'Staircase' square-wave jerks in early Parkinson's disease

Visually guided saccades and gaze-fixation ability were recorded in patients with early Parkinson's disease (PD).

Magnetic search coil system was used to measure horizontal and vertical eye positions.

'Staircase' visually guided saccades (multiple hypometric saccades separated by an intersaccadic interval) and 'staircase' square-wave jerks (SWJ) were present in all patients. The SWJ amplitude (total amplitude of the series of hypometric saccades comprising the SWJ) was abnormally large (mean 2°). SWJ frequency was also abnormally high (50-70 intrusions/min).

Loss of dopaminergic neurons in the substantia nigra pars compacta leading to phasic excitation of the substantia nigra pars reticulata and, in turn, phasic inhibition of the superior colliculus (SC), may account for 'staircase' visually guided saccades in these patients. This mechanism, however, does not explain abnormally large SWJ, which in the traditional view results from decreased inhibition upon SC. The abnormally large SWJ could be due to a compensatory increase in frontal eye field activity secondary to the increased inhibition upon the SC. Abnormally large and frequent SWJ are often used clinically to distinguish multi-system atrophy from idiopathic PD. Our study, however, suggests that idiopathic PD cannot be excluded if the patient has large-amplitude SWJ.