Joint examination of reflexive vertical saccades and small involuntary fixational saccades improves the classification of patients with progressive supranuclear palsy (PSP): a ROC study

A slowing of saccadic eye movements is one of the key symptoms of progressive supranuclear palsy and therefore represents a core functional domain of the current diagnostic criteria. However, there is considerable overlap between the saccade velocities of healthy people and patients in early stages. Therefore, a highly specific discrimination between patients and controls based on eye velocity often results in a considerable loss of sensitivity. Another symptom of progressive supranuclear palsy is a high frequency of square wave jerks formed by small involuntary fixational saccades. Using ROC analyses of 50 patients and 50 controls and focusing on points of 100 and 90% specificity or sensitivity, we investigated whether the velocity and gain data of visually guided reflexive saccades could be combined with each other and with parameters of fixational saccades to improve discriminability compared to considering saccade velocity alone. Both approaches were successful in patients with long disease duration but less so in cases of short duration. The displacement rate produced by square waves during fixation proved helpful because its frequency distributions in patients and controls had value ranges that were not shared by the two groups. This fact allowed an a priori classification of some subjects as either patients or controls. Modified ROC analyses using this a priori information are expected to work equally well in patients with short and long disease duration. In future studies it might be addressed if these methods can also improve the discrimination between PSP and other Parkinsonian disorders.