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.
Cause
The cause is unknown but allergic and auto-inflammatory mechanisms have been proposed.In a 1957 review of IH, Mattingly did not find evidence that the condition is inherited unlike Reimann who, in 1974, describes the condition as “heritable, non-inflammatory, and afebrile”.More recently, specific association with the Mediterranean fever gene, MEFV, has been proposed. So, with some individuals carrying gene mutations (MEFV and also TRAPS-related genes), the native immune system seems to plays a role in the development of IH, i.e. there is an auto-immune component to the condition. Pathophysiology
Involvement of mast cells has been reported  reflecting a possible immunoallergic aspect to IH. Mattingly suggests that IH may be an unusual variant of rheumatoid arthritis, and some patients may go on to develop RA. Joint damage however does not generally occur and only the synovial membrane is affected by a ‘non-inflammatory oedema’.With regard to the periodic nature of effusions, Reimann theorises that:
“…either an inherent rhythm or a feedback mechanism (Morley, 1970) excites bioclocks in the hypothalamus or in the synovial membrane (Richter, 1960). These Zeitgebers provoke sudden accumulation of plasma in the lining and spaces of joints, tendon and ligament sheaths.”

Diagnosis
There is no specific test for this condition. Diagnosis is based on signs and symptoms, and exclusion of other conditions. Differential diagnoses
Rheumatoid arthritis. Confusion with rheumatoid arthritis may be common even though IH is a non-inflammatory condition without the many signs and symptoms associated with RA. So far, an association with HLA-B27 gene in IH has not been reported.