Porokeratosis

The term porokeratosis refers to skin lesions with a thinned centre surrounded by a ridge-like border called the cornoid lamella. They arise from clones of cells with varying degrees of dysplasia. There are a number of different types of porokeratoses, those best defined are described below.

This chapter is set out as follows:

The distribution depends on the type of porokeratosis 

• Isolated lesions (syn. porokeratosis of Mibelli)
• ​Familial lesions are autosomal dominant in inheritance and arise in childhood, sporadic lesions have a later onset
• Lesions are single or a few in number and can be large
• They are most common on the limbs, especially acral, but can affect any body site

• Linear porokeratosis
• ​Usually presents at birth or in childhood
• Multiple lesions arise in a linear fashion along the lines of Blaschko, most commonly along a limb or one side of the trunk or head and neck 

•  Disseminated superficial actinic porokeratosis (DSAP)
  • ​This is the most common variety and arises in middle-aged individuals
  • It is an autosomal dominant condition, but a history of chronic UV exposure also appears to be needed
  • Lesions are predominantly found on sun-exposed areas of the legs, and less so the arms

Morphological appearance

• Lesions are annular with a well-demarcated, slightly raised keratotic rim known as a cornoid lamella. The centre may be scaly or thin and atrophic
• They grow gradually to between 3-10 mm. Grouped lesions will appear larger. Rarely, lesions of porokeratosis of Mibelli can be very large and are termed ‘giant’
• Dermoscopically there are parallel lines, sometimes referred to as 'tram lines' at the periphery 

• The diagnosis is normally clinical, although if a diagnostic biopsy this must be taken from the scaly edge of the lesion. Histology demonstrates the presence of a cornoid lamellae - a thin column of tightly packed parakeratotic keratinocytes within a keratin‐filled invagination of the epidermis through the stratum corneum

General measures 

• Historically there appears to be no effective treatment, many have been tried. Short-term studies on a small number of patients have shown significant improvement when using a topical mixture of 2% lovastatin (or 2% simvastatin) and 2% cholesterol cream. This is a private therapy, which can be difficult to obtain

• With regards DSAP:
  • Provide a patient information leaflet
  • UV protection - although half of the children of affected individuals will have a tendency to develop DSAP, only some will go on to develop the condition, the risk is likely to be less for those with better UV protection

Risk of malignant transformation

• Patients with DSAP appear to have a very low risk of malignant transformation - large lesions are said to have the highest malignant potential, and malignant change has also been reported in linear forms. SCC is the most common tumour

• Patients should be advised on UV protection and asked to report any significant change