The Pattern of Extra-Ocular Muscle Involvement in Ocular Myasthenia

Abstract

Background: The ocular motility (OM) deficit in myasthenia has not been studied systematically. Anecdotal reports yield no consensus, with the pattern mimicking infranuclear, internuclear or supranuclear disorders. The current study defines the pattern of extra-ocular muscle (EOM) weakness at presentation for a group of newly diagnosed untreated ocular myasthenics (OMG). Methods: Cases referred to one consultant neurologist (RAM) between 1991 and 2001 were reviewed. EOM weaknesses were quantified using an established scoring method in 49 patients with OMG and 49 age- and sex-matched controls, to differentiate age-related changes from those secondary to myasthenia. Results: Bilateral and multiple EOM weaknesses were typical. Only 6 cases mimicked isolated cranial nerve palsy. Weakness of the elevator muscles (superior rectus and inferior oblique) was common in both the OMG and control groups, but the amount of weakness was significantly greater for the myasthenia group (p = 0.003). All control subjects had a normal range of depression. EOM weaknesses for the control group were not confined to the older subjects. Discussion: This is the first detailed analysis of EOM involvement in ocular myasthenia. The EOM weaknesses for the control group were invariably bilateral and symmetrical, mainly involving the superior recti. The preponderance for involvement of the elevator muscles is discussed.

KEYWORDS:

• Myasthenia gravis
• ocular myasthenia
• extra-ocular muscles
• diplopia
• ptosis
• fatigability
• ocular motility