The clinical manifestations of HS are highly heterogeneous, but the disease tends to manifest with deep, painful, inflammatory lesions, including nodules, fistulas, and abscesses53 (Fig. 2 A-C). Double comedones are another characteristic finding and have been described as possible precursors of HS lesions in the skinfolds of children.54

Figure 2.

Grade III hidradenitis suppurativa in a 36-year-old man. A, Indurated, painful linear lesions in the armpit. B, Inflammatory nodules and abscesses leaking pus on the buttocks. B, Confluent abscesses in the right groin area.

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Flares are associated with increased pain and oozing of pus and are common in women just before menstruation (40% of cases). They tend to spontaneously improve without treatment in about 7 to 10 days.55

HS generally appears in the second to third decade of life, although cases of early onset have been described in children.56 Early onset has been associated with greater involvement of lesions and increased genetic susceptibility, with a family history reported in 55% of patients with early-onset HS, compared with in 34% of patients with postpubertal HS.57,58

The most common sites of involvement are the axillae, the groin, the buttocks, the perianal and perineal areas, and the mammary and inframammary areas. Location of disease varies according to sex. While inframammary, axillary, and inguinal lesions are more common in women, lesions on the buttocks, in the perianal region, and at atypical sites (e.g. nuchal scalp and retroauricular areas) are more common in men.58–60 In 2013, Canoui-Poitrine et al.13 identified 3 phenotypes of HS, which they called LC1 (axillary-mammary class), LC2 (follicular class), and LC3 (gluteal class). LC1 patients typically have axillary and mammary lesions and a higher prevalence of hypertrophic scars,while LC2 patients typically have lesions on the ears, chest, back, and legs, in addition to follicular lesions (pilonidal sinus and comedones), severe acne, and a family history of HS. This phenotype was found to be more common in men and smokers, and was also linked to greater disease severity. Finally, LC3 patients tend to have gluteal lesions, papules, and folliculitis, as well as a lower prevalence of obesity and less severe disease.61 A new phenotype—HS fulminans—was proposed more recently, and reported to be more common in Afro-Caribbean men and to occur in association with rheumatological symptoms (arthritis and/or spondylitis) and anemia, in the absence of increased body mass index.62 It has been suggested that a phenotype-based classification could be of help for personalized therapy in HS.63

Although HS was initially considered to be a disease of the apocrine glands, it is currently defined as a follicular disease.64,65 Von Laffert et al.,66 on investigating the histologic findings for surgically excised HS lesions from 60 patients, reported follicular hyperkeratosis in 82% of cases, hyperplasia of follicular epithelium in 77%, and perifolliculitis in 68%. These 3 features would appear to correspond to the phase preceding rupture of the follicle structure. Other relevant features included a subepidermal interfollicular inflammatory infiltrate (78%) and epidermal psoriasiform hyperplasia with rete ridges of a similar length (58%). These last 2 structures were both present in 36% of the specimens studied.

In the same study, immunohistochemical staining of the follicular and subepidermal inflammatory infiltrate showed a very similar mixture of inflammatory cells, consisting of lymphocytes, neutrophils, plasma cells, and histiocytes. Another noteworthy finding is the presence of CD8+ lymphocytes with striking follicular and epidermal epitheliotropism.67 Finally, increased mast cells have been described in the dermis of early lesions and perilesional skin, possibly explaining, at least in part, why so many patients with HS report itching.68

A characteristic histologic finding associated with more developed lesions is the presence of fistulas with stratified squamous epithelium surrounded by fibrosis and inflammation.69 No differences have been found between patients with HS and controls in terms of the expression of estrogen or androgen receptors in the apocrine glands.70

Finally, it is important to note that squamous cell carcinomas may arise in HS lesions, typically in men with severe, long-standing disease with gluteal and perineal involvement. Although these carcinomas have a good histologic prognosis (well-differentiated or verrucous carcinomas), they are clinically aggressive (5-year survival, 61%) and are frequently associated with high-risk human papillomavirus infection.71

The Hurley Staging system, proposed by Hurley in 1989, was the first classification model described for HS. It distinguishes between 3 levels of disease severity (Table 3).72–74 It is widely used because of its simplicity and speed, but it does have some limitations, including its qualitative, static nature. In other words, it does not take into account the number of body sites affected or the number of lesions at each site. It also contemplates certain fixed or invariable characteristics, such as scars and fistulas, and as such is not very useful for assessing treatment response.

