Introduction

Hidradenitis suppurativa (HS) is a chronic and progressive skin disorder characterized by the formation of inflammatory lesions particularly in the axilla, groin, buttocks, and inframammary regions (Jemec 2012). Initially, starting as inflamed nodules, these lesions can develop into painful abscesses, sinus tracts, and scarring. Given that HS has a significant and detrimental impact on patient quality of life (Alavi et al. 2015; Wolkenstein et al. 2007), there is a need for increased understanding regarding the prevalence, incidence, and epidemiology associated with this condition.

Prevalence of HS reported in the literature has significant variation, ranging from 0.03% to 4% (Jemec and Kimball 2015; Saunte and Jemec 2017; Delany et al. 2018; Ingram et al. 2018). A more precise understanding of the prevalence of HS would have implications in quantifying the resources required to meet the needs of this patient population and would influence healthcare provision. It is likely that there are multiple contributing factors to the significant variation in HS prevalence reported in the literature. Variation in study design is one such factor, including registry-based studies, prospective physical examination of patients, data reported from cross-sectional population studies, and diagnosis by patient-completed questionnaires.

One less studied factor is the influence of geography and ethnicity on the prevalence of HS. Recent studies from Korea show they have much lower rates of HS compared to Caucasians and African Americans based on population study data (Lee et al. 2018). We hypothesized that this significant variation may be due to different prevalence rates of HS according to geographical location as well as sex. To assess the impact of these factors, we performed a systematic review and meta-analysis of available population studies published and performed subgroup analysis to determine the impact of geographic region and sex on HS prevalence.

Methods

Search strategy

The current systematic review and meta-analysis was performed according to recommended PRISMA guidelines (Moher et al. 2009). As no human or animal subjects were involved in this study, ethics approval was not required. Electronic searches were performed using Ovid Medline, PubMed, Cochrane Central Register of Controlled Trials (CCTR), Cochrane Database of Systematic Reviews (CDSR), ACP Journal Club, Database of Abstracts of Review of Effectiveness (DARE), PsycINFO, and Embase, from their dates of inception to August 2018. To achieve maximum sensitivity of the search strategy and identify all studies, we combined the terms “hidradenitis suppurativa,” “acne inversa,” “velpeau,” “verneuil,” “prevalence,” “epidemiology,” and “incidence” as either keywords or MeSH terms (Additional file 1: Table S1). The reference lists of all retrieved articles were reviewed for further identification of potentially relevant studies. All identified articles were systematically assessed using the inclusion and exclusion criteria. A protocol was not registered for the present systematic review and meta-analysis.

Selection criteria

All studies reporting the prevalence of HS were included in the present review. All studies must have included the proportion of patients with HS in the population studied. All publications were limited to those involving human subjects. Language was not an exclusion criterion. Abstracts, case reports, conference presentations, editorials, and expert opinions were excluded. Review articles were omitted because of potential publication bias and duplication of results.

Data extraction

All data were extracted from article texts, tables, and figures. The data collected included in the study are as follows: the proportion of patients with HS in the population studied, the geography of the population studied, and proportion of males and females.

Statistics

A meta-analysis of proportions was conducted for the available main perioperative and postoperative variables. Firstly, to establish variance of raw proportions, a logit transformation was applied. To incorporate heterogeneity (anticipated among the included studies), transformed proportions were combined using DerSimonian-Laird random effects models. Finally, the pooled estimates were back-transformed. Heterogeneity was evaluated using Cochran’s Q and I2 test. All analyses were performed using the metafor package for R version 3.4. P values < 0.05 were considered statistically significant.

Results

A total of 740 studies were identified through electronic database searches and from other sources such as reference lists. After exclusion of duplicate or irrelevant references, 46 potentially relevant articles were retrieved (Fig. 1). After detailed evaluation of these articles, 16 studies (Delany et al. 2018; Ingram et al. 2018; Lee et al. 2018; Calao et al. 2018; Cosmatos et al. 2013; Garg et al. 2017; Ianhez et al. 2018; Jemec et al. 1996; Lookingbill, 1988; Reeder et al. 2014; Revuz et al. 2008; Richard et al. 2018; Theut Riis et al. 2018; Santos et al. 2017; Shahi et al. 2014; Sung and Kimball, 2013) remained for assessment. The study characteristics are summarized in Table 1. All included studies were population studies. There were two studies from the Asia-Pacific region, six studies from the USA, six studies from Europe, and two studies from South America (Additional file 2).

Fig. 1
figure 1

PRISMA flow chart of the search strategy used in the present study

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Table 1 Characteristics of included studies in the present systematic review and meta-analysis. −, not reported
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The overall pooled prevalence rate was 0.3% (0.2–0.6%) based on 118,760,093 HS cases available (Fig. 2). Subgroup analysis demonstrated prevalence differences, with the highest being in Europe 0.8% (0.5–1.3%), compared to the USA 0.2% (0.1–0.4%), Asia-Pacific 0.2% (0.01–2.2%), and South America 0.2% (0.01–0.9%).

