A large number of research studies have applied US to the assessment of enthesitis in spondyloarthritis . The commonest entheseal sites to be ultrasonographically examined in standard clinical practice include: Achilles tendon and plantar fascia insertions to the calcaneus, tibialis anterior tendon insertion, greater trochanter, pubis, patella (at insertions of the quadriceps femoris and patellar tendons), medial, and lateral epicondyles. Sonographically, enthesitis appear as loss of normal fibrillar pattern, increased fusiform thickness (Fig. 4.3), peri-tendinous fluid, a linear intra-substance tear, calcific deposits (Fig. 4.4), or intralesional focal changes at the tendon insertion. In addition, periosteal changes such as erosions or new bone formation may also be seen at the tendon insertion. Enlarged bursa is usually considered as a sonographic manifestation of bursitis. Enhanced vascularity assessed by power Doppler (PD) at the involved entheses may also add further information about the activity status at the assessed site as reflected by its degree of vascularity (Fig. 4.5). Table 4.1 summarizes the different sonographic patterns of enthesitis .

In a trial to standardize the quantitative assessment of enthesitis, numerous scoring tools have been developed . One of the most commonly used is the Glasgow Ultrasound Enthesitis Scoring System (GUESS) [19]. The GUESS was developed based on a study of 35 patients with spondyloarthritis (including seven psoriatic arthritis patients) who underwent independent clinical and ultrasonographic examination of both lower limbs at five entheseal sites: superior pole and inferior pole of patella, tibial tuberosity, Achilles tendon, and plantar aponeurosis. A total of 18 features, each given a score of 1, were scored on each lower limb, the maximum score being 36. The intraobserver k value for analysis of all sites was 0.9.

In another work, D’Agostino et al. [20] implemented a five-stage classification system of ultrasonography enthesitis according to B-mode and PD findings (Table 4.2) where stage 1: vascularization at the cortical junction without abnormal findings in B mode. This represents isolated vascular changes occurring during the initial phase of enthesitis; stage 2a: vascularization associated with swelling and/or decreased echogenicity at the cortical junction in B mode; stage 3a: same as stage 2a, plus erosions of cortical bone and/or calcification of enthesis, and optional surrounding bursitis. Stages 2a and 3a reflect increased vascular alterations associated with graded signs of morphologic alterations. Stage 2b: abnormal findings in B mode as in stage 2a, but without vascularization and stage 3b: abnormal findings in B mode as in stage 3a, but without vascularization. Stages 2b and 3b, which are devoid of vascularization, are suggestive of inactive lesions.

Two other scoring systems developed at the patients’ level were published. The Spanish Enthesitis Index (SEI) [21] entails, after giving the patient information about variable enthesis sites, the evaluation of global patient inflammatory activity or enthesis structural damage. The scoring system, however, used grayscale abnormalities only and did not differentiate between involvement of body of the tendon, enthesis, and bursa . The Madrid Sonographic Enthesitis Index (MASEI) [22] combined abnormalities detected by grayscale US and enhanced vascularity as assessed by PD (including bursal involvement). This scoring system was the first to evaluate enthesis in both lower limbs and upper limbs (the attachment of the triceps tendon to the olecranon). Also, the MASEI included assessment of bone erosions, enthesophytes, as well as PD signal .

Critical analysis of the four systems revealed that, although they all included assessment of enthesitis sites for both inflammatory and structural manifestations using grayscale US with or without PD, sensitivity to change was low making none of them a sensitive tool for monitoring the patient’s enthsitis activity status. Furthermore, there is inconsistency among the four scoring tools. While the GUESS and D’Agostino scoring systems were set up to grade enthesis affection (enthesitis level). The MASEI and the SEI were established as enthesitis indices at the patients’ level. Therefore, these scoring systems are uncomparable. Currently, there is an unmet need to agree on a system for use in research trials as well as standard clinical practice [23, 24] .

The value of US in the diagnosis of enthesitis is that it has been reported to be more sensitive than clinical examination in identify enthesitis, raising the query whether the presence of enthesitis could be missed during standard clinical assessment . A study was carried out in 2002 [25] comparing the detection of enthesitis based on clinical examination versus US. A total of 35 patients (27 AS patients, 7 psoriatic arthritis patient, and 1 patient with reactive arthritis) had their enthesitis clinically assessed in the lower limbs as well as using US and scored according to the GUESS. The study reported significantly higher enthesitis score in the US-assessed group in comparison to clinical assessment. It concluded that US is a good method to assess for enthesitis in spondyloarthritis patients. These findings were confirmed in another study carried out by by Spadaro et al. [23]. A total of 36 AS patients had both clinical and PD ultrasonography assessment of 432 enthesis sites, and 14.8 % were diagnosed to have clinical enthesitis in comparison to 44.4 % were considered abnormal on PD ultrasonography. A third study assessed enthesitis in the feet of 44 subjects with spondyloarthritis (19 AS patients, 5 with psoriatic arthritis, 8 with reactive arthritis, and 12 with undifferentiated spondyloarthritis) [26]. Pathological findings were reported in 25 patients, most of who did not have any foot problem or a complaint. Similarly, upper limb enthesitis was assessed in another study [27]. US was used to assess for enthesitis at the proximal insertion of the deltoid, which mimics clinically impingement syndrome, in 100 symptomatic consecutive spondyloarthritis outpatients, compared to 4 groups of control patients: 100 with rheumatoid arthritis, 100 with osteoarthritis, 100 with painful shoulder, and 50 with shoulders undamaged by local pathological processes. Results revealed the frequency of enthesitis in the course of spondyloarthritis was 9 %. The prognostic value of this subclinical enthesitis was assessed in a recent study [28] carried out to identify predictors of structural joint damage in patients with early spondyloarthritis. Results revealed an increased probability for structural progression in the presence of US verified enthesitis .

US plays also an important role in the differential diagnosis of enthesitis, in particular differentiating mechanical or metabolic enthesopathy from inflammatory enthesitis . Beside spondyloarthritis patients, enthesitis have been reported among athletes as a consequence of traumatic injuries. Distinguishing fibromyalgia tender points from enthesitis is another important challenge to be considered. However, in such cases, enthesitis is not associated with intra-articular inflammation (i.e., synovitis). Furthermore, PD technology has allowed the visualization of abnormal vascularity and soft tissue hyperemia in inflammatory conditions [28]. Therefore, that abnormal enhanced vascularity, detected by PD US, at the insertion of tendons, ligaments, fascia, and capsules into the bone is considered as a primary lesion which is seen exclusively in spondyloarthritis patients and may underlie all spondyloarthritis skeletal manifestations [20]. On the other hand, enthesophytosis cannot be considered an explicit manifestation for spondylarthritis-related enthesitis, as it has also been reported in mechanical, osteoarthritic as well as in normal asymptomatic subjects [29

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