Primary Lesion of Hidradenitis Suppurativa
The primary lesion of hidradenitis suppurativa is follicular occlusion (plugging of the hair follicle), which subsequently leads to the clinical manifestations of painful inflammatory nodules, abscesses, and eventually sinus tracts and scarring.
Pathogenic Sequence
The disease begins at the microscopic level with follicular hyperkeratosis and occlusion of the upper portion of the folliculopilosebaceous unit 1. This is the initiating pathologic event that sets off the cascade of inflammation and tissue destruction characteristic of HS.
The Progression from Primary to Secondary Lesions
- Primary pathologic event: Follicular occlusion occurs in the apocrine gland-bearing areas
- Follicular rupture: The occluded follicle dilates and eventually ruptures 1
- Inflammatory response: Rupture triggers perifollicular lymphohistiocytic inflammation 2, 3
- Clinical manifestations: This inflammation presents clinically as the painful nodules and abscesses that patients experience 4
- Chronic changes: Over time, this leads to sinus tract formation, granulomas, fibrosis, and scarring 1
Histopathologic Confirmation
While HS is primarily a clinical diagnosis, histopathology reveals the primary lesion clearly 1:
- Follicular hyperkeratosis (excessive keratin production in the follicle)
- Follicular hyperplasia (thickening of the follicular epithelium)
- Follicular occlusion with associated spongiform infundibulofolliculitis
- Follicular dilatation and rupture (secondary changes)
Clinical Implications
Understanding that follicular occlusion is the primary lesion explains several key features of HS:
- Post-pubertal onset: Increased androgen activity affects follicular keratinization 5
- Location in apocrine-rich areas: These regions have dense follicular units susceptible to occlusion
- Genetic associations: γ-secretase gene mutations (NCSTN, PSEN1, PSENEN) can cause follicular occlusion in animal models 1
- Why antibiotics alone are insufficient: The primary defect is mechanical (follicular plugging), not purely infectious 1
Common Pitfall
Do not confuse the primary pathologic lesion (follicular occlusion) with the primary clinical lesion (painful inflammatory nodule). The follicular occlusion occurs first at the microscopic level, but patients present clinically with nodules and abscesses, which are actually secondary inflammatory responses to the follicular rupture 6, 5.
The diagnostic criteria require "typical lesions" which include painful nodules, abscesses, sinus tracts, bridged scars, or open comedones 1, 7, but these are all downstream consequences of the primary follicular occlusion event.