If a Calcium Crystal Falls in the Wrist, Does it Make a Sound?

Today at EULAR, the uses of imaging in crystal arthropathies was discussed. Clearly, radiographs show calcification of soft tissues nicely, but specificity is low enough that radiography can't definitively rule out calcific crystalline disease. MRI can show lots of inflammatory features, but without a synovial fluid analysis demonstrating crystals, it's hard to differentiate from competing inflammatory diagnoses, infections, and tumoral tissues. Dual energy CT has shown great promise in diagnosing crystal arthritis, especially gout. Even though DECT can even provide quantitation of gouty deposits, I wonder what the clinical uptake will be given the radiation dose, high cost, and limited access? Less than exuberant, would be my guess.

Enter the ultrasound (US), an inexpensive, ubiquitous, and no-risk tool with good ability to detect crystals, at least in superficial joint areas where US waves can penetrate.

A few years ago, US was shown to have superior sensitivity to radiography for detecting calcium pyrophosphate crystal deposits in knee joints. However, the wrist is at least as common a site for CPPD disease, so the need to differentiate from autoimmune inflammatory arthritis diagnoses that CPPD loves to mimic (classically, RA) is particularly important. Unfortunately, the conventional radiograph isn't much better than 50% sensitive for picking up CPPD. It would be great if we had a more reliable method to confidently detect (or rule out) CPPD. This is especially true when joint fluid isn't readily accessible to make the diagnosis with the gold standard test, polarized light microscopy of the aspirate. My CPPD referrals often don't have fluid to aspirate because patients are usually taking or have completed high-dose prednisone for their flare by the time I see them.

Dr. Combier and colleagues aspirated fluid from any swollen joint (including wrists) in 58 patients, 32 of whom ended up having CPPD. They then compared the diagnostic performance of US vs. radiographs of the patients' wrists. Hyperechoic spots in the TFCC or radiocarpal joints visualized by US was able to detect CPPD with 94% sensitivity versus only 53% by radiography. Specificity was 84%, still fairly good, but outdone by the X-ray at 100%. Notwithstanding, CPPD can trick even the most seasoned rheumatologist whose raison d'être is never to miss a chance to treat RA. Given the pervasiveness of CPPD, and considering the downside of overdiagnosis/overtreatment of diseases like RA, the sensitivity discrepancy is the main message here.

On the other hand, it's important to note that technically you could have both RA and CPPD. I'm also not sure the presence of crystals in a knee aspirate (since any swollen joint was aspirated in this study, not just the wrist) definitively proves that wrist swelling is due to CPPD, which may have factored into the lower specificity calculation for US vs X-ray.

US takes longer to perform than ordering an X-ray and can slow down a busy rheumatology clinic when used indiscriminately. That said, in situations of diagnostic uncertainty, this is a pretty clear example where US can have great utility and an attractive risk/benefit profile. Like with most things, the right test for the right patient at the right time is always the best approach, a philosophy passed on by my mentors and strong among rheumatologists in general.

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