COMMENTARY

Ultrasound for Gout, RA, and Other Forms of Arthritis

Lisa Vasanth, MD, MS

Disclosures

May 14, 2015

Beyond Rheumatoid Arthritis

Ultrasound is changing the way we diagnose and monitor other rheumatologic diseases as well. Polymyalgia rheumatica (PMR) has traditionally been a clinical diagnosis related to symptoms of pain in the shoulders and hips of a person over age 65 years. Diagnosis is often supported by elevated inflammatory markers like erythrocyte sedimentation rate and C-reactive protein.[12] To increase the specificity of its classification criteria for PMR, ACR/EULAR added optional ultrasound criteria.[13] The ultrasound criteria involve ultrasound evaluation of the shoulder and hip for evidence of bursitis, tenosynovitis, and synovitis.[13] Macchioni and colleagues[14] did a study to test the ability of the ACR/EULAR provisional criteria for PMR to distinguish PMR from controls. Their study found that ultrasound increased the specificity of the classification criteria from 81.5% to 91.3% and also helped distinguish patients with PMR from patients with RA.[14] It is important to remember, however, that classification criteria are developed for inclusion of patients in clinical trials.[15] Because it is vitally important that all of the subjects in a clinical trial have the condition that is to be studied, classification criteria for clinical trials require higher specificity than diagnostic criteria, which are used in clinical practice.

Monitoring disease activity in spondyloarthropathy can be challenging, as patients may have back pain, tendonitis, and enthesitis rather than arthritis with tender and swollen joints. MRI findings including bone edema and osteitis are most frequently used to document early sacroiliitis.[16] Ultrasound can also effectively depict sacroiliitis in patients with early spondyloarthropathy.[17] The Madrid Sonographic Enthesitis Index (MASEI) is used to monitor enthesitis in clinical trials.[18] Because enthesitis is an underdiagnosed cause of patient morbidity, an enthesitis index like the MASEI could potentially be used clinically to differentiate psoriatic arthritis from psoriasis alone.[18]

The presence of monosodium urate crystals in synovial fluid or tophus aspirates remains the gold standard for diagnosis of gout.[19] However, given the lack of access to joint aspiration and polarizing microscopy in primary care, criteria to diagnose gout without crystal identification would be highly beneficial for clinicians.[20,21] To address these needs, ACR/EULAR funded The Study for Updated Gout Classification Criteria (SUGAR) to update the American Rheumatism Association gout classification criteria from 1977.[22] The proposed ACR/EULAR classification criteria include an imaging domain if crystal identification is not available.[23]

Ultrasound features of gout include the double contour sign that highlights the contrast of the bright, echogenic bony contour and a parallel echogenic line of monosodium urate crystals, which deposit on top of the darker cartilage lining the joint.[24] Tophi are often described as "wet clumps of sugar" as they are bright and often surrounded by a darker signal of fluid.[24] Ultrasound can also identify erosions related to gout.[25] Finally, ultrasound can monitor response to urate-lowering therapy as ultrasound can document the disappearance of monosodium urate crystals when serum urate levels are lowered.[24] The imaging domain proposed ACR/EULAR classification criteria for gout include ultrasound evidence of a double contour sign in a symptomatic joint or bursa or dual-energy CT evidence of urate deposition and/or imaging evidence of at least one x-ray erosion in the hands or feet.[23]

Although musculoskeletal ultrasound is still not universally available to rheumatologists in the United States, the potential clinical applications of ultrasound continue to grow. As the availability of comprehensive ultrasound training programs improves, more rheumatologists will be able to use ultrasound as a supplemental tool to help diagnose and monitor response to treatment in various rheumatologic diseases.

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