Macro Square Wave Oscillations (MSO)

Timothy C. Hain, MD and Heather Rudisill, AuD

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Also see SWJnystagmusopsoclonus

Macro Square Wave Jerks and oscillation in patient with a cerebellar degeneration due to breast cancer. Note how the size of the SWJ increases markedly in the dark. There is also an upbeating nystagmus in the dark. This is a clearly excessive # of square wave jerks.

Macro-saccadic oscillation (MSO) are very large square wave jerks that appear in a long series. They are always horizontal. They may occur when overshoot dysmetria is so large that a sequence of refixations occur, each overshooting. Or perhaps they occur when when the cerebellar regulatory process that governs the size of SWJ (in normal persons) goes haywire. While classically described in cerebellar syndromes, MSO can also occur when ocular function is suddenly changed, as for example, after edrophonium medication in an ocular myasthenic (Komiyama et al, 1999).

MSO differ from opsoclonus (a much more ominous finding), in that in MSO there is a delay between the eyes reversing directions (usually about 100-200 msec), while in opsoclonus or flutter, the saccades are "back to back".

MSO differ from vestibular nystagmus which has a slow phase and fast phase, causing a "sawtooth" pattern.

ENG Testing of MSO

During the ENG test, MSO can be seen superimposed on nearly all parts. Here is a series from the same patient shown above with a cerebellar degeneration due to breast cancer.

During the saccade test, the same SWJ are seen on an expanded time scale. Note that the vertical trace shows nothing at all.


During the vertical saccade test, vertical saccades look fairly normal, but horizontal saccades continue to oscillate.


MSO are superimposed on horizontal pursuit.


During vertical pursuit, the oscillation in the horizontal trace is not echoed in the vertical trace. Square wave jerks are always purely horizontal.