Nontraumatic,Painless,Unilateral Knee Swelling in a 4-Year-Old Patient

Clinical Course

Two weeks later at the pediatric rheumatology visit, the patient continued to have painless knee swelling that was unchanged with taking the Naproxen. On exam, the patient was noted to have leukonychia on multiple nails with 3+ left knee swelling with 2+ limitation to full flexion. His labs were reviewed at this visit and were within normal limits except for mild elevation of LDH of 386 IU/L (110-295 IU/L). The pediatric rheumatologist conducted an ultrasound of the left knee which demonstrated a striking amount of mostly hypoechoic structures in the suprapatellar recess with hyperemia in the surrounding synovial tissue (Image 1). The differential mentioned at this time was pigmented villonodular synovitis versus juvenile idiopathic arthritis (JIA). The pediatric rheumatologist referred him to orthopaedic surgery for further evaluation due to suspicion of pigmented villonodular synovitis and ordered an MRI of the knee. It was recommended that they continue the 250mg naproxen twice daily for 6 weeks and then transition to as needed only.

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Image 1.

This ultrasound image demonstrates a transverse view of the distended suprapatellar recess filled with numerous small structures with heterogenous echotexture, predominantly hypoechoic, indicated by the blue arrows. The surrounding synovial tissue appears slightly thickened with mild hyperemia demonstrated by the Power doppler and indicated by the orange arrow.

The subsequent left knee MRI demonstrated “innumerable tiny nonenhancing intraarticular filling defects, possibly loose bodies with thick synovial enhancement, suggestive of inflammatory arthopathy” (Image 2). In discussion with radiology, the differential diagnosis included chondromatosis, rice body synovitis, and infection, particularly fungal as it has been described with such filling defects. Due to multiple missed consultation appointments by the family, the pediatric orthopaedic surgeon first assessed the patient 2 months after MRI. Initially, there was discussion of deferring a biopsy given that his condition was still nonpainful and had not progressed. However, after a collaborative discussion with orthopaedic colleagues and rheumatology, the biopsy was scheduled. The patient subsequently developed COVID which necessitated his biopsy to be rescheduled. His knee swelling persisted but remained stable throughout this time, and ultimately the biopsy was completed 4 months after the MRI. Large amounts of white fluid were obtained from the left knee and the frozen section demonstrated “fibrin, synovium, and foci of chronic inflammation, consistent with rice bodies” (Image 2). The joint was closed without complication. Culture of the fluid for aerobic and anaerobic bacteria as well as fungus and acid-fast bacilli were negative. No cell counts of the fluid were obtained.

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Image 2.

This MRI image of the left knee demonstrates an effusion with filling defects and thick synovial enhancement (yellow arrows). The histology of the fluid sample demonstrates enlarged rice bodies consistent with fibrin and synovium (red arrows) and chronic inflammation with increased lymphocyte presence (blue arrows).

The patient wore a hinge-brace for a month and took part in physical therapy. His swelling resolved gradually and completely over the subsequent 5 to 6 months following his debridement and has not recurred in that joint or any other joints over 2 years of additional follow up. Parental consent and patient assent to publish this case report was obtained at the patient’s most recent primary care visit, and the University of Florida IRB acknowledged this project and approved its status as a case report.

Case and Literature

Rice body synovitis is a rare, nonspecific response to inflammation first reported in patients with tuberculosis and is more commonly seen in patients with rheumatoid arthritis and septic arthropathy.^1-3 The term “rice body” refers to its resemblance to pieces of white rice, and they consist mostly of fibrin, collagen, macrophages, and very few blood vessels. ³ Given their biochemical similarity to the synovium, rice bodies are likely derived from the synovial membrane.^4,5 The mechanism of origin is debated, and theories include synovial shedding versus proliferation and degeneration of the synovium.^4,5 Rice body synovitis has also been described as a response to foreign bodies like metal and balloon spacers in joints.^6-9 Although it has been noted in 34.9% of synovial fluid samples from adults with rheumatoid arthritis, rice body synovitis is rarer in children and its prevalence is not well documented in this population. ¹⁰ Due to the lack of documentation of the prevalence of rice body synovitis in children, this case adds to the literature of another example of a rare disease in this population and can help to quantify the prevalence in the future. It has primarily been described in the literature as a single episode in association with JIA.^11-23 Our review of the literature revealed only two pediatric cases of rice body synovitis in absence of JIA, with one being otherwise healthy after a follow up period of 2.5 years and the other of whom had neuropathy related to chronic insensitivity to pain syndrome.^14,24 One case describes the diagnosis of JIA being made following the recurrence of bilateral knee rice body synovitis after a 10-year period without symptoms. ¹⁶ Since rice body synovitis has only been reported in the absence of JIA twice before, it is important to add another case to the literature detailing this patient’s clinical course in order to provide more knowledge around the subject and benefit future patients. This case highlights a unique finding of rice body synovitis in an otherwise healthy child with no definitive cause or concurrent diagnosis with JIA.

In the pediatric population, rice body synovitis usually presents with painless or painful joint swelling that limits range of motion and is typically diagnosed via pathology examination and cell count analysis of the affected joint fluid that reveals rice-like bodies without evidence of granulomas or mycobacterial involvement. ¹⁸ Given that some cases of rice body synovitis are associated with JIA and some are not, nonspecific inflammatory markers such as ESR and CRP are not definitively helpful in clarifying whether a patient with joint swelling has rice body synovitis amongst other conditions on the differential. Definitive diagnosis requires imaging and fluid sampling. As in this patient, evidence of rice body synovitis can be seen on ultrasound and MRI imaging of the joint. ²⁵ Despite the lack of synovial fluid cell counts in this case, the knee imaging and frozen sections of the fluid were both consistent with rice body synovitis (Image 1). Diagnosis should also include work up for autoimmune and rheumatologic disease, as they are inextricably linked.

Treatment for rice body synovitis varies. Case reports detail remission inducing treatments ranging from naproxen and methotrexate monotherapy, joint aspiration followed by long-acting intraarticular corticosteroid injections, to radical debridement and radioablation of the synovium.^9,22,26 The current patient underwent joint debridement and physical therapy with close clinical follow up. Patients subsequently diagnosed with a rheumatologic condition typically start on medication treatment as well. The American College of Rheumatology issued conditional recommendations for treating JIA including a stepwise approach of starting with NSAIDs and intraarticular glucocorticoids followed by DMARDs then biologic DMARDs as indicated. ²⁷ Due to the lack of knowledge regarding optimum treatment for rice body synovitis, it is vital to demonstrate this patient’s positive recovery and prognosis with a trial of NSAIDs followed by joint debridement. The present case emphasizes that this treatment can lead to lack of recurrence without requiring long term treatment with anti-inflammatories or biologics. Future studies and patient follow-up are required to determine the optimal treatment for each patient, including which treatments have lower risk of recurrence.

Final Diagnosis

A diagnosis of monoarticular rice body synovitis was made based on the pathologist’s analysis of the synovial fluid coupled with an absence of other causes like TB or sarcoidosis. The history of painless knee swelling, ultrasound indicating joint effusion and synovial proliferation, and the knee MRI displaying intraarticular loose bodies further support this diagnosis.