Evaluating small eye movements in patients with saccadic palsies

Slow saccades are an important diagnostic feature of a range of degenerative, metabolic, and genetic diseases of the nervous system. Many affected patients have difficulty initiating saccades, and the movements themselves may be small, making it difficult to make comparisons with control subjects. A large-field optokinetic stimulus may elicit quick phases of nystagmus in patients who cannot initiate voluntary saccades, but these movements may also be small. We show that it is still possible to compare amplitude-duration and amplitude-peak velocity relations with controls if data are fit with a power function (rather than an exponential equation). When analyzed this way, the dynamic properties of small saccades and quick phases from patients with progressive supranuclear palsy (PSP) could be differentiated from fast movements made by patients with idiopathic Parkinson's disease or controls. Normal saccades show a fairly constant ratio: peak velocity/mean velocity (Q approximately 1.6 for vertical saccades). This ratio was abnormally high (Q >3) for some larger saccades made by patients with PSP, suggesting that either these movements were not entirely saccadic or that they were composed of a series of small saccades.