How to distinguish scalp psoriasis from seborrheic dermatitis?
Scalp seborrheic dermatitis and scalp psoriasis are the most difficult to distinguish clinically. They can be generally differentiated by the table below. If it is difficult to determine one from another clinically, a “sebopsoriasis” diagnosis will be temporary made due to the overlapping features between them. Dermatologists will wait and see if there are more typical lesions developed on other sites later on.
How to diagnose psoriasis?
The detective spirit: Don’t forget to check the hairline, ears, genital area, umbilicus, intergluteal folds, and nails.
The interesting part of being a dermatologist is that it is like being a detective who make a diagnosis based on two things which are quite challenging sometimes.
- Medical history
- Clinical findings
Clinically, differential diagnosis between psoriasis and many diseases such as seborrheic dermatitis or chronic dermatitis is required. However, psoriasis is much like a clumsy robber who leaves many clues waiting for the dermatologists to uncover.
Typical distribution sites (blue color) of plaque-type psoriasis are the scalp, the extremities (prone to injury or friction), and lower back. However, if psoriatic lesion is nowhere to be found at these sites, we may look for clues of psoriasis lesions from the hairline, ears, genital area, umbilicus, intergluteal folds, and nails (red color).
Nail manifestations are subdivided into two categories:
- Nail matrix lesions: red spots in lunula, leukonychia, pitting, or nail plate crumbing
- Nail bed lesions : onycholysis, subungual hyperkeratosis, splinter hemorrhages, or oil drops.
Pathologic examination shows the incomplete keratinization (parakeratosis) of the stratum corneum, when we use the wooden end of a swab to scrape a lesion, it is like scraping a candle where silvery white dander continues to fall off (candle sign). Additionally, due to the dilated capillaries of the papillary dermis and the thin epidermis layer above it, as we continue to scrape harder, we will see punctate bleeding spots (Auspitz signs). This is why when patients suspected of having psoriasis arrive in my clinic, I will take a cotton swab for scrapping.
Another special part about psoriasis is that it is found on a skin injury site. When it is difficult to determine clinically, the dermatologist’s ultimate secret weapon is skin biopsy.
The left picture below is a sunburned psoriasis patient who had psoriasis shows on the place where doesn’t cover by clothes . The right picture is a patient who had a skin biopsy. A dry sore grows at the slice position.
Author: Dr. Wan-Yi Chou
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