EULAR 2023: Ultrasound and DECT in Gout and Crystalline Arthropathies

Clinical Significance of the EULAR Document

EULAR 2023 recommendations fill a practical gap: how to use ultrasound, DECT, radiography, CT, and MRI in gout, CPPD, and BCP disease not in a research setting but in clinical practice. The document does not replace synovial fluid microscopy but shows when imaging helps confirm a diagnosis, assess crystal burden, inflammation, structural damage, and treatment dynamics.

For the ultrasound diagnostician, the main takeaway is that ultrasound is considered a first-line method for detecting superficial and periarticular crystalline deposits, especially in gout and CPPD. DECT is most specifically useful in gout for visualizing monosodium urate deposits but requires a standardized protocol and clinical interpretation.

General Principles: What EULAR Expects from Imaging

The document outlines 5 general principles. Their practical content boils down to four tasks: identify crystals, visualize inflammation, assess structural damage, and assist in invasive procedures. The choice of method depends on the clinical question, location, availability, specialist experience, and likelihood of an alternative diagnosis.

An important caveat from EULAR: imaging should not be interpreted separately from clinical context. The same hyperechoic focus may have different significance in typical gout, osteoarthritis, post-traumatic changes, or suspected infectious arthritis. In cases of diagnostic uncertainty and the possibility of arthrocentesis, synovial fluid analysis remains a priority.

Gout: Ultrasound and DECT as Key Methods

When gout is suspected, EULAR highlights ultrasound and DECT as the most informative methods for detecting monosodium urate deposits. On ultrasound, three groups of signs should be specifically sought: double contour sign, tophi, and aggregates. Additionally, erosions, effusion, synovial hypertrophy, and power Doppler are assessed as signs of inflammatory activity.

The double contour sign is a hyperechoic line along the superficial contour of the hyaline cartilage, corresponding to crystal deposition on the cartilage surface. It is important to distinguish it from an interface artifact: the true sign persists with changes in the insonation angle and is located on the cartilage surface, not within it. A tophus typically appears as a well-defined heterogeneous mass with hyperechoic and hypoechoic components, sometimes with an acoustic shadow and a peripheral anechoic rim.

When Characteristic Imaging is Sufficient and When Aspiration is Needed

According to EULAR logic, characteristic ultrasound or DECT signs in a patient with clinically probable gout significantly increase diagnostic confidence. This is especially important when a joint is difficult to aspirate, the patient is already receiving anti-inflammatory therapy, or crystals could not be technically obtained.

However, imaging does not negate aspiration when septic arthritis is suspected, in atypical presentations, in the first severe monoarthritis without a clear cause, or when clinical and imaging findings do not match. In such cases, ultrasound is useful not only for finding crystals but also for guiding the needle to minimal effusion, small joints, or periarticular collections.

CPPD: What to Look for on Ultrasound

In calcium pyrophosphate dihydrate crystal deposition disease, EULAR considers ultrasound and radiography as primary methods for visualizing peripheral manifestations. The classic radiographic finding is chondrocalcinosis, but ultrasound allows real-time visualization of deposits in hyaline cartilage, fibrocartilage, tendons, and ligaments.

The key difference from gout: in CPPD, hyperechoic inclusions are usually located within the cartilage or fibrocartilaginous structure, not on its surface. In the knee joint, it is advisable to assess the menisci and hyaline cartilage; in the wrist area, the triangular fibrocartilage complex; and other symptomatic areas as clinically indicated. CT may be useful for axial CPPD locations where ultrasound is anatomically limited.

BCP Disease: Role of Ultrasound and Radiography

BCP disease is associated with basic calcium phosphates, including hydroxyapatite deposits. Clinically, these are often periarticular calcifications, calcific tendinopathy, bursitis, or acute inflammatory episodes around the shoulder and other joints. EULAR points to ultrasound and radiography as practical methods for detecting such calcifications.

Ultrasound offers an advantage in assessing soft tissues: it can determine the localization of calcification in a tendon or bursa, the presence of perifocal effusion, Doppler hyperemia, and mechanical conflict. Radiography better demonstrates dense calcifications and is convenient as a baseline method. MRI is useful for complications and differential diagnosis but is not itself an optimal method for identifying crystal types.

Practical Table of Methods

Monitoring Gout Treatment

A separate practical value of the recommendations is the recognition of ultrasound and DECT as tools for monitoring the burden of urate deposits in gout. This does not mean that imaging replaces clinical assessment and laboratory urate control. However, during long-term urate-lowering therapy, reduction in the double contour sign, tophi, and deposits on DECT can serve as objective confirmation of crystal dissolution.

For correct dynamics, the protocol must be reproducible: the same joints and tendons, the same planes, fixation of tophi in two planes, description of Doppler activity and effusion. It is better to separate signs of deposits, inflammation, and structural damage in the conclusion, as they change at different rates.

How to Formulate an Ultrasound Conclusion

An optimal conclusion when suspecting crystalline arthropathy should not be a description of "hyperechoic inclusions" but an answer to the clinical question. For gout, indicate the presence or absence of the double contour sign, tophi, aggregates, erosions, and active synovitis. For CPPD, note the localization of calcifications relative to cartilage or fibrocartilage. For BCP, describe the relationship of calcification to the tendon, bursa, or periarticular tissues.

If the signs are not specific, this should be clearly stated: "Ultrasound findings of crystalline deposits are inconclusive," "signs of inflammatory synovitis without definitive deposits," "CPPD possible with corresponding clinical context." Such language aligns with the EULAR approach: imaging increases or decreases the likelihood of a diagnosis but should not create false certainty.

Key Interpretation Errors

• Mistaking a cartilage interface artifact for a double contour sign without checking in another plane and changing the probe angle.
• Describing any calcification as gout: in CPPD and BCP, deposit distribution is different.
• Excluding gout or CPPD based on negative radiography.
• Considering Doppler signal as proof of crystalline nature of arthritis: it is a sign of inflammation, not crystal type.
• Failing to perform navigational aspiration when infection is suspected or when diagnostically significant effusion is present.