Hidradenitis Suppurativa Differential Diagnosis and Mimickers

Introduction

The average length of time from onset to diagnosis in hidradenitis suppurativa (HS) patients worldwide is 7.2 years. This unfortunate delay is partly because of the nonspecific nature of many HS lesions—inflammatory papules, nodules, pustules, abscesses, and scarring—which may be confused with a variety of other cutaneous diseases. The diagnosis may also be confounded by coexistence or overlap with other inflammatory diseases presenting with similar morphology. Although there are a number of conditions that can mimic HS in their clinical appearance, HS can usually be accurately diagnosed on the basis of its chronic and recurrent history, typical anatomic locations, nonspecific bacterial culture results, and lack of systemic signs and symptoms.

However, a high index of suspicion is sometimes needed to make an accurate diagnosis, especially in early, localized disease. It is important to note that the differential diagnosis may differ for early HS compared to late HS, and for adult HS compared to pediatric HS. Overall, the differential diagnosis of HS includes both follicular and non-follicular conditions of infectious, inflammatory, and neoplastic origin.

Table 4.1 highlights the differential diagnosis for early HS lesions, and Table 4.2 the differential diagnosis for late HS lesions.

Infections

Acute lesions of HS are most often misdiagnosed as infections due to the presence of acute inflammation and significant pain. Therefore, infectious diseases must always be considered in the differential diagnosis, especially early in the disease when there are few lesions, and during acute flares of localized HS. Bacterial culture in HS usually reveals normal flora, albeit an altered microbiome compared to normal skin. Infections usually respond more rapidly and completely to antibiotic therapy than does HS. Cultures to rule out pathogenic or resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) and gram-negative bacteria should be considered in HS patients with unusually inflamed or painful lesions, or with systemic symptoms of infection.

Superficial cutaneous infections such as bacterial folliculitis ( Fig. 4.1 ) and furunculosis are common in the general population, may be localized to the flexures, and may be recurrent, mimicking HS; however, bacterial culture usually reveals pathogenic Staphylococcus aureus bacteria. The atypical HS follicular phenotype may be particularly difficult to distinguish clinically from infectious folliculitis, although the presence of numerous comedones, including double-headed comedones, and a spectrum of normal flora on culture may help indicate a diagnosis of HS. Superficial folliculitis may also be caused by normal skin microbes, including pityrosporum (Malassezia) and demodex mites, as well as common organisms such as herpes simplex and pseudomonas aeruginosa (hot tub folliculitis). Gram-negative folliculitis may occur as a result of long-term tetracycline antibiotic therapy for acne.

Fungal folliculitis may also resemble early HS. Candidal folliculitis is common in intertriginous areas, especially in diabetics or following antibiotic therapy. Follicular dermatophyte infection (Majocchi granuloma) presents as pustules/papules, nodules, or indurated plaques usually in areas of shaving or other superficial trauma in otherwise healthy individuals. Although it traditionally involves the lower legs of women after shaving, an area not commonly affected by HS, in recent years it has been reported following pubic hair removal by shaving and waxing, and may also be transmitted through sexual contact, with organisms varying depending on the geographic locale of acquisition. Deep dermatophytosis, presenting as dermal and subcutaneous nodules, usually in association with superficial dermatophyte infection, has been reported in immunosuppressed patients. Widespread infections due to candida and opportunistic fungal species (such as mucor, fusarium) may occur in immunocompromised patients, manifesting as cutaneous papules, nodules, and necrotic lesions. Biopsy and tissue culture are necessary to confirm the diagnosis.

Drug-induced folliculitis is commonly seen with systemic corticosteroid therapy, lithium, and increasingly with novel chemotherapeutic agents such as epidermal growth factor receptor (EGFR) inhibitors, and targeted therapies such as vemurafenib and dabrafenib.

Deeper cutaneous infections in typical HS locations, such as erysipelas and cellulitis, and perirectal/perineal abscess are usually due to common bacteria such as Staphylococcus aureus and Streptococcus . Patients residing in certain geographic areas, travelers, and immunosuppressed patients may acquire less common infections that may resemble HS, such as nocardiosis, actinomycosis, blastomycosis ( Fig. 4.2 ), cat scratch disease, and cutaneous tuberculosis. All may present with inflammatory lesions suggestive of HS; however, they often occur on exposed areas of skin that are subject to trauma. History of travel, previous trauma, and diagnostic work-up including biopsy for histopathologic examination and tissue culture should reveal the proper diagnosis.

Sexually transmitted diseases such as granuloma inguinale, lymphogranuloma venereum, and noduloulcerative syphilis may present with inflammatory or ulcerative anogenital lesions that resemble active HS ( Fig. 4.3 ). Granuloma inguinale may present as ulcerative nodular lesions or linear fissures. The absence of other signs and symptoms of HS, a detailed history of possible exposure, and serologic screening are essential to make the correct diagnosis. The coexistence of a sexually transmitted infection in a patient with HS should also be considered and patients who have an established diagnosis of HS but present with any unusual lesions should be screened appropriately.

Inflammatory Disorders

A number of inflammatory disorders, including autoinflammatory conditions, may present with lesions similar to those seen in HS. These include both follicular disorders (such as acne, folliculitis decalvans) and non-follicular disorders (such as pyoderma gangrenosum [PG], cutaneous Crohn’s disease).