The bithermal caloric test, first described by Robert Barany, assesses the function of the lateral vestibular canal. The brainstem vestibulo-ocular reflex (VOR) causes tonic eye deviation, opposite to that of the head turn, to maintain fixation on an object. This action is, however, counteracted by the saccades from the frontal eye field in a reverse direction to maintain fixation on a moving object, which results in the fast component of the horizontal nystagmus seen during the test. The caloric test is one of the tests included within the electronystagmography and is also a component of the brainstem reflexes while assessing the patient for brain stem integrity.
The slow water irrigation (flow rate of 350 +/- 30 ml per minute) of the ear (with an intact tympanic membrane) with water at 7 degrees above or below the body temperature for a period of 30 to 40 seconds, induces convection current as well as thermal stimulation of the vestibular apparatus. The warm water causes the ampullopetal movement of the endolymph, causing depolarization of the hair cells and stimulation of the vestibular nerve, resulting in a fast component of the horizontal nystagmus beating towards the stimulating ear. The cold water causes ampullofugal endolymph movement, causing hyperpolarisation of the hair cells and inhibition of the vestibular nerve, resulting in a fast component of the nystagmus beating away from the stimulating ear. The reduced vestibular response and the directional preponderance are calculated using the Jongkee's formula.
Types of caloric testing
The interaural variability in the caloric paresis of more than 22 to 25% and the directional preponderance of greater than 26 to 30% indicates asymmetric response.
Areas of concern
When a person is fully awake, the caloric reflex tests both the tonic vestibular stimulation with its reflexive slow deviation of the eyes and the fast corrective saccadic movement initiated by the frontal eye field. With warm water irrigation, the normal response consists of a slow movement of the eyes away from the side of the stimulus and the corrective fast saccade towards the side of the stimulus. Opposite responses occur with cold water irrigation. When the patient is comatose, there will not be any fast corrective saccade. As a result, an intact cold caloric response will only have the slow tonic phase of conjugate deviation of the eyes towards the cold stimulated ear and away from the warm stimulated ear. Intact caloric reflex in a comatose patient will indicate the intact integrity of the brainstem reflex pathways. The clinical utilization of the reflex will include the following:
Limitations of the test
The caloric reflex test is most helpful and utilized most frequently in the intensive care setting, especially when the patient is comatose. The nursing staff has to monitor the patient's vitals more carefully and frequently. The intensivist has to find out the cause of coma and assess the severity and prognosis. The pharmacist has to monitor and advise on the use of medications and side effects more carefully. Neurology consultation is often necessary as part of the interprofessional team of management. The caloric reflex test is an invaluable tool to assist in identifying any generalized or localized brainstem disorder and help in assessing the prognosis. The monothermic ice water cold caloric is an essential part of brainstem reflex assessment in any patient diagnosed to be in brain death. This evaluation is an important part of the activities of the interprofessional organ transplant team.
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