Introduction

Perianal fistulas (PF) are uncommon with an incidence varying from 1.04–2.23 patients per 10,000 persons per year [1]. PF are associated with considerable discomfort, morbidity, and decreased quality of life as patients can present with perianal pain, pruritus, bleeding, or purulent discharge [2]. Most frequently a PF is the result of a cryptoglandular anorectal abscess [3]. Other, less common causes comprise Crohn’s disease, surgery, or trauma [3]. However, a fistula in the perianal region may also be a sign of the underdiagnosed, chronic, auto-inflammatory skin disease hidradenitis suppurativa (HS). HS is characterized by recurrent abscesses, nodules, and draining fistulas in the intertriginous regions such as the axillae and inguinal areas but also the buttocks and perianal area. Anogenital involvement is fairly common and seen in 18.8% of HS patients [4]. The prevalence of HS in the European general population is approximately 1% with a male to female ratio of 1:3 [5]. The disease usually starts in the early twenties. Smoking and obesity are well-established risk factors for HS [5]. Even though HS is quite common and has a tremendous impact on the quality of life, it is still is relatively unknown for patients and physicians [6]. This results in a mean time to diagnosis of 7.2 years after consulting 3.9 health care professionals on average [6]. Since HS and PF can be clinically similar, the diagnosis of HS should be part of the differential diagnosis of perianal fistula. However, unfamiliarity with this diagnosis could mean that HS goes undiagnosed in a portion of the perianal fistula population. Therefore, the aim of this study was to evaluate the prevalence of HS in patients with PF.

Methods

All patients with a PF attending the specialized proctology clinic, Proctos, in Bilthoven, the Netherlands, between July and September 2017 were included. A validated diagnostic question for HS was used as a screening tool: “Have you ever had recurrent boils in your armpits, groins or on the buttocks except for the anus for the last six months?” [7]. Subsequently, all patients with a positive answer underwent physical examination of the axillary, inguinal, and perianal region to confirm the diagnosis of HS when nodules, abscesses, or sinus tracts were found. Subsequently, HS severity was classified according to the three-stage Hurley classification [5, 8]. As part of the routine care in the Proctos Clinic, all perianal fistulas were examined using a three-dimensional endoanal ultrasound and classified according to the Parks classification [9]. The Mann-Whitney U tests and Fisher’s exact tests were performed to analyze the difference between patients with perianal fistulas with and without HS. Data were analyzed by use of SPSS software (IBM, version 24.0). This type of study is exempt from medical ethical committee approval under Dutch law.

Results

A total of 122 patients with a PF were included. Twelve patients (9.8%) answered positively to the diagnostic HS question. Physical examination confirmed the diagnosis in eight of these patients, amounting to a prevalence of HS in patients with a perianal fistula of 6.6%. Four out of eight patients had not previously been diagnosed with HS. Patient characteristics of patients with HS are presented in Table 1. All patients with HS were classified as Hurley II severity due to the presence of the perianal fistula. Four patients with HS had a positive family history (first degree) of the disease. Skin regions affected by HS included the perianal (n = 8), inguinal (n = 4), gluteal (n = 3), and axillary (n = 2) areas. Three of these HS patients presented with a superficial fistula, two with a blind ending fistula, and two with a transsphincteric fistula. One patient had more than one type of fistula. Patients with HS were more frequently smokers compared with patients without HS, respectively, 62.5% and 23.6% (p = 0.03). Moreover, in four out of eight HS patients, a pilonidal sinus was present (Table 2).

Table 1 Characteristics of perianal fistula patients with HS
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Table 2 Patient characteristics perianal fistulas with or without HS
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Discussion

To our knowledge, this is the first study investigating the prevalence of HS in patients with PF. We found a prevalence of HS to be at least 6.6%. This estimate might be higher (9.8%) when incorporating the, possibly cases of mild, HS. Since mild HS can go in regression, it cannot be confirmed by physical examination. Four patients received a new diagnosis of HS, demonstrating the underdiagnoses of this disease. Early diagnosis of HS is important as it allows for early treatment with anti-inflammatory medication, which may reduce symptoms and delay of natural disease progression. Since the presumed prevalence of HS in the general population is 1%, we argue that there may be an association between HS and PF [6]. However, whether PF is the result of HS or that PF is a distinct entity present next to HS, with a similar pathogenic mechanism, remains unknown. HS sinus tracts usually do not penetrate the anal sphincter. We argue that the PF found in the majority of our patients represent a HS-related sinus tract rather than a separate entity. Interestingly, most patients that were found to have HS were male. Even though HS predominantly affects women, it is known that HS with anogenital involvement is most frequent in men [4]. Strengths of this study are the use of a validated HS diagnostic question in combination with physical examination in order to diagnose HS as the use of three-dimensional endoanal ultrasound for a more precise classification. Future research should investigate PF in HS patients versus non-HS patients using histology. In conclusion, we demonstrate an underdiagnosis of HS in patients presenting with perianal fistulas, emphasizing the importance of asking patients for HS symptomatology and subsequent physical examination of the intertriginous regions.