Myasthenia Gravis of Onset After the Age of 50

Its Comparison with that of Onset at Younger Age

Abstract

Myasthenia gravis (MG) of onset after the age of 50 was compared clinically with that of onset below 50. There were 48 patients of MG (14 males, 34 females) who visited the neurologic clinic, University of Gunma Hospital for the last seven years, and 6 another patients of MG (3 patients of onset after 50 years old) admitted to the geriatric ward, University of Tokyo Hospital. Of 48 patients of the former group those with onset after the age of 50 was 7 (50%) in males and 4 (12%) in females with the incidence according to age at onset being highest in the sixth decade in males and in the second to fourth decades in females. The distribution resembled to those in USA and European patients (Schwab et al., Osserman et al.) rather than to those in Japanese series in which onset was most common in younger adults in both sexes, and it suggested a scrutiny in the elderly group might bring forth the discovery of more male patients of MG.
Symptoms at onset, severity of MG according to Osserman's classification at our initial examination, and severity at the follow-up study more than one year after the initial examination were all similar in both older and younger onset groups, indicating the onset at older age implied no poor outcome. Antiacetylcholine receptor antibody titers were determined in serum in 9 patients of older onset and 24 of younger onset, revealing the relatively low levels below 10nMol/l significantly more frequent in the former group.
Thymus was examined by means of computed tomography (CT) of the anterior mediastinum, pneumomediastinography, and histological investigations after thymectomy or autopsy, though CT was useful only in the detection of thymic tumors. Hyperplastic or remaining thymus was disclosed in 2 (25%) of 8 patients with older onset and 15 (65%) of 23 with younger onset, with significantly lower frequency in the former group, thus indicating that thymus might play no important role in the pathogenesis of MG in the older age group except in the case of thymoma.
Three illustrating cases of onset at older age were presented. Case 1, a 78-year-old male developed ptosis and diplopia, followed by bulbar and neck muscle weakness. Because of poor response to anticholinesterase medications prednisolone was prescribed with dose of 25mg on alternate days, causing subsequent transient respiratory and cardiac arrest. Dysphagia persisted after the improvement of other muscles and debility progressed terminating in death due to paralytic ileus. By autopsy no thymic tissue was discovered both macro-and microscopically. Case 2, a male developed MG of Osserman IIA at the age of 52 and had a fluctuating course including four crises even after thymectomy for thymoma, dying of laryngeal cancer 9 years after the onset of MG. No remnant of thymus was found by autopsy. Case 3, a male with onset of MG of Osserman IIA at the age of 60 showed a hyperplastic or remaining thymus with pneumomediastinography, though thymectomy was not indicated because of good control with anticholinesterase drugs.