Lichen planus (LP) and other lichenoid eruptions tend to create characteristic primary lesions and patterns. The pruritic, purple, polygonal, flat-topped papules of LP can appear unmistakably shiny as they reflect light back at the keen diagnostician. These papules can be widespread and coalesce into plaques and streaks that demonstrate the Koebner phenomenon as another clue to the diagnosis. A thorough physical examination can reveal white lacy leukoplakia of the oral mucosa, annular lesions of the genitalia, or a variety of nail changes from ridging to pterygium formation. Lichen planus pigmentosus/actinicus is an often photodistributed LP variant that demonstrates more distinctive hyperpigmentation.
Lichen nitidus and lichen striatus are lichenoid conditions that are more common in children. With its almost pinpoint, shiny, flat-topped papules, lichen nitidus can appear slightly hypopigmented against darker skin. As in LP, papules can demonstrate the Koebner phenomenon and be either localized or widespread. Lichen striatus presents with a characteristically curvilinear plaque most commonly on the extremities of a child; however, lesions can be seen on the trunk and, rarely, the face. The eruption follows the lines of Blaschko and can involve the nail with longitudinal dystrophy. In the later phase, a hypopigmented or hyperpigmented patch will remain on the skin temporarily as a sign of the recently resolved plaque of lichen striatus.
Lichen sclerosus most commonly involves the genitalia, with a higher incidence in females. Genital lichen sclerosus classically presents with atrophic white patches that can appear wrinkled and erosive, and can obliterate and scar normal anatomic landmarks. In some patients, the presence of bruising can be misdiagnosed as trauma from sexual abuse, highlighting the importance of a careful physical examination with consideration of skin biopsy in unclear cases. Extragenital lichen sclerosus can present with atrophic white papules and macules, often on the trunk.
Skin biopsy can be diagnostic in many lichenoid conditions, demonstrating the typical bandlike inflammation at the dermal-epidermal junction, along with more specific differentiating features depending on the specific eruption. This portion of the atlas features many images to help clinically distinguish lichen planus and its many related conditions.