INTRODUCTION

Porokeratosis (PK) is an uncommon disorder of epidermal differentiation with an increased incidence in organ transplant recipients (OTR). There are different clinical manifestations of PK, but all share a distinct histopathology characterised by the “cornoid lamella,” a narrow dyskeratotic column, which interrupts the granular layer with associated parakeratosis and often hydropic degeneration of the corresponding basal layer (Figure 16.1).

CLINICAL FEATURES

Porokeratosis presents as a well-demarcated, irregular plaque that expands slowly in a centrifugal fashion with a prominent hyperkeratotic, ridged border corresponding to the cornoid lamella and some central atrophy. PK was first described by Mibelli in 1893, and the classic Porokeratosis of Mibelli (PKM) is a form found commonly in OTR. PKM presents as one or more localized asymptomatic annular plaques. These may become large (up to 20 cm diameter) and are usually, but not exclusively, located on the limbs, commonly the lower legs (Figure 16.2).

Other clinical presentations of PK include:

1. Disseminated superficial actinic porokeratosis (DSAP): Common in countries with high ultraviolet (UV) exposure and presenting with multiple small superficial, annular lesions symmetrically distributed on sun-exposed areas of skin. Individual lesions are less prominent than those of PKM and may be overlooked or ignored. Patients often describe symptomatic exacerbations following sun exposure.

2. Disseminated superficial porokeratosis (DSP): Similar to DSAP, but without UV light as a precipitating factor.

3. Linear porokeratosis (LPK): Linear systemized lesion reminiscent of an epidermal nevus.

4. Porokeratosis palmaris, plantaris et disseminata (PPPD): DSAP-like lesions initially limited to the palms and soles.

5. Punctate porokeratosis (PPK): Punctate keratotic spines on the palms and soles, which may mimic other punctuate palmar keratoses.