The VNG is a specialized diagnostic test that evaluates the function of the inner ear and the central nervous system pathways responsible for maintaining balance. It works by precisely measuring and recording involuntary eye movements, known as nystagmus, which are directly linked to the vestibular system. The VNG test uses lightweight goggles equipped with infrared cameras that track the pupils during various tasks, providing objective data on how well the balance system is functioning.
Symptoms and Conditions Diagnosed
Patients undergo a VNG test when they report symptoms suggesting a balance disorder, such as persistent dizziness, vertigo, lightheadedness, or unexplained imbalance. The test helps clinicians determine whether these symptoms originate from the peripheral vestibular system in the inner ear or from the central vestibular pathways in the brainstem and cerebellum.
VNG testing helps diagnose several specific conditions affecting the balance organs. These include Benign Paroxysmal Positional Vertigo (BPPV), Meniere’s Disease, labyrinthitis, or vestibular neuritis. Furthermore, the test can assist in detecting central nervous system involvement, such as lesions, stroke, or vestibular signs related to multiple sclerosis or acoustic neuromas.
Preparing for the Test
Patient preparation is required for accurate VNG results. Patients must discontinue certain medications that can suppress or alter the vestibular response, such as antivertigo medications, sedatives, tranquilizers, and some antihistamines, typically for 24 to 48 hours before the procedure. Patients should consult their prescribing physician before stopping any prescribed medication.
Dietary restrictions include avoiding alcohol and caffeine for at least 24 hours prior to the test. Fasting from solid foods for a few hours may also be recommended to minimize the risk of nausea. Patients are advised to wear comfortable clothing and to avoid eye makeup, as it can interfere with the camera’s ability to track the pupil accurately. Arranging for transportation home is also suggested, as the test can provoke temporary dizziness.
The VNG Procedure Explained
The VNG procedure takes place in a darkened room with the patient wearing specialized video goggles. These goggles contain infrared cameras that continuously record eye movements throughout the test. The test is systematically divided into three main phases, each designed to assess a different component of the balance system.
Ocular Motor Tests
The first phase consists of Ocular Motor Tests, which evaluate how well the eyes track moving visual targets. The patient follows lights or dots that move across a screen in different patterns while their head remains still. This section primarily assesses the function of the brainstem and cerebellum, which control the coordination between the visual and vestibular systems. Abnormalities here, such as impaired smooth pursuit, often point toward a central vestibular issue.
Positional Testing
The next phase is Positional Testing, where the examiner moves the patient’s head and body into specific positions. A common maneuver is the Dix-Hallpike test, which involves quickly moving the patient from a sitting position to lying down with the head turned. The purpose is to provoke nystagmus, particularly to identify conditions like BPPV, where displaced inner ear crystals trigger involuntary eye motion. The resulting eye movements determine the affected ear and the specific inner ear canal involved.
Caloric Test
The final phase is the Caloric Test, which assesses each inner ear separately. Warm and then cool air or water is gently introduced into the ear canal. This temperature change stimulates the fluid in the inner ear’s horizontal semicircular canal, mimicking a head movement. The stimulation should prompt a specific, measurable nystagmus, and the response from one ear is compared to the other. A significantly reduced or absent response indicates damage to the peripheral vestibular system on that side.
Understanding VNG Results
Interpretation of VNG results centers on the characteristics of the recorded nystagmus. A normal result shows symmetrical responses between the two ears and expected eye movements during tracking and positional changes, suggesting the balance system is functioning within normal limits. Abnormal results are identified by specific patterns of nystagmus that deviate from the norm.
Asymmetry in the caloric test, where one ear shows a weaker response, suggests a peripheral lesion within the inner ear. Conversely, certain types of nystagmus observed during the ocular motor tests, or nystagmus that changes direction, are indicative of a central vestibular disorder. The specialist analyzes these objective measurements to accurately determine the site of the dysfunction and guide further diagnosis or treatment.