Other Papulosquamous Disorders

Parapsoriasis

• Chronic, usually asymptomatic patches or thin plaques with fine scale whose color varies from pink to red-brown; may have associated epidermal atrophy and occasionally poikiloderma (large plaque parapsoriasis).

• Two major forms of parapsoriasis are small plaque (lesions usually <5 cm in diameter) and large plaque (usually >5 cm); digitate dermatosis is a form of the former, whereas retiform parapsoriasis is a variant of the latter ( Figs 7.1 and 7.2 ).

  
  

• While the distribution may be limited or more generalized, there is a tendency for an increase in extent over time; large plaque parapsoriasis can favor the sun-protected “girdle” area; both forms usually occur in adults.

• Controversy exists regarding the percentage of cases of large plaque parapsoriasis that eventually evolve into mycosis fungoides.

• Histologically, parakeratosis and nonspecific spongiotic dermatitis is seen in small and large plaque; large plaque may have a more lichenoid infiltrate.

• An infiltrate of CD4 ⁺ T lymphocytes, often clonal, in large plaque > small plaque parapsoriasis, leading to the term “clonal dermatitis”.

• DDx: small plaque – pityriasis rosea (PR), PR-like drug eruption, pityriasis lichenoides chronica, guttate psoriasis, secondary syphilis; large plaque parapsoriasis – patch stage mycosis fungoides (MF), MF-like drug eruption; if a few lesions, consider tinea corporis.

• Rx: topical CS, sunlight, phototherapy (e.g. NB-UVB).