Differing Views on Imaging Gout

In difficult-to-diagnose cases of gout, novel imaging techniques are capturing more detailed pictures of urate crystal deposition and joint damage than ultrasound (US).

Dual-Energy Computed Tomography (DECT) is able to measure the size of urate crystal deposits in key joints of patients with clinically suspected gout, according to Wolfgang A. Schmidt, MD, chief physician in rheumatology and clinical immunology at the Immanuel Hospital in Berlin.

“Ultrasound misses the small deposits, ‘ultrasound double contour sign,’ a carpet of urate crystals on the cartilage.” Schmidt said. “It also cannot confirm the presence or urate if aspirated samples are unclear under microscopy.” 

“Some patients don’t have fluid in the joint,” he added, “so aspiration isn’t very sensitive in this case. The DECT machine measures the load of urate deposits, Schmidt told the 2013 American College of Rheumatology annual meeting in San Diego.

His case-control study compared DECT to microscopy analysis and to comprehensive, bilateral US of feet, knees, hands, wrists, and elbows in 60 consecutive patients with suspected gout (most of them men, mean age 62).

Analyzing pooled clinical information, including polarization microscopy, maximum documented uric acid levels, and the presence of podagra, DECT confirmed a positive diagnosis of gout in 39 of the 60 patients, 16 of whom had gout plus another concomitant rheumatic disease.

Polarization microscopy of aspirated joint samples confirmed a diagnosis in only 46% of the patients. The specificity of DECT diagnosis is 85.7%, as compared with 76.2% for US. While US is 100% sensitive, DECT has a sensitivity of 84.6%. The positive predictive value is 0.92 for DECT and 0.89 for US.

Another study, which used DECT to measure urate in the feet and hands of 73 patients with tophaceous gout, found that it is less sensitive than US detecting changes in urate volumes and mean serum urate concentrations over one year in patients on urate-lowering therapy.

“DECT may be too much for daily practice,” Schmidt remarked. “In clinical practice, you don’t need to know if the patella has a big tophus; it doesn’t really change your treatment. It’s the tricky cases where DECT would be useful, such as those with an overlap of rheumatoid arthritis".

Such cases can include axial gout, frequently a missed diagnosis because it mimics other spine diseases and is under-recognized as a manifestation of gout.

But a small case study (n=2) of color-display DECT used the technology to color-code urate deposits and distinguish them from surrounding structures, diagnosing the conditionmore effectively than X-ray, MRI, or CT, which may not distinguish gouty erosions and tophi from other pathologies.

The study showed that DECT can be used to visualize axial urate deposition, aiding appropriate diagnosis and management of axial gout -- while avoiding invasive procedures and erroneous treatment.

Additionally, a prospective study of 92 patients with tophaceous gout found that the method can also help track radiographic damage. DECT analysis of the 10 metatarsophalangeal joints in these patients showed a very high correlation between the number of joints at each site affected by urate deposition and features of radiographic joint damage.

The study also supports the concept that urate crystals directly interact with articular tissues to influence the development of structural joint damage in this disease