Parapsoriasis describes a poorly understood and poorly distinguished group of diseases that share clinical features. Parapsoriasis is not related to psoriasis Psoriasis Psoriasis is an inflammatory disease that manifests most commonly as well-circumscribed, erythematous papules and plaques covered with silvery scales. Multiple factors contribute, including... read more 
; it is so-called because the scaly plaques sometimes appear similar.
There are 2 general forms:
Small-plaque type: Usually benign
Large-plaque type: A precursor of cutaneous T-cell lymphoma Cutaneous T-cell Lymphomas (CTCL) Mycosis fungoides and Sézary syndrome are uncommon chronic T-cell lymphomas primarily affecting the skin and occasionally the lymph nodes. (See also Overview of Lymphoma.) Mycosis fungoides... read more
(CTCL)
Small-plaque parapsoriasis transforms into CTCL extremely rarely. Large-plaque parapsoriasis transforms into CTCL in about 10% of patients per decade.
Symptoms and Signs of Parapsoriasis
The plaques are usually asymptomatic; their typical appearance is thin, scaling, dull, pink patches and plaques with a slightly atrophic or wrinkled appearance. In contrast, the plaques in psoriasis are well-demarcated and pink with thicker silvery scale.
Small-plaque parapsoriasis is defined by lesions < 5 cm in diameter, whereas large-plaque parapsoriasis has lesions > 5 cm in diameter.
This image shows small-plaque parapsoriasis (lesions 5 cm in diameter), which is benign.
Image courtesy of Susan Lindsley via the Public Health Image Library of the Centers for Disease Control and Prevention.
This photo shows large-plaque parapsoriasis on the buttocks, characterized by thin, dull, pink, and slightly scaly patches.
Image provided by E. Laurie Tolman, MD.
This photo shows dull, pink, scaly patches of large-plaque parapsoriasis on the back.
Image provided by E. Laurie Tolman, MD.
Sometimes digitate plaques develop along the dermatomes, especially on the flanks and abdomen, in small-plaque parapsoriasis. Although digitate plaques of parapsoriasis may be > 5 cm, transformation into CTCL is extremely rare in small-plaque parapsoriasis.
Prognosis for Parapsoriasis
Course for both types is unpredictable; periodic clinical follow-up and biopsies give the best indication of risk of developing CTCL.
Diagnosis of Parapsoriasis
Clinical evaluation
Sometimes biopsy and genetic and molecular testing to rule out cutaneous T-cell lymphoma (CTCL)
Diagnosis of parapsoriasis is based on clinical appearance and distribution.
Biopsy can be helpful if there is concern for CTCL (see diagnosis of CTCL Diagnosis Mycosis fungoides and Sézary syndrome are uncommon chronic T-cell lymphomas primarily affecting the skin and occasionally the lymph nodes. (See also Overview of Lymphoma.) Mycosis fungoides... read more ), but otherwise the diagnosis of parapsoriasis is clinical. Histologic findings may not be classic for CTCL, but immunophenotyping analysis and T-cell gene rearrangement studies can be done to identify a T-cell clone if it exists.
Differential diagnosis of small-plaque parapsoriasis includes the following:
It is most important to rule out CTCL because early CTCL can be hard to distinguish clinically from small-plaque parapsoriasis. Biopsy is informative and can be used to distinguish between small- and large-plaque psoriasis.
Differential diagnosis of large-plaque parapsoriasis includes the following:
CTCL
Treatment of Parapsoriasis
Treatment of small-plaque parapsoriasis is unnecessary but can include emollients, topical tar preparations or corticosteroids, phototherapy, or a combination.
Treatment of large-plaque parapsoriasis is phototherapy (narrowband UVB) or topical corticosteroids.
Key Points
Parapsoriasis is an etiologically heterogeneous group of disorders that tend to have a similar appearance—thin, scaling, dull, pink patches and plaques with a slightly atrophic or wrinkled appearance.
Parapsoriasis with plaques < 5 cm in diameter is usually benign; parapsoriasis with larger plaques transforms into cutaneous T-cell lymphoma in about 10% of patients per decade.
Diagnose based on clinical appearance; biopsy and other tests may be necessary to rule out cutaneous T-cell lymphoma.
Treat small-plaque parapsoriasis symptomatically and large-plaque parapsoriasis with phototherapy or topical corticosteroids.
