Videonystagmography (VNG): An Introduction

Table of contents

• What is videonystagmography (VNG)?
• Why do patients have VNG testing performed?
• Pre-test examinations
• Pre-test considerations
• Calibration process
• The different VNG sub tests
• VNG equipment

Related course: Getting started: VNG

What is videonystagmography (VNG)?

Videonystagmography (VNG) is a series of tests that measure eye movements. Some tests within VNG look at how well the brain can control and coordinate eye movements, while other tests look at how the brain uses vestibular sensory information to control and position the eye.

One of the main purposes of VNG testing is to check for the presence of – and measure – an eye movement called nystagmus. Nystagmus is a term used when there are eye movements that are rhythmical, repetitive, and involuntary. These nystagmic eye movements can move:

• Side to side (horizontal nystagmus)
• Up and down (vertical nystagmus)
• In a circle (torsional nystagmus)

VNG has emerged as the gold standard assessment of eye movement 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 and brisk eye movements that would otherwise be unobservable by direct observation at the bedside. You can also use VNG to quantify any changes in these measurements over time.

Why do patients have VNG testing performed?

VNG often covers:

• Spontaneous nystagmus testing
• Gaze testing
• Saccade testing
• Pursuit testing
• Optokinetic testing
• Positional and positioning testing
• Caloric testing

Let’s start by considering the sub tests of the oculomotor test battery. Oculomotor function is assessed by measuring spontaneous nystagmus, gaze stability, saccades, smooth pursuit, and optokinetic testing. 90% of central vestibular lesions are associated with oculomotor dysfunction [1].

Some central nervous system etiologies that you should be aware of are:

• Multiple sclerosis
• Cerebellar ataxias
• Alzheimer's disease
• Brainstem infarctions
• Space occupying lesions
• Ophthalmological conditions
• Progressive supranuclear palsy
• Transient ischemic attacks and strokes

The diagnosis of the central vestibular disorders is through the falsification of peripheral disorders, and we don't want to miss any of these conditions.

When managing vestibular patients, central conditions are actually more common than you might think. A study by Strupp et al, 2020 [2] looked at the distribution of conditions in different types of clinics. If we add the vestibular migraine to central vestibular conditions, a large proportion of patients in a neurology clinic have what can be diagnosed as a central vestibular condition.

However in an ENT clinic, a condition such as Benign Paroxysmal Positional Vertigo (BPPV) is often responsible for many of the patients’ symptoms.

This brings us to positioning and positional testing. Positioning testing, generally associated with tests for BPPV, would be considered with dizziness that's elicited by movement. Whereas positional testing tests for dizziness induced by a certain head and/or body position. These can either be of central cause or due to compression of nerves or even blood supply.

Often, the last test performed in VNG is the caloric irrigation test, and even after many years it is still considered the gold standard test of ear-specific peripheral vestibular function.

Pre-test examinations

Before you begin testing, there are certain pre-test examinations that are important to conduct.

1. Assess the health of the ear

First, assess the health of the ear using otoscopy and tympanometry. This is especially important when considering caloric testing later on. There is also value in performing audiometry, as any changes in the audiogram may give some indication of the involved ear, or whether a pathology is also affecting the auditory structures and nerve pathways.

2. Assess the eyes

Not only should you assess the health of the ear, but you can also benefit from performing some pre-test examinations of the eyes and their movement. Here, you are looking to get an understanding of the gross oculomotor function, and specifically, the function of the muscles involved in eye movement:

• Lateral rectus
• Medial rectus
• Inferior rectus
• Superior rectus
• Inferior oblique
• Superior oblique

Range of motion and smooth pursuit function

The first test you can perform to measure these is one that will also assess the range of eye motion and smooth pursuit function by tracing an imaginary ‘H’ in the air with your finger and asking the patient to follow with their eyes only (Table 1).

Table 1: Engaging the various eye muscles.

During this examination, you're not only looking to see if there's any nystagmus in a gaze-holding position. You're also looking to see:

• That the eyes can trace / follow your finger in that full range of motion.
• Disconjugate eye movements. This is where one eye moves less than the other.
• Endpoint nystagmus that is generated far earlier than it ought to be. For example, at just a 20-degree holding position.

Volitional saccades

By holding your hands in the air, and by alternating a finger on one hand or the other, you can ask the patient to quickly move their eyes from one finger to the other. By doing this, you can assess their ability to make a volitional saccadic eye movements following your instructions.

