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Influence of ectopic thymic tissue on clinical outcome following extended thymectomy in generalized seropositive nonthymomatous myasthenia gravis

^a Unit of Myasthenia Gravis, Hospital General Universitari Vall d’Hebron, Autonomous University of Barcelona, Barcelona, Spain
^b Department of Surgery, Hospital General Universitari Vall d’Hebron, Autonomous University of Barcelona, Barcelona, Spain
^c Department of Neurology, Hospital General Universitari Vall d’Hebron, Autonomous University of Barcelona, Barcelona, Spain
^d Department of Pathology, Hospital General Universitari Vall d’Hebron, Autonomous University of Barcelona, Barcelona, Spain

Received 25 February 2008; received in revised form 18 July 2008; accepted 21 July 2008.

^_* Corresponding author. Address: Unit of Myasthenia Gravis, Department of Surgery, Hospital General Universitari Vall d’Hebron, Passeig Vall d’Hebron 119–129, E-08035 Barcelona, Spain. Tel.: +34 93 2746000x6587; fax: +34 93 2746224. (Email: jmponseti{at}vhebron.net; 5012jpb{at}comb.es).

Objective: We determined the effect of detecting ectopic thymic tissue in thymectomy specimens on the long-term outcome of patients with myasthenia gravis. Methods: A total of 83 consecutive patients with generalized seropositive nonthymomatous myasthenia gravis underwent transsternal extended thymectomy (T-3b according to Myasthenia Gravis Foundation of America). Ectopic thymic tissue was only accepted when Hassal's corpuscles in the excised cervicomediastinal fat were documented. The primary endpoint was to assess differences in time to obtain complete stable remission (CSR) according to the presence or absence of ectopic thymus. Results: Thirty-five patients (42.2%) had ectopic thymic tissue. The mean follow-up was 88.4 ± 36.3 months (range 20–144). By the Kaplan–Meier analysis method, the estimated median follow-up to obtain a CSR in the group without ectopic thymic tissue was 32.9 months (95% confidence interval [CI] 21.1–44.8 months) and 117.8 months (95% CI 98.0–137.6 months) for the group with ectopic thymic tissue (log-rank test, p = 0.0002). The probability over time of obtaining CSR for the groups without and with ectopic thymic tissue was 65% vs 26% at 5 years. After stratification by hyperplasia or involution of the thymus as well as by post-thymectomy immunomodulating regimen (prednisone and prednisone–tacrolimus), the probability over time of obtaining CSR at 5 years was also significantly higher for patients without ectopic thymic tissue than for those with ectopic thymic tissue. Conclusions: The clinical outcome of patients with nonthymomatous seropositive myasthenia gravis is significantly affected by the presence of ectopic thymic tissue in the mediastinal fat.

Key Words: Myasthenia gravis • Thymectomy • Prednisone • Tacrolimus