In this test attempt is made to stimulate labyrinth with cold / warm water irritation to induce nystagmus. The temperature of fluid used for irrigation is 7 degrees centigrade above or below normal body temperature. If cold water alone is used then it is known as Kobrack test while if both warm and cold water is used it is known as bithermal caloric test. Caloric testing is a clinically useful bed side tool to isolate peripheral vestibular system and to rule out central causes of vertigo. It is also used to test for brain stem functions in comatose patients.
Before attempting to perform caloric testing otoscopic examination should be performed to ensure that there is no obstruction in the external canal ( cerumen impaction), there is no infection involving skin lining of the external auditory canal.
Position of the patient:
Patient is placed in supine position with head elevated to 30 degrees. This position ensures that the lateral canal is placed in a vertical plane thereby optimizing stimulation. Patient should clearly be explained about the procedure. A kidney tray or catch basin is placed under the ear which is to be irrigated. Water flowing out of the external canal will be caught in the kidney tray. An irrigation system is used to deliver 250 cc of either warm or cold water solution over a period of 25 - 30 seconds to the ear under test. The irrigated solution will freely dribble out of the external canal and can be collected in the basin. Nystagmus starts 30 seconds after the onset of water delivery and will build in intensity over the ensuin 30-45 seconds. After an interval of 5 minutes after cessation of nystagmus the other ear can be tested. The pnuemonic COWS helps in identifying the direction of nystagmus (cold opposite and warm same side).
Canal paresis - Caloric test indicates (based on the duration of nystagmus) after stimulation with water is less than that of the opposite side.
Directional preponderance - This takes into consideration the duration of the nystagmus to the left or right irrespective of the fact that it is elicited from the right or left labyrinth. If nystagmus is 30% more on one side then the other then it is known as directional preponderance to that side. Directional preponderance occurs towards the side of a central lesion and away from the side in a peripheral lesion. Canal paresis and directional preponderance can also be seen together in the same patient.
Modified Kobrak test:
This is a quick office procedure where cold water is used for the test. In this test the patient is seated with head tilted 60 degrees backwards to ensure that the horizontal canal is in vertical position. The ear is irrigated with cold water for 60 seconds. The temperature of the irrigated water should be 7 degrees centigrade below the normal body temperature. Initially 5 ml is used. If there is no response then the volume should be increased to 10, 20 and 40 ml if needed. In normal persons the nystagmus starts beating towards the opposite ear when 5 ml is irrigated. If higher volumes of water is needed to elicit nystagmus then the labyrinth is considered to by hypoacitve. If there is no response even after irrigating 40 ml of water then it indicates dead labyrinth.