psoriasis
Meaning of psoriasis: Neither an infection nor catching, it runs in flares and quiet spells and can involve the nails and joints as well as the skin.
Definition of psoriasis
Psoriasis is a long-term skin disease in which the immune system drives skin cells to multiply far faster than normal, so they pile up instead of shedding. The result is raised, scaly patches called plaques, often silvery on top, which look red or pink on lighter skin and purple, gray or dark brown on darker skin.
What is happening in the skin?
Skin renews itself constantly. Cells form in the deepest layer, travel upward, flatten and are shed, a journey that normally takes weeks. In psoriasis the immune system misreads healthy skin as a threat. Immune cells release signaling proteins that order the skin cells to divide, and the journey to the surface is finished in a matter of days. The cells arrive immature and cannot be shed cleanly, so they build up as scale, while the small blood vessels beneath widen and bring the inflammation that colors the patch and makes it warm.
What forms does it take?
| Form | What it looks like |
|---|---|
| Plaque | Thick, sharply edged patches on the scalp, elbows, knees and lower back; the commonest form |
| Guttate | Small drop-shaped spots over the trunk and limbs, often after a throat infection |
| Inverse | Smooth, shiny areas in skin folds, with little scale |
| Pustular | Small blisters of sterile pus, often on the palms and soles |
| Erythrodermic | Widespread redness and shedding over most of the body; uncommon and serious |
| Nail | Pitting, thickening, discoloration and lifting from the nail bed |
What causes it, and what sets off a flare?
The tendency is inherited. Psoriasis clusters in families, and a genetic contribution is well established, involving genes that affect how the immune system regulates itself. Carrying the tendency does not mean the disease will appear, which is where triggers come in. A streptococcal throat infection is the classic prompt for the guttate form, and injury to the skin is another, since a cut, a scratch or a tattoo can raise a new plaque along the line of damage, an effect named the Koebner phenomenon. Stress, cold dry weather, smoking, heavy alcohol use and certain medicines are also reported, among them lithium, some blood pressure drugs and the withdrawal of steroid tablets. Many flares have no identifiable cause. What does not cause psoriasis is worth stating plainly, because the misunderstandings persist: it is not the result of poor washing, it is not an infection of the skin, and it is not catching, so it cannot pass on by touch, by a shared towel or through shared water. The scale is not dirt. Because the inflammation reaches beyond the skin, other conditions travel with it, most notably psoriatic arthritis, which can begin before the skin gives trouble.
How is it told apart from eczema?
The two are often mistaken for one another, and a few features separate them.
| Psoriasis | Eczema | |
|---|---|---|
| Edges | Sharply defined | Blurred into normal skin |
| Scale | Thick and silvery | Fine, or weeping and crusted |
| Usual sites | Outer elbows and knees, scalp, lower back | Inner elbows and knees, neck, hands |
| Nails | Pitting and lifting common | Rarely involved |
Used in a sentence
The dermatologist identified plaques consistent with psoriasis on the patient's elbows and scalp.
Good to know
Psoriatic plaques are typically well demarcated and carry a silvery scale, which helps distinguish the condition from eczema, where patches have less defined borders, different scaling and a stronger tendency to itch.
Word origin
Where “psoriasis” comes from
From Greek psora, meaning itch, combined with the suffix -iasis, indicating a diseased condition.