An article published in Stat Pearls reported that facial melanoses—affecting the face and neck region—are of considerable concern to darker-skinned people. Although many facial melanoses, such as melasma are clinically well-defined and easy to diagnose, others have overlapping clinical morphology as well as histopathology.

The authors discussed that Riehl melanosis (RM) – commonly considered as a pigmented contact dermatitis (PCD), is one such condition with diverse yet overlapping clinical features with other facial melanoses—such as ashy dermatosis and lichen planus pigmentosus. RM is best defined as an acquired low-grade allergic contact dermatitis to fragrance and other cosmetic products. The condition manifests as abnormal hyperpigmentation involving the face and neck regions. The disorder predominantly affects Asian patients.

RM was first identified in several patients of both genders by Riehl, during the first world war. Characteristic features were:

• Striking dark brown to grayish-brown pigmentation on the forehead, temporal and zygomatic regions of the face.
• The involved skin was mildly indurated with a rough texture and appeared to mildly scaly without erythema or atrophy.
• The pigmentation was diffuse and uniform but without well-defined margins. It extended up to the ears and neck, with gradual fading off towards the chest as small discreet pigmented follicular-based macules.
• Other body parts like the axillae, arms, and forearms, submammary and umbilical regions also showed similar pigmentation, but it was much lighter than the face.
• The etiology remained unknown initially—food substitutes, mainly poor-quality flour, were considered as contributing factors.
• Hoffmann and Habermann emphasized that local chemical irritation due to the use of certain oils and hydrocarbons were responsible for melanosis similar to Riehl melanosis and labeled it melanodermatitis toxica.
• The inflammatory component of the early phase of RM was strongly suggested by Miescher and was confirmed on histology in 200 cases by Joulia et al. in 1950.
• The etiology of RM was unknown until the late 1940s and 1950s, when two studies from Argentina by Pierin et al. and Puente et al. attributed facial pigmentation similar to RM to cosmetic ingredients, typically aniline dye (orange II) found in facial powder.
• Positive patch tests to the dye were supported by the disappearance of early inflammation and progressive reduction in pigmentation on discontinuing the use of the facial powder.
• In 1970, the term "pigmented contact dermatitis" (PCD) was coined by Osmundsen, who documented several cases of peculiar facial hyperpigmentation resulting from contact dermatitis to an optical whitener used in washing powder.
• Until now, the etiology of RM is still controversial, although the majority of experts consider it synonymous with PCD of the face.
• A recent global consensus statement emphasized that finely reticulated hyperpigmentation on the face and neck region should be labeled “pigmented contact dermatitis” rather than RM if its appearance is preceded by contact allergy.
• On the other hand, if etiology is uncertain, the term RM should be used.

Source: In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2020.