Thread by @HarkerDavid

Describing rashes can be hard. I get it. You got like 1 lecture about it in med school and then us snooty dermatologists sneer when we get consulted for a "petechial rash" that's actually just crusted excoriations.

So, here's a refresher on describing rashes.

📷: @dermnetnz

First of all, why does it even matter? Why can't you just call it a rashy rash and refer to derm?

Well, skin disease is common. Dermatologists are not. If you're in an area with limited access to derm, building a correct ddx for skin disease relies on an accurate description.

Accurately describing a rash means you've correctly identified the primary lesion and relevant secondary features. If you can't correctly identify the primary lesion, your ddx could be completely off point.

Contrary to the diagnostic process in most specialties, in dermatology, the physical exam is king. History is secondary.

So, here goes!

Papules and nodules. Basically bumps on the skin. Papules are small, nodules are bigger. Cutoff is usually considered to be 1 cm.

Macules (small) and patches (bigger). Non-palpable changes in the skin. Usually referring to lesions that just show color change, but if you closed your eyes you couldn't find them by palpation. Again, cutoff is usually 1 cm.

Plaques are flat topped palpable lesions. I like to think of them as something that's wider than it is tall. Papules can sometimes coalesce into a plaque.

Vesicles and bullae. Basically small or large blisters. Cutoff 1 cm. These can be tricky because sometimes the patient has no intact vesicles or bullae on exam. A clue is round erosions with a collarette of scale.

That brings me to erosions and ulcers. An erosions is a defect that involves the epidermis only. The dermis is intact. Ulcers extend through the dermis.

Peteichiae and purpura. RBCs have extravasated into the dermis, so the lesions do not blanch with pressure. Usually a purple hue. Purpura can be macular or palpable. Palpable purpura can be a sign of vasculitis.

Scale is retained stratum corneum. There are various types of scale, but in general, when you see it, there is an epidermal process involved.

Obviously there's tons more that could be said (derm is its own residency for a reason!) but that seems like a good start.

If you want to learn more about skin disease, the @AADskin has an amazing FREE online derm curriculum. Link below:

aad.org/education/basi…