Clinical

Psoriasis is a systemic skin disease. It is characterized by pink to red plaques that are covered with silver-white scales. Lesions are commonly found on the elbows, knees, back, scalp and vulva, and may be exacerbated by stress. Lesions in the vulva can coalesce to form large areas of erythema with smaller satellite plaques. Pruritus is a common complaint. The nails are commonly involved, which show oncolysis and pitting. Severe psoriatic conditions will also have associated inflammatory bowel disease and arthritis.

Clinical signs useful in the identification of psoriasis include:

• Koebner phenomenon, which is the occurrence of new psoriatic lesions at the site of skin injury.
• Woronoff’s ring, which is a ring of peripheral blanching skin around a psoriatic plaque
• Auspitz’s sign, which are small bleeding points seen upon lifting of a psoriatic scale

If any question arises regarding diagnosis, a biopsy is necessary.

Histopathology

The histologic features commonly present in psoriasis include acanthosis (uniform elongation of the rete ridges), parakeratosis and orthokeratosis, loss of the granular cell layer and the formation of spongiform pustules and parakeratotic microabscesses.

In the epidermis, the rete ridges are narrow towards the surface and broad at the base. Bridges may form among some of these ridges. Inversely, the papillary dermis is broadened and clubbed near the surface. The capillary vessels within the superficial dermis are slightly dilated and may have associated chronic inflammation. Neutrophils extravasate from these capillaries and are found in the thinned superficial epidermis (spongiform pustules of Kogoj). These neutrophils eventually aggregate in the parakeratotic layer, forming the Munro microabscess, which is characteristic of this condition.

Mitotic activity, commonly seen only in the basal cells, is typically increased in psoriasis. Mitotic figures are present in the parabasal (prickle cell) layers.