Association Between Myasthenia Gravis and Sarcoidosis: A Case Report

Sarcoidosis is a systemic granulomatous disease of unknown cause.Its association with myasthenia gravis has mainly been documented in isolated case reports and small series.We report double-seronegative myasthenia gravis for the antibodies tested, associated with stage I thoracic sarcoidosis and thymic hyperplasia without thymoma.A 46-year-old woman was hospitalized in November 2021 for grade 3 exertional dyspnea according to the modified Medical Research Council scale, associated with fluctuating left-sided ptosis that worsened toward the end of the day.The neostigmine test was positive, with transient improvement in the ptosis.Lowfrequency repetitive nerve stimulation demonstrated a decrement compatible with a postsynaptic neuromuscular transmission disorder.Antibodies against the acetylcholine receptor and anti-MuSK antibodies were negative, leading to the diagnosis of double-seronegative myasthenia gravis for the antibodies tested.At the same time, thoracic imaging showed multiple bilateral mediastinal and hilar lymphadenopathies without pulmonary parenchymal involvement.Mediastinoscopy with lymph node biopsy revealed non-necrotizing epithelioid and giant-cell granulomatous lymphadenitis compatible with sarcoidosis.Systemic corticosteroid therapy with prednisone was initiated.The neurological course was marked by worsening myasthenic manifestations, with increased ptosis, diplopia, painful dysphagia, and predominantly left-sided weakness.A course of intravenous immunoglobulin resulted in marked clinical improvement.Thymectomy performed in July 2023 revealed thymic hyperplasia without thymoma.At the latest available follow-up, the patient was asymptomatic and remained under neurological follow-up.The coexistence of sarcoidosis and myasthenia gravis remains rarely described.In this case, the two diagnoses were supported by independent findings.Sarcoidosis was supported by radiological and histological findings, whereas the fluctuating and fatigable nature of the ptosis, the response to neostigmine, and the electrophysiological abnormalities supported the diagnosis of myasthenia gravis.Neurosarcoidosis was the main neurological differential diagnosis.The coexistence of the two diseases may be coincidental and, although an immune-mediated background may be considered, the available data do not demonstrate a specific or causal relationship between sarcoidosis and myasthenia gravis.This case describes a rare association between stage I thoracic sarcoidosis and double-seronegative myasthenia gravis, with thymic hyperplasia without thymoma.It highlights the importance of documenting each condition separately and of investigating a neuromuscular transmission abnormality in the presence of fluctuating ptosis, diplopia, or bulbar involvement in a patient with sarcoidosis, rather than immediately attributing these manifestations to neurosarcoidosis.