Parapsoriasis describes a poorly understood and poorly distinguished group of diseases that share clinical features. Parapsoriasis is not related to psoriasis; 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 (CTCL)
Small-plaque parapsoriasis transforms into CTCL extremely rarely. Large-plaque parapsoriasis transforms into CTCL in about 10% of patients per decade.
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.
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.
Diagnosis of parapsoriasis is based on clinical appearance and distribution.
Biopsy can be helpful if there is concern for CTCL (see diagnosis of CTCL), 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:
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.