Years after publication of the Hurley Staging system, Sartorius developed a new, more detailed, system for assessing disease severity in HS. This system was subsequently modified by both himself (modified Sartorius score, Table 4)35,74,75 and Revuz (Sartorius score modified by Revuz).76

In the original scoring system, each area affected by HS is evaluated separately, with different points assigned depending on the type of lesion present (abscess, draining fistula, nondraining fistula, inflammatory nodule, noninflammatory nodule, hypertrophic scar), the longest distance between 2 relevant lesions, and the presence of lesions separated by normal skin. An overall score is calculated by adding the total number of points.74,75

The modified Sartorius score is a simplified version that places more emphasis on the presence of inflammatory lesions in an attempt to make the system more useful for assessing treatment response (Table 4).76

Like the original system, the modified Sartorius score also takes into account the body areas affected, the number and type of lesions in each area, the distance between the 2 most relevant lesions, and the presence of normal skin separating these lesions. It also, however, takes into account the number of inflammatory lesions (nodules and fistulas) in 3 locations (axillae, groin, and buttocks). The system produces an overall score and a score per body area.76 Other recommendations include assessing pain using a VAS and considering the number of boils reported by the patient for the preceding month.76

Interobserver variability is low for the modified Sartorius score, and scores have been found to correlate positively with the presence of risk factors and other measures of severity, such as the Dermatology Life Quality Index.77 The use of this system, however, is limited in severe cases in which lesions that were originally separate eventually coalesce.77 While the modified Sartorius score is more dynamic than the Hurley Staging System, it also includes lesion characteristics that rarely vary in response to medical treatment (e.g., distance between 2 relevant lesions).74 A further limitation is that it does not assess the inframammary region as a separate region.

The modified system proposed by Revuz76 is similar, but assesses the perineal region, the inframammary and intermammary folds, and hypertrophic scars.

The HS-PGA is one of the most widely used classification models to assess response to medical treatment in clinical trials.

It classifies disease severity into categories based on the number of abscesses, fistulas, inflammatory nodules, and noninflammatory nodules in all areas.78 The latest version of the HS-PGA classifies disease severity into 6 levels78 (Table 5).

It is dynamic, simple, and quick to use, and is suitable for monitoring disease course. Its limitation is that it does not consider individual areas separately.

Rather than a classification model, the HiSCR is a measure for assessing response to medical treatment. It was recently validated and is designed to quantify disease severity and establish a meaningful clinical endpoint.79

The HiSCR is defined as a reduction of 50% of more in inflammatory lesion count (sum of abscesses and inflammatory nodules [AN]) and no increase in abscesses or draining fistulas when compared with baseline79 (Table 6).

The HiSCR therefore reflects a clinical endpoint based on total inflammatory lesion count in a patient with HS at a given moment. As a result, it allows for the calculation of percentage reductions in abscesses and inflammatory nodules with respect to baseline as follows: AN50 (50% reduction), AN75 (75% reduction), and AN100 (100% reduction).

The measure is particularly useful for assessing response to medical treatment, as it contemplates inflammatory lesions (not static lesions such as scars) and it is also quick to apply.

The Hidradenitis Suppurativa Severity Index includes objective and subjective categorical variables80 and has been used to assess the clinical efficacy of infliximab in 2 studies80,81 and of adalimumab in 1 study.82

The main ultrasound findings in HS include diffuse alteration of dermal echogenicity patterns, dermal thickening, dermal pseudocysts, widening of hair follicles, and identification of fluid collections and fistulous tracts (Figs. 3–5).83

Figure 3.

Hidradenitis suppurativa. A, Clinical image of inflammatory papular lesion. B, Ultrasound image of inflammatory pseudocyst.

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Figure 4.

Hidradenitis suppurativa. A, Clinical image of apparently insignificant nodular lesion. B, Ultrasound image showing a large underlying fluid collection. C, High inflammatory activity evidenced by Doppler ultrasound.

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Figure 5.

Hidradenitis suppurativa. A, Long erythematous lesion in the right armpit. The red circle shows the clinically evident area. The blue lines show the affected area as seen by ultrasound. B, Ultrasound image showing an underlying fistulous tract. C, Mild inflammatory activity evidenced by Doppler ultrasound.

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The ultrasound findings corresponding to the different stages of clinical disease progression are described below. The earliest finding is hair follicle widening, which appears to have a key role in the development of HS.

The next stage consists of alteration of the dermal pattern (first perifollicular and then diffuse) and dermal thickening, which reflects the marked underlying inflammatory process, largely brought about by the action of varying innate immune system mediators. Accordingly, both degree of hypoechogenicity and the extent of the hypoechogenic area can provide important clues regarding the degree of underlying inflammation (Figs. 3–5).83

Patients with high inflammatory burden will have dermal pseudocysts (Fig. 3 A and B), which appear on ultrasound as round or oval hypoechoic or anechoic nodular structures. The next stage involves the development of fluid collections, which are seen on ultrasound as hypoechoic or anechoic fluid deposits in the dermis or hypodermis that are typically connected to the base of the altered hair follicle (Fig. 4 A-C).

The final stage is the development of fistulous tracts, seen as bands of hypoechoic or anechoic structures crossing through different structures located in the different layers of the dermis or hypodermis, and connected to the base of altered follicular structures (Fig. 5 A-C).

Using the ultrasound findings for HS lesions in 34 patients, Wortsman et al.83 developed a clinical-sonographic scoring system (SOS-HS) for staging HS (Table 7). Although the system has not yet been validated, it may be useful for follow-up and monitoring in this setting.