Skew deviation

You can also assess for skew deviation with the cover and uncover tests. Cover one eye with an object or your hand for a couple of seconds and then uncover the eye. Observe for the eye realigning from a drift once the eye is uncovered or if the eye is able to stay focused at the point it was on before you covered it.

In summary, for somebody with no skew deviation, as you alternate the cover over each eye, you will notice there is no re-fixation / drift of either eye, as both are able to maintain that fixation on the target behind.

Clinical head impulse test (cHIT)

As a final bedside test, you can also perform a clinical HIT examination. This test works by testing the vestibulo-ocular reflex (VOR), which is needed to maintain corrective eye position during any change in head position and to correct the eye movement rapidly so that vision remains on the target [3].

During a head impulse, have the patient sitting in front of the tester with their eyes fixed on the examiner's nose or a distant target. Ask the patient to keep their eyes looking at the target while the examiner moves the patient’s head quickly and unpredictably to 10 to 15 degrees of neck rotation.

While doing so, look for any overt saccades, which are eye movements the patient might make to refocus on the target.

An abnormal result would be the appearance that the eyes are dragged off the target by the turning of the head, followed by a corrective saccade back to the target after the turning of head. If the eyes appear to remain in position, this is a normal response for the bedside clinical HIT [3].

Pre-test considerations

Before proceeding with the test, you should also have a gross understanding of the patient's history. Here, you're thinking about symptoms, syndromes, and medication.

Symptoms

Symptoms and signs that you should be looking out for and asking the patient are:

• Vertigo
• Dizziness
• Oscillopsia
• Nystagmus
• Ocular tilt reaction
• Vestibulo-ocular reflex failure

It is important to remember that patients often have different words to describe their dizziness symptoms. You can also consider tumbling sensation, unsteadiness, and disequilibrium, to name a few. It often takes a few questions to understand what the patient is trying to describe when reporting their symptoms.

Syndromes

The timing and frequency of a patient’s symptoms are often referred to as an episode or syndrome. We can categorize these syndromes into acute vestibular syndrome, episodic vestibular syndrome, and chronic vestibular syndrome. We can understand these symptoms more based on the patient's reported history.

Acute vestibular syndrome

An acute vestibular syndrome is characterized by an acute onset of symptoms, which reduce over hours and resolve within seven days. This could be caused by conditions which have suddenly affected the peripheral vestibular sensory system, such as vestibular neuritis, certain types of strokes, or more rarely, labyrinthitis.

Episodic vestibular syndrome

Episodic vestibular syndrome has brief peaks of symptoms which repeat over time. They can also be described as discrete episodes of symptoms. What you need to try and understand is any underlying pattern to these recurring episodes of symptoms, as this is often helpful in differentiating between different conditions.

Conditions like BPPV, vestibular migraine, and Meniere’s disease can produce episodic vestibular syndromes, but have their own discrete pattern of presentation, or triggering factors.

Chronic vestibular syndrome

Chronic vestibular syndrome is characterized by a steady increase of symptoms over a long period of time. This could be due to poorly compensated unilateral vestibulopathy, PPPD, or cerebellar degeneration. By asking the patient to describe the history of their symptoms, you can understand where they may fall within this categorization of syndromes.

Chronic symptoms are often challenging to investigate, as the patient may have more than one condition overlaying due to how they have responded to the longer duration of symptoms. For example, anxiety is often associated with patients who are experiencing chronic symptoms, and this can add its own subset of additional symptoms overlaying the original condition the patient experienced [4].

Medication

Finally, a point on medication before starting a VNG appointment. The medication should be reviewed by the referring physician. However, in general, it's accepted that any vestibular suppressants that have been taken for short term should be stopped 24 to 48 hours before testing to avoid the suppression of vestibular symptoms which are useful to see during testing.

However, if a patient has been taking vestibular suppressants for a long time, the patient may now have a new baseline or norm. Stopping the medication may make them more symptomatic before testing, or they may become anxious about stopping longer term medication.

As such, if they have been taking medications that suppress their vestibular symptoms for a long time, it is generally advised that they can continue to take these before the VNG appointment, and that you add a note to the VNG report on what medications the patient took before the VNG assessment.

Calibration process

The calibration process is a straightforward task, but it’s important to do it as accurately as possible. This involves explaining the process to the patient, and what is required from them to complete the task.

