A Case of Ocular Myasthenia Gravis Combined with a Tonic Pupil Mistaken for Partial Third Nerve Palsy

Title A Case of Ocular Myasthenia Gravis Combined with a Tonic Pupil Mistaken for Partial Third Nerve Palsy
Author, Co-Author Marjolaine Plourde, Stephanie Lebeau, Marie-Eve Corbeil, Danielle de Guise
Program Number
Great Hall Foyer
Abstract Introduction: Ocular myasthenia gravis may appear as cranial nerve palsy, especially third nerve palsy with a spared pupil.

Case report: A 66 year old Caucasian male came at the office complaining of an intermittent horizontal diplopia that he had been experiencing for two years. He had a medical history of arterial hypertension and hypercholesterolemia. The initial exam showed an intermittent alternating exotropia and a left hypertropia. A limitation of the left eye in adduction and upgaze was found. The patient reported the simultaneous occurrence of a left ptosis and a larger unreactive pupil three years ago. At this point an old and incomplete third nerve palsy with pupil involvement was suspected. However, according to the patient, the ptosis and the diplopia worsened when he was either tired or ill. The variability of the ptosis was confirmed during the exam. A second differential diagnosis then needed to be considered: ocular myasthenia gravis. The neuro-ophthalmology exam demonstrated a bilateral weakness of the orbicularis muscles and a fatigability of the medial and superior recti of both eyes during the saccades, which supported the diagnosis of myasthenia gravis. Once the third nerve palsy was ruled out, the anisocoria required its own evaluation. A light-near dissociation was noticed in the left eye as well as a denervation hypersensitivity assessed with 0.125% diluted pilocarpine. The anisocoria could therefore be explained by a left tonic pupil. The patient was sent to neurology to initiate myasthenia gravis treatment. Conclusion: Ocular myasthenia gravis combined with a tonic pupil is unique and could have led to the wrong diagnosis of third nerve palsy with pupil involvement.  To the best of our knowledge, this singular association has never been reported and was therefore considered as a coincidence.
Affiliation of Co-Authors Université de Montréal, Université de Montréal, Université de Montréal