Caloric Test: testing the activities of the three semicircular canals, otolith organs, and the brain stem

The caloric test is a clinical examination to evaluate the activities of vestibular function by applying thermal stimulation to the external ear canal. This test is widely performed to determine the causative side and the level of vertigo, since it used to be the only way to stimulate the right or left ear separately. The amount of thermal stimulation is determined by the amount of thermal energy transmitted to the inner ear, i.e., the medium (water or air), the temperature difference from the ear drum, the length of time of stimulation, and the amount of flow of the medium. The thermal stimulation induces nystagmus, neck torsion, nausea and sensations of vertigo. Because nystagmus is a pure reflexive response dependent on the strength of the stimulus, caloric nystagmus has been developed as an examination of vestibular system activities. For clinical examination, the procedure and the criteria for diagnosis need to be determined not only for patients, but also for the examiners. To apply a thermal stimuli, a small amount of cold water close to room temperature is recommended, i.e.. 5mL of tap water with a temperature of 20°C. The water should be used to fill the ear canal for 20 sec and then to bring the head 30°anteflex from the supine position (Uemura). If the caloric nystagmus is recorded using EOG or VOG, the maximal velocity of the slow phase is the best indicator to show the inner ear activity, since it is correlated with the strength of the stimulus. To obtain a stable and strong response, the recording must be done while the eyes are open and the subject is in complete darkness. During the caloric response, the examinee should look at a visual target for 10 seconds to check the activity of the central vestibular system; the slow phase velocity should decrease to about 50% (visual suppression test of Takemori.) Turning the head 45° to the right or left will suppress or potentiate the vertical component, indicating the participation of the vertical semicircular canals. When the caloric nystagmus is completed, the examinee should be asked to sit up at 90°; a horizontal nystagmus to the opposite side will then appear (“second phase”). This nystagmus shows the stored activity in the central vestibular system. If the inner ear activity is the same in both ears, the right-left difference of the ”second phase” will reflect the central vestibular asymmetry called the Direction Preponderance (DP). A slow phase velocity of 20-50°/s with right-left difference