Introduction

Hidradenitis suppurativa (HS) is an inflammatory, chronic, and recurrent skin condition that affects hair follicles in areas of the body with apocrine glands, predominantly in the axillary, inguinal, intermammary, and anogenital regions. It manifests more frequently after puberty and is characterized by painful, inflammatory lesions that vary from papules, double-ended comedones, and subcutaneous nodules that eventually evolve into fistulas and abscesses, with subsequent formation of sinus tracts and scars [1••, 2]. In advanced stages, fibrosis occurs together with contractures of the adjacent skin [1••].

HS used to be considered a disorder of apocrine glands; today, it is considered a disease of the pilosebaceous unit associated with an immune imbalance in a genetically predisposed individual. For the development of the disease, there must be a genetic predisposition, in addition to other acquired factors such as obesity, smoking, heat, exercise, the use of tight clothing, and the use of deodorants, which play an important role in the development or worsening of this condition [1••, 3].

To assess the severity of the disease and stratify its progression, multiple classifications have been proposed, yet Hurley classification is the one most widely used since its creation in 1989. This classification has 3 stages of disease: the first or stage I, isolated or multiple abscess formations, without the presence of fistulas or scars; stage II, formation of abscesses, fistulas, and scars, separated from each other; stage III, formation of interconnected, confluent abscesses, fistulas and scars, distributed throughout the affected area [1••, 4].

Additionally, association with certain autoimmune diseases that are of importance in the development and have prognosis of this disease has been reported. Among the most common are inflammatory bowel disease, hypothyroidism, pyoderma gangrenosum, psoriasis, metabolic syndrome, dyslipidemia, diabetes, rheumatoid arthritis, and follicular occlusion triad, and in some cases, there is even a relationship with cutaneous malignancies [1••, 3, 5].

Understanding clinical and epidemiological aspects of the disease has become more relevant, given the emergence of new therapeutic options that includes biological therapies, not to mention the arsenal of treatments that under development. This study aimed to provide a comprehensive and current state of the art about epidemiology and clinical aspects of the HS.

Epidemiological Data on Prevalence and Incidence

North America

The country with the highest number of studies reporting the prevalence of HS is the USA. One of them was carried out by Shlyankevich et al. in Massachusetts obtaining a prevalence of 0.078% [5].

At the same time, in 2014, Shahi et al. included 144,000 people from Olmsted County, MN, finding a prevalence of 0.13% in this population [6]. Finally, Savage et al. published in 2013 a study in which they included 34,187,919 patients, from the Burlington, MA, clinical database. The prevalence in this population was 0.147% [7].

Latin-American

In Argentina in 2019, Zimman et al. published a study based on a hospital population in Buenos Aires, determining a prevalence for this population of 0.02% [8]. Another South American country with prevalence data is Brazil, where a study was published in 2018, carried out by Ianhez et al. A total of 17,004 inhabitants were included, finding a prevalence of 0.41% in that country [4].

Europe

In Europe, HS prevalence ranges from 0.001 to 1.4%. The lowest prevalence is reported for the Polish population and Ireland has the highest prevalence [9,10,11,12,13,14].

In France, a study determined the prevalence of the most common dermatological disorders in the French population in a period of 12 months, finding a prevalence of 0.97% [9].

Kirsten et al. calculated a period prevalence between 2010 and 2015 in Germany. This study included a sample of 2.3 million insured people, which is equivalent to 40% of the country’s insured population, calculated a prevalence of 0.04% [10].

A Polish study carried out by Matusiak et al. including almost 90% of the country’s population, calculated an estimated prevalence for HS of 0.001% [12].

In Italy, Bettoli et al. reported an analysis of the possible variations between two consecutive Italian registries of patients with HS between the years 2009 and 2013 vs. 2015–2019 concluding that the disease frequency estimates ranged between 5:10,000 and 4:100 people [13].

Finally, in Ireland, a cross-sectional study was carried out in 4 selected hospitals, in which the prevalence of the disease was estimated at 1.4% [14].

Asia

In Korea, Lee et al. obtained data from the national health insurance database from 2007 to 2016, finding 28,516 patients diagnosed with HS, and taking into account an estimated population of 50 million, it was determined a prevalence of 0.06% [15].

Australia

In Australia, a population-based cross-sectional study estimated a prevalence of 0.67% [2].

Incidence

Among the articles included, only the study by Kirsten et al. included HS new cases data. An incidence in the German population of 0.03% was estimated [10].

Table 1 describes data on the prevalence and incidence of HS in various countries.

Table 1 Prevalence and incidence of hidradenitis suppurativa
Full size table

Determinants of the Disease

Age and Sex

In most studies, it is observed that HS can be twice more prevalent in women than in men, except for Korea where demographic data have been collected showing a higher proportion in men, which ranges from 61.3 to 71.7%, while in women, it ranges from 28.3 to 38.7%. This contrasts with the studies reported in America, Europe, and even Oceania [2, 15, 16].

