Merck Manual

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Shinjita Das

, MD, Harvard Medical School

Reviewed/Revised Aug 2021 | Modified Sep 2022
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Parapsoriasis refers to a group of skin diseases characterized by maculopapular or scaly lesions. Treatment may include a combination of various topical and oral drugs and phototherapy.

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 Psoriasis ; it is so-called because the scaly plaques sometimes appear similar.

There are 2 general forms:

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.

Manifestations of Parapsoriasis

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 non-Hodgkin lymphomas primarily affecting the skin and occasionally the lymph nodes. (See also Overview of Lymphoma and Non-Hodgkin... read more Diagnosis ), 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:

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.

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