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
Pruritus seems to occur in two phases:
An itch is identified with a local
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
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
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
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
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
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
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
Seborrheic dermatitis on the chest of a
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.
All content solely
developed by the American Academy of Dermatology
For an overview, visit
the AAD pamphlet
Psoriasis and Psoriatic Arthritis.