Bithermal caloric test stimulates which of the following?
Correct Answer: Horizontal semicircular canal
Description: Fitzgerald-Hallpike test (bithermal caloric test): In this test patient lies supine with head tilted 30 degree forward so that the horizontal (lateral) canal is veical and then ears are irrigated Caloric reflex test ('Vestibular caloric stimulation') It is a test of the vestibulo-ocular reflex that involves irrigating cold or warm water or air into the external auditory canal. It is commonly used to validate a diagnosis of asymmetric function in the peripheral vestibular system. Calories are usually a subtest of the electronystagmography (ENG) battery of tests. Cold or warm water or air is irrigated into the external auditory canal, usually using a syringe. The temperature difference between the body and the injected water creates a convective current in the endolymph of the nearby horizontal semicircular canal. Hot and cold water produce currents in opposite directions and therefore a horizontal nystagmus in opposite directions. In patients with an intact brainstem: - If the water is warm (44degC or above) endolymph in the ipsilateral horizontal canal rises, causing an increased rate of firing in the vestibular afferent nerve. - This situation mimics a head turn to the ipsilateral side. - Both eyes will turn toward the contralateral ear, with horizontal nystagmus to the ipsilateral ear. If the water is cold, relative to body temperature (30degC or below), the endolymph falls within the semicircular canal, decreasing the rate of vestibular afferent firing. The eyes then turn toward the ipsilateral ear, with horizontal nystagmus (quick horizontal eye movements) to the contralateral ear. Absent reactive eye movement suggests vestibular weakness of the horizontal semicircular canal of the side being stimulated. In comatose patients with cerebral damage, the fast phase of nystagmus will be absent as this is controlled by the cerebrum. As a result, using cold water irrigation will result in detion of the eyes toward the ear being irrigated. If both phases are absent, this suggests the patient's brainstem reflexes are also damaged and carries a very poor prognosis One mnemonic used to remember the FAST direction of nystagmus is COWS. COWS: Cold Opposite, Warm Same - Contralateral when cold is applied and ipsilateral when warm is applied
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