An Exceptional Preaortic Ectopic Type B2 Thymoma as the Cause of Refractory Myasthenia Gravis: A Case Report

Myasthenia gravis (MG) is a chronic autoimmune neuromuscular disorder mediated by autoantibodies directed against acetylcholine receptors (anti-AChRs), in which thymic pathology plays a central pathogenic role. While thymomas predominantly arise in the anterior mediastinum, ectopic locations represent a rare and clinically distinct entity. Among these, preaortic ectopic thymomas are exceedingly rare, and their association with refractory MG has not previously been described. We report a 48-year-old woman with generalized MG with bulbar predominance, presenting with an eight-month history, confirmed by markedly elevated anti-AChR antibodies (91.1 nmol/L), a positive neostigmine test, and a pathological decrement exceeding 10% on repetitive nerve stimulation across three nerve-muscle pairs. The patient experienced a life-threatening myasthenic crisis requiring ICU admission and failed to achieve sustained remission despite corticosteroids, pyridostigmine, intravenous immunoglobulins (IVIg), and two cycles of rituximab (1000 mg at day 1 and day 15, followed by 1000 mg at six months), fulfilling criteria for refractory MG. After repeated immunotherapy failures and a normal chest X-ray, a prior external CT, initially interpreted as normal or possibly representing ectopic thyroid tissue, was followed by a dedicated thoracic CT at our institution, which identified a left-lateralized anterior mediastinal soft-tissue mass in close preaortic contact with the ascending aorta (42 × 28 × 41 mm). Complete surgical resection via median sternotomy was performed. Histopathological investigation confirmed a WHO type B2 thymoma (2021 WHO classification), with identification of Hassall's corpuscles confirming the thymic origin of the mass. The patient achieved complete clinical remission (Besinger score 100/100; MGC score 0/50) following postoperative IVIg and was subsequently tapered off corticosteroids. This case highlights the necessity of systematic thoracic CT in all refractory MG regardless of plain radiograph findings and prior imaging reports and underscores the potential importance of complete surgical excision of preaortic ectopic thymic tissue in achieving disease control. Following a systematic search of PubMed, Scopus, and Google Scholar, we did not identify any previously reported case of a WHO type B2 preaortic ectopic thymoma described as the underlying cause of refractory generalized MG.