In this single-center, cross-sectional ultrasound study of dactylitis in PsA, soft tissue thickening and subcutaneous edema in combination with synovitis and/or flexor tenosynovitis were the most common lesions, and the reliability of scoring established ultrasound components of dactylitis in a clinical setting was good-excellent.
The contribution of different pathologies in dactylitis are mostly in line with other recent imaging studies (10, 21) where subcutaneous changes were also found to be the most frequent component (90–92%). Next after subcutaneous changes we found synovitis and flexor tenosynovitis - both occurring with similar rates. Other studies though, have reported flexor tenosynovitis to be more frequent than synovitis (8, 10, 21). We found more tenosynovial than synovial Doppler activity – in line with previous studies (21). We also found pathologies in both the flexor and extensor tendon entheses. Previous MRI studies have not found involvement of flexor tendon entheses (6, 10) and varying involvement of the extensor tendon entheses, ranging from 0% in earlier studies (6, 7) to 50% in more recent studies (10). No recent ultrasound dactylitis studies have reported numbers on these changes.
Tender digits had higher sum-scores and numerically higher prevalence of most pathologies than non-tender digits. In contrast, larger studies on this topic (21) found symptomatic dactylitis to have more extra-synovial features (flexor tenosynovitis, soft tissue edema and subcutaneous power Doppler) and asymptomatic dactylitis more synovitis (GS and power Doppler). Our numbers were probably too small to find this difference.
We calculated a simple dactylitis sum-score in order to evaluate the inflammatory burden of each digit and found that it was associated with tender more than non-tender dactylitis. We incorporated all possible sites, since each of them were involved in at least one of our patients. We weighted all components equally, but it could be argued that for example subcutaneous edema as the most frequent pathology should be scored semiquantitatively (22). This would increase its importance in a sum-score, and possibly enhance its sensitivity to change, although the applicability of such as score has been questioned, as subcutaneous findings seem to be highly variable also in a non-psoriatic population (23). Recently, a dactylitis score for psoriatic arthritis was published (12), including peritendon extensor inflammation, soft tissue edema, flexor tenosynovitis and synovitis. However, entheses, flexor tendon pulleys and collateral ligaments could potentially also be involved (10, 24) which could be evaluated in future studies.
Reliability of scoring individual dactylitis components was overall good to excellent both for established scores of synovitis and flexor tenosynovitis and for a simple present/absent score of subcutaneous edema with/without Doppler activity. Few enthesitis elementary lesions had poorer inter-reader agreement, even when low frequency of lesions was considered. Especially evaluation of hypoechogenicity of entheses proved challenging. Moderate -excellent agreement was found for all components of the published dactylitis score (12); however, this study did not include entheses. Reliability of scoring enthesitis of larger entheses in spondyloarthritis and PsA has been established(18); however, this is not validated for the small digital entheses and might not be directly transferrable, especially not for toes. The score of CD activity in entheses showed an excellent agreement, and the use of active enthesitis (requiring Doppler activity) could possibly be better option as sum-score component. The use of higher frequency probe could possibly improve agreement.
This study was performed using validated definitions present at the time of study initiation, in a clinical setting on consecutive patients, so results are readily applicable. The main inclusion criteria were PsA and dactylitis judged by a rheumatologist. Also, we included dactylitic fingers as well as toes and both tender and non-tender dactylitis. Inter-reader agreement was based on live scans and not stored images, which also makes it more applicable in a clinical setting. The primary limitation is the relatively small number of subjects.
Further work is required to establish the relevance of individual ultrasound elementary components for diagnosis and monitoring of dactylitis and to validate an ultrasound dactylitis score in a clinical setting and on both fingers and toes, as such a score would be important in future clinical studies.