On the other hand, the studies describe a wide range of the age of presentation of HS, from adolescence to adulthood. Generally, the prevalence of the disease increases after puberty, depending on the maturity of the apocrine glands, making HS more prevalent in adolescents and adults than in children and the elderly [4].

The age range where HS is most frequently found worldwide is between 15 and 40 years, with a higher prevalence in the third and fourth decades of life, even in the eastern region, where the most frequently reported age of consultation was 23.9 years, and the age of disease onset was 26.7 ± 12.7 years [11].

Regarding the age of presentation according to sex, Shahi et al. report a greater number of cases in the male population between 30 and 39 years old and in the female population between 20 and 29 years old [6].

Clinical Classification

Few studies encompassed in this review included the clinical classification of patients. Zimman et al. reported the clinical classification distribution for the Argentinean population. The highest percentage of patients were Hurley I 42.4%, followed by Hurley II 36.4%, and Hurley III to a lesser extent with 21.2% of all cases [8]. Similar percentages are present in the Korean population where the largest number of patients was found in Hurley stage I (64.6%) [15, 16].

In Greece, there is a higher percentage of patients in Hurley II stage 44.7%, followed by Hurley III with 29% and, lastly, Hurley I with 26.3% [11]. The same happens in Italy, where most of the patients were in Hurley stage II with 45.9% [13].

Most HS patients worldwide are classified as Hurley stage I and II.

Anatomical Location

The anatomical location is consistent throughout the different countries, with the most frequent being the inguinal, axillary, and perianal regions. In the study by Zimman et al. [8], the percentages were as follows: axillary region in 56.1%, vulvar/perianal region in 39.4%, and the inguinal area in 34.9% [8]. In the Greek population, the percentages were as follows: the inguinal region 61%, the axillary region 55.3%, and the buttocks 19.7% [11]. In contrast, with the Korean population, the most frequently affected areas were the buttocks in 48.4%, followed by the axillary region 35.8%, and the inguinal region 22.1% [16].

In the study carried out in Australia, they distributed the anatomical involvement by sex; in this way, they reported in the female population, involvement of the inguinal region in 56.9%, axillary 44.8%, inframammary 25.9%, genital 20, 7%, perianal 19%, and folds (abdominal/periumbilical) 17.2%. Similar to the male population, which had 43.3% involvement of the inguinal region, perianal 36.7%, axillary 30%, abdominal/periumbilical folds 26.7%, genital, and inframammary area 6.7% [2], being the inguinal region the most frequently affected in both sexes.

Comorbidities

Risk factors such as obesity, smoking, stress, hormonal factors, and genetic load that contribute to the development or worsening of the disease are described within the pathophysiology of HS. Moreover, associated comorbidities such as metabolic syndrome, atherosclerosis, dyslipidemias, thyroid diseases, and even psychiatric diseases such as anxiety or depression have been described [1••, 8, 15].

For example, the Argentinean study concluded that 59.1% of the patients were smokers, almost half of the patients were obese or overweight (47%), a third of the patients were dyslipidemic (34.8%), 6.1% had diabetes in addition to HS, 18.2% had some type of psychiatric disorder, 10.6% thyroid disease, 19.7% arterial hypertension, 15.2% anemia, and 27.3% acne [8].

Likewise, in Greece, it was found that up to 72.4% of patients with HS were smokers, 7.2% were ex-smokers, 75% were overweight or obese, and of these, 28.9% were moderately obese (BMI> 30–<35) [11].

In Korea, a lower proportion of patients were found to be smokers: 38.3%, overweight patients 30.3%, obese 12.4%, acne 12.6%, diabetes 6.4%, arterial hypertension 5.7% among other comorbidities [16].

Shlyankevich et al. report the association of comorbidities within patients with HS, obesity (OR 2.09), hypertension (OR 1.84), dyslipidemia (OR 4.06), thyroid disease (OR 2.18), psychiatric disease (OR 3.95), arthropathy (OR 9.41), and polycystic ovary syndrome (OR 13.7) [5].

Additionally, in the study by Lee et al. [15], associated comorbidities that were reported included the following: ankylosing spondylitis (OR 1.47), rheumatoid arthritis (OR 1.3), type I diabetes (OR 1.99), ulcerative colitis (OR 1.3), acne conglobata (OR 5.07), pilonidal cysts (OR 4.9), psoriasis (OR 4.45), pyoderma gangrenosum (OR 3.2), alopecia areata (1.35), and vitiligo (1.2). They also reported an increased risk for metabolic diseases, such as type 2 diabetes (OR 1.82), arterial hypertension (OR 1.2), and dyslipidemia (OR 1.36) [15].

Conclusions

Hidradenitis suppurativa is a complex dermatological disorder with a worldwide distribution. The age of onset, gender, clinical classification, and anatomical location is globally similar with very few exceptions. The identification of risk factors and comorbidities associated has allowed for a better understanding of the disease and thus a more efficacious treatment.