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LESSON 157

Oculomotor tests

Year 2
Spring semester
Vestibular Assessment I
Vestibular assessment
Duration: 11:49

This lesson covers the oculomotor tests of the VNG battery: saccade latency, velocity and accuracy, smooth pursuit gain, optokinetic nystagmus, gaze nystagmus and fixation suppression. We explain which findings suggest a central problem and which factors can distort the results.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will see the oculomotor part of the videonystagmography, or VNG, battery: saccade, pursuit, optokinetic and gaze tests.
For each test, we will cover what is measured and which findings suggest a central cause.
Oculomotor tests assess not the inner ear but the central pathways that control eye movements.
These pathways include nuclei in the brainstem, the cerebellum and some areas of the cerebral cortex.
An abnormality in these tests suggests that there may be a central cause behind the dizziness.
Also, if there is an oculomotor problem, the interpretation of nystagmus in caloric and positional tests changes.
That is why oculomotor tests are done before the vestibular tests.
The findings do not make a diagnosis; results that suggest a central pattern are passed on to the physician.
A saccade is the eye movement that shifts gaze very quickly from one point to another.
In the test, the patient follows with the eyes a dot that jumps from one place to another on a screen or light bar.
In the graph, the gray dashed line shows the target and the blue line shows the eye movement.
The time between the target's jump and the start of the eye movement is called latency.
In adults, saccade latency is usually around two hundred milliseconds; normal limits vary with age and device.
The position and timing of the targets are randomized so the patient cannot predict the jump.
The saccade test looks at three measures: latency, velocity and accuracy.
Prolonged latency may be linked to reduced attention, older age and problems affecting cortical pathways.
Peak saccade velocity rises with saccade size; this natural relationship is called the main sequence.
Marked slowing of saccades may suggest brainstem problems, or the effect of some drugs and fatigue.
Slowing in one eye or one direction may point in particular to an eye muscle or nerve pathway problem.
Accuracy shows how exactly the eye lands on the target; missing the target is called dysmetria.
This graph shows two different saccades made to the same target.
On the blue line, the eye falls short of the target and reaches it with a small second saccade; this is called hypometria.
Mild hypometria is common in healthy people too and is not meaningful on its own.
On the red line, the eye overshoots the target and comes back; this is called hypermetria.
Marked, consistent hypermetria or severe hypometria are among the findings that suggest cerebellar problems.
A finding matters more if it is consistent in one direction over repeated trials; a single faulty saccade is not interpreted.
Smooth pursuit lets us follow a small, slowly moving target with the eyes without interruption.
In the test, the patient follows a dot moving sinusoidally right and left on a screen, without moving the head.
The main value measured is pursuit gain, that is, the ratio of eye velocity to target velocity.
For slow targets, gain in a healthy adult is close to one; many laboratories expect it to be above about 0.8.
When gain falls, the eye lags behind the target and makes small saccades to catch up; this stepped look is called saccadic pursuit.
The red trace in the graph shows low-gain, stepped pursuit in this example.
Pursuit gain decreases with age, so age-appropriate norms are used.
As target frequency and velocity increase, gain falls in healthy people as well.
A mild reduction in both directions is often nonspecific; attention, fatigue, medications and alcohol also impair pursuit.
In an acute peripheral loss, strong spontaneous nystagmus can also distort the pursuit trace, so the spontaneous recording is checked first.
Marked, repeated impairment in one direction, however, is a more meaningful finding that suggests a central problem.
The pursuit result is always interpreted together with the saccade and gaze tests.
Optokinetic nystagmus, or OKN, is a normal nystagmus that occurs while watching stripes moving across the visual field.
As the stripes move right, the slow phase follows them to the right, and the fast phase brings the eyes back to the left.
The eye movement we make while looking out of a train window is an everyday example.
A true optokinetic stimulus fills the whole visual field; a small stimulus from a light bar largely tests the pursuit system.
Normally, responses to rightward and leftward movement are similar, that is, symmetric.
