The first thing that comes to mind is a condition called juvenile idiopathic arthritis or JIA (previously known as juvenile rheumatoid arthritis (JRA). Usually only one or two joints are affected (knee and ankle). Girls are affected five times more often than boys.
Symptoms of joint swelling and loss of motion show up between ages one and three. The child doesn’t avoid putting weight on the leg and doesn’t usually complain of fever, fatigue, pain, or much more than a bit of tenderness.
Besides ruling out Lyme disease, your physician will look for evidence that this could be some other type of infection, lupus, tuberculosis, or inflammatory bowel disease. Each of those conditions has its own set of symptom pattern (the actual signs and symptoms and what makes them better or worse).
For example, pain that is worse after activity is more likely to be mechanical — meaning a tendon or muscle problem like patellofemoral syndrome. Pain that is worse at night after going to sleep for several hours is a red flag for tumor or growing pains.
As mentioned, pain with weight-bearing is not a typical pattern with juvenile idiopathic arthritis (JIA). Pain that moves around from joint to joint is another tip-off that the problem isn’t JIA. A hot, tender, swollen joint is more common with infection or trauma. And the presence of extra-articular symptoms points to other conditions.
Extra-articular means “outside the joint” and includes such things as fever, nausea, vomiting, weight loss, elevated blood pressure, skin rash, sores in the mouth, redness of the eye(s), or sudden muscle weakness. When these kinds of signs and symptoms are present, blood tests and urinalysis may be more valuable in identifying the underlying cause.
Diagnosing either Lyme disease or juvenile idiopathic arthritis (JIA) in children is a challenging task. The physician spends more time making sure it isn’t something else before being certain just what it is.
After reviewing the history, examining the child, and taking all the tests, the diagnosis is tested out by treating the problem and waiting to see what happens. For JIA, symptoms that do not respond to nonsteroidal antiinflammatory drugs (NSAIDs) signal a need to re-evaluate and test further.