Selective
atrophy of type II muscle fibres has been long recognized as an enigmatic but consistent feature of
myasthenia gravis (MG) muscle; however, the pathophysiology and the mechanism of that change have remained obscure. In the present study, the results of histomorphometric analysis performed on muscle biopsies from 207 thymectomized seropositive MG patients were correlated with clinical features of MG to demonstrate possible pathophysiological associations and potential prognostic impact. The
atrophy of type II fibres was verified in 35 cases (16.9%), being more pronounced in fibres of IIB subtype. It was neither significantly associated with the duration and severity of MG nor with the age of the patients. On the other hand, we demonstrated that the
atrophy associated with long-term treatment with
corticosteroids, and correlated with increasing doses. Thus, we suppose that the
atrophy of type II muscle fibres in seropositive MG is
steroid induced rather than MG-associated event. Although the MG patients with
atrophy of type II fibres did not differ from the remaining MG cases in terms of improvement in the disease during the follow-up period, our analysis provides clear evidence that they presented a significantly slower tendency to reach an
asymptomatic state after
thymectomy. Therefore, the
steroid-induced
atrophy of type II fibres in MG muscle might be considered to be an unfavourable prognostic factor.