Myasthenia Gravis - a Critical Assessment of the Contribution of Muscle Biopsy Performed During Thymectomy
Abstract
Surprisingly, structural changes outside the neuromuscular junctions were reported in limited series of myasthenia gravis (MG) muscles previously - mainly small collection of interstitial lymphocytes and atrophy of type II muscle fibers. However, the pathophysiological mechanisms remained unclear.
We performed a systematic study on MG muscle biopsies to contribute to understanding of the association to MG and to demonstrate possible prognostic value. Methods: Sternothyroid muscle biopsies from 207 MG patients taken during thymectomy were analyzed histopathologically and by histochemistry and morphometry and correlated to the clinical features and the outcome of MG patients.
Results: In 143 MG patients, no obvious pathological changes were observed in muscle biopsies. In 20 patients we observed an interstitial infiltration by mature naive CD45RA+lymphocytes.
The lymphocytic infiltration associated with the thymus pathology, mainly with thymoma. The morphometry disclosed selective atrophy of type II muscle fibers in 35 cases.
The atrophy of type II muscle fibers was related to long-term corticosteroid treatment and the potential prognostic value could be demonstrated for its detection: the MG patients with type II fibers atrophy presented slower tendency to reach remission. Further, in 12 cases we demonstrated mild changes in mitochondrial metabolism, in 34 cases we disclosed angular atrophic fibers; no prognostic value was demonstrated for these changes.
Conclusions: The previously reported histopathological changes in MG muscle are rare and mostly do not represent a feature of the MG, rather a consequence of the thymic disorder or of the long-term steroid therapy. The value of muscle biopsy performed during thymectomy for the particular MG patient remains controversial.