Videonystagmography (VNG) is a diagnostic procedure used to evaluate patients experiencing symptoms like dizziness, vertigo, or instability. The test employs specialized video goggles equipped with infrared cameras to precisely record and measure eye movements. The primary goal of VNG is to objectively assess the function of the vestibular system, which is the body’s internal balance apparatus located within the inner ear. Analyzing how the eyes move in response to various stimuli helps clinicians determine if a balance problem originates in the inner ear or in the brain pathways that process balance information. This comprehensive examination guides subsequent treatment decisions.
Evaluating the Vestibular System
VNG testing relies on the close relationship between the inner ear and the eyes, mediated by the Vestibulo-Ocular Reflex (VOR). The VOR is an involuntary reflex that functions to stabilize vision on a fixed point even while the head is moving. When the head turns, the inner ear’s semicircular canals sense the rotation and immediately send signals to the eye muscles. This causes the eyes to move an equal distance in the opposite direction, ensuring the image remains steady on the retina.
If the VOR is impaired due to a disorder in the inner ear or connecting nerves, the stabilizing reflex becomes faulty. This malfunction results in nystagmus, which is an involuntary, repetitive, and rhythmic movement of the eyes. A compromised VOR can also lead to oscillopsia, a sensation where objects appear to jump or oscillate in the visual field. VNG technology meticulously tracks and documents the presence, direction, and intensity of nystagmus to provide an objective measurement of the VOR’s status.
The Sequential Steps of VNG
A complete VNG examination is a battery of tests divided into three phases, each designed to challenge different parts of the balance system.
Oculomotor Testing
This first phase evaluates the brain’s ability to control eye movements independent of the inner ear. The patient performs specific visual tasks, such as tracking a smoothly moving light or rapidly shifting gaze between two stationary targets. These tests assess the functional integrity of the brainstem and cerebellum, the central nervous system structures responsible for coordinating visual and motor control.
Positional Testing
The second phase involves Positional Testing, where the patient is moved into various head and body positions while wearing the recording goggles. The purpose is to determine if certain postures provoke nystagmus or symptoms of vertigo. Specific maneuvers, such as the Dix-Hallpike test, are used to check for Benign Paroxysmal Positional Vertigo (BPPV), a common inner ear condition. The characteristics of the nystagmus, including its direction and duration, are carefully recorded to localize the problem.
Caloric Testing
The final component is Caloric Testing, the only VNG subtest that evaluates each inner ear separately. Warm and cool air or water is gently introduced into the ear canal, causing a temperature change that stimulates the horizontal semicircular canal. This thermal stimulation mimics head movement, inducing a temporary nystagmus in a person with normal vestibular function. Comparing the eye responses from the cool and warm stimuli in both ears helps the clinician identify if one ear is significantly weaker or less responsive than the other.
Distinguishing Peripheral and Central Findings
VNG differentiates between dysfunction in the peripheral vestibular system (inner ear and vestibular nerve) and the central vestibular system (brainstem and cerebellum). Peripheral findings point to issues within the inner ear structures, such as conditions like vestibular neuritis or Meniere’s disease. A common peripheral finding is unilateral weakness on the caloric test, indicating a reduced response from one inner ear compared to the other.
Nystagmus associated with peripheral disorders usually has a horizontal direction and is suppressed when the patient visually fixates on a target. Positional nystagmus that is provoked, fatigues quickly, and is accompanied by intense vertigo suggests Benign Paroxysmal Positional Vertigo (BPPV). The pattern of eye movement often follows predictable rules, which helps in localizing the affected semicircular canal.
Conversely, central findings indicate the problem lies in the brain’s processing centers, which integrate balance and visual information. Abnormalities during the oculomotor tests, such as poor smooth pursuit tracking or inaccurate saccadic eye movements, are strong indicators of central nervous system involvement. Nystagmus with purely vertical or torsional movements often signals a central lesion. Unlike peripheral nystagmus, central nystagmus is frequently not suppressed by visual fixation.