Update on Hidradenitis Suppurativa
Hidradenitis suppurativa (HS) is a chronic disease presenting with recurrent, painful nodules and abscesses that heal with scarring, sinus tract formation, and fistulae (Figure 1). Estimates of prevalence range from 0.053% to 4.1%.1,2 However, the disease is likely underreported, because patients feel embarrassed and are hesitant to seek medical help.3 HS is often misdiagnosed as furunculosis or “boils.”4 Although the pathogenesis is not fully understood, the prevalent theory posits that HS is caused by occlusion of the terminal portion of the hair follicle.5 HS is not caused by poor hygiene and is not contagious.
HS is also known as acne inversa, because it has a predilection for the intertriginous or inverse areas, classically the axillae, groin, inguinal folds, inner thighs, mammary and inframammary areas, and buttocks.6 However, the term acne inversa is a misnomer, because HS is not a type of acne. Although hyperkeratosis of the hair follicle occurs in both conditions, HS involves the deep part of the follicle rather than superficial part, and there is no hyperseborrhea or change in density or activity of the sebaceous glands.7 HS is psychologically and physically debilitating, given that the lesions are painful and that patients are embarrassed by the location of the lesions and the chronic, malodorous drainage.3
The diagnosis of HS is based on clinical appearance and history. Skin biopsy is rarely needed for diagnosis but may be required if there is suspicion for malignancy.8 Criteria for diagnosis include classic localization of disease (axillae and groin), a chronic, relapsing course, and the presence of deep-seated, painful nodules and abscesses with or without sinus tracts and scarring.7 When considering the diagnosis of HS, the differential diagnosis should also include granuloma inguinale, Crohn disease, acne, actinomycosis, inflamed epidermal cysts, and staphylococcal abscesses.
Onset of disease occurs anytime after puberty until 40 years of age but is most common in the second and third decades of life. The disease may remit after menopause in women. HS affects women more often than men, with a 1 to 3 ratio.7 The axillae (most common location), groin, and inner thighs are affected in both men and women. Women are more likely to have upper torso involvement, whereas men more often have gluteal or perianal disease (Figure 2).9 Disease is common in both sexes at sites of increased friction, such as the beltline or bra strap.
The characteristic lesions begin as inflammatory, painful, deep nodules. Some nodules heal spontaneously, while others expand and merge with adjacent nodules to form coalescing lesions, which can rupture and drain seropurulent fluid.8 Drainage of fluid relieves the pain, but lesions usually recur. Longstanding HS results in the formation of scarring, contractures, and interconnected sinus tracts that may drain fluid. Scarring can range from small, atrophic scars to thick, fibrotic tracks.4 Large, multiheaded, open comedones are the end stage of damage and represent loss of the sebaceous gland and hair follicle.10
Longstanding HS is associated with a number of complications. Squamous cell carcinoma (SCC) can develop within scars in patients who have had the disease an average of 25 years.5 Case reports have attributed a number of deaths of HS patients to metastatic SCC. It is hypothesized that the constant inflammation of HS results in metaplasia and carcinoma formation, which can be difficult to identify against the background of scarring and sinus tracts.6 Extensive scarring in the groin can cause lymphatic obstruction and vulvar or scrotal lymphedema (Figure 3).11,12 Strictures and contractures may limit movement.5 Anemia has also been noted in a number of patients with HS.13 Osteomyelitis and epidural abscesses are the most serious consequences of secondary bacterial infection of the skin lesions.14,15
HS ranges widely in severity. There is no consensus for a grading system, but the Hurley staging system is most commonly used. The Hurley system divides HS into 3 stages16:
Stage 1: Single or multiple abscesses without sinus tract formation or scarring (Figure 4). Stage 1 disease is most common.
Stage 2: Recurrent abscesses with one or more sinus tracts and scarring widely separated by normal skin.
Stage 3: Diffuse involvement with multiple sinus tracts and no intervening normal skin (Figure 5).
