Thymus pathology in myasthenia gravis with anti-acetylcholine receptor antibodies and concomitant Hashimoto's thyroiditis. A four-case series and literature review.

Abstract

Objective: Identifying the morphological features of thymus in patients with myasthenia gravis (MG) with anti-acetylcholine receptor (AChR) antibodies and concomitant Hashimoto’s thyroiditis (HT), which were recruited from a single surgical unit of a tertiary referral hospital located in the North-Eastern region of Romania, over a period of 11 years. Patients, Materials and Methods: We retrospectively reviewed clinical, imaging, laboratory, thymic pathology, and outcome data that were obtained from medical records of patients with MG and concomitant HT, to whom a thymectomy was performed for a suspected thymic lesion. All the surgical interventions were done in the Third Clinic of Surgery, St. Spiridon Emergency County Hospital, Iaşi, Romania, for an 11 years’ period, i.e., from January 1, 2000 and December 31, 2010. Results: Four patients (three females and one male) were included. The mean age of the patients at the time of their thymectomy was 40.25 years. Of all patients, 75% had moderate or severe MG, 100% had anti-AChR antibodies, and an electromyographic decrement greater than 25%. All patients have been diagnosed with HT in their past medical history by a full thyroid panel [high thyroid-stimulating hormone (TSH) values, low free thyroxine (fT4) values, and the presence of the anti-thyroid antibodies] and all of them have been treated with Euthyrox. Our four patients expressed different MG subtypes, each of them being associated with different thymus pathology. Thoracic computed tomography (CT) scan revealed heterogeneous mediastinal masses and established the correct diagnosis only in 25% of cases. The pathological exams also revealed a heterogeneous pattern of thymic lesions. In contrast with other studies, our patients with MG with anti-AChR antibodies and concomitant HT presented atrophic thymus more frequently (50%), but with particular morphological changes of Hassall’s corpuscles. Also, 25% of cases were diagnosed with thymic lympho-follicular hyperplasia (TLFH) associated with thymic epithelial hyperplasia. In B2 thymoma, neoplastic epithelial cells expressed cytokeratin 19 (CK19) immunoreactivity, high Ki67 labeling index and strong p63 immunopositivity. Conclusions: In our series, MG and HT occurred simultaneously, or one of them was diagnosed before the other, raising some new questions regarding the immune mechanism of these two autoimmune diseases. Due to the heterogeneous morphological changes of the thymus that we found in this study, we can hypothesize that thymus is involved in the pathogenic mechanism of MG with anti-AChR-antibodies and concomitant HT development.