Overlap syndromes.

Many connective tissue diseases share common signs and symptoms, which frequently makes the diagnosis of a specific rheumatic disease difficult. Rheumatoid arthritis, systemic lupus erythematosus, systemic sclerosis, polymyositis, dermatomyositis (DM), mixed connective tissue disease, and Sjögren's syndrome can present with similar clinical features, particularly during the first 12 months of symptoms. Overall, a rheumatic disease can appears in conjunction with features of one or more other connective tissue diseases, for example, patients can have a combination of rheumatoid arthritis and systemic lupus erythematosus ("rhupus"), or systemic sclerosis and polymyositis, defining an "overlap syndrome", where the diseases comply with the diagnosis criterias. Finally, when a person has symptoms of various connective tissue diseases without meeting the full criteria for any one of them, it is often called Undifferentiated Connective Tissue Disease.