Marked asymmetry suggests a central problem; the finding is usually consistent with an abnormality on the pursuit test.
In the gaze test, the patient looks straight ahead, then right, left, up and down, and holds gaze there.
The gaze angle is usually about twenty to thirty degrees; in each direction, recordings are taken with and without fixation.
Nystagmus that beats in the direction of gaze and changes direction with gaze is called gaze-evoked nystagmus.
This finding may suggest cerebellar and brainstem problems; antiepileptics, sedatives and alcohol can produce the same picture.
End-point nystagmus, seen on extreme lateral gaze, lasting a few beats and symmetric on both sides, is usually normal.
Peripheral nystagmus, by contrast, does not change direction; following Alexander's law, it increases on gaze toward the fast phase.
There are other findings to watch for in gaze and saccade recordings.
Square wave jerks are small saccades that take the eye off the target, followed shortly by a return.
Occasional square wave jerks are considered normal, especially in older adults; very frequent, prominent ones are centrally meaningful.
If brief nystagmus in the opposite direction appears on returning to center after long lateral gaze, this is called rebound nystagmus.
Pure upbeating or downbeating nystagmus brought out by gaze is also a central sign.
These findings are marked on the recording and noted separately in the report.
Watching the video image during recording also makes it easier to tell these small movements from artifacts.
Fixation suppression is the reduction of nystagmus when the eyes fixate on a point.
In this example, slow-phase velocity in darkness was measured at twelve degrees per second.
When the patient looked at a light, velocity fell to three degrees per second, so the nystagmus was clearly suppressed.
Peripheral vestibular nystagmus usually falls by more than half with fixation; this suppression is largely provided by the cerebellum.
Nystagmus that does not decrease, or increases, with fixation is an important finding that suggests a central problem.
This measurement is made for spontaneous nystagmus and in the caloric test, by turning on a light at the response peak.
Now let us gather the oculomotor findings in a single table.
In a peripheral vestibular problem, saccades are normal; slow or markedly dysmetric saccades raise central suspicion.
Smooth pursuit is usually preserved in peripheral problems; marked impairment in one direction is more centrally meaningful.
The optokinetic response stays symmetric in peripheral problems; marked asymmetry is a central sign.
Nystagmus that changes direction with gaze fits a central pattern, while unidirectional nystagmus fits a peripheral one.
Nystagmus that is not suppressed by fixation is a centrally important finding.
Oculomotor findings are affected not only by disease but also by test conditions.
With age, saccade latency lengthens and pursuit gain decreases, so age-appropriate norms are used.
In a drowsy or distracted patient, saccades and pursuit may be impaired, so the patient is kept alert throughout.
The patient must be able to see the target clearly; severe vision problems and removing glasses can affect the result.
Sedatives, antiepileptics and alcohol impair pursuit and gaze tests in particular.
Finally, a faulty calibration can make every numerical measurement misleading.
A clinical tip: when you see an abnormal oculomotor finding, first rule out technical and patient-related causes.
Check the calibration, make sure the patient is alert and focused on the target, and review their medications.
If the finding is consistent across repeated trials, for example pursuit always impaired one way, state this clearly in the report.
Consistent findings that suggest a central pattern are passed on to the physician, regardless of the caloric result.
The saccade test measures latency, velocity and accuracy; slow or markedly dysmetric saccades raise central suspicion.
Pursuit gain is affected by age and attention; marked impairment in one direction is more meaningful.
Direction-changing gaze-evoked nystagmus and pure vertical nystagmus are central signs.
Peripheral nystagmus decreases with fixation; failure to decrease suggests a central problem.
Test yourself: which of the following oculomotor findings suggests a central problem?
The correct answer is C: nystagmus that does not decrease with fixation.
Mild hypometria, end-point nystagmus and a mild age-related drop in pursuit can also be seen in healthy people.
In the next lesson we will learn the best-known part of the VNG battery, the caloric test, with its bithermal procedure and interpretation.
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Prerequisites

LESSON 156 VNG and ENG

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