It has been suggested that enthesitis could be the primary lesion accounting for the development of all musculoskeletal features of spondyloarthropathies.2,3
For this reason, an increasing number of experts support inclusion of enthesitis in the classification criteria for AS and related syndromes.4
To assess the actual involvement of enthesitis in the pathogenesis of AS, as well as its contribution to disease activity, progression or affect on quality of life in the disease, a systematic examination of entheseal areas with a sensitive tool would be ideal. Furthermore, with new treatments able to control activity and progression of spondyloarthropathies becoming available, the evaluation of entheseal injury could help to select candidates and to measure response to treatment.29
In this regard, several reports have found improvement of enthesitis after treatment with tumour necrosis factor α blockers.30,31,32
It is thought that microtrauma on fibrocartilage structures is at the origin of enthesitis in spondyloarthropathies.4,29,33
In this regard, the evaluation of target areas at anatomical locations prone to trauma injuries, such as the foot and the knee, could be enough to reflect “total” enthesitis.34
In our study, up to 25% of total entheses examined in this AS cohort showed abnormalities, and only 3 of 44 patients had a normal study. The results confirm both the common involvement of the selected sites, as already reported, and the high prevalence of entheseal alterations in AS.19,20,22
As a whole, our findings were comparable to previous reports, although in contrast with the study from Balint et al
we found scant alterations at the inferior patellar tendon insertion. At this entheseal area, several anatomical factors, such as tendon widening and the sharp changing of fibre orientation, account for characteristic anisotropic artefacts that may be misleading. Thus, perhaps this entheseal area should be avoided in a relatively subjective scoring system such as SEI. On the other hand, the plantar fascia and the Achilles tendon were the entheseal sites found more frequently altered in our study and in previous studies.19,20,21,22
With regard to manual methods of entheseal evaluation, we did not expect to find a correlation between MEI and the SEI, since the target areas evaluated in each case were not coincidental. However, considering the entheseal targets of the foot, we found a remarkable dissociation between sensitivity to local pressure and US findings. There was a striking number of asymptomatic lesions, even between those classified as acute inflammatory signs. On the other hand, we also found normal US images in symptomatic entheses. In this sense, it has been suggested that structures in proximity, such as bone marrow, rather than the enthesis itself could account for the pain.29
In fact, studies conducted with MRI have shown that bone marrow oedema precedes the appearance of inflammation at soft tissues.13
In turn, enthesitis could be more linked to symptoms such as swelling, discomfort, gelling or stiffness.
Enthesitis in AS consists of local, destructive microscopic inflammatory lesions, which evolve towards fibrous scarring and new bone formation. Perhaps the major novelty introduced by the SEI in this study is that it distinguishes between acute injury and chronic lesions, a differentiation that, at least in the case of rheumatoid arthritis, has therapeutic implications. Nonetheless, our data showed only a vague relationship between the SEI and disease measuring tools. Although this could possibly be overcome by the recruitment of a larger sample, the absence of correlation between measures has also been found by other authors. The poor ability of systemic parameters, such as ESR and CRP, to assess disease activity in AS has also been recognised.35,36,37
On the whole, this reminds us that several independent markers have to be measured to accurately assess AS, ideally one of them specifically targeting entheseal involvement.
In conclusion, we have developed an SEI that is easy to perform by rheumatologists experienced in US, which may help to characterise entheseal injury better in AS patients. Our results suggest that an SEI could be of interest for decision making in these patients. We highlight the convenience of including an evaluation of the calcaneal insertions, and also a classification of lesions into acute (potentially reversible) and chronic (possibly inactive) lesions. With new US devices of higher resolution becoming available, a more sensitive SEI could be developed.