Eye movement recordings have helped identify several forms of saccadic intrusions (Fig 9-4). Two classes of saccadic intrusions are distinguished by either the presence or the absence of an intersaccadic interval—the temporal separation between sequential saccades that lasts 180–200 milliseconds.
Saccadic Intrusions With Normal Intersaccadic Intervals
The most common saccadic intrusions are square-wave jerks, which have a normal intersaccadic interval (amplitude, typically <2°; latency to refixation, 200 ms) and may occur normally at low frequencies (4–6 per minute) in older patients. Pathologic square-wave jerks occur at a rate greater than 15 per minute. Macrosquare-wave jerks are much less common but also include an intersaccadic interval (amplitude, 5°–15°; latency to refixation, 70–150 ms). The larger-amplitude macrosquare-wave jerks tend to have a slightly higher frequency and are always pathologic. They are observed most frequently in patients with cerebellar disease or MS. Lesions of the cerebellum, brainstem, or their interconnecting fibers may alter the discharge of omnipause cells in the pons and burst cells in the paramedian pontine reticular formation (for horizontal saccades) (see Chapter 8). The inhibition of omnipause cells and the stimulation of burst cells lower the threshold for saccadic initiation, resulting in inappropriate saccadic intrusions that drive the eyes off of fixation.
Figure 9-4 Schematic representation of saccadic intrusions and oscillations. The baseline of each graph represents on-target fixation. A, Square-wave jerks: small, intrusive saccades away from and back to the target position. B, Macrosquare-wave jerks. C, Macrosaccadic oscillations: hypermetric saccades to either side of the target position. D, Ocular flutter: back-to-back, to-and-fro saccades without an intersaccadic interval.
(Modified with permission from Leigh RJ, Zee DS. The Neurology of Eye Movements. 3rd ed. Contemporary Neurology Series. New York: Oxford University Press; 1999.)
Macrosaccadic oscillation is another saccadic intrusion that breaks fixation. Macrosaccadic oscillations differ from square-wave jerks in that they consist of large saccades that straddle and overshoot fixation without foveation of the target. The saccades are separated by an intersaccadic interval. The oscillations occur in bursts in which the saccadic amplitude increases and then decreases; eye movement recordings show a characteristic spindle shape. Macrosaccadic oscillations occur from cerebellar lesions (eg, tumors, MS, stroke, paraneoplastic syndromes) that alter the calibration of saccadic size, resulting in hypermetric primary and corrective saccades that oscillate around the desired eye position.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.