Post-inflammatory erythema (PIE) is a relatively new term used to describe the erythema (redness) that occurs after inflammatory acne. It is considered a form of acne scarring but is different from post-inflammatory hyperpigmentation (PIH), atrophic scars, and hypertrophic scars.
Acne is caused by multiple overlapping factors and is primarily an inflammatory disease. When a hair follicle (pore) becomes clogged, oil accumulates inside which provides an ideal breeding ground for the p-acnes bacteria. This causes in an immune response and results in inflammation.
Inflammation begins with the clogged pore, continues throughout the acne lesions life-cycle, and remains even after the acne lesion has cleared. Post-acne inflammation is visible in individuals with dark skin as brown marks (hyperpigmentation) and in individuals with fair skin as red marks (erythema).