Fig. 2
figure 2

Forest plot demonstrating the pooled prevalence of hidradenitis suppurativa, stratified according to geographic region

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Pooled analysis was also performed to determine whether the prevalence of HS in males was lower or higher compared to females. The overall meta-analysis demonstrated that the prevalence in males was similar to that compared to females (OR 0.58, 95% CI 0.33–1.04) (Fig. 2). Subgroup analysis was performed according to geographical region. Prevalence in males was lower compared to females in the USA (OR 0.403, 95% CI 0.37–0.439, P < 0.001) as well as in Europe (OR 0.635, 95% CI 0.397–1.015, P = 0.08) but not in the Asia-Pacific region (OR 0.936, 95% CI 0.319–2.751, P = 0.78) (Fig. 3).

Fig. 3
figure 3

Forest plot demonstrating odds of hidradenitis suppurativa in males compared to females, stratified according to geographical region

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Discussion

There remains a lack of consensus as to the overall prevalence of HS in the general population. The large variation in reported prevalence rates of HS is likely contributed by differences in the ethnicity of the populations studied. A recent Japanese population study (Kurokawa et al. 2015) reported an HS prevalence of 0.06%, which was comparatively lower compared to HS prevalence rates described in Western populations (0.1–4%). As such, we hypothesized that ethnicity and hence geographical location is a potential contributing factor to the prevalence rates of HS.

Our meta-analysis supports our hypothesis, with an overall pooled HS prevalence rate of 0.3%. Subgroup analysis demonstrated highest prevalence in Europe 0.8%, compared to the USA 0.2%, Asia-Pacific 0.2%, and South America 0.2%. Prevalence in males was lower compared to females in the USA and Europe, but not in the Asia-Pacific region. These differences likely have a multifactorial underlying etiology, including a combination of genetic differences and immune and hormonal differences among the ethnicities. It has been previously reported that there are differences in gamma-secretase gene mutations occurring between Asian and European studies (Ingram 2016; Liu et al. 2016), implicating the role of genetics in the differences in the prevalence of HS observed.

Our meta-analysis also demonstrates that the sex predisposition of HS may vary according to geographical location and ethnicity. We found that in Asia-Pacific countries, the prevalence of HS in males appeared to be comparable to females, where as in Western and European countries, the prevalence was significantly lower in males compared to females. This suggests that there is an increased predisposition of HS in males in countries of Asian-Pacific ethnicity. Our findings are consistent with those of Lee and colleagues (Lee et al. 2018) who studied a Korean population. The authors reported a prevalence rate of 68.3 patients per 100,000 males, compared to 43.2 patients per 100,000 females, with male-to-female ratio of 1.6:1. Similar findings were reported by a Japanese epidemiological study by Kurokawa et al. (2015), with male-female ratio reported 2:1. The odds of HS prevalence were significantly lower in males compared to females across American populations and European populations included in this meta-analysis, with pooled male-female ratio of 0.4 in American patients and 0.64 in European patients. This implicates the significance of geography and ethnicity on the prevalence of HS.

The distribution of HS lesions also appears to differ based on geographical location and ethnicity. In the Japanese population as reported by Kurokawa et al. (2015), the body areas predominantly affected by HS were the buttocks. In contrast, HS in European patients tend to affect the groin and axillae, rather than the buttocks (Kurokawa et al. 2015; You et al. 2016; Schrader et al. 2014). This may be due to differences in the distribution of hairs and sweat glands in Asians compared with Europeans. A morphometric analysis of Korean adults demonstrated that the concentration of glands was lowest in the mammary areas for both females and males (Hwang and Baik, 1997), in contrast to studies in Europeans (Thomson, 1954). There are likely contributing factors to this association; for example, it is known that obesity rates in Asian countries such as Japan and Korea are significantly lower compared to Western countries.

The present meta-analysis is constrained by several limitations. Firstly, we were not able to stratify HS prevalence according to the severity of HS, as this data was not available in the majority of included studies. For studies reporting prevalence based on clinical assessment, there is selection bias in terms of people attending the clinical assessment. For data using population databases and registries, missing data is less likely for patients with closer follow-up with their family physicians and increased follow-up with their dermatologist. Inaccuracies and incomplete coding of diagnoses is another limitation of the use of population databases. The outcome definitions among studies vary significantly, which limit the interpretation of results. In some studies, outcome was defined as a self-reported HS diagnosis; in others, it was a valid administrative code for HS diagnosis; and in others, it was a physician-confirmed diagnosis of HS. There is also the possibility of confounding factors across geographical populations, such as lifestyle factors that may influence the occurrence of disease, race, ethnicity, obesity rates, and social determinants of health, which were not assessed by the included studies.

Conclusion

In conclusion, our meta-analysis demonstrates that the prevalence of HS varies significantly according to the geographical population. This variation is likely attributed to different ethnicity concentrations on different continents. Particularly, Asian populations were associated with lower prevalence compared to their European counterparts. Additionally, a higher relative prevalence of HS was found in Asian males compared to European and American males. These findings have implications in understanding the epidemiology and management of HS.