Onset and Evolution of Clinically Apparent Myasthenia Gravis After Resection of Non-myasthenic Thymomas

Patients with thymoma and without clinical or electromyographical myasthenic signs may occasionally develop myasthenia several years after thymectomy. Hereby, we investigated the predictors and the evolution of this peculiar disease. We performed a retrospective analysis in 104 consecutive patients who underwent thymectomy between 1987 and 2013 for thymoma without clinical or electromyographic signs of myasthenia gravis. Predictors of post-thymectomy onset of myasthenia gravis were investigated with univariate time-to-disease analysis. Evolution of myasthenia was analyzed with time-to-regression analysis. Eight patients developed late myasthenia gravis after a median period of 33 months from thymectomy. No significant correlation was found for age, gender, Masaoka's stage, and World Health Organization histology. Only high preoperative serum acetylcholine-receptor antibodies titer (>0.3 nmol/L) was significantly associated with post-thymectomy myasthenia gravis at univariate time-to-disease (P = 0.003) analysis. Positron emission tomography was always performed in high-titer patients, and increased metabolic activity was detected in 4 of these patients. Surgical treatment through redo-sternotomy or video-assisted thoracoscopy was performed in these last cases with a remission in all patients after 12, 24, 32 and 48 months, respectively. No patient under medical treatment has yet developed a complete remission. In our study the presence of preoperative high-level serum acetylcholine receptor antibodies was the only factor significantly associated with the development of post-thymectomy myasthenia gravis. The persistence of residual islet of ectopic thymic tissue was one of the causes of the onset of myasthenia and its surgical removal was successful.