Transcript Related Guidelines

Differential Diagnosis of Psoriasis

Jennifer Soung, MD · University of California at Los Angeles

Disclosures

July 26, 2021

Key Takeaways:

  • Psoriasis is an inflammatory skin disease characterized by red, erythematous plaques with well-demarcated margins. Nearly any part of the body can be affected, but extensor surface areas and the scalp are frequently impacted

  • Because psoriasis can resemble several other skin conditions, careful differential diagnosis is important

  • Conditions to consider in the differential diagnosis of psoriasis include seborrheic dermatitis, lichen simplex chronicus, atopic dermatitis, superficial fungal infections, subacute cutaneous lupus erythematosus, pityriasis rubra pilaris, scabies, and T-cell lymphoma

  • Location, skin color, chronicity, and response to treatment are important clues in the differential diagnosis of psoriasis

This transcript has been edited for clarity.

The diagnosis of psoriasis is really a clinical diagnosis. It is a red, erythematous, well-demarcated plaque with a classic silvery scale. Psoriasis can affect any part of the body, but generally spares the central face. Classic areas include the scalp, elbows, and knees. 

There are several other inflammatory skin conditions that may look like psoriasis, so I’m going to cover some of the most common ones and give you a couple of pointers on how to distinguish among them. One of them is seborrheic dermatitis.[1] This is a condition that mostly affects the scalp, ears, and can affect the central face. Most of these plaques are a little bit thinner. They have thin, scaly, erythematous patches in areas like the eyebrows or the nasal labial folds, even chest, and I mentioned behind the ears. 

Another skin condition that may look like psoriasis sometimes is lichen simplex chronicus.[1] So, in fact, if a patient has been scratching their psoriasis for a long time, it can become lichenified. These are very thickened plaques but usually do not have the classic silvery scale, so tend to be a little more smooth or rough, but no silvery scale. 

Another skin condition in the differential diagnosis of psoriasis is atopic dermatitis.[1] It’s probably one of the most common diagnoses that my patients will ask me—“Is this eczema or is this psoriasis?” In atopic dermatitis, these patches and plaques tend to be more ill-defined and are rough, and do not have that coarse, silvery scale. Also, it usually does not have sharp, raised borders. There are situations where atopic dermatitis, if it’s very chronic, can become more well demarcated and sharp, but classic atopic dermatitis is considered more ill-defined. 

Also, location is always super important in dermatology. So, the location of lesions is really important, and the pattern of distribution across the body can also give you clues to distinguish among these different skin conditions. And in kids, in particular, atopic dermatitis tends to affect the fold areas, for example. 

Other skin conditions that are included in the differential diagnosis of psoriasis are superficial fungal infections.[1] Often, these are also erythematous, slightly scaly plaques, but tend to be more annular. You want to look for the clue of a leading edge or border that tends to be a little more rough or scaly. 

Other conditions that are less common but definitely are included in the differential [diagnosis] of psoriasis include subacute cutaneous lupus, pityriasis rubra pilaris, and scabies, of course, is a great mimicker.[1] Also, a rare skin condition but definitely in the differential [diagnosis] is cutaneous T-cell lymphoma.[1] So, for each one of these, location is really important. Chronicity and response to treatment can also help you distinguish among them. Also, looking for clues like location, I mentioned. In psoriasis, you can look at the nails to check for nail involvement. Look in hidden areas like the belly button, and even the gluteal fold can help you with the diagnosis. 

Lastly, another factor is skin of color. And this can sometimes mask that classic red erythema that we see in psoriasis, and it can appear more violaceous. Also, from some of the limited studies that we are aware of for psoriasis in skin of color, is that the plaques sometimes, in darker skin types like Black skin or [Fitzpatrick] type IV Asian skin, tend to be thicker and even have more white, thick, inherent scale to them. 

So, in these situations, I don’t hesitate to do a skin biopsy to help me with the diagnosis. Even though most of the time you can make the diagnosis clinically, in situations where it’s difficult to differentiate, I definitely do a biopsy to confirm my diagnosis or rule out other diagnoses, as well.[2] And so that can be very important, also—for example, in cutaneous T-cell lymphoma, where the histological findings are very subtle. 

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