A different classification system relies on disease phenotype and includes axillary-mammary, follicular, and gluteal disease patterns. Axillary-mammary disease describes the classic presentation of HS with lesions in the axillae and mammary and inframammary regions with hypertrophic scarring. Disease is most common in women and patients with a higher body mass index (BMI). Patients with the gluteal disease pattern are more likely to be men, current smokers, and have a lower BMI. This pattern is associated with less severe disease presenting with gluteal involvement, papules, and folliculitis. Patients with the follicular pattern of disease have severe and early onset disease. They often have a family history of HS and present with acne, pilonidal sinuses, and comedones. Lesions are more frequently located in atypical areas such as the ears, chest, legs, and back. The follicular pattern is more common in men and current or former smokers.17
Several other grading systems have been designed, but they are mainly used as research tools. These systems include the modified HS Lesion, Area, and Severity Index, the HS Severity Index, and the Sartorius score.6
Follicular occlusion. Research into the pathogenesis of HS is ongoing. As suggested by its name, HS was originally believed to be a disorder of the apocrine (hidro-) glands (aden-), because disease occurs in apocrine gland-bearing areas. Apocrine glands, which are concentrated in the axillae, groin, buttocks, and inframammary regions, empty into the follicular canal above the entrance of the sebaceous gland. In comparison, eccrine sweat glands are present throughout the entire body and open directly onto the skin surface.3 However, research has disproved this theory and instead has demonstrated that occlusion of the terminal hair follicle is the key event in disease pathogenesis.18
HS is part of the follicular occlusion tetrad, which consists of HS, acne conglobata, pilonidal sinus, and dissecting cellulitis of the scalp. Patients may simultaneously present with 2 or more of the diseases within the tetrad.5 In HS, hyperplasia of the follicular epithelium causes follicular hyperkeratosis and follicle plugging. The follicle dilates and expands, releasing antigens that stimulate the immune system, causing a lymphocytic perifolliculitis.19 If repair does not occur, the follicle may rupture secondary to mechanical stress on the skin.5 Rupture of the follicle releases bacteria, sebum, and hair follicles into the connective tissue, which results in further inflammation.7
Infection. Bacterial infection does not cause the formation of HS lesions but is instead a secondary feature of the disease.7 Studies have failed to isolate a single dominant organism in cultures from the surfaces of the lesions. Cultures from deeper aspiration of lesions were frequently sterile (51% of cultures), with Staphylococcus epidermidis and Staphylococcus aureus otherwise most frequently isolated.3 Treatment with antibiotics does not cure the disease but often results in temporary improvement. Infection further increases inflammation, which contributes to scarring and sinus tract formation.
Genetics. HS can be familial or sporadic, with one-third of patients reporting a relative with the disease.4 A familial form of HS has been described with an autosomal dominant mode of transmission. Incomplete penetrance and hormonal factors explain why fewer than 50% of patients report having a first-degree relative with HS.20 A single culprit gene has not been identified.5
Patient characteristics. A clear association exists between obesity and HS.6 In one study, 69% of women were overweight (33% obese) and 77% of men were overweight (26% obese).21 Obesity is not causative but contributes to and exacerbates the disease through several mechanisms, including increased areas of friction between body folds, relative androgen excess, and irritation from sweat retention.3,5 In addition to obesity, HS patients have a higher prevalence of metabolic syndrome compared with controls, with increased rates of hypertriglyceridemia and insulin resistance.22
Smoking is also closely linked to HS. Smoking was significantly more likely in patients with HS (88.9%) than controls (46%).23 Within a subset of patients suffering from HS, disease was more severe in smokers compared with nonsmokers.7 It has been proposed that nicotine may release toxins into sweat and alter neutrophilic granulocytes and sweat gland activity, contributing to follicular plugging and inflammation.5
Literature reviews have revealed that a number of other diseases are commonly comorbid with HS, including inflammatory bowel disease (mainly Crohn disease), spondyloarthropathy, SCC, and genetic keratin disorders (eg, paronychia congenita).6 Crohn disease may present similarly to HS with perianal fistulae and sinus tracts. The diagnosis is particularly challenging if skin disease precedes gastrointestinal tract symptoms.
Hormones. There is conflicting evidence about the role of sex hormones in HS. An association with acne vulgaris and hirsutism suggests that androgens contribute to disease. Moreover, postpubertal onset, premenstrual flares, and improvement during pregnancy and after menopause argue for a role of hormones in disease pathogenesis. However, other studies have found normal androgen profiles in women with HS. Additionally, androgens principally affect the sebaceous glands, which have not been implicated in the development of HS.3 Further research is needed to clarify the role of hormones in HS.
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