Axial spondyloarthritis (AxSpA) is a chronic inflammatory disease that most often involves the axial skeleton, although it may also debut with peripheral manifestations in the form of arthritis, enthesitis, or dactylitis. It has an estimated prevalence of 0.25% in Europe1 predominantly in males (2:1 ratio). It frequently begins in the second or third decade of life and may be associated with other extra-articular manifestations, such as uveitis, psoriasis, and inflammatory bowel disease (IBD).

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease with onset typically after puberty. In Europe, it has a prevalence rate of 0.4–1.1%2 and affects females more frequently than males (3:1 ratio). It is associated with other non-dermatological inflammatory diseases such as IBD.

In the last few years, several studies have linked AxSpA to HS after detecting a greater prevalence than expected in the general population.3–6 Both conditions share the same aetiopathogenic mechanisms, inasmuch as there are changes in both innate and adaptive immunity in which the proinflammatory cytokines, such as TNF-, IL-17, IL-17, IL-17, IL-17, IL-17, IL-17, IL-17, and IL-17, play a major role. Moreover, one of the mainstays of treatment in both diseases is biological treatment with anti-TNF.7–9 Other arguments in favour are that the prevalence of IBD is higher in both AxSpA and HS9 patients and that smoking and being overweight or obese are known to constitute poor prognostic factors for both disorders.5

Early diagnosis of HS in individuals with AxSpA could improve prognosis and assist in selecting appropriate treatment, with a beneficial impact on the quality of life in these cases.

The main objective of this study was to ascertain the prevalence of HS in people with AxSpA and, with the secondary aim of describing the clinical, analytical, and radiographic characteristics of patients diagnosed with HS and to compare them with the AxSpA group without HS.

Of the 265 cases enrolled in the cohort, we excluded those who could not be contacted by telephone after at least five calls on different days and at different times (32 of 265 individuals; 12%), those who refused to give consent (28 of 233 individuals; 12%), and those who failed to answer the screening questionnaire (57 of 205 individuals; 27.8%). Finally, 148 people (55.8% of the total) who answered the HS screening questionnaire were included for analysis (Fig. 1).

Fig. 1.

Flow chart of the patient selection process and distribution of patients into groups. AxSpA: axial spondyloarthritis; HS: hidradenitis suppurativa.

A total of 9/148 patients (6.1%) met the criteria for a positive screening and were asked to attend a face-to-face appointment with dermatology. The diagnosis of HS was confirmed in four of the patients evaluated; consequently, the prevalence rate of HS in our sample was determined to be 2.7% (4/148).

Three participants were diagnosed during the study period, while the remaining person, despite having been previously diagnosed by axillary nodule biopsy, had never been seen by a dermatologist. One of the four individuals was selected in spite of not satisfying the time criteria, inasmuch as he was taking adalimumab for his AxSpA.

The characteristics of the four patients diagnosed with HS are provided in Table 2. Most presented lesions beginning at an age close to adolescence, with a mean diagnostic delay of 14.25 years. All subjects had mild HS (Hurley I) and only one person had a family history of HS.

The results of the comparative study between participants with and without a diagnosis of HS are displayed in Table 3. It is worth pointing out the higher percentage of smokers among those with HS (75% vs. 18%; p = 0.005), as well as greater clinical activity as determined by the BASDAI in this group (5.6 ± 2.3 vs. 3.2 ± 2.1; p = 0.026). In contrast, patients with HS had a shorter time of evolution of AxSpA (14.7 ± 2.6 vs. 27.8 ± 13.5 years; p < 0.001) and less structural damage as quantified by the axial BASRI on plain x-ray (1.75 ± 1.3 vs. 5.5 ± 3.5; p = 0.018).

As for demographic characteristics, no statistically significant differences were detected with regard to sex, albeit there was a trend toward a higher percentage of females in the group with HS (50% vs. 32%). In both groups, most participants had a BMI > 25 kg/m2 (67.3% without HS, 75% with HS), with overweight accounting for 45.9% and obesity for 21.6% of all cases.

No differences were noted between the two groups as concerns the rest of the variables examined.

The number of patients included in the HS group was insufficient for multivariate analysis.

