Timothy C. Hain, MD
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- Microsaccadic oscillation (MuSO)
- Macrosaccadic oscillation (MSO)
- Ocular flutter
- Square Wave Jerks (SWJ)
Microsaccadic oscillation denotes a tiny (0.2 deg) back-back saccadic oscillation. It is generally benign.
MSO is a disorder where the eye makes saccades back and forth about a target. This is usually a sign of a serious cerebellar disorder.
Ocular flutter denotes an instability of the eye where after the main saccade, the eye makes another saccade in the opposite direction. Ocular flutter is generally caused by cerebellar disorders or brainstem disorders (Bergenius, 1986; Schon, 2001). The most common clinical syndrome is the "benign-encephalitis" syndrome with ocular flutter. These persons are typified by ataxia and shimmering eye movements. They can be later recognized because their eyes shimmer under closed eyelids (one can see the eye movement under the lids).
Ocular flutter due to disease is rare. About 8% of the normal population can voluntarily produce ocular flutter, usually during convergence. This can be spotted because the pupil constricts during the convergence effort. When voluntarily induced, it is called "voluntary nystagmus". Most often ocular flutter is a variant of voluntary nystagmus as cerebellar disorders are much rarer.
No treatment is available for ocular flutter. As ocular flutter may be an early variant of opsoclonus (see following), some caution must be exercised, if the syndrome is recent in onset.
Opsoclonus denotes chaotic back-back saccadic eye movements. It is a dramatic syndrome, usually indicating either cancer or a brainstem encephalitis. Opsoclonus is classically caused by neuroblastoma or a paraneoplastic syndrome, especially with the anti-Ri antibody. These diagnoses are very rare and accordingly opsoclonus is rarely seen and even more rarely recognized. A full-scale workup for neoplasm is generally indicated and productive in persons with opsoclonus. Lung cancer is the commonly found tumor, and thus a chest-Xray (rather than an MRI) is usually the most productive first test.
SQUARE WAVE JERKS
Causes of Square Wave Jerks
Square-Wave Jerks (SWJ) are inappropriate saccades that take the eye off the target, followed by a nearly normal intersaccadic interval (approximately 200 msec), and then a corrective saccade that brings the eye back to the target (Leigh and Zee, 1983). Multiple sources have been suggested as generators for square-wave jerks including the cerebral hemisphere (Sharpe et al 1982), the cerebellum (Alpert et al 1975; Zee et al 1976; Dale et al 1978), and superior colliculus (Hikosaka and Wurtz, 1983). The table above shows the commonly reported clinical associations.
As square-wave jerks are universally found in normal subjects, the main criteria for abnormality is frequency. There are two factors that can affect frequency: age and fixation. Increasing age is associated with increasing frequency. Herishsanu and Sharpe (Herishsanu and Sharpe, 1981) reported a mean frequency of 4.7/min in young and 27/min in elderly. Another factor influencing frequency is the state of fixation. Shallo-Hoffmann and associates ( Shallo-Hoffman et al, 1989) reported that, for normal young subjects, the mean frequency was 4.4/min when recorded in light with visual fixation, 8.5/min when recorded in dark without visual fixation, and 5.4/min when recorded with eyes closed.
The clinical utility of square-wave jerks is to point towards the possibility of a central disorder. In young normal persons, square-wave jerks occur infrequently. Accordingly, when frequent SWJ are found in a young patient (more than 1/sec), this should bring up the question of a cerebellar disorder. In the elderly, square-wave jerks are common and are rarely of significance. However, in certain conditions such as progressive supranuclear palsy, the diagnosis cannot be made without finding frequent square-wave jerks (Troost and Daroff, 1977). The illustration above is from a patient with PSP.
Saccadic intrusions are unintended saccades, not necessarily followed by a return movement as is the case for square wave jerks. Saccadic intrusions are very common and have very little diagnostic significance. Horizontal saccadic intrusions are most commonly attributed to psychiatric disease such as schizophrenia. In the vertical and torsional planes, saccadic intrusions are instead attributed to neurological disorders. Vertical or torsional saccadic intrusions may arise from irritibility of burst neurons in the midbrain (Bentley et al, 1998).