Practice parameter: Thymectomy for autoimmune myasthenia gravis (an evidence-based review) [RETIRED]: Report of the Quality Standards Subcommittee of the American Academy of Neurology

Abstract

The Quality Standards Subcommittee of the American Academy of Neurology is charged with developing practice parameters for neurologists for diagnostic procedures, treatment modalities, and clinical disorders.The selection of topics for which practice parameters are used is based on prevalence, frequency of use, economic impact, membership involvement, controversy, urgency, external constraints, and resources required.This paper addresses the role of thymectomy in the treatment of nonthymomatous autoimmune MG.In 1939, Blalock et al. 1 reported the remission of generalized MG in a 21-year-old woman after removal of a cystic thymic tumor.Subsequently, Blalock et al. 2,3 performed thymectomy on MG patients without thymoma, found hyperplasia in the thymus glands, and reported improvement in at least half of their patients.Since these reports, thymectomy, with or without the presence of thymoma, has gained widespread acceptance as a form of treatment for MG.Because a definitive study of the effectiveness of thymectomy has never been done, 4,5 the role of thymectomy in the management of MG remains uncertain.Nevertheless, physicians have to advise their patients regarding the benefits of thymectomy based on the existing literature.Our goal is to develop evidence-based recommendations for clinicians considering thymectomy for patients with nonthymomatous autoimmune MG by performing a systematic review and analysis of the literature. Process. Identification and selection of studies.We searched the National Library of Medicine's Medline database from 1966 to February 1998 using the medical subject headings "myasthenia gravis" (restricted to the surgery subheading) and "thymectomy."To identify articles published before 1966, or missed by our original search strategy, we reviewed the references of the identified articles.We classified the resulting articles into the following categories: Class II, controlled but nonrandomized studies describing outcomes in MG patients with and without thymectomy; and Class III, uncontrolled case series describing outcomes in patients with or without thymectomy.Within a given controlled (Class II) study, we planned to describe the therapeutic effect of thymectomy by comparing the rate MG patients receiving thymectomy achieved desirable outcomes with the rate MG patients not receiving thymectomy achieved desirable outcomes.The nonrandomized nature of Class II studies often introduces differences in the baseline characteristics of the thymectomy and nonthymectomy patient groups that can effect MG outcomes independent of thymectomy.To the extent possible, we planned to adjust for these confounding differences in baseline prognostic characteristics.In the absence of randomized controlled trials, such adjustments are critical for estimating the actual therapeutic effect, if any, of thymectomy.6 Between uncontrolled (Class III) studies, comparisons of outcome rates in thymectomy and nonthymectomy MG patients introduce additional confounding baseline differences.6 These additional confounders include institutional, geographic, and historical variations that potentially impact MG patient outcomes independent of thymectomy.Because these additional confounders are difficult to identify and adjust for, outcome comparisons between uncontrolled studies are particularly unreliable.7 For this reason, we excluded Class III studies from further consideration. Abstracted study characteristics.From the controlled studies describing outcomes we abstracted the following characteristics: method and setting of cohort assembly, years during which patients were enrolled in the cohort, number of subjects assembled, duration of follow-up, proportion of subjects lost to follow-up, and the thymectomy techniques employed.