Tophi
The bony prominences or mass-like lesions in a patient with gout are called tophi (singular: tophus), which are deposits of monosodium urate (MSU) crystals in and around joints and soft tissues. 1
Clinical Characteristics
Tophi represent the most advanced clinical stage of gout and develop from chronic MSU crystal deposition when serum uric acid remains elevated above the saturation point of 6.8 mg/dL. 2, 3
Diagnostic Features
Definite tophi have very high specificity (99%) for gout diagnosis, with a positive likelihood ratio of 39.95, making them nearly pathognomonic when present. 1
Tophi appear as eccentric or asymmetrical nodular soft tissue masses, which may or may not have calcifications visible on imaging. 1
They can present in multiple morphological forms including periarticular subcutaneous tophi, disseminated intradermal tophi, ulcerative forms, and miliarial tophi. 4
Tophi may discharge chalky white material through the skin when ulcerated, which is pathognomonic for the condition. 4
Common and Unusual Locations
Most commonly located on or around joints, particularly the first metatarsophalangeal joint, hands, feet, elbows, and knees. 2, 5
Unusual locations include intradermal sites, nasal regions, and other soft tissues distant from articular regions, though these are rare presentations. 2, 6
The European League Against Rheumatism guidelines emphasize that ultrasound and DECT imaging should always include the first MTP joints and knees, as these are common sites for MSU crystal deposition even when asymptomatic. 1
Imaging Characteristics
On ultrasound, tophi appear as hyperechoic masses with a "wet clumps of sugar" appearance, often surrounded by an anechoic halo, with 65% sensitivity and 80% specificity. 7
Dual-energy CT (DECT) can detect and quantify tophi with 85-100% sensitivity and 83-92% specificity, showing MSU crystal deposits as color-coded masses. 1, 7
Both ultrasound and DECT are useful for assessing tophus resolution in response to urate-lowering therapy. 1
Clinical Significance
Tophi indicate chronic, inadequately treated gout and typically develop after 10 years or more of recurrent polyarticular gout, though earlier development can occur with aggressive disease or triggering factors like corticosteroid and diuretic use. 2, 4
The presence of tophi warrants more aggressive urate-lowering therapy targeting serum uric acid below 5 mg/dL (300 μmol/L) rather than the standard target of <6 mg/dL. 8
Treating to target sUA levels results in shrinkage and eventual disappearance of tophi, along with reduction in gout flares. 3