Description

• Parapsoriasis describes a group of cutaneous diseases characterized by scaly patches or slightly raised plaques that resemble psoriasis.
• There are three main types of parapsoriasis:
  • Pityriasis lichenoides
  • Large plaque parapsoriasis (see separate chapter)
  • Small plaque parapsoriasis (SPP)
• SPP is a rare, commonly benign chronic dermatitis most often seen in middle-aged men.
• SPP presents as well-circumscribed, round/oval, erythematous/salmon-colored, scaly patches or plaques 1 to 5 cm in length and scattered over the trunk and extremities (sun-protected areas). It is generally asymptomatic or mildly pruritic.
• Patches exhibit parakeratosis and a sparse perivascular cutaneous lymphoid infiltrate primarily composed of CD4+ T cells.
• Digitate dermatosis is a variant of SPP that consists of elongated, fingerlike patches that are symmetrically distributed along dermatomes of the flanks and may exceed 5 cm in length. This form may be underreported because of lack of symptoms and subtle presentation.
• A hypopigmented variant of SPP, characterized by a CD8+ predominant T-cell infiltrate, may be seen especially in darker skinned individuals (1).
• SPP may persist, resolve spontaneously (sometimes after several years), or rarely may progress to mycosis fungoides (MF), the most common form of primary cutaneous T-cell lymphoma (CTCL) (2). Digitate dermatosis does not progress to MF.

Epidemiology

Incidence
SPP most often presents in middle-aged men in the 5th decade of life, with a 3:1 ratio of men to women.

Etiology and Pathophysiology

SPP is likely a reactive process in which a specific subset of T cells multiplies in response to a corresponding antigen. This can produce an excess of one type of T cell or monoclonality of the lymphocytic infiltrate. Specific exposures or antigens have not been identified, although a viral etiology is suspected.

Risk Factors

Specific risk factors have not been identified.

General Prevention

No known preventative measures

Commonly Associated Conditions

CTCL, most often MF