Caloric testing has long been the standard for determining chronic labyrinthine asymmetry and dysfunction. With the advent of active and passive tests of the vestibular ocular reflex (VOR) such as vHIT, Active Head Rotation and Rotational Chair, caloric results have become a less critical part of an overall picture of vestibular function.
At the same time, caloric testing is difficult to perform and there are many opportunities for technical error. Patients do not like caloric stimulation, and nausea is a common byproduct. Reimbursement has decreased to the point that the importance of and techniques for caloric testing deserve reconsideration.
The long-time standard test procedure included four total irrigations, warm stimulation to each ear, then cool stimulation to each ear. The purpose of doing stimulation both above and below body temperature was to reduce the possibility of missing a pre-existing bias in nystagmus direction, often referred to as “directional preponderance.” The existence of a directional preponderance when no spontaneous or positional nystagmus is present is possible, but unlikely. Add that VOR tests mentioned above show no asymmetry or dysfunction, and the likelihood decreases further.
So, if we can get out of our rigid structure and think through how we can best use the patient’s time and resources to arrive at a diagnosis, when is it okay to skip calorics? And, our topic for today, when is it okay to perform monothermal calorics?
This question has been explored on several occasions. It is a relatively easy calculation because all you need to do is retrospectively review the results of bithermal caloric tests, and determine how often the third and fourth irrigations (or the second temperature) made a significant difference.
In reviewing a handful of these papers, it seems that all agree that it is reasonable and safe at times to limit the testing to monothermal irrigation. The percentage of acceptable difference in response between ears seems to be the determining factor.
Murnane et al. (2009) found that using an acceptable difference of 10% or less resulted in virtually no difference between monothermal and bithermal results. They calculated that, using this criterion, 40% of patients would not require all four irrigations.
Bush et al. (2013), using the same 10% interear difference criterion, found this technique to be 95% sensitive for detecting labyrinthine asymmetry. By changing the standard to 25% inter-ear difference, the sensitivity decreased to 87%.
Finally, Shupak et al. (2010) loosened up the standard to 32% or less inter-ear difference, and reported sensitivity of 90% and specificity of 92% in predicting those that would have abnormal results on bithermal caloric testing. They added the qualifier that these results pertained to patients with otherwise normal VNG exams.
I am not aware of any studies that added in the element of normal VOR function tests. My assumption is that a patient with normal rotational chair and vHIT or AHR tests, and a monothermal inter-ear difference of 10% or less would be very unlikely to benefit from the additional irrigations. For patients with low suspicion of vestibular dysfunction, a higher inter-ear difference may be acceptable.