VNG is a diagnostic tool used to evaluate the function of the vestibular system, the balance mechanism located in the inner ear. The test records and analyzes involuntary eye movements, known as nystagmus, which are linked to inner ear function via the vestibulo-ocular reflex. VNG helps determine if symptoms like dizziness or vertigo are caused by a peripheral issue (inner ear) or a central issue (brainstem or cerebellum). The procedure requires the patient to wear specialized goggles equipped with infrared cameras that precisely track eye movements.
Patient Preparation Before the Test
To ensure the VNG test yields accurate data, specific preparations must be followed. The most important step is discontinuing certain medications that can suppress or alter the vestibular system’s response, leading to misleading results. Patients are typically instructed to stop taking sedatives, tranquilizers, antivertiginous drugs, and antihistamines for 24 to 48 hours before the examination. Maintenance medications for conditions like heart disease or diabetes should be continued unless otherwise advised by a physician.
Dietary restrictions are also necessary, particularly avoiding stimulants and heavy meals. Caffeine and alcohol should be avoided for at least 24 to 48 hours before the test, as they affect the body’s natural responses. Patients should refrain from eating solid food for two to four hours before the test. Any meal consumed should be light to minimize the potential for nausea during the procedure.
The specialized goggles require a clear field of vision, so patients must not wear eye makeup, especially mascara or eyeliner, on the day of the test. These dark cosmetics interfere with the infrared camera’s ability to focus on the pupil, compromising the recording quality. Wearing comfortable, loose-fitting clothing is also recommended, as the test involves various body and head position changes.
The Three Phases of the VNG Examination
The VNG is divided into three distinct phases, each testing a different aspect of the balance system. The examination begins with the patient seated in a darkened room, wearing the infrared goggles. This initial setup allows the clinician to detect spontaneous nystagmus, which are involuntary eye movements occurring when the head is still.
The first phase, Ocular Motor Testing, evaluates how well the eyes and brainstem coordinate visual tracking and movement. The patient keeps their head still while following small lights or dots that move across a screen in different patterns. This includes saccades (rapid eye movements) and smooth pursuit (tracking a slow-moving target). Abnormalities in these movements often suggest a central nervous system issue rather than a peripheral inner ear problem.
The second phase, Positional Testing, assesses how gravity influences the inner ear and whether certain head or body positions provoke dizziness. The clinician quickly moves the patient into specific positions, such as lying back with the head turned to one side. This maneuver is often used to diagnose Benign Paroxysmal Positional Vertigo (BPPV). The infrared cameras track the resulting nystagmus, providing objective evidence of a positional disorder.
The final phase is Caloric Testing, which isolates and stimulates each inner ear separately to determine if one is weaker than the other. While lying on the back, the patient’s head is positioned to angle the horizontal semicircular canal for optimal stimulation. Controlled bursts of warm and cool air are introduced into the ear canal, creating a temperature difference that stimulates the inner ear’s fluid. This thermal stimulation induces a brief sensation of vertigo and nystagmus, allowing the clinician to compare the responsiveness of the left and right vestibular nerves.
Analyzing Results and Post-Test Effects
Following caloric testing, patients often experience temporary dizziness or mild nausea due to inner ear stimulation. Because of this possible discomfort, it is recommended that patients arrange for someone else to drive them home after the procedure. The full examination typically takes about an hour, and any side effects are short-lived.
The VNG equipment records a large volume of data, capturing the speed, direction, and intensity of eye movements as waveforms. The results are not instantaneous; the audiologist or physician must analyze these recorded waveforms to form a diagnostic report. By examining the patterns of nystagmus elicited during each phase, the clinician determines if the problem is peripheral (e.g., weakness in one inner ear) or central (an issue with the brain’s control of eye movement). This analysis usually takes several days, after which the report is used to plan treatment or suggest further testing.