Your child has been diagnosed as having an irritable hip. This condition is also known as observation hip or transient synovitis. It is very worrying for a parent if your child suddenly cannot walk or has a marked limp. However, these names emphasize the benign and short-lived nature of the condition.
WHAT IS AN IRRITABLE HIP?
There are probably two sorts of irritable hip. The most common type is caused by fluid in the hip joint that interferes with the normal movement of the hip. This fluid can be shown by an ultrasound scan of the hip, though the findings on examination are usually enough to make the diagnosis.
The second type probably results from inflammation of glands in the tummy from a viral infection.
WHAT CAUSES AN IRRITABLE HIP?
Fluid in the hip is most commonly caused by a fall onto the hip or excessive activity, particularly jumping, which puts high loads onto the hip. This is why this type is more common in boys. It has been suggested that a viral infection of the hip could make fluid in the joint but no one has ever grown a virus from the hip so this is unlikely. However there is often a history of a recent viral infection in the child with an irritable hip. Perhaps the virus makes the lining of the hip joint more susceptible to a minor injury that would not otherwise have produced an irritable hip.
The second type results from a viral infection that causes inflammation of the glands in the tummy. Some of these glands lie next to the muscles that make the hip bend up toward the tummy and turn in. Irritation of these muscles by the inflamed glands has the same effect.
ARE THERE SERIOUS CAUSES OF AN IRRITABLE HIP?
Yes. Infection of the hip joint should be excluded. A child with a septic hip is usually very unwell with a high temperature. Any movement of the hip is extremely painful so they lie completely still. If there is any doubt your child will be admitted for an operation to take the infected fluid off the hip and for antibiotic treatment. Blood tests and X-rays will also be taken. Another rare cause of an irritable hip is a condition called Perthes' Disease. An X-ray will make the diagnosis, though it can sometimes take 6 to 8 weeks to show up on X-ray. An irritable hip gets better with time and rest but Perthes' Disease does not. Repeat X-rays will be needed if the limp and pain do not settle. Other very rare conditions, such as a slipped upper femoral epiphysis or a tumour, would show on an X-ray so any child with a limp that does not settle should have an X-ray.
HOW IS AN IRRITABLE HIP TREATED?
If your child is well and can move their hip without severe pain they can be treated at home with strict rest and anti-inflammatory painkillers. The pain usually settles over a few days. The limp can take longer to go, especially if the child starts to run around too soon. If there is some doubt, perhaps because of a slight temperature, X-rays and blood tests may be needed. If these are OK your child can be treated at home with rest. When the hip is quite painful or the blood tests are abnormal, your child will be admitted to hospital for observation. Occasionally we have to take the fluid off the hip under an anaesthetic to be certain that infection is not present. This will help the pain by taking the pressure off the hip and confirm the diagnosis.