You need to ask the patient to look at the center dots (target) on the TV screen. You use the dots to direct the patient’s eye into different horizontal and vertical positions so that you can calculate the distance their eyes are moving.

Instruct them that the dot is going to move left, right, up, down, and then back to the center. Ask your patient to follow the dot with their eyes only, and it’s important they keep their head still.

In the VisualEyes™ software, there is a default calibration. If a patient isn't able to perform the calibration task, then you can use the default calibration. We always recommend that, if possible, you try to get an individualized calibration as this is going to give you the most accurate results.

Further reading: Eye calibrations

The different VNG sub tests

There are many different VNG sub tests to choose between when building your VNG test protocol. The following descriptions provide short introductions to each test, with links to further and dedicated reading for a greater deep dive.

Oculomotor eye movement testing

Firstly, we’ll cover tests belonging to the category of oculomotor eye movement testing.

Spontaneous nystagmus

The spontaneous nystagmus test is the first test to do in any VNG test battery. That's because you want to ensure that there aren't any underlying eye movements which are going to impact later tests in your VNG protocol.

The spontaneous nystagmus test determines if the patient has any nystagmus, with or without fixation. The patient is seated with their head and eyes looking straight ahead.

A patient with no spontaneous nystagmus will generally produce an eye recording which looks like a straight line. It’s normal to have some small eye movements when the patient is in the dark with no fixation. What you’re trying to rule out is an abnormal test result which would reveal the presence of nystagmus in the horizontal and/or vertical VNG eye recordings.

If you detected that nystagmus was present, you would also record the patient’s eye movement with a light turned on in the goggle to allow fixation. You would also extend the recording time and ask the patient to look towards the right and the left to check for changes in nystagmus velocity and direction.

Further reading: Spontaneous nystagmus test

Gaze test

The next test in the VNG test battery is the gaze test. The gaze test doesn't require the cover on the goggles because you’re going to ask the patient to look at a target (dot) on the screen. This is similar to the task we perform in calibration, however this time you’re going to ask the patient to hold their eyes in different positions for a longer time.

The gaze test evaluates a patient's ability to maintain steady gaze without making any extraneous eye movements, and also without experiencing nystagmus. It helps to identify central or peripheral vestibular system lesions, with gaze positions tested at horizontal and vertical angles.

Instruct the patient that, during this test, you’re going to ask them to look at the target on the screen. The target is going to move leftwards, rightwards, upwards and downwards. They will need to move their eyes to the new position as soon as the target moves. We must also instruct them to keep looking at that target while it's in that position.

Once completed, you’ll see the results for the five gaze positions the patient’s eyes moved to. As with the spontaneous nystagmus test, if there was any nystagmus present, this would be recorded and displayed in each of the average slow phase velocity graphs, which you can see to the right of each test.

Normal gaze produces eye positions that record 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

Saccades

Following the gaze test, the next test in the VNG test battery is the saccade test. In the saccade test, the patient follows a randomly moving target. That random movement can be horizontal, vertical, or a combination of horizontal and vertical movements. Most clinics routinely assess horizontal eye movements and use a randomized horizontal saccade test sequence.

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 allows for the accurate measurement of the latency, velocity, and accuracy of eye movements during the saccade tests. Responses for each saccade are represented on accuracy, velocity, and latency graphs, which can indicate whether responses are within or outside of expected normal limits.

Further reading: Saccade test

Smooth pursuit

The smooth pursuit test is often completed after the saccade test. It assesses a patient's ability to accurately track a visual target that is moving in a smooth, controlled manner in the horizontal or vertical plane.

As with the other tests in oculomotor testing, you can use this to determine if there is central pathology that is stopping accurate tracking of moving targets by the eyes. It is worth noting that the smooth pursuit test is the most susceptible to an age effect and attention.

It’s recommended that isolated abnormalities in the smooth pursuit test are repeated and can be cross checked with the optokinetic test performance.

Further reading: Smooth pursuit eye movement testing

Optokinetic nystagmus

The optokinetic reflex allows for the eyes to follow a moving target while the head remains still. The most common stimulus that we use with adults is the checkerboard or vertical stripes. However, depending on the age of the patient, you can make things a little bit more interesting. If testing children for example, you could change the stimulus to images and pictures.

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 disorder.

