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Pityriasis is one of any of a number of skin diseases that have in common lesions that resemble dandruff-like scales without obvious signs of inflammation.

Types of pityriasis include: pityriasis alba (also called pityriasis streptogenes, pityriasis simplex, erythema streptogenes); pityriasis rosea; pityriasis rubra pilaris; and pityriasis versicolor (tinea versicolor).

Description of Pityriasis

Pityriasis alba

This is characterized by hypopigmented, round to oval, scaling patches on the face, upper arms, neck, or shoulders. The patches vary in size, usually being a few centimeters in diameter. The color is white or light pink. The scales are fine and adherent.

Usually, the patches are sharply demarcated; the edges may be erythematous and slightly elevated. As a rule, pityriasis is asymptomatic. However, there may be mild pruritis. The disease occurs chiefly in children and teenagers.

The cause is unknown. Excessively dry skin following exposure to strong sunlight appears to be contributory. Efforts to find an infectious agent – either bacterial, viral, or fungal – have been unsuccessful.

Pityriasis rosea

This is a mild, inflammatory exanthem of unknown origin, characterized by salmon-colored papular and macular lesions that are at first discrete but may become confluent. The individual patches are oval or circinate and covered with finely crinkled, dry epidermis, which often desquamates, leaving collaterate scaling.

The disease usually begins with a single herald or mother patch, usually larger than succeeding lesions, which may persist a week or more before others appear. By that time involution of the herald patch has begun. The efflorescence of new lesions spreads rapidly, and after three to eight weeks they usually disappear spontaneously.

The incidence is highest between the ages of 15 and 40 years, and the disease is most prevalent in the spring and autumn. Women are more frequently affected.

The eruption is usually generalized, affecting chiefly the trunk, and sparing sun-exposed surfaces. At times it is localized to a certain area, such as the neck, thighs, groins, or axillae.

Moderate pruritis may be present, particularly during the outbreak, and there may be mild constitutional symptoms prior to the onset. The cause is unknown.

Pityriasis rubra pilaris

This is a chronic skin disease characterized by small follicular papules, disseminated yellowish pink scaling patches, and often, solid confluent palmoplantar hyperkeratosis.

The papules are the most important diagnostic feature, being more or less acuminate, reddish brown, about pinhead size, and topped by a central horny plug. In the horn center a hair, or part of one, is usually embedded. The disease generally manifests itself first by scaliness and erythema of the scalp.

The eruption is limited in the beginning, having a predilection for the sides of the neck and trunk and the extensor surfaces of the extremities. Then, as new lesions occur, extensive areas are converted into sharply marginated patches of various sizes, which look like exaggerated goose-flesh and feel like a nutmeg grater. The involvement is generally symmetric and diffuse with, however, characteristic small islands of normal skin within the affected areas.

Pityriasis Versicolor (tinea versicolor)

On the upper trunk and extending onto the upper arms, finely scaling, guttate or nummular patches appear, particularly on young adults who perspire freely. The individual patches are yellowish or brownish macules in pale skin, or hypopigmented macules in dark skin, with delicate scaling. Mild itching and inflammation about the patches may be present.

This common fungal disease is most prevalent in the tropics where there are high humidity and high temperatures and frequent exposure to sunlight.