What it is
A short-lived inflammation of the lining of the hip joint, with extra fluid in the joint, that causes pain and a limp. The NHS calls it the most common cause of hip pain in children.
Nobody knows the cause. It often follows a viral illness, such as a cold, a sore throat or diarrhoea and vomiting, or a minor injury, but that isn't proven: a Merseyside study found no seasonal pattern, which casts some doubt on the virus theory, and a Glasgow department saw different seasonal peaks in two years. A child with irritable hip usually holds the hip bent and turned outwards, the position the extra fluid makes most comfortable.
Sources 1, 2, 3, 4, 5, 6, 7
Who gets itStrong evidence
Usually children aged about 3 to 8, most often around 5 or 6, and boys 2 to 3 times as often as girls. In Merseyside it was diagnosed in about 25 in every 100,000 children aged 0 to 14 each year, at an average age of 5.4. A Glasgow children's emergency department diagnosed about 360 a year, at an average age of 3.3. In a Swedish study about 0.2% of children were diagnosed each year, a 3% chance of at least one episode in childhood, and Dutch GPs recorded about 1 in 1,000 children a year.
Under 3, doctors are extra careful, because infection and non-accidental injury are more likely causes of a limp at that age.
Sources 7, 5, 6, 8, 9, 10
How it shows
A limp, or not wanting to walk, crawl or stand, usually on one side; pain in the hip, groin, thigh or knee; and at most a mild temperature, in a child who is otherwise well.
Sources 3, 4, 11, 2, 12
How it's told apart from infection, and what tests meanStrong evidence
The NHS says hip pain in children should always be checked, because it could be a sign of something serious. A doctor examines the hip and may arrange an X-ray, blood tests or an ultrasound scan. The worry is septic arthritis, a bacterial infection of the joint, which can damage it quickly and needs surgery to wash it out, ideally within 24 hours of diagnosis.
The best-studied clues are four: a history of fever, refusing to put weight on the leg, a raised erythrocyte sedimentation rate (ESR, a blood test for inflammation) and a raised white blood cell count. In the original US study, the chance of infection was under 0.2% with none of them and 99.6% with all four; in a new group of 154 children the same four signs held, with somewhat lower accuracy. In another study of 48 children, a temperature over 38.5°C and a raised C-reactive protein (CRP, another inflammation test) were the strongest signs, but in a 2024 review of 11 studies (1,810 children), fever, not bearing weight, the ESR and the white cell count stood out and CRP didn't. NICE (the National Institute for Health and Care Excellence) asks doctors to consider a joint or bone infection in a child under 5 with a fever who has a swollen limb or joint, or won't use a limb or bear weight.
An ultrasound shows fluid in the hip, which is there in irritable hip too, so on its own it can't tell irritable hip from infection, and a British Orthopaedic Association standard says no single set of tests is completely reliable. An X-ray is normal in irritable hip; it is used to look for Perthes disease, a slipped growth plate or a break. Routine repeat X-rays at 3 months found little in a Swedish study.
Sources 1, 13, 14, 15, 16, 17, 18, 19, 8
How it usually goesEmerging evidence
The NHS says it usually gets better in 1 or 2 weeks and doesn't cause lasting problems; one NHS trust says between about 3 days and 2 weeks, and another that some cases take up to 4 weeks. In a review of 25 studies, almost all children were free of symptoms at 2 weeks, and symptoms lasted more than a month in 3% to 8%.
It can come back: in about 0% to 18% in the better studies (up to 26% in others), mostly in the first 6 months, and sometimes in the other hip. In 39 children who had repeated episodes, none developed Perthes disease or another hip problem over about 4 years, though 3 were later diagnosed with an inflammatory condition. In a UK emergency department, no serious problem had been missed when 80 children were reviewed 7 years later.
Sources 1, 2, 4, 7, 20, 21
Perthes disease found later: about 3 in 100Emerging evidence
In a review of 19 studies of mixed quality, about 3 in 100 children first diagnosed with irritable hip (2.7%) were later found to have Perthes disease, rising to about a third of those whose symptoms lasted or kept coming back. In a Swedish study, Perthes disease was found 1 to 5 months later in 3.4%, and most of those children had small signs of it on their first X-ray.
Irritable hip doesn't seem to cause Perthes disease: the overlap is mostly early Perthes disease first labelled as irritable hip. That is why a limp that lasts, or keeps coming back, is checked again.
Sources 22, 8, 7
