Unusual and bizarre abnormalities of ocular movement are often valuable localizing signs in central nervous system diseases. Square wave jerks (SWJ) are sporadic horizontal conjugate saccades away from the intended position of fixation, followed some 200 msec later by saccadic return to the fixation. SWJ are recognized as a normal phenomenon when present only under closed eyelids. SWJ during fixation are considered to be a pathological sign, indicative of cerebellar disease. Reports of SWJ in progressive supranuclear palsy, multiple sclerosis, posttraumatic cerebral syndrome, presenile dementia, and cerebral tumors suggest that brain stem or cerebral hemispheric disease may also be responsible. The mechanism of SWJ, however, remains unceretain. The present paper reports six cases of SWJ in two patients with cerebellar infarction, one with cerebellar hemorrhage, two with spinocerebellar degeneration, and one with dentato-pallidal degeneration. The localizing value of SWJ and its mechanism of action are discussed.
SWJ were conjugate, horizontal, symmetrical, occurred in burtst of several seconds, had amplitudes of 4° to 35°, and were evoked whenever the patient attempted to shift visual fixation or pursue a moving target. Electronystagmographic recordings in one patient with cerebellar hemorrhage showed the following features of this disorder of saccadic eye movement : 1) SWJ was composed of saccades, 2) their frequency was 0.5 to 2 Hz, 3) bursts occurred with decreasing amplitude and 4) the eye position did not exhibit systematic drift during the intersaccadic period. These features reflect the increased gain and instability of the visually guided saccadic system. Our resuls suggest that the loss of the calibrator function of the cerebellum accounts for the development of the abnormality underlying SWJ, and that SWJ are the result of unwanted supranuclear trigger signals that interrupt the saccadic pulse-step control system thereby releasing saccadic burst units.