The direct cause is a reduction in blood flow to the joint, though what causes this is unknown. Genetics does not appear to be a determining factor, though it may be involved. Some evidence suggests that parental smoking may be a factor, though this is not yet proven, or more recently that a deficiency of some blood factors used to disperse blood clots may lead to blockages in the vessels supplying the joint.
The first signs are complaints of soreness from the child, which are often dismissed as 'growing pains', and limping or other guarding of the joint, particularly when tired. The pain is usually in the hip, though can also be felt in the knee (so-called 'referred pain'). It is predominantly a disease of boys (around 4 in 5 cases), and is generally diagnosed between 5 and 12 years of age. Typically the disease is only seen in one hip, though appearance in both hips is not uncommon (about 10-15% of cases).
Diagnosis is made predominantly by X-ray study, together with physical examination (MRIs have also been found useful for judging the extent of the deformity). Sufferers typically have limited range of motion in their hip, particularly when rotating the joint.
Treatment has traditionally centered on removing pressure from the joint until the disease has run its course. Options have included bed rest and traction (to separate the femur from the pelvis and reduce wear), often for several months or even years. Braces were also popular, again to isolate the joint. Recent evidence suggests that these methods are not effective, and treatment seems to be moving towards a mixture of careful monitoring, and surgical intervention when needed.
Perthes is a long-term problem - treatment is aimed at minimizing damage while the disease runs its course, not at 'curing' the disease. As sufferers age problems in the knee and back can arise, as a result of the abnormal posture and stride adopted to protect the affected joint. The condition is also linked to arthritis of the hip and other joints, though this appears not to be an inevitable consequence. Hip replacements are relatively common as the already damaged hip suffers routine wear; this varies by individual, but generally is required any time after age 50.
Perthes is a relatively unusual condition, with only 1 in 9,000 children being affected. Because of this relatively low profile, much of the available literature on the subject is written from a medical viewpoint. A useful, understandable first step for parents or others affected by the disease is the 'Parents of Perthes Support Group': http://www.audi44.freeserve.co.uk/main%20mk6%202003.htm