The bithermal caloric test, first described by Robert Barany, assesses lateral vestibular canal function. The brainstem vestibulo-ocular reflex (VOR) causes tonic eye deviation, opposite to that of the head turn, to maintain fixation on an object. However, this action is 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 Jongkee's formula.
Types of caloric testing
- Bithermal caloric test (by water at 7 degrees above or below the body temperature or by air at 24 or 50 degrees Celcius)
- Monothermal caloric test
- Ice caloric test
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.
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:
- In patients with suspected peripheral vestibular problems, the bithermal caloric test, coupled best with ENG, will be able to define the side with peripheral vestibular hypofunction. Caloric areflexia may indicate bilateral vestibulopathy due to vestibular toxins, e.g., use of aminoglycoside or cisplatinum.
- In comatose patients: the first step in assessment is generally the bedside vestibulo-ocular (VOR) testing (doll's eye signs). If this is negative, a monothermic caloric (cold caloric) test provides a much stronger stimulus than the VOR. An absence of tonic deviation of eyes to the side of cold water irrigation confirms a problem in the brainstem. Differences in the position of the eyes during the monothermic cold caloric may provide additional clues to the underlying brainstem problems. When cold water is irrigated into the ear of a comatose patient, we should see the deviation of both eyes to the side of the cold stimulus. If there is a loss of abduction of the ipsilateral eye, it indicates an ipsilateral lateral rectus (abducens nerve) palsy. If there is a loss of adduction o the contralateral eye, it indicates a contralateral lesion of the medial longitudinal fasciculus or internuclear ophthalmoplegia.
- Brain stem death- there will be an absence of both the slow (vestibulo-ocular reflex) as well as the fast component ( saccades from the frontal eye field) of the horizontal nystagmus
- Persistent vegetative state- only slow component (intact brain stem) with no fast component of the nystagmus (absent cortical functioning)
- In patients suspected to be suffering from conversion disorder with pseudocoma, a monothermic cold caloric testing will result in a normal cold caloric reflex testing with the fast phase beating towards the contralateral side. The patient should wake up immediately with severe nausea and vomiting; this is a really sensitive and specific test for pseudocoma, yet it should not be performed often due to the severe discomfort associated with the test.
Limitations of the Test
- High interrater bias
- Poor compliance from the patient (vertiginous feel)
- The size of the external ear canal and the pattern of generation of the convection current have a significant influence on the results
- Stimulates only lateral semicircular canal at low frequencies
Nursing, Allied Health, and Interprofessional Team Interventions
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 underlying 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 to help determine if a patient has brain death. This evaluation is an important part of the activities of the interprofessional organ transplant team.