Introduction
A previously healthy 13-month-old African American boy was referred to the rheumatology clinic for persistent right knee swelling for 2 months. In the mornings, the child had stiffness of the right knee lasting approximately 10-15 minutes. Once this resolved, he was able to resume his normal level of activity, although he walked with a limp. According to the mother, the right knee was mildly warm to the touch, but there was no redness or tenderness to palpation. There was no history of ill contacts or preceding trauma, and the child had not had recent illness, fever, weight loss, malaise, cough, diarrhea, vomiting, muscle pain or weakness, or joint or bone pain. He had been a term infant and had had normal growth and development with no previous hospitalizations or surgeries. He lived in Dallas, Texas with his mother and grandmother and had never traveled outside of the city. He did not attend day care but was cared for by his grandmother at home. The patient's father was in prison, and the family visited him frequently. There was no history of animal or tick exposure. The child was not taking any medications, and his immunizations were up-to-date.
In the rheumatology clinic, the patient had a temperature of 36.9°C, heart rate of 126 beats/min, respiratory rate of 28 breaths/min, and blood pressure of 106/55 mm Hg. He was a well-nourished, well-appearing boy. Examination was notable for a small right knee effusion with minimal warmth and without erythema or tenderness. In addition, the right knee had a 35-degree flexion contracture and the right thigh appeared larger than the left thigh. The remainder of the physical examination was normal.
Laboratory studies revealed a white blood cell count of 13.1 x 10/mm (differential of 50% neutrophils, 5% bands, 27% lymphocytes, 10% monocytes, and 8% eosinophils), hemoglobin of 11.7 g/dL, platelet count of 426 x 10/mm, and erythrocyte sedimentation rate of 49 mm/h. Antinuclear antibody was negative, antistreptolysin O titer was <56 IU/mL, and angiotensin converting enzyme level was normal at 4 units. Plain radiograph of the right knee revealed marked soft tissue swelling but no bony abnormalities. Magnetic resonance imaging of the right knee showed marked thickening of the synovium and a small joint effusion, but no cartilaginous or osseous abnormalities (Fig. 1, http://links.lww.com/A534). Several small lymph nodes were noted posterior to the right knee.
Synovial fluid aspiration and synovial tissue biopsy were performed. The synovial fluid cell count showed 18,600 red blood cell/mm, 11,500 white blood cell/mm with 43% neutrophils, 47% lymphocytes, and 10% monocytes. The synovial fluid was not tested for glucose and protein. The histopathology of the biopsy specimen and cultures of the synovial fluid revealed the diagnosis.