Global Rather Than Vertical‐Selective Saccadic Abnormalities in Progressive Supranuclear Palsy

OBJECTIVE: To test whether vertical saccades are preferentially affected in Progressive Supranuclear Palsy (PSP). METHODS: PSP patients (n = 24) were compared to age-matched controls (n = 94) and two degenerative groups (Alzheimer's disease, n = 20; Lewy body disease, n = 50). Video-oculography (sampling rate: 1000 Hz) was used to measure visually guided saccades (15.3° amplitude) in vertical and horizontal planes. Traditional corrected velocity alongside novel metrics of velocity interruption, vacillation, and directional instability were compared across diagnostic groups. RESULTS: Vertical saccades were slower than horizontal in all groups including controls. Velocities were significantly slowed in PSP; however, the magnitude of slowing was comparable in vertical and horizontal directions. Similarly, vertical and horizontal velocities were strongly correlated in PSP (r = 0.77, p < 0.001), as they were in all other groups. Likewise, velocity interruption, vacillation, and directional instability were more disordered in the vertical compared to the horizontal axis in both controls and PSP (p < 0.05). Z-scores of all metrics in PSP were abnormal but with no predilection for vertical saccades. Vacillation was a predictor of saccade velocity. INTERPRETATION: Contrary to conventional belief, vertical saccades were not selectively impaired in PSP. Instead, the findings indicated a more global disruption of the saccadic system. The clinical observation of impaired vertical saccades in PSP appears to be because vertical saccades are slower and more disordered than horizontal in the general population; thus, when abnormalities of saccades develop in all directions, it first reaches a threshold to be noticeable at the bedside in the vertical plane.