Diagnosing disconjugate eye movements: phase-plane analysis of horizontal saccades

Saccades are fast eye movements that conjugately shift the point of fixation between distant features of interest in the visual environment. Several disorders, affecting sites from brainstem to extraocular muscle, may cause horizontal saccades to become disconjugate. Prior techniques for detection of saccadic disconjugacy, especially in internuclear ophthalmoparesis (INO), have compared only one point in abducting vs adducting saccades, such as peak velocity.

We applied a phase-plane technique that compared each eye's velocity as a function of change in position (normalized displacement) in 22 patients with disease variously affecting the brainstem reticular formation, the abducens nucleus, the medial longitudinal fasciculus, the oculomotor nerve, the abducens nerve, the neuromuscular junction, or the extraocular muscles; 10 age-matched subjects served as controls.

We found three different patterns of disconjugacy throughout the course of horizontal saccades: early abnormal velocity disconjugacy during the first 10% of the displacement in patients with INO, oculomotor or abducens nerve palsy, and advanced extraocular muscle disease; late disconjugacy in patients with disease affecting the neuromuscular junction; and variable middle-course disconjugacy in patients with pontine lesions. When normal subjects made disconjugate saccades between two targets aligned on one eye, the initial part of the movement remained conjugate.

Along with conventional measures of saccades, such as peak velocity, phase planes provide a useful tool to determine the site, extent, and pathogenesis of disconjugacy. We hypothesize that the pale global extraocular muscle fibers, which drive the high-acceleration component of saccades, receive a neural command that ensures initial ocular conjugacy.