Quantitative analysis of transitory alternating saccadic abnormal eye movements

Although transitory alternating saccade (TAS) is classically-observed as one of the cerebellar ocular-motor signs, its occurrence has been noted only in isolated cases in the literature. In the present study, we quantitatively examined a total of 29 TAS waveforms with spectral analysis by discrete Fourier transformation. TAS was characterized by periodic eye movements over a time course of several seconds coupled with fast eye movement that reverses its direction. That is, the low frequency component of TAS was a periodic eye drift with a frequency of 0.36±0.09 Hz and an amplitude of 11.4±4. 4°, whereas the high frequency component was a nystagmus-like eye movement with an amplitude of 1.41±0.4°. Routine computerized electronystagmography (ENG) testing revealed that in only 3 cases TAS was observed during the testing of spontaneous nystagmus. In another 26 cases, TAS was evoked during head position-change, neck stimulation such as neck-torsion and carotid-compression, or after optokinetic and rotational stimulation. In 48% of the 29 cases, disorders in visual fixation, smooth pursuit, and optokinetic nystagmus were also seen. TAS may be a result of the pathological oculomotor findings associated with lesions in the vicinity of the midline of the cerebellum.