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Autoimmune myasthenia gravis (MG) is a heterogenous disorder. In young women, the thymus gland is often hyperplastic, and the patients respond well to thymectomy. However, in the increasing number of patients over the age of 40 years, predominantly men, thymic hyperplasia is uncommon, and there are no clear aetiological clues.1
We diagnosed MG in three male patients who had undergone cardiac surgery between three and ten weeks before developing symptoms. Table 1 summarises their main features. The patients presented with ocular, bulbar, and mild generalised weakness (Osserman classification grade I to IIb). None of them was taking antiarrhythmic agents or other drugs impairing neuromuscular transmission and none of them had had any thyroid or other autoimmune disorder. Two showed typical decremental responses to repetitive stimulation on EMG, and all three had positive levels of antibodies to acetylcholine receptors, which are diagnostic for MG.1 None had had postoperative recovery difficulties that might have suggested pre-existing MG. All three patients were treated with cholinesterase inhibitors, two also required corticosteroids (for the plan of treatment, see Sanders and Scoppetta2), and one required plasma exchange. The patients have now been followed up for up to seven years and all still require some medication and have positive AChR antibodies.
The development of MG within a few weeks of cardiac surgery is intriguing, and has not been reported before. The presentation of MG in these subjects may be purely coincidental but, because of the short delay between surgery and presentation, it is difficult to escape the conclusion that the cardiac surgery precipitated the condition or exacerbated subclinical disease. During cardiac surgery there is damage to the atrophic thymic remnants that are present in the anterior mediastinal fat. The thymus contains muscle-like cells, called myoid cells, which express whole AChR molecules.3 Release of thymic AChR could increase an existing subclinical antibody response, or allow AChR to be presented de novo to the immune system; the lack of postoperative difficulties, which are commonly encountered in undiagnosed MG, suggests that the second hypothesis is most probable. The AChR is very immunogenic; for instance, in mice, intraperitoneal injection of purified murine AChR without adjuvants can result in the typical antibodies and clinical expression of the disease.4
Further studies should investigate the presence of muscle weakness and positive titres of acetylcholine receptor antibodies after cardiac surgery, and could compare with similar measurements after other forms of major surgery in this age group. Equally, as other autoantigens are also expressed in the thymus,5 the presence of other autoantibodies or signs of other autoimmune diseases should be sought.
Competing interests: none declared.