PsoriasisNet Article
Itching: Causes and Treatment

Itching is a sensation common to a number of skin disorders, including psoriasis. It is a common sensation not specifically associated with any particular skin disorder, and may sometimes seem entirely random - for example, the spontaneous “itchy spot” on the nose, cheek, etc. It is a frequent manifestation of systemic diseases such as chronic renal disease, biliary obstruction, cirrhosis, some types of thyroid disease, Hodgkin’s Lymphoma, and some cancers.

Itching is so common one would think it is well understood. To the contrary, itching is not well understood at all. There isn’t even a good definition of itching. The working definition of itching from a standard dermatology textbook is, “a sensation that causes a desire to scratch.”

The medical term for itching is pruritus. By using medical terminology we can get a step away from the popular conception of itching as a minor or even funny phenomenon. When pruritus is severe, it is anything but humorous. As anyone with severe pruritus can attest, the sensation closest to severe pruritus is severe pain. Pain and pruritus do share some neurophysiologic pathways.

Pruritus seems to occur in two phases:

• An itch is identified with a local site

• A subsequent more diffuse area of “itchy skin” that can be stimulated to intense pruritus by a light touch—for example, by fabric brushing against the skin.

The dual-phase sensation is similar to pain: (1) a sharp, relatively short-lived intense pain at the site of injury, and (2) a later, more generalized area of pain that can be intensified by pressure against the skin.

Pruritus seems to be associated with the spinal cord circuits involving so-called “A” and “C” fibers in the skin. The A and C fibers give rise to free nerve endings in skin that respond to mechanical stimulus such as pressure, heat, cold, and certain chemicals. The responses are associated with pain. Those that respond to histamine (a potent itch-producing chemical) may contribute to the sensation of itching. Other itch-producing chemicals include opiates, papain, and members of the chemical group called kinins.

The apparent similarities between pruritus and pain have led a number of investigators to look for specific “pruritus nerve fibers” in the skin that may parallel specific pain pathways. However, neither A nor C fibers seem to be a specific “itch fiber.” To date, the search for a specific “itch fiber” similar to a heat-responsive or cold-responsive fiber has been unrewarding.

Why does scratching temporarily relieve pruritus? Scratching seems to send volleys of signals to the spinal cord “control box,” overloading the circuits so to speak and thereby inhibiting the response of spinal cord nerve cells to signals from A and C fibers.

Pruritus has a central nervous system component that is subject to control from the brain. Under certain conditions, pruritus can be produced by suggestion, but this should not be interpreted to mean that pruritus is “in your head.” Pruritus is a genuine physiologic phenomenon. As the complex neurophysiologic and neurochemical processes involved in pruritus are better understood, it is likely that effective anti-pruritus drugs can be developed.

Treatment of Pruritus
The treatment of pruritus is based on the identification of the cause, insofar as that is possible. Your dermatologist will ask you to be as specific as possible about such questions as: When do you itch - morning, evening, all day, etc? Where do you itch? Are there identifiable events that trigger your itching? How much and how deeply do you scratch? Do you perceive your itching as a major problem that must be relieved?

General measures to relieve pruritus include:·

• Keep your skin cool; warmth tends to make pruritus worse;

• Wear light clothing for coolness and to avoid scratchy pressure against your skin; and,

•  Keep your skin moisturized with a light emollient recommended by your dermatologist, as dry skin tends to be more pruritic

Specific drugs to relieve pruritus associated with psoriasis are best prescribed by your dermatologist, based on his/her knowledge of your case. Drugs with side effects that could trigger psoriasis should be avoided.

Seborrheic Dermatitis and Psoriasis
Seborrheic dermatitis is a common, chronic skin condition that shares some features with psoriasis:

• The lesions of seborrheic dermatitis and psoriasis can appear as light red to pink patches with scales over the face and ears

• Seborrheic dermatitis is believed to evolve into psoriasis in some patients.

The two peak periods for seborrheic dermatitis to develop are the first three months of life and ages 40 years and older. In infants the disease is commonly known as “cradle cap” when it occurs on the scalp as oily, thick crusts. The infant form of seborrheic dermatitis is usually self-limiting and does not persist into childhood. It is not known whether the infant form of seborrheic dermatitis predisposes a person for the condition in later life.

Adult seborrheic dermatitis is a chronic condition. In its mildest form, adult seborrheic dermatitis is eczema-like in appearance—oily, scaling, mildly inflamed and reddened skin, and moderate to intense pruritus (itching). Seborrheic scales flaking from the scalp can look like dandruff.

In moderate to severe adult form, seborrheic dermatitis has characteristic thick, oily, yellowish-brown crusts, mild to severe skin inflammation and redness, and intense pruritus. The pruritus may be especially intense on the scalp, ears and eyelids.

When lesions are on the scalp, seborrheic dermatitis and psoriasis may be almost indistinguishable from one another. In patients with a genetic predisposition to psoriasis, seborrheic dermatitis is believed to trigger psoriasis or evolve into psoriasis.

The following photos show some typical presentations of seborrheic dermatitis:

Seborrheic dermatitis on the scalp and face of a 29-year-old woman. The reddish, scaly patches were difficult to correctly diagnose.

Seborrheic dermatitis on the scalp and face of a man.

Seborrheic dermatitis on the face of a woman.

Seborrheic dermatitis on the chest of a man.

Seborrheic dermatitis presenting as “cradle cap” on the scalp of an infant.

(Photos used with permission of the American Academy of Dermatology
National Library of Dermatologic Teaching Slides)

The occurrence of seborrheic dermatitis is thought to be associated with a reaction to the overgrowth of yeast on the surface of the skin in genetically susceptible individuals. The treatment of adult seborrheic dermatitis is directed toward decreasing yeast on the scalp and affected areas, loosening and removing the thick scales, preventing secondary bacterial infections, and reducing inflammation and pruritus. There is no known cure for seborrheic dermatitis, however the routine treatments available from your dermatologist control the disease in the majority of patients.

An educational program brought to you by the American Academy of Dermatology.

For an overview, visit the AAD pamphlet Psoriasis and Psoriatic Arthritis.

Supported by an educational donation provided by Amgen and Wyeth.