Histological features of Riehl melanosis have rarely been compared between lesional and perilesional normal skin and have not been precisely described using quantitative or immunohistochemical analysis or electron microscopic findings.

A recent study published in The American Journal of Dermatopathology investigated the histopathological and immunohistochemical features of lesional and perilesional normal skin in patients with Riehl melanosis.

This study retrospectively evaluated electronic medical records and skin biopsy specimens of 48 patients with Riehl melanosis. Additionally, electron microscopy was performed in one case.

The findings revealed that although the difference was statistically insignificant—melanin pigment was increased in the epidermis of the lesional skin compared to that of perilesional normal skin. In addition, the number of melanocytes and their activity were significantly increased in the lesional epidermal skin; melanin pigment was also significantly increased in lesional dermis. Meanwhile, pigmentary incontinence, basal cell liquefaction, dilated vessels, epidermal spongiosis and colloid bodies were found in lesional skin, as well as in the perilesional normal skin to a lesser extent. Under electron microscopy of a randomly selected subject, many fibrocytes contained numerous melanosome particles in the cytoplasm of lesional dermis. Whereas, in the perilesional normal skin, fibroblasts also contained melanosome particles; however, the number of melanosome-containing cells were less than that in the lesional skin.

The results were summarized as Riehl melanosis is characterized by increased epidermal melanocytes and pigmentation, primarily involving the dermis, with histologically typical changes at the interface. Unlike that in other pigmentary diseases, most perilesional normal-appearing skin in Riehl melanosis shows typical histopathological changes, although to a lesser extent.

Source: The American Journal of Dermatopathology. 2020 Feb;42(2):117-121. doi: 10.1097/DAD.0000000000001515.