A Rare Case of Juvenile Idiopathic Arthritis following a Rup
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A 4-year-old girl presented to the Emergency Department (A&E) with a 3-week history of right calf swelling after her usual, nontraumatic dancing. The swelling had worsened over 48 hours. There was no history of trauma to the area, no erythema, and she had been apyrexial. She regularly went dancing and participated in acrobatics. There was no family history of a clotting disorder or joint arthropathy.

On examination, her weight was just above the 75th centile at 20 kg, with a BMI of 17.9. She was unable to fully extend her right leg or weight bear due to her calf swelling. Her right calf measured 28.6 cm compared to the left calf of 26.4 cm. She was able to perform a straight leg raise test with all ligaments intact, no demonstrable joint effusion, and no issues with joints above and below.

Three weeks prior to her presentation, she was seen in the A&E with pain and swelling in her right calf but was able to weight bear at this point. No imaging was conducted then, and she was discharged home with the diagnosis of soft tissue injury. On further questioning, the patient's mother revealed that she had been experiencing leg pain for over 6 months without noticeable leg swelling. Family history consisted of Crohn's and coeliac disease on the maternal side.

The initial X-rays of her right knee, tibia, and fibula on presentation showed no bony injury and unremarkable soft tissue swelling when compared to her previous left knee/tibia/fibula X-rays. A urine dipstick was negative to blood, protein, and cells with a normal specific gravity. Blood results showed an erythrocyte sedimentation rate (ESR) of 26 mm, C-reactive protein (CRP) < 4 mg/L, and a normal white cell and neutrophil count. Her prothrombin time was slightly increased at 13.4; however, the extended clotting profile for factor VII, factor VIII, factor XIII, and von Willebrand factor were all normal. The complement levels (C3 and C4) were within normal ranges, including the anti-nuclear antibodies (ANA), and rheumatoid factors were also negative. An ultrasound (US) of the affected joint showed a 43 × 10 mm collection within gastrocnemius with a possible haematoma which raised the suspicion of a venous thromobosis. Another differential was the possibility of a fast growing tissue sarcoma, but her lactate dehydrogenase was unremarkable.

She was commenced on conservative management and discharged home on nonsteroidal anti-inflammatory medication pending her MRI investigations. Three weeks later, her calves were unchanged in size and she had full and painless range of movement in both knee joints, but an effusion was still evident in her right knee with a firm swelling in the posteromedial aspect of the proximal calf. The MRI showed an inflammatory arthropathy with a large Baker's cyst (4.3 × 2.2 × 9.4 cm). There was extensive synovial proliferation throughout the knee joint with large joint effusions. There was also a clear communication within the Baker's cyst, with numerous septations within the encapsulated lesion and associated soft tissue edema tracking superiorly and inferiorly along with the medial head of the gastrocnemius and anteriorly along the tibia,

She was seen in our rheumatology clinic and was diagnosed with juvenile idiopathic arthritis, a monoarthritis of her right knee on a background of a ruptured Baker's cyst. Ophthalmology review showed bilateral anterior uveitis, and she was commenced on topical eye drops. In addition, a short course of oral steroids was prescribed, and an intraarticular steroid injection was given into her right knee joint (triamcinolone hexacetonide). This was continued with weekly subcutaneous methotrexate with a referral to physiotherapy.

Since her initial acute presentation, the pain within the patient's right knee had improved significantly and the calf swelling had subsided.
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