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S4C1 - PITYRIASIC-LICHENOID REACTIONS (tier 1)
(Erythema Multiforme-like & Pityriasic Reactions)
SECTION 4
Chapter1 (Home)
Richard J. Reed, M.D., New Orleans, LA
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Lichenoid Lymphocytic Vasculitides
(Pityriasic-lichenoid Reactions, including Lues, Erythema Multiforme, Pityriasis Lichenoides, & others)
In the development of histologic models of inflammation, a category of lichenoid lymphocytic vasculitides has utility. In this category, the patterns include: 1.) an interface
reaction which satisfies the criteria for the recognition of a lichenoid reaction; and 2.) a perivenular component which extends along vessels into the upper (or even deeper) portions of the reticular dermis. In
addition, these disorders generally are associated with lichenoid patterns affecting appendageal (both follicular and syringeal) components; the lichenoid qualities, affecting the adnexae, are comparable to those at
the dermal-epidermal interface. An hyperplasia of the basal unit of the epidermis sets the lichenoid lymphocytic vasculitides apart from the lichen planus-like category. LP-like reactions are associated with a
phenotypic shift in which most, or all, of the keratinocytes of the affected epidermis in a lesion take on the qualities associated with terminal differentiation. Terminal differentiation is descriptive of
the programmed death of cells in the superficial unit of the epidermis. It has application to physiologic processes leading to the formation of a normal keratin layer. In pathologic processes, changes in the nature
of the programmed process are manifested in an altered keratinized product along the surface of the lesion. In lichen planus-like lichenoid reactions, the superficial unit of the epidermis undergoes hyperplasia and
the product at the surface consists of compact orthokeratin.
In the pityriasic-lichenoid category, the basal unit, although damaged by the interaction with activated T lymphocytes, shows regenerative hyperplasia. The preservation of a basal
unit impacts on the kinetics of repair in response to the erosive effects of a lichenoid reaction; the delivery of cells from the basal unit to the superficial unit lessens the need for compensatory hyperplasia of
the superficial unit. In addition, with the preservation of a basal unit and continued replication of cells in the basal unit, defects in the epidermis are repaired by regenerating keratinocytes; there is no need
for fibroblasts to invade defects (where they might then inlay newly formed fibrous tissue). Accretive fibrosis generally is not a prominent feature. Finally, the regenerating keratinocytes proliferate around and
then sequester necrotic keratinocytes. Having sequestered the necrotic cells within whorls of regenerating keratinocytes, both the whorls and the necrotic keratinocytes are delivered to the surface of the skin;
there is little opportunity for the necrotic cells in lytic defects to become entrapped in newly formed fibrous tissue, as is the rule in the LP-like disorders. In view of these distinctions, one might anticipate
that basement membrane changes are less severe in the erythema multiforme-like and pityriasic categories.
Having proposed these distinctions, it must be admitted that all of these pityriasic-lichenoid disorders may, at certain stages, show epidermal patterns which overlap with those of
the lichen planus-like category; the distinctions are not exclusive at all stages during the life history of any given lesion. In the pityriasic-lichenoid categories, the basal unit, which suffers to the point of
extinction in lichen planus-like reactions, tends to be hyperplastic. At certain stages in the life history of a lesion in the pityriasic category, the patterns may be erosive at the dermal-epidermal interface; the
patterns then become lichen planus-like. As a consequence, some of the disorders in this category may, at some stages in their evolution, have lichen planus-like qualities. The band-like quality of the inflammatory infiltrates in classic lichen planus is characterized by little, or no, extension of the infiltrates along vessels into the upper portion of the reticular dermis. If a problematic lesion, with lichen planus-like features, in both the epidermis and the papillary dermis, shows significant perivascular extensions into the reticular dermis, then the lesion should be qualified as “lichen planus-like” rather being simply dismissed as lichen planus; a lichen planus-like drug eruption, or one of the pityriasic disorders in a lichen planus-like phase then should be considered in the differential diagnosis.
The hyperplasia of the basal unit in the pityriasic-lichenoid category offers opportunities for the migration of lymphocytes and histiocytes into the epidermis along widened inter-cellular spaces (expanded, mucoid
avenues). In addition, this type of epidermal response is likely to be associated with a more rapid delivery of keratinocytes from the basal unit into the superficial unit; keratinization of cells of the superficial
unit, as they migrate to the surface, is likely to be compromised (parakeratosis is the likely result). The mucoid avenues may be open in the epidermis close to the level at which keratinized cells of the horny
layer first appear. This is a common feature of pityriasic disorders, such as lues or pityriasis lichenoides. On the other hand, lesions of erythema multiforme show less hyperplasia of the basal unit, and more
confinement of the infiltrates to the immediate vicinity of the basal layer (perhaps in erythema multiforme, the basal layer is the primary site of antigen fixation to, or within, keratinocytes). The epidermis of a
lesion of erythema multiforme shows hyperplasia of the superficial unit, but commonly shows an almost normal keratin layer; the latter attribute may be related to the acuteness of the process. Older, or more severe,
forms of erythema multiforme may show a plate of parakeratosis between a relatively normal, loosely laminated, orthokeratotic layer and the granular layer. Some of the zones of parakeratosis over lesions of erythema
multiforme represent the contributions provided as whorls of necrotic keratinocytes are carried along within whorls of keratinocytes to be discharged into the keratinized debris at the surface.
Variants of the lichenoid reaction as seen in disorders such as erythema dyschromicum perstans, lichen and macular amyloidosis (epithelial amyloidosis), and lichen aureus are also discussed in this section.
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