Tophaceous gout of the ankle joint

Clinical History

A 57-year-old man presented with a painful soft-tissue swelling at the right ankle for several months. There was no history of acute trauma or previous malignancy.

Imaging Findings

Magnetic resonance imaging (MRI), including post-gadolinium images, and ultrasound were performed.
MRI demonstrates a soft tissue mass at the posterior talocrural joint capsule. The lesion is slightly heterogeneous and is of low signal intensity on T1-weighted images (Fig. 1) and intermediate signal intensity on fat-suppressed T2-weighted images (Fig. 2). Gradient echo sequences show no blooming artefacts (Fig. 3). After IV administration of gadolinium contrast, there is marked enhancement of the lesion (Fig. 4). Note also a talocrural joint effusion and subtle bone erosion and bone marrow oedema at the talus and fibula.
Ultrasound confirms the presence of a soft tissue mass with increased power Doppler at the periphery (Fig. 5).
Additional blood testing shows a serum urate level of 9.1 mg/dL, which is above the upper reference limit of 6.0 mg/dL [1].

Discussion

Gout is a common arthritis caused by deposition of monosodium urate (MSU) crystals within joints following chronic hyperuricaemia. It affects 1-2% of adults in developed countries and predominantly affects middle-aged and elderly men [2].
Acute gouty arthritis initially involves one single joint in the lower limbs, usually the first metatarsophalangeal joint. The affected joint is erythematous, warm, swollen, and tender. Untreated gout mostly resolves within a few days. Subsequent attacks frequently last longer, affect multiple joints, and spread to the upper limbs, especially the elbows and hands [2].
When left untreated, acute attacks of gout can lead to chronic gout, which is characterized by chronic destructive asymmetric polyarticular involvement with low-grade joint inflammation and tophus formation. A tophus consists of a soft tissue mass composed of MSU crystals surrounded by chronic mononuclear and giant-cell reactions. It may cause bone erosion and tissue remodelling. Tophi are frequently seen around the ear, olecranon, Achilles tendons, toes, fingers and knees [2].
Until recently, the definite diagnosis of gout required demonstration of MSU crystals in synovial fluid or tophus aspirates. However, the new 2015 gout classification criteria from the American College of Rheumatology and the European League Against Rheumatism state that, when the examination of synovial fluid or tissue samples is not feasible, a diagnosis of gout can be supported by a combination of clinical, laboratory and imaging findings. Laboratory findings include a serum urate level > 6.0 mg/dL. Imaging findings include well-defined cortical erosions with sclerotic margins and overhanging edges on conventional radiography, and/or the presence of MSU crystal deposition on ultrasound or dual-energy computed tomography [1].
Magnetic resonance imaging (MRI) is usually not needed to diagnose gout and is usually nonspecific. MRI is not able to directly demonstrate MSU crystals. Tophi are observed on MRI as amorphous and sometimes nodular regions of low-intensity signal on T1-weighted images, variable intensity on T2-weighted images and variable, patchy enhancement after intravenous contrast [3].
In our patient, concomitant cortical erosions and tophus formation at the first metatarsophalangeal joint (Fig. 6) and the history of alcohol abuse were further indicators of gout.
The main differential diagnosis is pigmented focal villonodular synovitis (PVNS). However, the age of the patient and absence of blooming artefacts on gradient echo sequences make a diagnosis of PVNS less likely [4]. Another differential diagnosis is synovial chondromatosis, which generates typical chondroid signal characteristics on MRI including high signal intensity on T2-weighted images [5].

Differential Diagnosis List

Final Diagnosis

Tophaceous gout of the posterior talocrural joint

URL:	https://www.eurorad.org/case/13515
DOI:	10.1594/EURORAD/CASE.13515
ISSN:	1563-4086