Square wave jerks in Friedreich's ataxia.

We observed a brother and sister with Friedreich's ataxia during steady fixation. They both had abnormal eye movements consisting of randomly occurring, small amplitude, conjugate horizontal saccades away from the fixation target. The saccades were followed by maintenance of this eccentric position for a brief period, and finally, a corrective saccade back to the original position. Electro-oculographic recordings showed these eye movements to be square wave jerks. They had an amplitude ranging from 1.5 to 10.7 degrees and a duration averaging less than 200 msec. Square wave jerks were also superimposed on pursuit movements in both patients. Square wave jerks are an additional ocular finding that may signify chronic cerebellar dysfunction. More specifically, they are an additional neuro-ophthalmological finding that may be observed in Friedreich's ataxia.