Mixed connective tissue disease, overlap syndromes, and eosinophilic fasciitis.

While the cause of connective tissue diseases (CTDs) remains unknown the classification of individual patients will continue to depend on identifying certain patterns of clinical and laboratory features. This is the basis of the various classification criteria for systemic sclerosis,'l systemic lupus erythematosus (SLE),2 and other connective tissue diseases. As many as 25% of patients with CTD, however, present with overlapping clinical features. The term 'overlap syndrome' is applied to what appears to be a very heterogeneous group of disorders, though features associated with systemic sclerosis are often a major component. Frequently, patients have a mild CTD characterised by Raynaud's phenomenon, oedema of the hands, and, possibly, acrosclerosis and arthritis, which has been termed 'undifferentiated CTD'.3 Other patients present manifestations of more than one defmiite CTD and commonest among these are disorders with features of systemic sclerosis combined with those of SLE4 or polymyositis,5 or both, with features of systemic sclerosis or SLE and rheumatoid arthritis,6 7 and features of Sjogren's syndrome in association with any of the other CTDs. Overlapping features may occur concurrently, but more commonly over time one syndrome takes on the features of another. It is still contentious whether or not overlap syndromes represent the coexistence of separate diseases, the broad clinical expression of one of the rheumatic diseases, or distinct clinical entities with distinctive aetiology and pathogenesis. In some cases overlap syndromes are associated with a particular serological marker (table 1). Antibodies to the RNA binding proteins Ro (SSA) and La (SSB) are associated with Sjogren's syndrome developing in association with other CTDs, such as systemic sclerosis or SLE. Recent reviews suggest that Sjogren's syndrome in this context resembles primary Sjogren's syndrome and differs significantly from secondary Sjogren's syndrome accompanying rheumatoid arthritis.8 Severe xerophthalmia, xerostomia, and parotid gland enlargement are often