Videonystagmography (VNG)
What is videonystagmography (VNG)?
Videonystagmography (VNG) is a vestibular assessment of peripheral vestibular systems located in the inner ear and of the central motor functions of eye movement. VNG testing uses goggles with infrared cameras to track eye movements, and can help to determine if the patient is experiencing an abnormality from the peripheral or central vestibular system, or from an impairment in the oculomotor system.
Benefits of VNG
Videonystagmography (VNG) has emerged as the new standard over electronystagmography (ENG) due to its convenience, efficiency, and accuracy in tracking eye movement. Using high-resolution video goggles allows clinicians to observe very small eye movements – that would otherwise be unobservable on the bedside – and quantify any changes in these measurements.
In doing so, you can:
- Determine the origin (sensory, motor output, and/or neural pathways) which may be responsible for your patient’s reported symptoms.
- Assess your patient’s functional ability to use the system inputs in an integrated fashion.
- Determine the appropriate intervention for your patient to experience relief, such as vestibular rehabilitation.
The different VNG tests
There are many different VNG tests to choose between when building your test protocol. The following provide short introductions to each test, with links to further and dedicated readings.
Active Head Rotation (AHR)
The Active Head Rotation (AHR) test assesses the residual function of the vestibular system, which can be useful in patients with bilateral caloric weakness. The test involves active head movements to a metronome beat while fixating on a target. Results are shown in plots for gain, phase, and symmetry, with a normal gain being 100% and symmetrical responses expected.
Further reading: Active Head Rotation
Advanced Dix-Hallpike
The Advanced Dix-Hallpike test, used to detect Benign Paroxysmal Positional Vertigo (BPPV), includes a 3D head model guide and torsional nystagmus analysis for improved accuracy. As in the standard Dix-Hallpike test, the patient is positioned with their head turned 45 degrees and laid down with their head hanging 10 degrees.
Eye movements are graphed for horizontal, vertical, and torsional movements, with active BPPV indicated by a red diamond. The slow phase velocity for torsional eye movements is also graphed in this test, with eye movements greater than zero plotted as right slow torsion, which is counterclockwise (CCW) to the examiner. Eye movements less than zero are plotted as left slow torsion, which is clockwise (CW) to the examiner.
Further reading: Advanced Dix-Hallpike
Caloric irrigation
The caloric test is used to test the responsiveness and symmetry of the lateral semicircular canals in the vestibular system. It involves water or air irrigation of each end organ separately to identify any weaknesses or asymmetries in vestibular function. Abnormal results can show unilateral or bilateral weakness or directional preponderance. The test is a valuable tool for assessing vestibular function and allows for independent evaluation of each peripheral vestibular end organ.
Further reading: Caloric test
Gaze
The gaze test evaluates a patient's ability to maintain steady gaze without extraneous eye movements. It helps to identify central or peripheral vestibular system lesions, with gaze positions tested in various angles. Normal gaze produces a straight line, while abnormal tracings may exhibit square wave jerks, nystagmus, or gaze decay. An average nystagmus slow phase velocity exceeding 6 degrees per second for horizontal positions indicates an abnormal response.
Further reading: Gaze test
Lateral Head Roll
The Lateral Head Roll test is a diagnostic tool used to identify horizontal canal BPPV. It involves turning the patient's head 45 degrees to the right and left while recording for at least 20 seconds in each position. The test can be aided by a 3D head model guide and proper head placement instructions to ensure accurate positioning. The results are summarized on a graph, with any nystagmus over 6 degrees per second indicating an abnormal finding.
Further reading: Lateral Head Roll
Ocular Counter Roll
The Ocular Counter Roll (OCR) is a Vestibulo-Ocular Reflex (VOR) characterized by torsional eye movements in response to the lateral tilt of the head. There are two components of the OCR.
The first is a dynamic part that shows up as nystagmus on the tracings. This part occurs during the head tilt and is largely driven by activity from the semicircular canals and the otolithic organs.
The second component of the OCR is the static part, which is the ocular tilt that occurs after the head movement. This tilt comes primarily from the utricles and should be approximately 10 to 25% of the gain of the head movement.
Further reading: What is the Ocular Counter Roll (OCR) test?
Optokinetic nystagmus
The optokinetic nystagmus test measures the ability to track objects in motion with the eyes while the head is still. The responses should be symmetrical for both directions, and abnormal responses can suggest a central vestibular disorder. A video projector or LCD screen is needed to stimulate the patient's entire visual field. If the patient has a normal optokinetic response in one direction but not the other, it can suggest a lesion on that side. Optokinetic nystagmus testing helps to determine if there is central pathology that hinders reflexive response to moving targets.
Further reading: Optokinetic nystagmus test
Positional nystagmus
Positional nystagmus testing is used to determine if a change of position of the patient's vestibular systems in space provokes nystagmus. Central and peripheral vestibular lesions can cause positional nystagmus and vertigo, and the examination focuses on distinguishing between them. Observations of direction, intensity, latency, and fatigability of nystagmus are important for diagnostic purposes.
Further reading: Positional nystagmus testing
Saccades
The saccade test measures the patient's ability to accurately move the eyes from one designated focal point to another target in a single, quick movement. It quantifies the latency, velocity, and accuracy of eye movements. Responses for each saccade are represented on accuracy, velocity, and latency graphs, which show whether responses are within or outside of normal limits.
Further reading: Saccade test
Saccadometry
Saccadometry is an advanced oculomotor protocol used to diagnose central vestibular injuries and consists of the prosaccade and antisaccade tests.
The prosaccade protocol is similar to the random saccade task, except the patient always returns to the center target and the target location is always equidistant to the center target. The antisaccade is much more cognitively complex, as it requires the patient to suppress their reflexive eye movement toward the target and they are instructed to look equal and opposite from the target.
In prosaccade and antisaccade tasks, normal saccade parameters are evaluated along with directional error rate. The fatigability and increased error rates further assist in the diagnosis of central abnormalities.
Further reading: An introduction to Saccadometry
Smooth pursuit
Smooth pursuit testing is an oculomotor test used to assess a patient's ability to accurately track a visual target that is moving in a smooth, controlled manner in the horizontal or vertical plane. It is used to determine if there is central pathology that is precluding accurate tracking of moving targets by the eyes. It is susceptible to an age effect and attention.
Further reading: Smooth pursuit eye movement testing
Spontaneous nystagmus
The spontaneous nystagmus test determines if a patient has any nystagmus, with or without fixation, when sitting with head and eyes in a neutral position. It is essential to identify the presence of spontaneous nystagmus before performing any other VNG tests. A patient with no spontaneous nystagmus will produce a straight line. An abnormal test result will reveal the presence of nystagmus in the horizontal and/or vertical channels.
Further reading: Spontaneous nystagmus test
