Repeated, periodic joint effusions of the knee. Usually one knee is affected but sometimes both knees. Other joints may also be involved along with the knee. Effusions are large, restricting range of motion but significant pain is not a feature. There is usually stiffness. Tenderness of the joint may or may not be present. Aspirated synovial fluid is usually sterile but will sometimes show elevated cell count (>100 cells/mL) with 50% being polymorphonuclear leukocytes.

Onset of effusions are sudden with no particular trigger or stimulus. Each episode lasts for a few days to about a week and recurs in cycles of 7 to 11 days with extremes of 3 days to 30 days also reported. Sometimes the joint may begin to swell again as soon as the fluid has subsided. Where both knees are affected concurrently, as one joint ceases to swell the other may become involved.

The cycle of joints swellings have been reported as being very regular, even predictable. This has been a characteristic feature of IH in many case reports. However, over the longer-term especially, these cycles of effusion and recovery may not be as constant as first reported.

In women, many cases seem to begin at puberty. Episodes of knee swelling may coincide the menstrual cycle. In nearly all case reports, pregnancy seems to suppress the condition but after birth, during lactation, it returns.

In the main, patients are mostly free of other symptoms. Fever is rare. There no signs of local inflammation or lymphatic involvement. Laboratory tests are generally normal or within reference limits.

Intermittent hydrarthrosis (IH), also known as "periodic synoviosis", "periodic benign synovitis", or "periodic hydrarthritis", is a chronic condition of unknown cause characterized by recurring, temporary episodes of fluid accumulation in the knee. While the knee is mainly involved, occasionally other joints such as the elbow or ankle can additionally be affected. Fluid accumulation in the joint can be extensive causing discomfort and impairing movement, although affected joints are not usually very painful. While the condition is chronic, it does not appear to progress to more destructive damage of the joint. It seems to affect slightly more women than men.

Episodes of swelling last several days or longer, can occur with regular or semi-regular frequency, typically one or two episodes per month. Between periods of effusion, knee swelling reduces dramatically providing largely symptomless intervals. Unlike some other rheumatological conditions such as rheumatoid arthritis, laboratory findings are usually within normal ranges or limits.

Clear treatment options have yet to be established. NSAIDs and COX2-inhibitors are generally not effective. Where this condition has been correctly diagnosed, various anti-rheumatic drugs as well as colchicine may be trialled to find the most effective option. More aggressive intra-articular treatment such chemical or radio-active synovectomy can also be helpful although benefits beyond 1 year have not been reported in literature.