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.

Table 4.1

Differential Diagnosis of Early Papulopustular and Nodular Hidradenitis Suppurativa Lesions

Adapted from Saunte DM, Jemec GBE. Hidradenitis suppurativa: advances in diagnosis and treatment. JAMA. 2017;318(20):2019–2032.

Condition	Clinical Presentation	Differentiating Features from HS	Associated with HS
Infectious
Bacterial folliculitis and furunculosis	Pustules, nodules, abscesses, purulent drainage	• Culture: Staphylococcus	No
Candidal folliculitis	Pustules in intertriginous areas	• KOH, culture Candida	No
Cellulitis and erysipelas	Painful localized diffuse dermal/subcutaneous redness, swelling	• Preceding trauma • Possible systemic symptoms • Tends to be unilateral	No
Gram-negative folliculitis	Pustules, papules in acne areas	• History of antibiotic therapy • Culture: gram-negative organisms	No
Herpes folliculitis	Tender vesicles, pustules	• Self-limited • Culture: Herpesvirus	No
Hot tub folliculitis	Pustules on trunk, limbs	• History of hot tub use • Culture: Pseudomonas	No
Perirectal/ischiorectal abscess	Perianal pain, swelling	• Acute onset • Culture: Aerobic or anaerobic bacteria	No
Pityrosporum folliculitis	Pustules on head, upper trunk	• Pruritic • Scraping: Malassezia, pityrosporum	No
Sexually transmitted diseases • Chancroid • Granuloma inguinale • Lymphogranuloma venereum • Noduloulcerative syphilis	Nodules, ulcers, draining abscesses, lymphadenopathy Late—possible scarring, lymphedema	• Biopsy, culture, serologies: organism-specific changes	No
Subcutaneous mycoses	Papules, nodules, abscesses, ecthyma-like lesions	• History of immunosuppression • Possible systemic symptoms • Biopsy, tissue culture: organism-specific changes	No
Sycosis barbae	Indurated plaque studded with pustules in beard area	• Contact with animals if fungal • Culture positive for bacteria ( Staphylococcus aureus ) or dermatophyte fungus ( Trichophyton verrucosum or Epidermophyton mentagrophytes most commonly)	No
Inflammatory
Acne	Papules, pustules, nodules, cysts, abscesses, scars, comedones on face and upper trunk	• Distribution on face, upper trunk, sparing flexures • Single comedones • Onset early adolescence • Usual good response to isotretinoin	Yes
Acne keloidalis nuchae	Follicular pustules, keloid scarring on nuchal scalp	• Localization to nuchal scalp • Absence of comedones	Yes
Disseminate recurrent folliculitis	Pustules, comedones, follicular scarring especially on buttocks	• Involvement of extensor trunk, limbs more than flexures	Yes
Drug-induced folliculitis	Papules, pustules on head/neck and trunk	• Corticosteroids • Lithium • EGFR inhibitors • Vemurafinib/dabrafenib	No
Frictional folliculitis	Pustules on opposing skin surfaces	• Superficial, no comedones • Culture: normal flora or Staphylococcus	No
Pseudofolliculitis	Follicular pustules, nodules, abscesses	• Presence of embedded hair • History of hair removal	No
Cysts
Bartholin’s cyst	Unilateral vulvar swelling, pain	• Solitary • Localized to posterolateral introitus	No
Epidermoid cyst	Cystic nodule, often with a visible punctum	• Typically solitary • Contains whitish keratinous debris • Not localized to intertriginous areas	Yes
Pilonidal cyst/sinus	Cystic nodule, sinus, possible drainage	• Solitary • Typical location in superior midline gluteal cleft	Yes
Steatocystoma multiplex	Cystic nodules	• Involvement of both extensor and flexural areas • Rarely inflammatory	No
Neoplastic
Langerhans cell histiocytosis	Inflammatory papules and ulcers in inguinal, genital region	• Onset in childhood usually • Biopsy: CD1a, S-100, and Langerin (CD207) positive • Birbeck granules on EM	No

CD, Cluster of differentiation; EGFR, epidermal growth factor receptor; EM, electron microscopy; HS , hidradenitis suppurativa ; KOH, potassium hydroxide; PCR, polymerase chain reaction.

Table 4.2

Differential Diagnosis of Late Hidradenitis Suppurativa Lesions Including Scars and Sinus Tracts

Adapted from Saunte DM, Jemec GBE. Hidradenitis suppurativa: advances in diagnosis and treatment. JAMA . 2017;318(20):2019–2032.

Condition	Clinical Presentation	Differentiating Features from HS	Associated with HS
Infectious
Actinomycosis	Draining fistulae, sinus tracts	• Sites of trauma • Culture: Actinomyces • Pathology: Clumped bacteria, sulfur granules.	No
Atypical mycobacterial infection	Indurated, ulcerated plaques with exudate	• Sites of trauma • Pathology: Intracellular bacteria; AFB and PAS stain positive • Tissue culture: Organism-specific such as M. marinum , M. kansasi , M. ulcerans • Positive mycobacterial PCR	No
Blastomycosis	Pustules, sinus tracts, purulent drainage	• History of trauma • Favors extensor surfaces	No
Cat scratch disease	Papulopustular, suppurating lesions, +/- regional adenopathy	• History of scratch/bite • Culture: Bartonella	No
Cutaneous tuberculosis (lupus vulgaris)	Indurated plaques, gelatinous nodules, scarring	• Pathology: Intracellular bacteria, positive with AFB and PAS stains • Positive TB skin test, serology	No
Cutaneous tuberculosis (scrofuloderma)	Nodules, draining, ulcerating abscesses over lymph nodes and bone	• Painless • Presence of pulmonary TB • Positive TB skin test, serology	No
Nocardiosis	Draining fistulae, sinus tracts	• Culture: Nocardia • Pathology: Filamentous bacteria, AFB negative, Fite stain positive	No
Inflammatory
Cutaneous Crohn’s disease	Nodules, ulcers in perianal/genital region, characteristic “knife-cut” ulcers	• Absence of comedones • Pelvic MRI: Fistulae in CD may communicate with anal sphincter more commonly than in HS • Thickening of bowel wall in CD • Presence of epithelioid granulomas on pathology	Yes
Dissecting cellulitis	Pustules, nodules, sinus tracts, and scarring on scalp	• Localized to scalp	Yes
Folliculitis decalvans	Follicular-based pustules and scarring with tufted hair usually confined to the scalp	• Culture: Staphylococcus aureus • Absence of comedones	Yes
Pyoderma gangrenosum	Pustules, nodules, ulceration in sites of trauma; may heal with cribriform scarring	• Pathergy response • Often in sites of trauma • Vasculitis may be seen on pathology	Yes
Neoplasia
Squamous cell carcinoma	Indurated nodule/plaque, ulceration	• Pathology: nests of malignant epithelial cells extending into the dermis consistent with squamous cell carcinoma	Yes
Other
Lymphedema due to infection, obesity, neoplasia, congenital lymphedema	Scrotal, penile, vulvar lymphedema	• Absence of typical lesions of HS	Yes