Over the past few weeks, we have been discussing the Oculomotor battery, which is a section of the VideoNystagmoGraphy (VNG) exam. Let’s take a closer look at the subtests of the Oculomotor battery. This week we will discuss Saccades. This will be a little dry if you are reading this from a patient’s perspective, but it might be useful to practitioners that want more detail about this test.
Saccades are rapid eye movements made to bring an object of interest into the center of the line of sight (foveal vision). Saccadic eye movements are used both voluntarily and reflexively to initiate eye movement quickly toward an object of interest and to stop and ‘‘lock on’’ to the target accurately. Saccades allow us to refixate our gaze with minimal duration of retinal slip. Saccades are tested for accuracy, velocity (eye speed during movement), and latency (the difference in time between the presentation of a new target and the initiation of eye movement). Age-weighted normative values for each of these components of saccadic eye movement have been established.
The speed of initiation (latency) of a saccade is 150 to 250 ms when the target is unpredictable (random) and about 76 ms with a predictable target.1 Latency of saccades is considered abnormal when there is a consistent delay of about 260 to 270 ms or longer.
Accuracy of saccades can be affected both before and after the new target has been obtained. Keeping in mind that the goal of a saccadic eye movement is to fixate visually both quickly and accurately on a new object, eye movement that is equal in amplitude to the distance between the former object of interest and the new target is desired. Normal individuals will often ‘‘undershoot’’ the target by about 10% to 15%, referred to as hypometria and is considered abnormal if saccades are consistently performed at less than about 70% of the target amplitude. Patients with normal cerebellar function will also occasionally overshoot the target, referred to as hypermetria. Hypermetria is considered abnormal if saccades are consistently more than 15% to 20% over target amplitude. Even when a saccadic eye movement equals the amplitude of the target, for it to be considered normal the eye must be able to remain fixated on the new target. If the eye drifts from or toward (as sometimes occurs in hypometria) the target, it is referred to as a glissade.
Saccadic velocity is measured as the peak speed of eye movement when refixating gaze from one target to another. Saccadic velocity is involuntary and has no abnormal upper limit. Peak velocities of saccades have been measured as high as 700 degrees per second and are considered abnormal if consistently slower than 430 degrees per second for large-amplitude saccades (30 degrees) and slower than about 200 degrees per second for small-amplitude saccades (10 degrees) (personal communication, GN Otometrics Staff).
The saccade test is performed much like the calibration procedure, with the patient seated and facing a light bar. Unlike the calibration procedure, true saccadic testing must involve a randomly moving target. The patient is instructed to ‘‘follow the lights’’ as accurately as possible while keeping the head still. The lights are moved in a random fashion as controlled by the ENG/VNG software. Targets appear on the light bar for 1 to 4 seconds before changing position. The targets may appear anywhere within a range of 30 degrees from center.