Abstract
INTRODUCTION: Gouty tophi and pigmented villonodular synovitis (PVNS) may both present as an isolated mass with similar imaging findings. We present a case of gout presenting as an isolated tophus in the absence of any joint inflammation. This patient was initially referred to orthopaedics with a suspicion of PVNS and then to oncology/plastic surgery for suspected sarcoma before being diagnosed with gout. This case highlights differential diagnoses for tophi and reminds clinicians that tophi may be the initial presenting feature of gout in the absence of overt arthritis. CASE DESCRIPTION: A 43 year old male with no significant past medical history was referred to the orthopaedic department with a 3 year history of progressive swelling of his right lateral forefoot. He described discomfort at the site of the lesion on activity but no significant pain. An X-ray of his foot was normal. An ultrasound scan of the swelling prior to orthopaedic referral showed a heterogenous area overlying the tarsometatarsal joint 23x20x10mm. Synovial thickening was noticed, without increased vascularity and a suspicion of pigmented villonodular synovitis (PVNS) was raised. An MRI scan of his foot showed a dorso-lateral soft tissue mass arising from the proximal 4(th) intermetatarsal space. Erosive and proliferative changes were noted at the third and fourth tarsometatarsal joints. These changes were reported as consistent with a local reaction to a benign process such as PVNS but potentially concerning for sarcoma. He was referred to the tertiary sarcoma multi-disciplinary team meeting. The lesion was excised by plastic surgeons and found to be comprised of uric acid crystals, in keeping with a gouty tophus. He was referred to rheumatology. No history of joint inflammation typical of gouty arthritis was established. He disclosed excessive alcohol intake of 20-30 units a week but no other risk factors for gout were identified. On examination he had no evidence of synovitis and there were no tophi identified other than the lesion on his right foot. Uric acid was elevated at 475. He was therefore commenced on allopurinol 100 mg daily as initial treatment for gout with a plan to up titrate allopurinol until serum urate <300. DISCUSSION: Approximately 12% of people with gout develop tophi. The prevalence of tophi as a presenting feature of gout without classical gouty arthritis is unknown but has been extensively reported in the literature. Therefore, clinicians ought to be mindful of the possibility of gout in patients presenting with subcutaneous lesions particularly in the context of risk factors for gout. Differentials for the ultrasound lesion in this case include an atypical ganglion cyst, synovial cyst, PVNS, gout, rheumatoid nodule, lipoma and neoplasia. Fibromas, sarcomas, PVNS and gout all appear, as in this case, as heterogenous masses. Tophi and PVNS have similar features on USS. Tophi typically have hypoechoic foci representing uric acid crystals which were not seen in this case. Unfortunately, the underlying joints were not assessed in our case; it was not clear if any double contour sign was present which would have favoured gout at an early stage. Synovial hypertrophy would be present in both gout and PVNS. PVNS is generally hypo-vascular whereas a tophus may have variable vascularity depending on chronicity. In suspected PVNS MRI imaging (as obtained in this case) is recommended. Both PVNS and gout could cause the erosions seen on MRI whereas sarcoma would typically cause destruction of the bone and soft tissue. PVNS classically causes low T2 signal due to haemosiderin deposition, gout is variable but can be low due to crystal deposition. Sarcoma usually causes high T2 signal. Another key feature of PVNS is marked blooming on susceptibility weighted imaging which would not typically be seen in gout or sarcoma but was not undertaken in this case. Ultimately, histology was required in this case for definitive diagnosis which confirmed gout. The hallmark of PVNS on histology is abundant haemosiderin deposition within macrophages. Numerous multinucleated giant cells and synovial proliferation are also typically seen. KEY LEARNING POINTS: • Clinicians should be mindful that a tophus may be the presenting feature of gout in individuals who do not describe symptoms suggestive of gouty arthritis. • Differentials for gouty tophi include rheumatoid nodules, lipomas, sarcomas and PVNS. • There are several overlapping features between PVNS and tophi on ultrasound imaging including heterogeneity of the lesion, avascularity and synovial hypertrophy. • Both gout and PVNS may cause erosive change and low T2 signal on MRI. PVNS causes classic blooming artefact on gradient weighted or susceptibility weighted MRI. Urate crystals can cause blooming but not to the same extent. Unfortunately, these sequences were not undertaken in our patient but if undertaken may have prompted consideration of a diagnosis other than PVNS at an earlier stage. • Our case did undergo any serological testing until he was referred to rheumatology after biopsy confirmation of gout. Although biopsy would probably still have been undertaken due to the possibility of malignancy, testing of inflammatory markers and serum urate at an earlier stage may have prompted consideration of gout at an earlier stage and we would advocate for screening tests for rheumatoid arthritis and gout to be undertaken in similar cases. • This case required input from radiology, orthopaedics, oncology, plastic surgery, histopathology and rheumatology before receiving a definitive diagnosis highlighting the need for effective multi-disciplinary collaboration in cases of diagnostic uncertainty.