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Pigmented Lichenoid Reactions (tier 2)
Lichenoid reactions, in their cytopathic effects, release melanin pigment from the affected keratinocytes and melanocytes; the pigment is phagocytized by histiocytes. The pigmented cells (melanophages) collect in the
papillary dermis. The presence of pigmented, melanin-rich histiocytes in the dermis qualifies the resulting patterns as melanoderma. Incontinence of melanin is especially common in lichenoid drug reactions.
Often in melanodermas, keratinocytes also will be pigmented (epidermal hypermelanosis). As pigmented keratinocytes move to the surface and commit to terminal differentiation, the pigment will condense, as the
keratinizing cell contracts. In the keratin layer, clumped melanin will be found among, and within, some of the lamellae; these pigment deposits in the keratin layer may be mistaken for areas of parakeratosis.
Erythema dyschromicum perstans (ashy dermatosis) is a lichenoid reaction, but the lytic changes generally are mild; they tend to be focal at the dermal-epidermal interface. Inflammatory infiltrates are
mild in the upper portion of the dermis. Vacuolar changes at the dermal-epidermal interface are spotty in distribution; the patterns are cell-poor. Rete patterns are focally effaced. Occasional colloid bodies may be
found in the papillary dermis. The epidermis in the areas showing vacuolar changes is likely to be hypopigmented (focal epidermal hypomelanosis). The papillary dermis is widened and fibrotic; characteristically, it
contains melanophages (S4C23P15-1-4).
Other disorders showing a cell-poor lichenoid reaction, with some degree of melanoderma, are the primary forms of cutaneous amyloidosis, including lichen and macular amyloidosis. Melanoderma is a more
prominent feature in the macular variant. In lichen amyloidosis, the epidermis is hyperplastic, particularly the superficial unit. The lichenoid (vacuolar) changes are spotty in distribution (it is worth
repeating that the lichenoid reaction is an erosive, or cytopathic process mediated at the level of cellular immunity with keratinocytes of the basal unit of the epidermis as target cells). The patterns in these two
variants of cutaneous amyloidosis are cell-poor. In the areas showing vacuolar changes, the basal layer may be interrupted. Amyloid collects in the form of rounded globules at the dermal-epidermal interface (S4C24P16-1-4). Amyloid deposits may also be found in the cytoplasm of keratinocytes in transit to the surface of the skin
(this phenomena might be characterized as a variation of “transepidermal elimination”). The amyloid shows congophilia (a positive reaction with congo red, or a comparable, stain) (S4C24P16-5); it is birefringent (with a congo red stain, a section containing amyloid will show apple green birefringence in
the areas of amyloidosis). In macular amyloidosis, the epidermis usually is not significantly hyperplastic; rete patterns may be partially preserved. The changes at the dermal-epidermal interface are similar
to those seen in lichen amyloidosis. There are alternating areas of epidermal hyper- and hypo-melanosis. Melanoderma also is a feature. The amyloid deposits are not as prominent in the macular forms as in the lichen
forms.
The nodular deposits of colloid milium differ from the deposits of amyloid of the primary cutaneous variants (S4C25P17-1).
The amyloid of macular and lichen amyloidosis might be characterized as epithelial type (presumably it is the product of keratinocytes). Similar deposits, probably also of epithelial origin, occasionally may be
found in the stroma of low grade epithelial tumors of the skin, such as some inflamed seborrheic keratoses and many low grade (well differentiated) basal cell carcinomas. They may be encountered in the stroma of
trichoepitheliomas.
Not all of the pigmented “lichens” are examples of melanoderma. The pigmented purpuras of the lower extremities often have lichenoid qualities at the dermal-epidermal interface. In this group of “progressive
pigmented purpuras,” one variant is characterized as lichen aureus. The lesion histologically has a papular quality with a confluent infiltrate of lymphocytes and histiocytes in a widened papillary dermis.
Extravasated rbc’s are a variable feature, but hemosiderin deposits in histiocytes of the lymphoid infiltrates provide a marker for sites in which extravasated red blood cells have been phagocytized and then
converted to hemosiderin (S4C26P18-1-4).
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