In a similar way to the smooth pursuit test, the optokinetic nystagmus test helps to determine if there is a potential central pathology present that is reducing the patient’s reflexive response to tracking moving targets.

As with the smooth pursuit test, it is susceptible to an age effect and attention. For some patients who may describe sensitivity to movement in their visual fields, they may try to suppress the reflexive eye tracking response.

It’s recommended that isolated abnormalities are repeated and can be cross checked with the smooth pursuit test.

Further reading: Optokinetic nystagmus test

Positioning testing

Now, we’ll cover tests belonging to the category of positioning testing.

Dix-Hallpike testing

In the Dix-Hallpike test, you take the patient from sitting with their head turned to the right or left, to lying down with their head extended backwards, below the body if possible. Hold this position for up to 60 seconds and then assist the patient back to sitting upright.

The purpose of the Dix-Hallpike test is to look for BPPV. You perform this test with the VNG goggle cover on so that fixation is removed. You want to see if the patient has crystals (otoconia) floating around in the posterior semicircular canal. If so, the patient would feel dizzy when:

• They lie down
• They roll over in bed
• They tilt their head backwards
• Sitting up from being laid down

With the traditional Dix-Hallpike protocol conducted with VNG eye recording, you’ll see the horizontal and vertical eye movements plotted on the VNG recordings. At time zero, with the patient's head laying down, you can check for nystagmus and symptoms.

If there’s no nystagmus, you can hold the Dix-Hallpike test position for up to 60 seconds. With some cases of BPPV, the nystagmus can have a delay before the free-floating crystals start to abnormally simulate the posterior semicircular canal.

Once you’re confident that you have held the position long enough to see any abnormal nystagmus, or if nystagmus has been triggered, until the nystagmus resolves, you then sit the patient upright.

Advanced Dix-Hallpike

The Advanced Dix-Hallpike test is conducted using the same technique as the traditional Dix-Hallpike, but uses an additional accelerometer and includes a 3D head model to guide the head position, with torsional nystagmus analysis for improved accuracy in recording eye movement. As in the standard Dix-Hallpike test, position the patient with their head turned 45 degrees and laid down with their head hanging 20 degrees where possible.

Eye movements are recorded and displayed for horizontal, vertical, and torsional movements, with active eye movements that could be BPPV indicated by a red diamond in the VisualEyes™ software.

The slow phase velocity for torsional eye movements is also measured and displayed 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

Lateral Head Roll

The Lateral Head Roll test is an assessment you can use to identify horizontal semicircular canal BPPV. For this assessment, lay the patient supine with their head lifted approximately 30 degrees, either by inclining the examination couch or using a pillow.

Then ask them to turn their head as close to 45 degrees to either the right or left, recording the eye movement for at least 30 seconds in each position. As with the Dix-Hallpike test, repeat the maneuver to the opposite side.

A positive test for horizontal semicircular canal BPPV would be the triggering of nystagmus. As with the Advanced Dix-Hallpike test, the test can be aided by using the accelerometer and 3D head model to guide optimal head placement and positioning.

Within the VNG software, the results are summarized on a graph, with any nystagmus over 4 degrees per second indicating an abnormal finding [5].

Further reading: Lateral Head Roll

Positional nystagmus

For positional nystagmus testing, you start by having the patient supine with their head in the center. Record the eye movement with the goggle cover in place, as this removes fixation and allows for more accurate eye movement assessment, especially for eye movements that might be suppressed during fixation.

If nystagmus is triggered, you can assess the nystagmus behavior to fixation by turning on a fixation light inside the goggle. Then repeat the test in other positions such as with the head to the right and then to the left. You may also ask the patient to roll on onto their right and left sides.

You can use positional nystagmus testing to determine if a change of position of the patient's head and by association their peripheral vestibular system 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, as is any nystagmus behavior with and without fixation.

Further reading: Positional nystagmus testing

Caloric test

The caloric test is usually the last test that we perform in the VNG test battery.  It tests the responsiveness and function of the lateral semicircular canals by measuring them and comparing the symmetry of their response.

It involves water or air irrigation of each ear to stimulate the peripheral vestibular end organ separately to identify any weaknesses or asymmetries in vestibular function of the lateral semicircular canals.

The patient is laid supine with their head angled upright at 30 degrees, as this is the correct angle to have their horizontal / lateral canals to stimulate the ears effectively with caloric irrigation.

