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Ann Thorac Surg 2000;69:314-315
© 2000 The Society of Thoracic Surgeons
a Department of General and Thoracic Surgery, Hospital "La Paz", c/Don Ramon de la Cruz 97, 6 B, 28006 Madrid, Spain
e-mail: iprieto{at}intermic.com
To the Editor
First things first, I would like to thank Dr Kirschner for the chance he gives us to clear up some issues in our paper entitled "Prognostic Factors for Myasthenia Gravis Treated by Thymectomy: Review of 61 Cases" (Ann Thorac Surg 1999;67:1568–71), as well as to congratulate him for his shrewd criticism to the aforementioned article.
To begin on one’s subject, we in no way state in our paper that thymomas present the highest incidence in Osserman classification III and IV groups, which are, no doubt, the most severe cases. We state rather that these patients present a lower rate of clinical remission (18.1%) as well as higher morbility rate, as compared with patients with other postthymectomy histologic findings. However, this difference did not reach statistical significance, presumably due to the small size of our subsample of patients with thymoma (n = 11) and the fact that we did not consider histologic type as a dychotomous (thymomatous myasthenia gravis [MG], nonthymomatous MG) independent variable, so we can just hypothesize that thymoma patients were more likely to be classified in the highest Osserman groups, a variable that did reach statistical significance in our study. This has been pointed by other authors too [1], who consider that the association of MG with thymoma is an adverse prognostic factor for the former. On the contrary, some authors feel that the presence of thymoma could contribute to earlier detection of MG, thereby providing a better chance of early diagnosis and, as a consequence, of survival [2].
On the other hand, we are afraid that we are not experienced enough, as far as immunologic patterns involved in MG are concerned, to carry on a discussion with Dr Kirschner about this subject. Also, we did not measure the levels of receptors for antibodies to acetylcholine in our patients because the procedure was not easily available as a routine test in our hospital when we carried out the investigation. Fortunately, today it is of easy access for both clinicians and investigators, hence, we are planning to further investigate the immunologic issues in MG in the near future.
Dr Kirschner is likely to be right when he states that the term "extended or maximal thymectomy" is ambiguous in the way we have used it in our paper. For us, maximal or extended thymectomy refers to removal of the fat and tissue surrounding the thymus along with the gland [3]. In this respect, it is noteworthy that ectopic thymic tissue has been reported in up to 72% of the fat from the surgical specimen of anterior mediastinum, which is undoubtfully associated with poor outcome of the operation [4].
Dr Kirschner is "shocked" by the impressive number of cases with thymolipoma in our series (9.8%). We have no conclusive explanation for this, except that this histologic type might be more prevalent in Spain than in the United States, for reasons we do not know currently. Unfortunately, we have no data about the incidence of thymolipoma in the general population of Spain. Of course, we are sure that the histologic diagnosis of thymolipoma was correct in all the cases, and that all thymolipoma patients in our series certainly had MG.
On the other hand, we did not use the newer classification system that Dr Kirschner calls for (cortical, medullary, and well-differentiated carcinomas) because it was not, unfortunately, available when we conducted our investigation.
Finally, we fully agree with Dr Kirschner that one should underscore the key difference between thymomatous and nonthymomatous MG, because such a distinction may be of great concern for MG prognosis. However, it is necessary to keep in mind that our study was not designed to directly compare thymoma versus nonthymoma tumor prognostic influence on MG. In addition, our subsample of MG patients was too small (n = 11) and, as a consequence, our investigation did not have enough detection power in this respect. It is logical to assume that thymoma is an adverse prognosis factor for MG, because thymoma patients were likely to present worse clinical course and lower complete clinical remission rate, and apparently fall into the highest groups in the Osserman staging system, and this latter variable did prove to be a statistically significant prognostic factor in our investigation. Therefore, we assume that histologic type and Osserman stage are, in part, redundant variables, but we could not estimate the exact degree of redundancy, due to limitations inherent to the study design. We were able to conclude only that time from onset to thymectomy and Osserman classification stage are both significant prognostic factors for MG. Of course, further investigation is needed to assess the exact prognostic significance of the variable histologic type by including a larger number of patients with thymoma. Thus, we favor Dr Kirschner’s point of view that "stricter adherence to these distinction would further our knowledge of this vexing group of diseases, the myasthenias." We simply regret not having adhered in a stricter way to distinctions Dr Kirschner urged. However, we feel it is not necessary to employ the term "myasthenia" in plural, as Dr Kirschner suggest; rather, we consider it is sufficient to emphasize that thymomatous and nonthymomatous myasthenias are two clinical variants, presumably with different prognostic implications, with specific investigation being needed in this respect, using the dichotomy thymomatous MG and nonthymomatous MG as a specific independent variable, and, of course, it was not the aim of our investigation.
References
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