Introduction to the Video Head Impulse Test (VHIT)

The Video Head Impulse Test (VHIT) is a rapid and objective assessment tool used to evaluate the function of the semi-circular canals, which are critical components of the vestibular system. This non-invasive test measures the vestibulo-ocular reflex (VOR) in response to high-acceleration head movements, providing valuable insights into peripheral vestibular function. Understanding VHIT test norms is crucial for accurate diagnosis and clinical interpretation. The VHIT effectively assesses all six semi-circular canals, distinguishing it from older, less specific methods.

Key Metrics and VHIT Norms

Two primary metrics are evaluated during a VHIT: VOR gain and the presence of corrective saccades. These measurements are compared against established VHIT norms to determine if vestibular function is within the normal range or indicates a deficit.

VOR Gain

VOR gain represents the ratio of eye velocity to head velocity during the head impulse test. In a healthy individual, the VOR should generate eye movements that precisely compensate for head movements, keeping the gaze fixed on a target. Therefore, a normal VOR gain is typically expected to be close to 1.0 (or within a range such as 0.8 to 1.2, depending on specific clinical guidelines and equipment). A VOR gain significantly below this normal range suggests impairment of the ipsilateral semi-circular canal. Reduced gain indicates that the eyes are not moving fast enough to keep the target in focus, leading to retinal slip.

Saccades

When the VOR is impaired, the eyes cannot maintain fixation during a head impulse. To compensate for this loss of gaze stability, patients will generate corrective eye movements called saccades. These saccades are a key indicator of vestibular dysfunction when interpreting VHIT norms.

• Covert Saccades: These are quick, compensatory eye movements that occur during the head impulse itself, often before the head has returned to the center. They are subtle and not visible to the naked eye but are readily detectable by the VHIT system. The presence of covert saccades, particularly when associated with reduced VOR gain, strongly suggests peripheral vestibular hypofunction.
• Overt Saccades: These are more noticeable, corrective eye movements that occur after the head impulse has finished and the head has returned to the center. Overt saccades are also indicative of an impaired VOR but may sometimes be seen in less severe cases or as a secondary compensation mechanism.

In a normal VHIT, neither covert nor overt saccades should be present, as the VOR alone should be sufficient to maintain stable gaze. The absence of saccades, coupled with a normal VOR gain, confirms healthy vestibular function.

Factors Influencing VHIT Norms and Interpretation

While general VHIT norms provide a robust framework, several factors can influence the results and require consideration during clinical interpretation. These include patient age, the specific equipment used, and the technique of head impulses. For instance, some studies suggest slight age-related changes in VOR gain. Consistency in head impulse technique is also vital to obtain reliable and comparable results. Understanding these variables helps refine the application of diagnostic criteria for vestibular disorders.

Clinical Significance of VHIT Norms

Comparing a patient's VHIT results against established VHIT norms is fundamental for diagnosing unilateral or bilateral peripheral vestibular hypofunction. Abnormal VOR gain and the presence of corrective saccades, particularly covert saccades, are hallmark signs of vestibular weakness. This information is crucial for guiding treatment decisions and rehabilitation strategies for patients experiencing dizziness and imbalance. The VHIT offers an objective measure of semi-circular canal function, making it an indispensable tool in vestibular assessment protocols.