Goggles with cameras illuminated by infra-red lighting are placed on the patient. The darkest spot of the eye, the pupil, is tracked by the software, so a tracking threshold must be established. Once tracking is established, a calibration procedure begins in which the patient is asked to visually track a moving target as it travels over fixed parameters (or degrees), in order to quantify any existing nystagmus.
During this test, the patient is reclined at 30 degrees to place the horizontal semi-circular canal perpendicular to gravity. The caloric vestibular test is performed by irrigating the ear canal with either water or air for 30 to 60 seconds and recording the eye movement response (nystagmus) for approximately two minutes following the irrigation. Once the recording and analysis are complete, the patient is given sufficient time, typically five minutes, before the opposite ear can be irrigated. This eliminates any significant chance of residual thermal effect from the previous irrigation. During this time, the examiner must delete artifact such as eye blinks from the recorded data, and measure the slow phase velocity of any pathologic nystagmus.
In order to provide accurate information, the test must meet several minimum standards. The stimulus and recording parameters must be precise and consistent ,as the clinical value of this test involves comparing differences in responses to (hopefully) identical stimulus and recording parameters. For this test to be meaningful, at least two and typically four irrigations are performed on the same day. After two or four irrigations, the results are calculated through a mathematical formula to arrive at a conclusion of normal versus abnormal response.
To read more about patient preparation, please see the previous blog on the Gaze and Spontaneous Nystagmus Test.