overlap syndrome

Overlap syndrome: refers to the overlap of disease manifestations between two or more connective tissue diseases, also known as overlap connective tissue disease. This overlap can occur simultaneously, with the patient meeting the diagnostic criteria for two or more connective tissue diseases at the same time; it can also occur sequentially at different times, with another connective tissue disease developing later; or a patient may first have one connective tissue disease, which then transitions into another CTD. This transition can be continuous or occur after a certain interval. OS typically occurs among the six diffuse CTDs, namely SLE, RA, DM/PM, PSS, polyarteritis nodosa (PN), and rheumatic fever (RF), and can also involve overlap of these six CTDs with related diseases such as Behçet's disease, Sjögren's syndrome, and panniculitis. Additionally, it may overlap with other autoimmune diseases such as chronic thyroiditis and autoimmune hemolytic anemia.