Tophi (plural of tophus) usually appear as lumps on the skin over affected joints due to deposits of monosodium urate crystals in patients with longstanding high levels of serum uric acid (hyperuricemia). Tophi are a pathognomonic feature of gout.
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Clinical presentation
Tophi are firm, yellow, painless lesions 1. They have a mass-like monosodium urate crystal core, enveloped by a fibrovascular zone of chronic granulomatous inflammation. Tophaceous gout is a form of chronic gout.
Tophi typically involve the joint capsules and juxta-articular soft tissues such as tendons or ligaments. The first metatarsophalangeal joint is the most commonly affected. They may also be found formed over the olecranon, prepatellar bursa, and the helix of the ear 1,2.
Radiographic appearance
Plain radiograph
Tophi are typically encountered as eccentric, juxta-articular soft tissue nodules. Bony erosions with sclerotic margins and overhanging edges are commonly seen in their proximity 2.
CT
More sensitive the plain film, with dual-energy CT color coded visual representation and quantification of tophus burden are also feasible 2,3.
Ultrasound
Tophi appear heterogeneously hyperechoic, surrounded by hypoechoic halo caused by the zone of chronic inflammation. Color or power Doppler may demonstrate increased vascularity in this peripheral zone 2,4.
MRI
Tophi typically show moderate to low signal on T1 and heterogeneous high signal on T2 weighted sequences. The peripheral, fibrovascular zone may demonstrate enhancement depending on the severity of inflammation 2,5.
History and etymology
Tophus means "stone" in Latin.
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