Myasthenia Gravis Presenting as Inferior Oblique Paresis

Abstract

A 53-year-old male presented with left ptosis and diplopia beginning 4 years ago. His ptosis resolved spontaneously 2 years later, but his diplopia persisted until now. The patient also cited intermittent dysphagia of solids and liquids as well as intermittent limb weakness. Ophthalmic examination showed a visual acuity of 6/8.6 OD and 6/6 OS. Intraocular pressures were normal. Eye fissure heights were reported as 11 mm in the right eye and 10 mm in the left eye, which did not change after a 2-min fatigue test. A prism cover test showed 10 prism diopters (PD) esotropia and 30 PD of left hypertropia on primary gaze. The esotropia increased to 40 PD on right gaze and remained 10 PD on left gaze. The left hypertropia decreased to 4 PD on right gaze and remained 30 PD on left gaze. It decreased to 20 PD on right tilt and increased to over 30 PD on left tilt. Three-step test indicated that it could result from paresis of the right inferior oblique muscle. Extraocular movements showed mild limitation of supraduction in the right eye which got worse in the supero-nasal direction (Fig. 32.1). A Maddox double rod test did not reveal image torsion. Slit lamp examination revealed mild cataracts in both lenses. Fundoscopic examination showed mild intorsion of the right eye and mottling of the retinal pigment epithelium in the left eye (Fig. 32.2). The clinical feature is compatible with right inferior oblique paresis. Anti-acetylcholine receptor antibodies were positive (11.0 nmol/L). He also received a chest CT scan which revealed a 3.7 × 2 cm lobulated soft tissue mass in the anterior mediastinum consistent with thymoma.