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ERYTHEMA MULTIFORME (tier 2)
(a representative of the general category of lichenoid lymphocytic vasculitis)
In the category of lichenoid lymphocytic vasculitides, erythema multiforme is distinguished by the formation of clusters of
necrotic keratinocytes in the basal layer of the epidermis and by the regeneration of keratinocytes, in whorling patterns, about the clusters of necrotic cells. In a addition, in the toilet of such a process, the
necrotic cells, in concert with the whorls of regenerating keratinocytes, first are carried from their site of origin in a hyperplastic basal unit into the population of terminally differentiating keratinocytes of
the superficial unit of the epidermis. They then are transported to the surface to be discharged into the keratin layer (in toto, this toilet of the epidermis qualifies as whorled transepidermal elimination) (S4C11VA3-1-5).
The classic pattern of a lesion of erythema multiforme is a lichenoid reaction affecting both the basal layer and the basal unit of
the epidermis in combination with perivenular infiltrates of lymphoid cells in the dermis. The cells of the basal unit focally undergo lysis and coagulation necrosis (apoptosis). The necrotic cells may not show
significant shrinkage; often they appear polygonal in outline. They may be solitary, or clustered in the basal unit. In response to this injury, the adjacent, viable keratinocytes proliferate in concentic patterns
about the nidi of necrotic cells. The regenerating cells often appear spindle shaped, but curved, with the concavity accommodating a defect containing the clusters of necrotic cells (S4C12P4-1-3). In addition, there is a strong tendency for the viable cells of the basal unit to make the phenotypic switch from regenerating cells of a
basal unit to terminally differentiating cells. They make the transition from cells of basal unit-type to cells of superficial unit-type. In the process the dimensions of the superficial unit are expanded (S4C12P4-2); those of the basal unit are correspondingly diminished. The regenerating cells (i.e., those forming the whorls) tend to have pale cytoplasm.
The epidermal patterns sometimes are complicated by lysis of keratinocytes. In S4C12P4-5, the patterns, in part, appear to be those of a spongiotic change. On closer inspection (S4C12P4-5-6),
the patterns are something more than simple inter-cellular edema. They take on the qualities of ballooning degeneration. In areas, the defects are reticulated; this quality speaks for a component of ballooning
degeneration - a process which, with progression, results in lysis of the affected cells (these are herpes virus-like [cytopathic] changes, even though intra-nuclear inclusions are seldom identified). The resulting
defects are reticulated. Occasionally, in the clinical setting of erythema multiforme, it is possible to identify herpes simplex-like inclusions in the nuclei of some of the cells in areas of reticulated
degeneration (S4C12P4-7). Regardless of the presence, or absence, of suggestive inclusions, an evaluation of some of the patterns in the epidermis in a
lesion of erythema multiforme often will show features in common with those of herpes virus infection (S4C13P5-1-3). See also (S4C14P6-1-6)
Some examples of erythema multiforme centrally are characterized by marked papillary dermal edema (S4C12P4-4). In such lesions, an examination of the patterns at the periphery will usually show the features of the “epidermal” pattern of erythema
multiforme. In general, the presence of marked papillary edema will be associated with a reduction in the intensity of cellular infiltrates at the dermal-epidermal interface. This reduction in the density of
cellular infiltrates in areas of marked papillary dermal edema is a feature seen in a variety of inflammatory processes. The edematous papillary dermis forming the floor of some sub-epidermal vesicles is
usually less infiltrated with inflammatory cells than the neighboring papillary dermal beyond the zone of edema and vesiculation. Eosinophils occasionally are a feature of the infiltrates of erythema multiforme.
The skin appendages are affected in lesions of erythema multiforme; the changes are basically similar to those at the
dermal-epidermal interface (S4C12P4-5).
Patterns in this category are not always diagnostic of erythema multiforme. The upper migration of keratinocytes, and an epidermis
showing a shift to patterns of terminal differentiation qualifies as erythema multiforme-like qualities, without identifying the process as erythema multiforme (S4C12P4-6). Occasionally, in classic examples of erythema multiforme, patterns may overlap with other lichenoid processes such as lichen planus; lytic
defects may be found at the dermal-epidermal interface in such examples (S4C13P5-1-7).
The changes in drug eruptions and in toxic epidermal necrolysis overlap with those of erythema multiforme (S4C15P7-1-5, & S4C16P8-1-2). For some problematic lesions, it may be difficult to
make a distinction between erythema multiforme and pityriasis lichenoides.
Darragh TM, et al: Identification of herpes simplex virus DNA in lesions of erythema multiforme by the polymerase chain reaction. J
Am Acad Dermatol 1991;24: 23-6.
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