Probable Overlapping Myasthenia Gravis and Myositis: A Case Report

Polymyositis and myasthenia gravis (MG) are both autoimmune disorders with different pathophysiological mechanisms but similar clinical features.Polymyositis is one of the idiopathic inflammatory myopathies, characterised by endomysial inflammation and presents with symmetrical proximal muscle weakness, fatigue, and elevated muscle enzyme levels.MG is a neuromuscular junction disorder with clinical findings of fatigable weakness with ocular and bulbar involvement.Their coexistence is rare and may lead to significant diagnostic delay, hence, presenting this case can raise awareness of the concomitant occurrence of polymyositis and MG.We report a 68-year-old man who presented with progressive proximal muscle weakness and myalgia with markedly elevated creatine kinase levels of 6,435 U/L.He also developed fluctuating diplopia and evidence of left eye adduction failure on examination.These ocular features raised suspicion for neuromuscular junction involvement.Initial neurological, ophthalmological, and rheumatological assessments, together with imaging and negative serological testing, including acetylcholine receptor (AChR) and muscle-specific kinase antibodies (anti-MuSK), favoured polymyositis.Electromyography and MRI revealed classical myopathic changes in the pelvic girdle, thigh, and shoulder girdle musculature consistent with inflammatory myositis.Despite oral corticosteroids and methotrexate, the patient deteriorated with worsening ophthalmoplegia affecting both eyes, left eye blepharoptosis, and significant muscle weakness, requiring hospital admission, reflecting a suspected corticosteroid-induced exacerbation of MG.A repeat serological test subsequently revealed AChR antibody positivity, suggesting a probable diagnosis of MG with ocular predominance and myositis overlap.Following administration of intravenous methylprednisolone and intravenous immunoglobulin (IVIG), there was significant clinical improvement.After the initial response, rituximab was introduced as long-term immunosuppression.Ocular involvement is rare in inflammatory myopathies and should prompt evaluation for coexisting MG.This case report adds to the limited existing literature on MG-myositis overlap, highlighting the value of a multidisciplinary team approach in avoiding misdiagnosis and guiding treatment.