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
Mechanical irritation such as by friction from tight clothing or shaving is often reported as a trigger.
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. A less frequent association to keep in mind is increased risk of 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.