Bilateral pseudo-internuclear ophthalmoplegia in a patient with myasthenia gravis

Purpose: To report a case of myasthenia gravis presenting with a false localizing sign, a bilateral pseudo internuclear ophthalmoplegia. Observations: A 61 year-old male presented with a five-week history of painless binocular oblique diplopia that was associated with fatigue, vocal hoarseness and bilateral ptosis, the latter two of which worsened through the course of the day. Exam was remarkable for an apparent bilateral internuclear ophthalmoplegia (INO). MRI of the brain with and without contrast and MRA of the head and neck were within normal limits. Lab work was positive for anti-AChR binding, blocking, and modulating antibodies, as well as anti-striated muscle and anti-titin antibodies. The patient was initiated on therapy for myasthenia gravis. Conclusion and importance: Although myasthenia gravis often presents with ptosis or diplopia, rarely patients may develop pseudo-INO secondary to extraocular muscle weakness. True INO occurs with damage to the medial longitudinal fasciculus, a myelinated tract of fibers that controls yoked horizontal eye movements. Clinicians should be suspicious of the false localizing sign of a pseudo-INO associated with myasthenia gravis when more common causes of INO have been excluded.