AOSD (Adult-onset Still’s disease) is a rare type of inflammatory arthritis that causes joint damage similar to that seen in rheumatoid arthritis. It is thought to be the adult form of SJIA (systemic juvenile idiopathic arthritis, which is also called Still’s Disease). Like SJIA, AOSD affects the whole body but its symptoms are very variable. Some people may have only one bout of the illness and then recover, while in others the disease persists and may lead to permanent joint destruction and other serious health problems.[1,2] AOSD mainly develops in young adults between the ages of 16-35. Men and women are equally at risk.[3,4]
AOSD affects only a few people in every million. Estimates in Japanese and European populations suggest there are between 1 and 34 cases per million people. However, because the disease is so rare and the symptoms vary so much, AOSD often goes undiagnosed or is confused with other conditions, making it difficult to work out how common it really is. AOSD does not run in families but research has shown that people with certain types of genes may be more likely to develop it.
There are a trio of typical symptoms of AOSD: daily spiking fevers, skin rash and painful, swollen joints. However many people have other symptoms too and the disease often starts off with a sore throat. The most common symptoms are:[2,3,4]
The signs and symptoms of AOSD can mimic those of other conditions, including other autoinflammatory syndromes, systemic lupus erythematosus (lupus) and a type of blood cancer called lymphoma.[1,3]
The specific symptoms of AOSD, its severity, and the frequency of episodes vary from one person to another and the progression of the disorder is difficult to predict. In some individuals, AOSD develops suddenly, disappears almost as quickly, and may not return for many years, if ever. In other people, AOSD becomes a chronic condition and can cause long-term, severe, and disabling complications. If untreated, chronic inflammation can result in deterioration and destruction of the affected joints. If excess fluid builds up around the lungs, it can occasionally cause breathing problems.[1,3] Another serious but very rare complication in people with AOSD is macrophage activation syndrome (MAS). MAS can lead to symptoms of blood poisoning and internal organ failure.[7, 8]
AOSD is idiopathic which means the cause is unknown. Researchers believe that an abnormal or excessive response to an infection or toxic substance may trigger the disorder in people who also have a genetic susceptibility. AOSD was only recently classified as an auto-inflammatory condition but it differs from other auto-inflammatory conditions in that it is not inherited. Some researchers think AOSD is a cross between an auto-inflammatory condition and an autoimmune disease like rheumatoid arthritis.