What does a fast phase of nystagmus toward the opposite side indicate when cold water is flushed into the ear?

When cold water is flushed into the ear, it induces a temperature change that stimulates the vestibular system, specifically the horizontal semicircular canal. This results in a phenomenon known as the caloric reflex test, which is a part of the neurological examination to assess the function of the vestibular system.

In a normal response, flushing cold water into one ear stimulates the opposite side of the vestibular system, causing a fast phase of nystagmus (the quick movement of the eyes) toward the ear that is away from the side being stimulated. This fast phase represents a compensatory response to maintain visual stability. Therefore, the appearance of nystagmus with a fast phase toward the opposite side in response to cold water indicates that the vestibular system is functioning normally.

This normal response helps clinicians evaluate the integrity of the brainstem pathways involved in the vestibular reflexes. Observing that the fast phase is directed away from the irrigated ear confirms proper function and indicates that the vestibular system is responsive to thermal stimuli. Consequently, this interpretation aligns with understanding how the vestibular system processes directional information and how nystagmus should manifest under normal conditions during the caloric test.