A Rare Case of Triple-Negative Ocular Myasthenia Presenting as Pupillary Sparing Third Cranial Nerve Palsy.

Ocular Myasthenia Gravis (OMG) is a localised autoimmune disorder characterised by fatigable weakness of extraocular muscles, typically presenting with ptosis and diplopia. While most cases are associated with autoantibodies against acetylcholine receptor (AchR), muscle-specific kinase (MuSK), or low-density lipoprotein receptor-related protein 4 (LRP4), a subset of patients remains seronegative, complicating diagnosis. We report a case of a 48-year-old woman who presented with a 20-day history of right-sided ptosis and horizontal diplopia. Neurological examination revealed complete third cranial nerve palsy with pupillary sparing. Brain and orbital MRI with contrast and a comprehensive metabolic and autoimmune workup were unremarkable. Initially managed as possible vasculitic neuropathy with intravenous methylprednisolone, she showed partial improvement. On further evaluation, the presence of diurnal variation in symptoms, a positive ice-pack test, and Cogan's lid twitch sign raised clinical suspicion for ocular myasthenia. Serologic testing for AchR, MuSK, and LRP4 antibodies was negative. A diagnosis of seronegative ocular myasthenia gravis was established based on clinical features. The patient was treated with pyridostigmine and a tapering course of oral corticosteroids, resulting in marked improvement in ptosis and diplopia within two weeks.