Abduction saccades in unilateral internuclear ophthalmoplegia

Abstract

Horizontal eye movements were investigated in 60 consecutive patients with unilateral internuclear ophthalmoplegia utilizing direct current electrooculography. In nine patients additional conjugated slowing of ipsiversive saccades indicated the diagnosis of a one-and-a-half syndrome. Slowing of abduction saccades was bilateral in two patients and unilateral in 20 (ipsilateral to the MLF lesion in 17 patients and contralateral in three). Slowing of abduction saccades was attributed to impaired inhibition of the tonic resting activity of the antagonistic medial rectus muscle. On the eye contralateral to the lesion 70% of the patients had abduction nystagmus and 66.7% hy permetric abduction saccades. Hypermetricity of abduction saccades is attributed to an increased medial rectus activation to overcome paresis by which commensurately the activation of the agonistic contralateral lateral rectus muscle is increased. This view is confirmed by the increasing prevalence of hypermetric abduction saccades with increasing severity of the medial rectus paresis.

Key Words:

• internuclear ophthalmoplegia
• abduction nystagmus
• slowed abduction saccades
• hypermetric abduction saccades
• medial rectus inhibition