The epidermal nevus syndromes are conditions in which epidermal nevi coexist with other features of skeletal, cerebral, ocular, or other systemic involvement. Among these is phakomatosis pigmentokeratotica.
It is characterized by the presence of:
- A sebaceous nevus in a linear pattern
- A speckled lentiginous nevus in a checkerboard pattern
- Other systemic anomalies, most commonly hemiatrophy, dysesthesia, and hyperhidrosis affecting a segment of the body
A sebaceous nevus is an overgrowth of sebaceous gland and hair follicle tissue, along with other components of the epidermis. It presents at birth, most often on the scalp, and appears as a single smooth patch without hair, becoming more prominent at puberty.
A speckled lentiginous nevus is a congenital melanocytic nevus, appearing light brown with darker spots inside. It is a flat lesion.
Hemiatrophy is the most common among the extracutaneous manifestations of the syndrome. This may present with differing degrees of muscle weakness on the affected side. Mental retardation, sensory abnormalities over a particular segment of the body, epilepsy, ptosis, strabismus, and hearing disability may all be present.
In common with other epidermal nevus syndromes, phakomatosis pigmentokeratotica shows features of mosaicism i.e. both normal and mutated cell lines are present in the same individual. The condition is thought to be due to a genetic change termed the ‘twin spot phenomenon.’ Different cell lines are derived from the same common cell, but migrate to different segments of the body during the period of embryonic life. The twin spot phenomenon is thought to arise from the occurrence of an early post-zygotic mutation on an already mutated chromosome pair, one occurring on each chromosome.
The new mutation results in crossing over and recombination leading to the emergence of two different homozygous cell lines, forming a mosaic. In phakomatosis pigmentokeratotica, HRAS mutations have been reported. It is now also considered to be another form of the Schimmelpenning syndrome; therefore, Phakomatosis pigmentovascularis is another example of a twin-spot mutation.
The sebaceous and lentiginous nevus need not be adjacent to be due to a twin spot phenomenon. The organoid or sebaceous nevi occurs in a linear pattern, as usual, while the sebaceous linear nevus occurs in a checkerboard pattern, again as predicted. The timing of the mutation determines which stage of differentiation is affected, which in turn leads to specific phenotypes.
Diagnosis and Management
The characteristic appearance of the lesions should prompt a search for other systemic manifestations, which may be supplemented by appropriate testing. Management is largely symptomatic. Large and unsightly lesions may be excised, but scarring is often significant and the recurrence rate is high.
The dark-colored nevus may lead to malignant change in a very few cases for which reason any change indicative of cancer should lead to an excision biopsy and histopathological examination.
Reviewed by Afsaneh Khetrapal BSc (Hons)