Clinical Scientist Trainee
There are various conditions that can impair our vestibular system, leading to severely debilitating symptoms. Vestibular function tests allow clinicians to accurately diagnose what has happened to the vestibular system, which can help plan treatment.
The vestibular system is a collection of tiny structures within the inner ears that detect head movement. Each structure detects head movements in different directions. When the vestibular system detects a movement, a message is sent along the vestibular nerve to the brain, which allows us to perceive movement and maintain visual focus and postural stability.
Since the vestibular system is deep within the head, it is not possible to directly observe and examine the integrity of the structures. Therefore, we use vestibular function tests to measure how well the vestibular system is functioning. To do this, we utilise the various reflexes that exist between the inner ears and other parts of the body.
The vestibulo-ocular reflex (VOR) connects the inner ears to the eye muscles and makes rapid corrective eye movements when we move our heads. This reflex is essential for everyday life as it allows us to see things clearly when we move. For example, if you move your head to the right, your eyes will move to the left to maintain your focus on a visual target. The vestibulo-collic reflex connects the inner ears to the neck muscles and allows us to keep our heads stable when we move.
A common vestibular function test is the video head impulse test (vHIT). This test involves placing a pair of goggles on a patient and recording their eye movements in response to small but rapid head movements in various directions. This is a quick, non-invasive, and comfortable test and is widely used. If a patient has a problem with their vestibular system, their eyes will not move as quickly in response to a head movement as they should. By measuring the eye movements in response to different movements, the clinician can identify which part of the vestibular system is not functioning normally.
The vestibular evoked myogenic potential (VEMP) test measures how well the vestibulo-collic reflex is working. To do this, we stimulate the inner ear using sound and measure the muscle activity in the neck using sensors. In a normally functioning vestibular system, we expect to see a change in the muscle activity of the neck in response to a loud sound. We expect both inner ears to produce similar muscle activity, so by comparing the responses on each side we can identify if there is an asymmetry which can be suggestive of a vestibular abnormality.
The caloric test stimulates each inner ear using temperature. By warming or cooling the ear canal, a convection current can be induced in the inner ear which leads to movement of the inner ear fluid. This simulates head movement, which in turn causes the eyes to move (the vestibulo-ocular reflex). By measuring the eye movements during each stimulus, we can identify if one side is functioning less than the other, which helps diagnose certain conditions.
By performing a range of vestibular function tests, clinicians can gather a comprehensive picture of how the vestibular system is functioning. Different patterns of results across the various tests allow differentiation between the various conditions that can affect the vestibular system.