What is Folliculitis?
Folliculitis is an inflammatory reaction in the superficial aspect of the hair follicle that can involve the follicular opening or the perifollicular hair follicles, presenting clinically as 1mm-wide vesicles, pustules, or papulopustules in acute cases. 1
Anatomical Classification
The pilosebaceous unit is divided into three compartments that help classify folliculitis 1:
- Infundibulum (superficial part, outlined by the sebaceous duct)
- Isthmus (between the sebaceous duct and arrector pili protuberance)
- Inferior segment (stem and hair bulb)
Most follicular conditions can be classified according to their anatomical location and histopathologic patterns. 1
Clinical Presentation
Acute folliculitis manifests as small (1mm) vesicles, pustules, or papulopustules, while chronic folliculitis shows hyperkeratosis and keratotic plug formations. 1
The presence of superficial pustules does not always indicate an infectious origin, as there are many noninfectious types of folliculitis. 1
Distinction from Deeper Infections
Folliculitis differs from furuncles in that inflammation is more superficial and pus is limited to the epidermis. 2 In contrast, furuncles (boils) are infections where suppuration extends through the dermis into the subcutaneous tissue, forming a small abscess. 2
Etiologic Categories
Folliculitis can be classified into three main groups based on histologic findings 3:
- Infectious folliculitis (bacterial, viral, fungal)
- Noninfectious folliculitis (folliculitis decalvans, eosinophilic pustular folliculitis, lichen planopilaris)
- Perifolliculitis
Microbial Colonization
In patients with clinically diagnosed non-infectious folliculitis, bacterial colonization is found in approximately 26% of inflamed follicles, with Staphylococcus species predominating (18% of cases). 4 Fungal colonization occurs in approximately 8% of cases. 4 Both Staphylococcus species and Propionibacterium acnes can form macrocolonies/biofilm structures within follicles. 4
Common Pitfalls
The diagnosis of folliculitis occasionally requires histologic confirmation and cannot be based solely on clinical appearance of scalp lesions, particularly when differentiating between infectious and noninfectious variants. 3 Misdiagnosis can occur when assuming all pustular follicular lesions are infectious in nature, when in fact many represent inflammatory reactions without microbial involvement. 1