Perhaps the most important diagnostic contribution to be made by recording eye movements in parkinsonian disorders concerns tests of vertical saccades (7,8). Reliable measurement of horizontal or vertical saccades requires methods with adequate bandwidth (0-150 Hz), sensitivity (0.1°), and linear range (±30°). DC-amplified electro-oculography (EOG) is adequate to signal horizontal eye position and timing at the beginning and end of a saccade, but is unreliable for measuring vertical movements. Infrared methods provide better bandwidth but inferior range to EOG, and also cannot be used to measure vertical movements. The most reliable method is the magnetic search coil which, in our experience, is well tolerated by frail and elderly subjects, and has the added advantage of being calibrated independently of the patient's voluntary range of movements (1). Fast frame-rate video-based techniques are suitable for measuring dynamic properties of horizontal and vertical saccades (9), but their calibration depends on the ability of the patient to look at visual targets, and this may be impaired, for example, in PSP.
Saccades show consistent relationships between their size, speed, and duration (2,10). Thus, the bigger the saccade, the greater its peak velocity and the longer it lasts. Examples of the "main sequence" relationships between peak velocity, duration, and amplitude are provided from normal subjects in Fig. 1; exponential or power-function equations have been used to describe these relationships and define prediction intervals for normal subjects (2,10,11). Deviations of measured eye movements from these relationships indicate either abnormal saccades, or nonsaccadic eye movements. Thus, in Fig. 1, we also provide an example of abnormally slow vertical saccades from a patient with PSP.
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