The caloric test is one of the few tests in the VNG test protocol where we expect the patient to experience some dizziness. This does vary between patients but preparing them that this might be something they experience for 1 to 2 minutes allows you to complete the test successfully.

You can also use distraction techniques, such as asking the patient to count aloud, recite children’s names, or recite places they have visited on holiday. This distracts them from the spinning sensation they might be experiencing, but also reduces the patient’s instinct to subconsciously try to suppress the dizziness and nystagmus you have triggered with the simulation.

An abnormal result in the caloric test can indicate a unilateral or bilateral weakness or directional preponderance. The test is a valuable tool and remains the gold standard for assessing low frequency peripheral vestibular function and allows for independent evaluation of lateral canal function in the peripheral vestibular end organ.

Further reading: Caloric test

Additional sub tests

Finally, we’ll cover additional sub tests that you can conduct within a VNG test protocol.

Ocular Counter Roll

The Ocular Counter Roll (OCR) is a more recent introduction to the VNG test protocol. It is an additional assessment of the 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?

Active Head Rotation (AHR)

The Active Head Rotation (AHR) test is another recent addition to the VNG test protocol. This 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 keeping the eyes 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

Rotational chair testing

With the addition of equipment such as a rotational chair to the VNG test protocol, you can extend the testing of the vestibular system at frequencies beyond the caloric test. The rotational chair test is a mid-frequency test of vestibular function, testing a range of different frequencies from 0.01 Hz to 1.28 Hz.

In the rotational chair, you’re moving the patient’s head and therefore their peripheral vestibular organs at different speeds. You can then compare this movement to the triggered eye movement and use this to assess the VOR.

Patients are tested with their eyes open without fixation. They wear VNG goggles with the cover on or seated inside a dark enclosure. Mental alerting tasks are performed, for the same purpose as we described in the caloric testing, and the nystagmus is recorded and measured.

Rotational chair testing is often used when assessing populations such as infants and children who do not tolerate the caloric assessment, as well for adults when caloric testing may be contraindicated due to ear pathology.

For vestibular impairments that affect both peripheral vestibular organs, such as bilateral lateral semicircular canal hypofunction, the rotational chair test is considered the gold standard assessment.

Further reading: An introduction to rotary chair testing

Saccadometry

Saccadometry is another new addition to the VNG protocol. It is an advanced oculomotor protocol used to assess eye movements to diagnose central vestibular injuries and consists of a combination of the prosaccade and antisaccade tests.

The prosaccade protocol is like 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 in equal and opposite directions from the target.

In the 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 abnormalities in the central nervous system pathways.

Further reading: An introduction to Saccadometry

VNG equipment

Within this VNG guide, we might have referenced some equipment that you have not used or encountered in your clinical practice. The table below will assist in helping you to discover the equipment needed to perform VNG testing as described in this article.

Table 2: VNG equipment offered by Interacoustics.

Summary

As we have covered in this article, VNG testing covers many sub tests with assessments that have applications in various fields such as vestibular audiology, physical therapy, and neurology.

With VNG systems such as VisualEyes™ and any complementary pieces of equipment, you’re in a good position to tailor the vestibular test battery to the testing needs and specific patient population of your clinic.

Related courses

• Getting started: VNG
• Advances in VNG
• Beyond the basics: Identifying and minimizing artifacts in VNG

References

[1] Strupp, M., Hüfner, K., Sandmann, R., Zwergal, A., Dieterich, M., Jahn, K., & Brandt, T. (2011). Central oculomotor disturbances and nystagmus: a window into the brainstem and cerebellum. Deutsches Arzteblatt international, 108(12), 197–204.

[2] Strupp, M., Dlugaiczyk, J., Ertl-Wagner, B. B., Rujescu, D., Westhofen, M., & Dieterich, M. (2020). Vestibular Disorders. Deutsches Arzteblatt international, 117(17), 300–310.

[3] Halmagyi, G. M., & Curthoys, I. S. (1988). A clinical sign of canal paresis. Archives of neurology, 45(7), 737–739.

[4] Jacobson, G. P., Shepard, N. T., Barin, K., Janky, K., & McCaslin, D. L. (Eds.). (2020). Balance function assessment and management. Plural Publishing.

[5] McCaslin, D. L. (2019). Electronystagmography and videonystagmography (ENG/VNG). Plural Publishing.