CASE #1: Disseminated superficial actinic porokeratosis

Porokeratosis encompasses a group of genetic disorders that share the common characteristic clinical feature of a thin, raised, threadlike border called a cornoid lamellae. The most common clinical forms of porokeratosis are disseminated superficial actinic porokeratosis (DSAP), porokeratosis of Mibelli, linear porokeratosis, porokeratosis palmaris et plantaris disseminata, and punctate porokeratosis. These variants differ in size and distribution of lesions.1-3

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Porokeratosis can be inherited in an autosomal-dominant manner but is most commonly sporadic, with many patients lacking a family history of the condition.1 The pathogenesis of porokeratosis is not entirely clear, but the most widely accepted hypothesis is that the lesions represent a mutant clone of keratinocytes that multiplies and expands outward. The raised, threadlike border that is seen clinically represents the border between normal keratinocytes and the advancing edge of the mutant proliferation. Such factors as UV exposure or immunosuppression may trigger the proliferation of mutant keratinocytes in those individuals who are genetically predisposed.1

DSAP is characterized by numerous 2- to 7-mm, annular, keratotic papules on sun-exposed skin. On close examination, the thin, threadlike border can usually be visualized.1,2 The lesions occur most commonly on the extremities in a symmetric fashion and spare the palms and soles. The usually asymptomatic lesions4 typically appear in the third to fourth decades of life and are more prominent in the summer months.1

The differential diagnosis of DSAP includes such small hyperkeratotic lesions as actinic keratoses (AKs), seborrheic or stucco keratoses, flat warts, pityriasis lichenoides chronica, excoriated insect bites, and guttate psoriasis as well as more annular lesions such as tinea corporis. DSAP is most frequently confused with AK, and the two entities may coexist. One distinguishing feature is that DSAP frequently involves the legs but rarely the face, whereas AKs are very common on the face.1 If the diagnosis is not clear clinically, it can be confirmed easily with skin biopsy.

All types of porokeratosis have a very distinctive histopathologic picture. The threadlike border is the cornoid lamella, and it has a very specific microscopic appearance; there is a distinct parakeratotic column overlying an area of epidermis with an absent granular layer and occasional dyskeratotic cells.3 It is very important for a biopsy to be taken across the edge of the peripheral raised rim in order to see this characteristic pathology. A biopsy that only contains the central portion of a larger lesion may only show nonspecific findings.

The lesions of porokeratosis generally increase in size and number with time.4 Malignant degeneration, especially the appearance of squamous cell carcinoma (SCC), has been reported in all types of porokeratosis except for punctate keratosis. SCC is more common in the larger longstanding lesions of porokeratosis of Mibelli and linear porokeratosis than in DSAP.4

Many treatments have been reported for porokeratosis because no option is uniformly effective. If the abnormal clone of keratinocytes is not completely destroyed, lesions will recur. Reported treatment options include cryotherapy, topical 5-fluorouracil, topical retinoids, topical imiquimod, ablative lasers, curettage, excision, and dermabrasion. Widespread lesions have been treated with oral acitretin (Soriatane), but the condition recurs after discontnuation.1 Porokeratosis can be extremely difficult to treat and very frustrating for both the practitioner and the patient.

A biopsy was performed in this patient to confirm the diagnosis. The patient returned for his results, and after a thorough discussion of the condition and treatment options, he chose to observe the condition in lieu of treatment.