Hidradenitis suppurativa (HS), or acne inversa, is a chronic destructive inflammatory disorder of the terminal follicular epithelium. While the pathogenesis is unclear, it is thought that follicular rupture initiates interaction between the follicular microbiome and innate immune system, triggering an inflammatory response. HS is more common in women, adults between the third and fourth decades of life, Black individuals, and the socioeconomically disadvantaged. With a prevalence of up to 1% in some population-based studies, HS is a common disease.

The diagnosis is clinical and based on 3 criteria:

• Typical lesions: nodules, abscesses, and tunnels (previously referred to as sinuses and fistulae)
• Recurrence: more than 2 lesions over a 6-month period
• Typical locations: primarily intertriginous

HS is seen most commonly on the buttocks, breasts, groin, and axillae. Usually, the onset of the disease occurs soon after puberty, and patients typically report recurring "boils." Pain is consistently reported as the most bothersome symptom, but pruritus, drainage, odor, fatigue, and arthralgias are frequent additional concerns.

Mechanical irritation such as by friction from tight clothing or shaving is often reported as a trigger. For many women, the week before menses can trigger disease flares, and pregnancy and the postpartum period can be associated with either disease improvement or flaring.

Obesity and cigarette smoking are associated with HS severity but are unlikely to be the main trigger of disease in most patients. Prevalence of metabolic syndrome, major cardiovascular events, cardiac death, and diabetes (type 1 or type 2) are increased in HS compared to the general populations. Some of the most frequently associated comorbidities are related to mental health, and depression, anxiety, and risk of suicide are major burdens for the population. Regional ileitis (Crohn disease) has a statistical association with HS, while ulcerative colitis does not.

HS is often accompanied by the other disorders of the "follicular occlusion tetrad," which includes acne conglobata, dissecting cellulitis, and pilonidal sinus. Less frequent associations to keep in mind are increased risk of polycystic ovarian syndrome and lymphoma. People with Down syndrome are known to be at increased risk of HS.

A positive family history is reported in 30%-50% of patients in recent cohorts, and familial risk studies indicate a 20-fold risk of disease development for those with an affected first-degree relative. Pathogenic variants affecting the gamma-secretase complex genes PSENEN, PSEN1, and NCSTN have been reported in familial cases.

Associated syndromes include PASH (pyoderma gangrenosum, acne, and hidradenitis suppurativa), PAPASH (pyogenic arthritis, pyoderma gangrenosum, acne, and hidradenitis suppurativa), and PsAPASH (psoriatic arthritis, pyoderma gangrenosum, acne, and hidradenitis suppurativa), and these may in some cases be associated with mutations in PSTPIP1.