Case 1: The key diagnostic feature of porokeratosis is cornoid lamella, which is a column of parakeratosis overlying hypogranulosis and dyskeratosis. The columns of cornoid lamella often slant or lean toward the center of the lesion.

A pink column of cornoid lamella is seen to the right. The epidermis in the central portion of the lesion may be hyperplastic, normal or atrophic.

Case 2: A low power view of the column of parakeratosis.

Note the diminished granular layer giving rise to the parakeratosis.

Another example Image

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Porokeratosis is a disorder of keratinization characterized by the formation of one or more atropic patches surrounded by a raised border of cornoid lamella, a key diagnostic feature of this entity. The pathogenesis of these lesions are still not entirely known, but there is an inherited predisposition and immune host response plays a role.

Five clinical variants of porokeratosis have been described (McKee, Spencer):

• Classic porokeratosis of Mibelli (PM): Presents in childhood or in adults following immunosuppression. Most commonly affects the extremities. Basal cell carcionma or squamous cell carcinoma can arise within the lesions.
• Disseminated superficial actinic porokeratosis (DSAP): A skin condition inherited in an autosomal dominant fashion, DSAP usually affects women in their 3 or 4th decade of life. It can be clinically confused with solar keratosis ("sun spots"), however, there is a questionable association with sun exposure. Solar keratoses is more likely arise in faces and hands, whereas DSAP arises in the arms and legs.
• Porokeratosis palmaris et plantaris disseminata (PPPD): Develops on palms and soles and can affect the mucous membranes. Most commonly affects adolescents or young adults with a male predilection (M:F ratio of 2:1).
• Punctate porokeratosis: Also occurs on the palms and soles. Affects adults.
• Linear porokeratosis: Seen in early infancy and childhood, with reddish plaques or papules distributed along the lines of Blaschko.

Clinically, an annular lesion with a "threadlike elevated border" is seen (Rapini). This threadlike border corresponds to the cornoid lamellae seen histologically.

Treatment depends on the type of porokeratosis and includes topical medications such as 5-Fluorouracil. Oral retinoids are sometimes used. For lesions with malignant transformation, surgical excision will be necessary.

→Although cornoid lamellae can be seen in other conditions (e.g. actinic keratosis), for an exam, think porokeratosis.

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