Lichen planus is the example stereotypical of lichenoid dermatitis. Fully developed lesions of it are characterized by compact orthokeratosis, wedge-shaped hypergranulosis (each wedge centered in an acrosyringium or an “acrotrichium”), jagged acanthosis, a sprinkling of necrotic keratocytes in the lower part of the epidermis, vacuolar alteration along the dermoepidermal junction, a bandlike infiltrate of lymphocytes that fills the upper part of the dermis and obscures the dermoepidermal junction, and a superficial perivascular infiltrate of lymphocytes. Those findings histopathologic, in toto, are diagnostic of lichen planus and are not found in any other lichenoid dermatitis. Several other lichenoid dermatitides can be differentiated histopathologically from lichen planus by findings specific and what follows, in brief, are the criteria we employ to accomplish that in lesions formed fully unable that desideratum to be accomplished.
The lichenoid purpura of Gougerot and Blum is typified by an infiltrate of lymphocytes that surrounds venules of the superficial plexus and fills the upper part of the dermis. Often it fails to obscure the dermoepidermal junction. Extravasated erythrocytes or siderophages, or both of them together, are present in the upper part of the dermis. That constellation of findings is specific for the lichenoid expression of persistent pigmented purpuric dermatitis.
Lichenoid discoid lupus erythematosus is marked usually by a superficial and deep perivascular lymphocytic infiltrate that assumes a bandlike configuration in the upper part of the dermis and obscures the dermoepidermal junction, at least in loci, in conjunction with vacuolar alteration, and, sometimes necrotic keratocytes. The epidermis is thinned focally and covered mostly by an increased number of orthokeratotic corneocytes arranged compactly and often by parakeratosis, too. At the dermoepidermal junction itself, in addition to vacuolar alteration, resides a basement membrane of variable degrees of thickness. That latter finding is observed in but two diseases, to wit, discoid lupus erythematosus and dermatomyositis, both of which may harbor abundant mucin in the reticular dermis.
Lichenoid photodermatitides, such as may be induced by thiazides, are typified by a superficial and deep perivascular infiltrate of lymphocytes that creates a lichenoid design in the upper part of the dermis. The infiltrate obscures the dermoepidermal junction where it is affiliated with vacuolar alteration. Necrotic keratocytes are either few or absent. The epidermis is neither thinned in loci nor accompanied by a thickened basement membrane. Mucin is not increased in amount in the reticular dermis.
A lichenoid drug eruption may be associated with either a superficial or a superficial and deep perivascular infiltrate of lymphocytes mostly. In some instances, a bandlike infiltrate that fills the upper part of the dermis consists of lymphocytes solely; in others, lymphocytes are joined by eosinophils. As is the case for all lichenoid infiltrates of inflammatory cells that obscure the dermoepidermal interface, vacuolar alteration is present, too. The epidermis may be unevenly acanthotic, and focally hypergranulotic, but keratohyaline granules are not arranged in a distinctly wedge shape. The pattern formed by corneocytes in a thickened stratum corneum, when orthokeratotic, may be basket-weave or compact. Parakeratosis is limited to loci, but those may predominate over ones of orthokeratosis. When clusters of necrotic keratocytes, sometimes as many as fifty of them or more, are noted in high power fields in the lower part of the epidermis and the uppermost part of the dermis, the likeliest diagnosis is lichenoid drug eruption. A drug that commonly induces necrosis of keratocytes in the distinctive fashion just described is gold; another is procainamide, and others surely are capable of it. Not every lichenoid drug eruption, however, is marked by clusters of necrotic keratocytes in the vicinity of the dermoepidermal junction, and not every cluster of necrotic keratocytes is a sign of a lichen planus-like drug eruption.
Lichenoid infiltrates of lymphocytes in the upper part of the dermis may be present in conditions other than dermatitides, e.g., lichen planuslike keratosis and plaques of mycosis fungoides, the former being a solar lentigo/reticulated seborrheic keratosis and the latter a lymphoma. Even when clusters of necrotic keratocytes are present in the lower part of the epidermis of a lichen planuslike keratosis, clues to specific diagnosis of that condition can be found at the periphery of the lesion where a hint of the original solar lentigo or reticulated seborrheic keratosis may be spotted still. As a rule, clusters of necrotic keratocytes are not present at the dermoepidermal junction in plaques of mycosis fungoides; in fact, individual necrotic keratocytes are encountered uncommonly in that condition.