Two things are left behind after inflammatory acne settles, they look similar at arm's length, and they need opposite advice. Post-inflammatory hyperpigmentation is melanin in the epidermis or dermis. Post-inflammatory erythema is dilated vessels. Patients describe both as 'the marks', and so do a lot of prescriptions.
The bedside test takes two seconds. Press the lesion with a glass slide or a fingertip, or stretch the skin: erythema blanches and refills, pigment does not. Dermoscopy makes the same call — a vascular pattern against a brown structureless or pseudo-network pattern. Wood's lamp adds depth for pigment: epidermal melanin sharpens under it, dermal melanin does not.
The advice then diverges. Pigment responds to sun protection, topical retinoid, azelaic acid, and tyrosinase inhibitors, and it takes months. Erythema does not respond to any of them; it settles on its own over weeks to months, and where it persists the answer is vascular — pulsed dye or potassium titanyl phosphate laser — not a lightening cream. Treating erythema with a hydroquinone-containing combination is a common way to produce irritation, then post-inflammatory hyperpigmentation on top of the erythema, which is precisely the outcome the patient came to avoid.
In darker skin the two frequently coexist on the same face, so say which mark you are describing when you write the plan.
- Blanch-test every residual mark before prescribing — erythema blanches, pigment does not
- Use Wood's lamp to judge epidermal versus dermal pigment when planning depigmenting treatment
- Do not prescribe lightening agents for post-inflammatory erythema; they irritate and can make things worse
- Give a timeline: weeks to months for erythema, several months for pigment, and photoprotection for both
- Treat the active acne first — new lesions keep producing new marks whatever you apply to the old ones
Why it matters
The two residual marks of acne look alike and need opposite treatment, and getting it wrong creates the pigmentation the patient came about.
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