Vertical saccadic velocity and force development in superior oblique palsy

Abstract

Vertical saccadic movements recorded with electrooculography (EOG) and force development measured by means of a contact lens-strain gauge technique were investigated in 13 patients with a clinical diagnosis of unilateral superior oblique palsy (SOP) and 10 normal subjects. Upward and downward movements from the horizontal level to 5, 10 and 20deg of deviation with monocular fixation were made in the 20 deg abduction and adduction fields of gaze. Peak velocity (Vp) and the ratio of downward over upward movement (Vp D/U) of the paretic eye (PE) and the sound eye (SE) of the patients were compared with the values of the covered, non-fixating eye (CE) and the fixating eye (FE) of the normals respectively. Peak tension (Fp), steady-state tension (Fs) and the ratios of downward over upward movement (Fp D/U, Fs D/U) of PE were compared with the values of CE. Comparisons were done (a) between normals and the whole group of 13 patients, (b) between normals and two subgroups of five patients with acquired and five with congenital SOP, (c) and between the subgroups. Mean Vp for the whole group of SOP showed reduced values from normals in many directions of gaze. However, mean Fp and ratios of Vp D/U and Fp D/U were not different in SOP and normals and such measures could not be used to differentiate SOP from normal eye muscles in the sample of patients studied. Clear differences were seen in the comparisons between normals and the subtypes of SOP. Vp was significantly reduced in PE for the downward movements in the adduction field of gaze, more marked in acquired than in congenital SOP. Slowing of upward saccades was also found in the congenital group. This may be due to adaptational changes in concomitization of congenital paretic strabismus. Fp for downward saccade and Vp D/U and Fp D/U were lower for PE in the acquired than in congenital SOP and normals in the lower adduction field of gaze, where the superior oblique has its main action. A linear relation was found between Vp and Fp of downward saccades for the congenital group in the lower adduction of gaze, but the slope was steeper than in normals. However, in acquired SOP the relationship was non-linear, which indicates a force production different from that of a normal muscle. Vertical saccadic velocity and force development measured at 20 deg adduction could reveal weakness of the superior oblique muscle and the imbalance of agonist-antagonist actions in the paretic eye. The characteristics of saccadic velocity and force development in acquired and congenital SOP are different, which is helpful in differentiating the subtypes of SOP. The measurements can also assist in analysis of the oculomotor adaptation after muscle paresis. © 1994.