IgG4 ‐Related Disease Overlapping With Anti‐Acetylcholine Receptor Antibody–Positive Myasthenia Gravis

Background IgG4‐related disease (IgG4‐RD) is a systemic, immune‐mediated, fibroinflammatory disorder with multiorgan involvement and variable clinical presentation, often creating diagnostic difficulty. Myasthenia gravis (MG) is an antibody‐mediated autoimmune disorder of the neuromuscular junction, most commonly associated with anti‐acetylcholine receptor (AChR) antibodies. Although both conditions involve immune dysregulation and affect the ocular region, their coexistence is extremely rare. We report a rare case of anti‐AChR antibody–positive MG overlapping with IgG4‐RD. Case Presentation A 66‐year‐old man developed bilateral diplopia, ptosis, and mild eyelid swelling over 3 months. Anti‐AChR antibodies were positive, and an edrophonium test improved ptosis. Single‐fiber electromyography showed abnormal jitter, leading to a diagnosis of ocular MG. Chest CT revealed an anterior mediastinal nodule, and thymectomy showed follicular hyperplasia. Despite pyridostigmine, tacrolimus, and surgery, the patient's ocular symptoms progressively worsened and he was admitted for re‐evaluation. Although serum IgG4 was within the normal range, imaging showed bilateral enlargement and enhancement of the lacrimal glands and extraocular muscles. Lacrimal gland biopsy showed dense lymphoplasmacytic infiltration, fibrosis, approximately 20 IgG4‐positive plasma cells per high‐power field, and an IgG4/IgG ratio > 40%. The patient fulfilled the 2019 ACR/EULAR classification criteria for IgG4‐RD. Intravenous methylprednisolone followed by oral prednisolone markedly improved eyelid swelling, although ocular movement limitation persisted at 6 months. Conclusions IgG4‐RD may closely mimic ocular MG and delay diagnosis, particularly in seronegative cases. Unexplained eyelid or lacrimal gland swelling in refractory MG should prompt consideration of IgG4‐RD and early histopathological evaluation.