Parapsoriasis is a disorder that is similar to psoriasis Psoriasis Psoriasis is a chronic, recurring disease that causes one or more raised, red patches that have silvery scales and a distinct border between the patch and normal skin. A problem with the immune... read more 
. Both disorders cause scaly patches (plaques) to form on the skin. The difference between the two disorders is that in parapsoriasis, the plaques are thinner than they are in the most typical form of psoriasis.
There are two forms of parapsoriasis:
Small-plaque form
Large-plaque form
Small-plaque parapsoriasis is usually noncancerous (benign). However, extremely rarely, small-plaque parapsoriasis turns into cutaneous T-cell lymphoma Cutaneous T-cell Lymphomas Cutaneous T-cell lymphomas (CTCL) are a rare, persistent, very slow-growing type of non-Hodgkin lymphoma, a cancer of a type of white blood cell called lymphocytes. Lymphomas (see also Overview... read more 
(CTCL), which is a type of skin cancer that is caused by specific types of white blood cells called lymphocytes White Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more 
.
Among people with large-plaque parapsoriasis, 10% will develop CTCL every 10 years.
Symptoms of Parapsoriasis
The plaques usually do not cause symptoms. They typically look like thin, scaly, dull, pink patches and may have a slightly wrinkled or thinned appearance.
In the small-plaque form, plaques are less than 2 inches (5 centimeters) in diameter. In the large-plaque form, plaques are greater than 2 inches (5 centimeters) in diameter.
Sometimes the plaques of small-cell parapsoriasis develop on areas of skin served by specific nerves that come out from the spinal cord, especially on the sides of the back and abdomen. These plaques are called digitate plaques because they look like fingers (digits) and can be over 2 inches (5 centimeters) in size. These plaques rarely become CTCL.
Dull, pink, thin, slightly scaly plaques on the trunk of a person with large-plaque parapsoriasis.
Image provided by E. Laurie Tolman, MD.
Thin, pink, slightly scaly plaques on the buttocks of a person with large-plaque parapsoriasis.
Image provided by E. Laurie Tolman, MD.
This image shows small-plaque parapsoriasis.
Image courtesy of Susan Lindsley via the Public Health Image Library of the Centers for Disease Control and Prevention.
Diagnosis of Parapsoriasis
A doctor's evaluation
Sometimes skin biopsy
Doctors base the diagnosis of parapsoriasis on how the scales and plaques look and where they appear on the body.
Sometimes, doctors take a sample of skin tissue and examine it under a microscope to rule out other disorders.
Prognosis for Parapsoriasis
The progression of both forms is unpredictable. People undergo periodic follow-ups and biopsies to determine whether they are at risk of developing CTCL.
Treatment of Parapsoriasis
Includes phototherapy, corticosteroids, or both
Treatment of small-plaque parapsoriasis may not be needed but can include skin moisturizers Moisturizing agents (emollients) Topical drugs (drugs applied directly to the skin) are a mainstay of treating skin disorders. Systemic drugs are taken by mouth or given by injection and are distributed throughout the body... read more (emollients), tar preparations, or corticosteroids Anti-inflammatory agents Topical drugs (drugs applied directly to the skin) are a mainstay of treating skin disorders. Systemic drugs are taken by mouth or given by injection and are distributed throughout the body... read more applied to the skin, phototherapy Phototherapy Psoriasis is a chronic, recurring disease that causes one or more raised, red patches that have silvery scales and a distinct border between the patch and normal skin. A problem with the immune... read more (exposure to ultraviolet light), or a combination.
Treatment of large-plaque parapsoriasis is phototherapy (narrowband ultraviolet B [NBUVB]) or corticosteroids applied to the skin.
