination and calculated various response measures. Now what? The next step is to understand what your pattern of results actually means. The caloric exam is the only test that can provide independent information about each horizontal semicircular canal (hSCC). All other tests of the hSCCs stimulate both canals at the same time. Why? Because all the other tests involve actually moving the head: you would have a hard time rotating around the y axis (i.e., yaw), but leaving one hSCC stationary! Let's take a look at some common caloric findings and what they can tell us about a patient's vestibular system.
When using the forumlae below to calculate caloric responses, refer back to this table to determine whether the response is abnormal or within normal limits. The data in the table below are norms reported in Jacobson et al. (1993), but note there are various limits reported by different studies. Each clinic should collect its own normative data to ensure a set of published norms is appropriate for its procedures, equipment, population, and calibration.
|Bithermal Unilateral Weakness||UW% > 20% is abnormal|
|Monothermal Unilateral Weakness||Monothermal UW% > 11% is abnormal|
|Directional Preponderance||DP% > 27% is abnormal|
|Gain Asymmetry||GA% > 25% is abnormal|
|Fixation Suppression||FI% > 0.6 is abnormal|
|Bilateral Weakness||Total Responses < 22 deg/sec is abnormal|
|Hyperactivity||1. Total Warm > 146 deg/sec is abnormal
2. Total Cool > 90 deg/sec is abnormal
3. Total Warm and Cool > 221 deg/sec is abnormal
The presence of a unilateral weakness (UW) reveals an imbalance in the peripheral vestibular system; that the responses from one side are significantly weaker than the responses from the other side. In other words, both calorics to one ear (e.g., right warm, right cool) are weaker than the tests involving the opposite ear (e.g., left warm, left cool). Refer to Figure 1 for an example of a 52% right UW.
An important question to ask is what structure(s) is responding weaker? The hSCC is an obvious choice, but there are other structures that could be involved:
Unfortunately, calorics cannot isolate which peripheral structures are impaired, but we do know something is wrong with the pathway that responds to y-axis rotation, and which side the lesion is on. We also know that a significant portion of hair cells have to be damaged, or the caloric test would not be able to notice a weakness. Specifically, Jacobson and Shepard (2016) note that roughly 30% - 40% of hair cells must be lost before a weakness shows up on caloric testing.
Another limitation of the caloric exam is that it doesn't let us know anything specific about what caused the lesion. That means we should be familiar with the common pathologies that are known to damage the vestibular hair cells, and/or the vestibular nerve so we can make an educated guess based on the other test results. Table 1 below offers a brief list for review.
|Trauma||Labyrinthine concussion, rupture||Stretching, cutting|
|Ischemia||Anterior vestibular artery|
|Toxic Agents||Aminoglycosides, chemotherapeutics|
|Demyelinating Disease||Multiple sclerosis|
|Other||Meniere's disease||Neurovascular compression|
Similar to the UW, the presence of a directional preponderance (DP) also indicates an imbalance, but does not mean one side is weaker than the other. Instead, a DP tell us that the nystagmus in one direction (e.g., right warm and left cool) is stronger than in the opposite direction (e.g., right cool and left warm). This is an important distinction compared to the UW (which compares responses between the ears). A DP compares the tests that are supposed to evoke right-beating nystagmus to the tests that are supposed to evoke left-beating nystagmus.
There are two mechanisms that can lead to a DP, with very different implications. The first, and most common, reason for a DP is due to spontaneous nystagmus (SN) interfering with the caloric nystagmus. This is a DP due to a baseline shift. More specifically, if your patient has SN that beats to the left at 10 deg/sec, then your left-beating caloric nystagmus will be enhanced (added to) that 10 deg/sec SN. Conversely, the right-beating caloric nystagmus will have to fight against the SN, and will be 10 deg/sec less. The outcome is a shift in the baseline of the caloric responses. That is, instead of starting at 0 (i.e., no nystagmus), now the caloric responses start at 10 deg/sec to the left. The figure below shows a 10 deg/sec baseline shift to the right, producing a 45% DP to the left.
Figure 2 shows a baseline shift of 10 deg/sec to the right. At first glance the SPV seems quite different between the left-beating (38 and 36 deg/sec) and the right-beating (-15 and -13 deg/sec) nystagmus. However, if we subtract the SN out (i.e., subtract 10 deg/sec) we end up with 28 and 26 deg/sec for the left-beating nystagmus and 25 and 23 for the right-beating nystagmus. We corrected for the SN and the responses are now normal! (A left DP of only ~6%.) In other words, we need to move all the data points in Figure 2 down 10 deg/sec, and when we do, we'll see the responses are in fact symmetrical.
The second mechanism for a DP is called gain asymmetry, and is caused by an actual asymmetry in the SPV. Figure 3 shows a 54% gain asymmetry to the left. Just like in Figure 2, the left-beating nystagmus is greater than the right-beating nystagmus, but there is no baseline shift. This means there is nothing to correct: those weaker responses to the right are real. This finding is extremely rare, and little is known about the underlying cause.