Ectopic thymic tissue in the mediastinum: limitations for the operative treatment of myasthenia gravis.

OBJECTIVES: The aim of the study was to investigate the distribution of ectopic thymic tissue in the mediastinum and to evaluate the possible relevance of this distribution to the therapeutic yield of thymectomies in patients with myasthenia gravis.

METHODS: In this prospective autopsy study, mediastinal dissections were performed on 50 cadavers without any previously known intrathoracic pathology. The initial dissection was performed in the same way as during the maximal thymectomy. The second stage consisted of dissecting areas of fatty tissue located out of reach of the standard maximal thymectomy, such as the perithyroid, periaortic, peritracheal and retrotracheal areas, as well as the areas adjacent to the right and left phrenic and recurrent laryngeal nerves. Each specimen was independently examined by two pathologists for ectopic thymic tissue.

RESULTS: There were 41 (82%) male and 9 (18%) female cadavers, with a mean age of 44.3 years (range: 15-75). Ectopic thymic tissue was detected in 32 out of 50 cadavers (64%). In 10 (20%) cadavers thymic foci were found in locations accessible to the standard surgical intervention and in 22 (44%)-in inaccessible locations. Thymic tissue incidence in individual locations was as follows: retrothyroid, 3 (6%); peritracheal, 5 (10%); retrotracheal, 1 (2%); right phrenic nerve, 2 (4%); left phrenic nerve, 14 (28%); right recurrent laryngeal nerve, 2 (4%); left recurrent laryngeal nerve, 2 (4%) and periaortic, 0.

CONCLUSIONS: The incidence of ectopic thymic tissue in the mediastinum is common. Although some improvements in the results of thymectomies may be expected with more extensive dissection, the frequent presence of thymic foci in anatomical locations hardly accessible to surgical intervention may be the true limitation for surgical treatment of myasthenia.