General Applications of Ultrasound in Rheumatology Practice

The use of US to find subclinical synovitis in patients with RA considered to be in clinical remission is a new issue. ¹⁶ Some reports have demonstrated progressive joint damage in these patients with evidence of active inflammation on PDUS despite clinical remission. ^17,18 More prospective studies are required to provide a better understanding of the long-term effects of residual inflammation and the proper long-term treatment of these patients. Furthermore, the PD signal has been shown to be superior to the Disease Activity Score 28 (DAS-28) in evaluating disease activity, particularly in predicting joint damage. ¹⁸

Ultrasound may be considered the gold standard imaging tool for the assessment of tendons in inflammatory arthritis and includes the detection of tenosynovitis and anatomical damage represented by the loss of the normal fibrillar echotexture and loss of definition of the tendon margins, which may occur in early disease. ^19,20 Tenosynovitis of the extensor carpi ulnaris (ECU) detected by US has been shown to be an independent predictive factor of erosive joint damage, suggesting that ECU tenosynovitis represents a useful ultrasonographic landmark in the diagnosis of early RA. ²¹

The availability of new nonbiologic and biologic therapies for inflammatory arthritis has raised the importance of identifying early changes, such as the detection of early erosions, which portend a poor long-term prognosis. The capability of US in identifying this lesion at an earlier stage compared with conventional radiography (CR) has allowed the early diagnosis and treatment of these patients before irreversible joint destruction occurs. ²² In spite of all the supportive evidence of US utility in RA, it is not considered among the mandatory diagnostic criteria in the ACR/EULAR classification criteria for RA. ⁵ Still, the addition of US findings to these criteria has increased the number of patients who fulfilled the 1987 ACR classification criteria for RA after 18 months of follow-up. ²³ Despite extensive evidence of its utility in the diagnosis and monitoring of RA, further studies are still needed.

Spondyloarthritis

Similar to RA, SpA discloses sonographic findings of inflammatory arthritis; however, with more entheseal and tenosynovium involvement. Ultrasound has also been used in the early identification of characteristic changes of the skin and nail tissues, which can aid the global assessment of this heterogeneous disease, especially in psoriatic arthritis (PsA). The most common locations of enthesitis in SpA are the quadriceps and the Achilles enthesis. ^24,25

Although US offers detailed imaging for the assessment of both tendons and enthesis, there is a lack of literature evaluating dactylitis. The OMERACT group recently released a composite measure of activity and severity of US dactylitis, which included newly defined elementary US lesions that may discern dactylitis of a digit. ^26,27 Ultrasound has been compared with MRI in the detection of SpA-related synovitis of the hands and feet and has demonstrated competitive diagnostic sensitivity. ²⁸ Ultrasound also shows higher sensitivity in detecting synovitis of the hands and feet compared with clinical examination and CR in PsA. ^28,29 Unfortunately, there are no strongly validated US findings that can aid in the differential diagnosis of PsA against other chronic inflammatory arthritides. The presence of peritendinous extensor tendon inflammation was a highly specific sonographic feature of PsA, because it was present in 66% of metacarpophalangeal (MCP) joints as the only US sign of inflammation compared with patients with RA. ³⁰