To the Editor:

We have read with interest the paper by Ridley et al1 on the radiographic damage discordance between the sacroiliac joints (SIJs) and the spine in patients with ankylosing spondylitis (AS) from the Prospective Study of Outcomes in AS (PSOAS) cohort, which includes patients from the United States and Australia. Their results have encouraged us to investigate the prevalence of severe SIJ disease without evidence of concomitant syndesmophytes in the spine in our patient cohort from Spain.

Ridley et al1 selected patients with bilateral grade IV SIJ disease, who had ≥ 20 years of disease duration (n = 354), and divided them in 2 groups: those who had syndesmophytes (n = 331, 93.5%) and those without syndesmophytes (n = 23, 6.5%) along the vertebral spine. Using univariate logistic regression modeling, female gender (odds ratio [OR] 0.17, 95% CI 0.07-0.41) was the strongest predictor of the absence of syndesmophytes in patients with severe SIJ disease. Age of symptom onset ≤ 16 years was also an important factor. Univariate analysis showed that patients with HLA-B27 positivity and older age at symptom onset (17-45 yrs, OR 2.72, 95% CI 1.15-6.45) had a greater risk of syndesmophyte presence.

Our cohort comprises 340 patients with axial spondyloarthritis who meet the Assessment of Spondyloarthritis international Society criteria for classification, and whose data have been retrospectively collected from specific records at a tertiary care hospital (database approved by the ethics committee of the Bellvitge University Hospital, no. PR053/20). Inclusion criteria were patients (1) diagnosed with AS who met the modified New York criteria, rather than those with bilateral grade IV sacroiliitis, as we believe that such a sample may be equally informative; and (2) who have ≥ 20 years of disease duration. Further, we have stratified sacroiliitis by grades and described the proportion of patients in each category. We have performed a cross-sectional evaluation of data relating to demographics, clinical information, inflammatory markers, disease activity measures, and radiographic evaluation from the last follow-up visit. We have divided the patients into 2 groups, according to the presence of syndesmophytes.

A total of 222 patients were included, of whom 76.1% (n = 169) were male, with a BMI (calculated as weight in kilograms divided by height in meters squared) of 27.2 (SD 4.5), of whom 136 (61.3%) had smoking history (either former or current smokers). The mean age of the population was 63.5 (SD 12.5) years, and 55.9% (n = 124) had grade IV SIJ disease (Table).

General characteristics of the population and comparison between patients with and without syndesmophytes.

There were 121 (54.5%) patients with syndesmophytes and 101 (45.5%) patients without syndesmophytes. In the Table, along with the general characteristics of the population, a comparison between groups is shown. Among the group of patients without syndesmophytes, the proportion of women was higher than in the group with (40 [39.6%] vs 13 [10.7%]), patients were younger (58.7 [SD 12.6] vs 67.6 [SD 10.9] yrs), and had a lower BMI (26.1 [SD 4.5] vs 28.0 [SD 4.3]). There was also a lower proportion of smokers (54.5% vs 66.9%). Further, in the group of patients with syndesmophytes, higher disease activity (AS Disease Activity Score based on C-reactive protein 2.53 [SD 0.96] vs 2.17 [SD 0.93]) and worse functionality (Bath AS Functional Index 5.01 [SD 2.63] vs 3.47 [SD 2.38]) were observed. Up to 33.7% (n = 34) of the patients without syndesmophytes had grade IV SIJ disease, compared to 74.4% (n = 90) in the other group. There were no differences between groups regarding the age at symptom onset, HLA-B27 positivity, or other disease symptoms.

These results show a lower prevalence of syndesmophytes among women, consistent with the work of Ridley et al.1 However, there were no differences between groups regarding the age of symptom onset or HLA-B27 positivity in our cohort. It is noteworthy that in our study, the proportion of patients with grade IV SIJ disease without syndesmophytes was considerably higher (n = 34; 33.7%), in contrast with the 6.5% (n = 23) reported in the study by Ridley et al.1 Neither of the 2 studies found a differing proportion of both musculoskeletal and extra-musculoskeletal manifestations between groups. Discrepancies could be attributed to population heterogeneity and varying patient stratification, since our cohort included patients with AS, not just those with bilateral grade IV sacroiliitis. The fact that women appear to have less radiographic involvement in AS has already been described in a previous study.2

In conclusion, women seem to exhibit a distinct phenotype in AS compared to men. The underlying reasons for these differences remain unclear and could involve genetic, hormonal, biomechanical, or environmental factors. Additional studies in this line of research may contribute to a better understanding of the variations in AS disease differences within subgroups.