Thermal labyrinth testing (synonym: caloric labyrinth testing) is a diagnostic method used in otolaryngology to test the vestibular apparatus (balance apparatus) and thus detect balance disorders. Vertigo and vestibular disorders are very common complaints and can be caused by a variety of diseases. Since the exact cause often remains unknown, differentiation into central and vestibular (originating from the vestibular organ) vertigo is often very helpful. Central vestibular dizziness is most often caused by lesions (damage) in the brainstem or cerebellum (e.g., circulatory disorders, infections, inflammations, tumors, etc.). Vestibular vertigo, on the other hand, is due to a dysfunction of the vestibular apparatus, often affecting only one side. Anatomically, the vestibular organ belongs to the inner ear or labyrinth along with the cochlea (hearing cochlea) and is located in close proximity to the middle ear. This anatomical relationship makes it possible to stimulate the vestibular organ indirectly via a thermal stimulus in the external auditory canal and to check its functionality. In the case of vestibular vertigo, thermal labyrinth testing can often be used to determine on which side and to what extent a vestibular organ is diseased.
Indications (areas of application)
Vertigo and vestibular disorders are indications for performing thermal labyrinth testing followed by nystagmus recording. Other methods such as rotatory or optokinetic stimulation may also be used to elicit nystagmus and thereby verify proper function of the vestibular apparatus. The main advantage of thermal labyrinth testing is that each vestibular organ is examined individually. Peripheral excitability is compared between left and right so that unilateral dysfunction or loss of function can be detected. Thermal labyrinth testing may be helpful for the following peripheral vestibular dysfunctions:
- Acute unilateral vestibular loss (the affected labyrinth is under/unexcitable in thermal labyrinth testing).
- Meniere’s disease (triad of vertigo attacks, tinnitus (ringing in the ears), and seizure-like hearing loss; during the attacks, vestibular nystagmus to the healthy side; in the course, hypofunction of the labyrinth of the affected side and thus no nystagmus in thermal stimulation).
- Bilateral peripheral vestibular loss (the nystagmus is only very weakly pronounced).
In central vestibular dysfunction, thermal excitability is usually equally diminished bilaterally or unremarkable bilaterally.
Contraindications
Thermal labyrinth testing must exclude tympanic membrane perforation. If perforation is known, warm/cold air irritation can be performed as an alternative.
The procedure
Water irrigation of the external auditory canal is used to excite the horizontal arcade of the vestibular apparatus. As either cold or warm water is used to irrigate, cooling or heating of the endolymph (inner ear fluid) in the arcade occurs. The temperature difference changes the density of the endolymph, causing a flow in the semicircular canal under the influence of gravity. This flow is registered in the ampulla (extension of the semicircular canal) by sensory cells and is transmitted as a neuronal impulse via the vestibular nerve (vestibular nerve) to the brainstem, where eye muscle nuclei are eventually excited, resulting in nystagmus (eye movement).
- Warm water: heating results in ampullopedal (toward the ampulla) movement of the endolymph, which results in depolarization of sensory cells, increase in vestibular nerve impulse frequency, and enhancement of resting tone in the vestibular center. Objectively, one can register a nystagmus to the side of the flushed ear.
- Cold water: a cold stimulus, on the other hand, causes ampullofugal (away from the ampulla) flow, hyperpolarization of sensory cells, reduction of impulse frequency, and attenuation of resting tonus in the vestibular center. Objectively, one can register a nystagmus away from the flushed ear.
Examination technique
- The patient’s head must first be placed in an optimal position. For this purpose, the head is raised by 30° when the patient is lying down and tilted back by 60° when the patient is sitting.This ensures that the horizontal arcades are as vertical as possible.
- Each ear canal is rinsed with warm and cold water for about 30-40 seconds each. A total of four rinses are required, between which breaks of a few minutes should be taken.
- Cold rinse: 30 °C (method according to Hallpike) or 17 °C (according to Veits).
- Warm rinse: 44 °C (Hallpike) or 47 °C (according to Veits).
- The thermally induced nystagmus can be registered with the help of Frenzel glasses, elektronystagmographisch or videonystagmographisch.
Possible complications
Due to thermal irritation of the vestibular apparatus, reactions other than eye movements may occur:
