What is the ultrasonic appearance of gout?

Ultrasound Appearance of Gout

Gout on ultrasound is characterized by three cardinal features: the double contour sign (a hyperechoic irregular band over cartilage surfaces), tophi (hypoechoic to hyperechoic inhomogeneous masses with an anechoic rim), and hyperechoic aggregates within the joint space or synovium. 1, 2

Primary Diagnostic Features

Double Contour Sign (DCS)

• The double contour sign appears as a hyperechoic, irregular enhancement over the superficial margin of hyaline cartilage, representing monosodium urate (MSU) crystal deposition on the cartilage surface 1, 3
• This finding has pooled sensitivity of 74% and specificity of 88% for diagnosing gout 1
• The DCS is highly specific for urate deposits with 95% specificity and is considered pathognomonic when present 2
• Found in 92% of crystal-proven gouty joints in validated studies, and absent in control patients with other arthropathies 3
• Sensitivity ranges from 37% to 78% depending on disease duration, with lower sensitivity in very early-onset gout 1

Tophi

• Tophi appear as hypoechoic to hyperechoic, inhomogeneous material surrounded by a small anechoic rim 3
• These deposits were found in 100% of gouty metatarsophalangeal and metacarpophalangeal joints in crystal-proven cases 3
• Tophi were detected in 24.64% of joints in a large cohort study of gout patients 4
• The presence of tophi increases with disease duration, appearing in 40% of joints in patients with more than 15 years of gout history 4

Hyperechoic Aggregates (HAG)

• HAG represents an early manifestation of MSU crystal deposition in joints 4
• Found in 7.80% of gouty arthritis joints and represents floating crystal material within synovial fluid 4
• HAG is associated with synovial inflammation but not with bone erosion 4

Associated Ultrasound Findings

Inflammatory Changes

• Synovial lesions (synovitis/effusion) occur in 17.74% of joints in gout patients 4
• Ultrasound can visualize synovitis, which is particularly useful for monitoring inflammation during treatment 1
• Power Doppler can detect active inflammation in gouty joints 5

Structural Damage

• Bone erosions appear adjacent to tophaceous material in 65% of metatarsophalangeal joints and 25% of metacarpophalangeal joints 3
• Erosions are strongly associated with the presence of tophi and double contour sign 4
• Erosions typically have overhanging edges characteristic of gout, distinguishing them from rheumatoid erosions 5

Clinical Utility and Diagnostic Performance

Diagnostic Accuracy

• When characteristic ultrasound features (double contour sign or tophi) are identified, synovial fluid analysis is not needed to confirm gout diagnosis according to 2023 EULAR guidelines 1
• Overall ultrasound sensitivity ranges from 37% to 100% and specificity from 68% to 97%, depending on which signs are assessed and disease duration 1
• The combination of effusion, tophus, erosion, and double contour sign may obviate the need for joint aspiration in appropriate clinical contexts 5

Disease Monitoring

• Ultrasound can effectively demonstrate decrease or disappearance of urate deposits during urate-lowering therapy 6
• Crystal deposition features (tophi, double contour sign, aggregates) are sensitive to change over 1 year of treatment 1
• Correlation between ultrasound findings and achievement of serum uric acid targets is excellent (κ=0.875) 6

Important Caveats

Limitations in Early Disease

• Sensitivity is significantly lower in patients with recent-onset acute gout (as low as 37%), as crystal burden is typically lower 1
• DECT is less sensitive in patients with shorter history of flares compared to those with longer disease duration 1

Specificity Concerns

• The double contour sign, while highly specific, may occasionally appear in CPPD and asymptomatic hyperuricemia 1
• Interpretation requires clinical context: the higher the number of typical lesions and the greater variety of findings, the more certain the diagnosis 1

Detection in Asymptomatic Hyperuricemia

• MSU crystal deposition is found in 25.58% of asymptomatic hyperuricemic patients on ultrasound, indicating subclinical disease 4
• Among joints with MSU crystal deposition in gout patients, 77.37% had a history of attacks, meaning some deposits are in clinically silent joints 4

Recommended Imaging Approach

Ultrasound should be the first-line imaging modality for suspected gout when crystal identification is not possible, as it provides superior resolution for detecting smaller deposits and can simultaneously assess inflammation 1. The examination should focus on commonly affected sites including the first metatarsophalangeal joints, knees, and other symptomatic areas, specifically looking for the double contour sign on cartilage surfaces and tophaceous deposits 2, 3.