What is the saccade test in electronystagmography (ENG)?

Updated: Aug 06, 2019
  • Author: Angela G Shoup, PhD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

Answer

The saccade test, also called the calibration test, evaluates the saccadic eye movement system. [7] This system is responsible for rapid eye movements and refixation of the target on the fovea.

For saccadic testing, one may place dots on the wall or ceiling at specified distances from each other (usually center and 10, 20, and 30 degrees off center) and then instruct the patient to look back and forth between the dots, keeping the head fixed.

More current technology allows the examiner to control (via computer) the presentation of pinpoints of light on a light bar, on a projection screen, or in an oculomotor stimulator. This allows randomized presentation of the stimuli and more specific assessment of eye movements. With older technology, a clinician could only visually assess the tracings of eye movements to look for gross deviations from normal. With computer-generated stimuli and analysis, the clinician can assess latency and velocity of eye movements.

Saccadic test results are influenced by patient cooperation and visual acuity. Various drugs can also affect performance. Accuracy, latency, and velocity should all be taken into consideration when interpreting saccades.

The following accuracy issues may be noted.

Also known as calibration overshoot, hypermetric saccades occur when a patient has difficulty measuring the distance required for the muscular act necessary for following the target. In these cases, the clinician may note an overshoot of the target followed by a correction. Patients who consistently exhibit hypermetric saccades may have ocular dysmetria, which is suggestive of a central nervous system (CNS) lesion at the level of the cerebellum.

Ocular flutter is evidenced as a type of spiky overshoot; the patient overshoots the target several times with a short duration between overshoots. This is also suggestive of CNS pathology.

With hypometric saccades, the patient undershoots the target. Occasionally undershooting a target is normal; the undershoot must be reproducible and must occur frequently to be considered abnormal. If extreme, hypometric saccades are suggestive of basal ganglia pathology.

Multistep saccades occurs when a patient undershoots the target and then attempts to correct with multiple small saccades. This pattern is suggestive of CNS pathology.

Also called glissade, postsaccadic drift is seen as a drifting of eye movement after the saccade. This pattern is consistent with cerebellar pathology.

Pulsion is seen in vertical saccades in patients with posterior inferior or superior cerebellar artery syndrome. The pattern includes a pulling to the left or right of the eyes when completing vertical saccades.

Disorders in latency are primarily due to prolongation of saccades. Saccades characterized by short latency are usually due to an artifact or the patient anticipating the position of the target. If prolongation of saccades is evident, the examiner should rule out inattention or uncooperative behavior. Prolongations of more than 400 ms in attentive and cooperative patients may be suggestive of CNS pathology.

Asymmetrical latencies can occur in patients with lesions in the occipital or parietal cortex. For these patients, saccades in one direction may be normal with a prolongation of saccades in the opposite direction.

The following velocity issues may be noted.

If saccadic slowing is observed, the examiner should first rule out drowsiness or drug effects. In the absence of these, saccadic slowing is consistent with various CNS or ocular disorders, including oculomotor weakness, degenerative conditions, basal ganglia pathology, and cerebellar disorders.

As with saccades that evidence an abnormally short latency, abnormally fast saccades are usually an artifact and may be due to technical difficulties. However, in some cases, abnormally fast saccades may suggest CNS or ocular pathology (ie, ocular flutter). [8]

Asymmetry in saccadic velocity can be observed as an asymmetry between the eyes or between directions. When asymmetry is evident, the clinician may suspect ocular nerve or muscle pathology (ie, lesions, palsies). CNS pathology may also be suspected. A lesion in the medial longitudinal fasciculus that causes internuclear ophthalmoplegia may evidence as asymmetrical saccadic velocity.


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