Case: An 8-year-old boy with a history of leg pain

You have just started your hematology/ oncology rotation when you get a call from the general pediatrics service. They are transferring a patient to your service for a leukemia workup. He is an 8-year-old Caucasian boy with a history of leg pain, who now refused to put weight on it. As the senior resident on service, with morning rounds not beginning for another hour, you decide to get started early.

As you look over his prior records, it appears that his current illness started about six weeks ago, when his mother noticed that he started limping. He then developed discomfort of the left leg severe enough that he could not participate in baseball practice. There wasn't any swelling or redness, and there was no history of trauma. An initial radiograph of the foot was unremarkable. He was treated with ice, rest, and ibuprofen, but the pain worsened. He was unable to pinpoint a specific joint or area of pain, just that his leg hurt.

This prompted a visit to an orthopedist. There it was found that he had full range of motion of the affected leg, except for some restricted internal rotation of the left hip. He had no swelling or effusion, and only mild discomfort of the hip when internally rotated. No palpable tenderness could be elicited. Additional radiographs of both hips and legs were normal, except for some generalized osteopenia. A CBC (complete blood count), ESR (erythrocyte sedimentation rate), C-reactive protein (CRP), ANA (anti-nuclear antibody), antistreptolysin O (ASO) titer, RF (rheumatoid factor), Lyme antibody, parvovirus antibody, HLA-B27 antigen, and urinalysis were obtained. All were normal except for an elevated ASO titer at 1679 IU/mL (normal <150) and a mild thrombocytosis (444 thous/mcL). Without a throat culture, he was started on amoxicillin for possible post-streptococcal reactive arthritis (PSRA).

Over the next week he tried naproxen for pain without any relief, and was switched to indomethacin. After several days on this anti-inflammatory, the pain was still getting worse. His mother desperately called the office reporting that he was again refusing to walk, and had missed the last five days of school. She had resorted to carrying him around the house. At this point, it was decided that he should be admitted to the hospital for further evaluation.

What could this be?

Most rheumatologic problems were less likely, since RF, ANA, HLA-B27, ESR, and CRP were all normal. Acute rheumatic fever was unlikely, since his arthritis was not typical of the migratory arthritis of ARF. Plus, other than evidence of a recent streptococcal infection he did not meet any other Jones criteria. PSRA was still a possibility. The likelihood of an orthopedic problem seemed low since radiographs had ruled out fractures or any lytic lesions, and rest, ice, and ibuprofen had no effect on the pain