Video-oculographic documentation of fatigable exotropia in probable seronegative ocular myasthenia gravis: a case report

The diagnosis of ocular myasthenia gravis (OMG) can be challenging in seronegative patients presenting with fluctuating diplopia without ptosis. We report a 62-year-old woman with intermittent binocular diplopia that worsened with fatigue and improved with rest. Neurological examination revealed intermittent exotropia without ptosis or fixed ocular motor deficits. Repetitive nerve stimulation, the edrophonium test, and serologic testing for acetylcholine receptor and muscle-specific kinase antibodies were negative. To objectively assess ocular motor fatigability, video-oculography (VOG) was performed during repeated horizontal saccades and demonstrated progressive disconjugacy with increasing outward deviation of the left eye. Treatment with pyridostigmine resulted in marked reduction of diplopia and improved ocular alignment on follow-up VOG. The clinical features, dynamic ocular motor findings, and treatment response were consistent with probable seronegative OMG. This case highlights the role of VOG in objectively documenting ocular motor fatigability and supporting the diagnosis of seronegative OMG.