Parapsoriasis is a disorder that is similar to psoriasis. 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 parapsoriasis is usually noncancerous (benign). However, extremely rarely, small-plaque parapsoriasis turns into cutaneous T-cell lymphoma (CTCL), which is a type of skin cancer that is caused by specific types of white blood cells called lymphocytes.
Among people with large-plaque parapsoriasis, 10% will develop CTCL every 10 years.
The plaques usually do not cause symptoms. They typically look like thin, scaly, dull, pink patches and any 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.
Treatment of small-plaque parapsoriasis may not be needed but can include skin moisturizers (emollients), tar preparations or corticosteroids applied to the skin, phototherapy (exposure to ultraviolet light), or a combination.
Treatment of large-plaque parapsoriasis is phototherapy (narrowband ultraviolet B [NBUVB]) or corticosteroids applied to the skin.