Since 1982, several authors have reported the association between AxSpA and HS in case series.12–18 More recently, Richette et al.3 and Fauconier et al.4 examined the prevalence of AxSpA in people with HS, obtaining a proportion of 3.7% (24/640) and 28.2% (11/39), respectively. Rondags et al.5 performed the only study to date that has looked at the prevalence of HS in patients with AxSpA. In their cohort of 592 patients, they document a prevalence rate of 6.9–9.1% by means of a screening questionnaire and a telephone confirmation by dermatology. These values are consistent with the prevalence of other extra-articular manifestations of AxSpA, such as psoriasis or IBD.19

As yet, there have been no studies that have addressed the prevalence of HS in individuals with other inflammatory rheumatological diseases. Nevertheless, a greater incidence of rheumatoid arthritis and psoriatic arthritis has been confirmed in patients with HS.20–22

The prevalence of HS in our cohort was 2.7%, higher than the estimated prevalence in the general population (0.4–1.1%).2 Of the total, 6.1% of the patients screened positive, which is on a par with the percentage reported by Rondags et al.5 The screening questionnaire identified individuals who were highly suspicious of having HS and the diagnosis was confirmed in person by dermatology in four of the nine patients (44.4%). This result highlights the importance of a face-to-face dermatological evaluation in cases of high diagnostic suspicion, given that the diagnosis of HS is based on clinical criteria.

Seventy-five percent of the confirmed cases (3/4) were diagnosed during the study. In the series by Rondags et al.,5 this proportion was 92.7% (38/41). Forty-eight participants in our cohort were receiving anti-TNF treatment for AxSpA, a circumstance that might have reduced the number of HS flare-ups, hindering its detection at the time of evaluation. Nonetheless, one patient in this group exhibited the typical clinical signs of HS, enabling the diagnosis to be confirmed.

In half of the cases, HS lesions preceded the beginning of AxSpA, with an early onset of symptoms and significant diagnostic delay. These findings support the hypothesis that HS is an underdiagnosed disorder and highlight the value of meticulous anamnesis and physical examination to increase detection rates.

All HS cases manifested a mild form of the condition. Only one person had a known family history, although it is estimated that 40% of those affected typically also have a history in the family.23

Patients with HS displayed greater AxSpA activity as gauged by the BASDAI index; three of them (75%) had active disease (BASDAI ≥ 4). This finding could have to do with the higher percentage of smokers in this group, taking into account the impact of smoking as a poor prognostic factor in both conditions.

With respect to radiographic characteristics, the subjects with HS only presented lesions at the sacroiliac level. This accounts for their more limited structural damage measured by axial BASRI and could be due to a shorter time of evolution of AxSpA in this group.

Overweight and obesity were prevalent in both groups: 67.3% among participants without HS and 75% among those with HS. These figures are higher than those estimated for the general Spanish population in 2020 (46.1% among females and 61.4% among males according to the Spanish National Institute of Statistics24). Similarly, Rondags et al.5 identified 62% of obesity in their cohort. We also detected a trend towards more women in the HS group (3:1 ratio), a result that is consistent with the female predominance observed in this disease.

The main limitation of the study is the small sample size. Patients who signed the informed consent form and completed the screening questionnaire were included, which comprised 55.8% of the initial cohort. This resulted in a small comparator group that was insufficient for multivariate analysis. Furthermore, there is sampling bias derived from the fact that patients treated at a reference centre were included.

Among the strengths of this work, the diagnostic method for HS is especially salient. Although there is no validated screening questionnaire in Spanish, we used a literal translation of validated questions in English with high sensitivity and specificity and supplemented it with photographs of typical HS lesions to facilitate identification. Face-to-face confirmation by dermatology provided greater diagnostic confidence. The response rate of the questionnaire was relatively high at 72.2% (148/205). Finally, this is the first Spanish study to assess the prevalence of HS in patients with AxSpA in our setting.

The prevalence of HS among individuals with AxSpA in this study was greater than that observed in the general population. Further studies, with larger patient samples, should be conducted to determine the actual strength of this association. Awareness of this association should encourage clinicians to actively explore symptoms and lesions, although specific studies are warranted to determine the most appropriate screening method in these cases.

AI has not been used in the drafting process of this manuscript.

None.

The data were collected and used solely for this study and are not available for other uses.