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NON LICHEN PLANUS-LIKE VARIANTS (tier 2)
The designation, toxic epidermal necrolysis, has acquired specificity following a clear delineation of the staphylococcal scalded skin syndrome (SSSS) (29). In toxic epidermal necrolysis, features of a reaction of erythema multiforme-like type are represented in an extreme fashion. The lichenoid reaction at
the dermal-epidermal interface is cell-poor. The insult to the basal layer of keratinocytes progresses to a loss of cohesion between epithelium and dermis, and to the shedding of broad expanses of the damaged
epidermis. Within the necrotic roof over the subepidermal clefts, clusters of necrotic keratinocytes provide a marker for the initial lichenoid reaction. In reparative phases, the regenerating epithelium along the
dermal-epidermal interface is thin; most of the cells of the regenerating epithelium have the characteristics of regenerating basal keratinocytes. Vacuoles among basal keratinocytes (vacuolar degeneration) are a
feature of the regenerating epithelium. Whatever the clinical setting, regenerating squamous epithelium characteristically shows intense cytoplasmic basophilia.
The graft-versus host reaction is lichenoid. Lymphocytes migrate into the epidermis, and produce lysis and coagulation of keratinocytes (8). A common
and prominent feature is the presence of a lymphocyte in close apposition to fragments of necrotic keratinocytes (satellite cell necrosis; a pattern of apoptosis). In some examples, the reaction resembles a
cell-poor, lichenoid process as seen in lupus erythematosus. Features of an erythema multiforme-like reaction may be represented. Connective tissue changes may mimic scleroderma, or may have the features of a dermal
mucinosis.
In Rothmund-Thomson syndrome, skin lesions show the pattern of a cell-poor lichenoid process.
In the clinical category of lichenoid eruptions, linear lesions may be spontaneous, or may be distributed along areas of local trauma (Koebner phenomenon). In lichen striatus, lesions are nevoid and linear; a
common histologic pattern is that of a primary lichenoid reaction (3). Lichen striatus is characteristically a disease of childhood, but occasionally is
seen in adults. It is a self-limited process of variable duration. The papules are rarely vesicular. A single lesion of lichen striatus sometimes demonstrates the entire spectrum of a lichenoid reaction (3). The inflammatory infiltrates in the epidermis and papillary dermis usually are spotty in distribution. Within some lesions, papillae are not always
involved in a regular pattern. The infiltrates extend to the apices of the involved papillae and into the overlying epidermis. If they are band-like in the papillary dermis, it is not uncommon for the epidermis to
have well developed psoriasiform qualities, although the rete may be irregularly effaced. Lymphocytes intermingle among hyperplastic, basal keratinocytes, but lytic defects are small and inconspicuous. The keratin
layer is focally parakeratotic. The patterns, as described above, are prototypic; the patterns are those of a primary lichenoid reaction. Focally or diffusely, the reaction, in some lesions, qualifies as an
established lichenoid pattern; it is indistinguishable, but for focality, from the patterns seen in lichen planus. In other lesions, focal senescent patterns with effaced rete ridges and a widened papillary dermis
are represented. The three phases of a lichenoid reaction may be combined in a single lesion.
The lesions of skin appendages in lichen striatus are also distinctive (3). The follicles are commonly involved by a primary, established, or senescent
lichenoid reaction. In the primary pattern, the hyperplastic, follicular epithelium has a widened, mucinous interstitium (follicular mucinosis, unrelated to CTCL). For some, but not all examples, the sweat ducts and
sweat glands are involved by a lichenoid reaction. The sweat glands and neighboring portions of the coiled duct may be surrounded by dense lymphoid infiltrates (the resulting patterns have a lymphoepithelial
quality. Lymphoid cells migrate into sweat glands; they are associated with a
hyperplasia of glandular epithelium. For some portions of the coils, the epithelial hyperplasia produces solid nest with lymphoid cells among the epithelial cells; such alterations can be compared to the lymphoepithelial islands of salivary gland lesions in Sjogren’s syndrome. The epithelial cells of the altered sweat glands have plump, irregular, hyperchromatic nuclei. Loss of nuclear polarity is a prominent feature of the altered epithelium. The epithelial cells have an increased amount of granular, faintly acidophilic cytoplasm. The epithelial islands in the lymphoid infiltrates are reminiscent of epimyoepithelial islands in Sjogren's syndrome. The proliferative changes in the eccrine glands might also be compared to similar proliferative changes in the epithelium of bile ducts in primary biliary cirrhosis. The significance of the proliferative changes in these disorders is not known, but may express a response by target cells to the products of activated lymphocytes (immunostimulation?) (15). Occasionally, similar changes are seen in sweat glands in lesions which do not qualify clinically as lichen striatus.
The linear distribution of papules in lichen striatus may be confused clinically with other linear dermatoses. As a group, linear epithelial nevi are histologically heterogeneous. A large group of these nevi have
features of epidermolytic hyperkeratosis; others are verrucous. A few resemble lentiginous nevi with diffuse melanocytic hyperplasia.
Porokeratosis of Mibelli is a variant of a linear nevus (30). A random biopsy of the center of a lesion of disseminated superficial actinic
porokeratosis often shows a nonspecific, senescent, lichenoid pattern. The distinctive cornoid lamellae, in a proper setting, are diagnostic.
Linear dermatitic nevus or inflammatory linear verrucous epidermal nevus (ILVEN) is psoriasiform (31). A parakeratotic cap which may contain
neutrophils often is a feature. The papillary dermis contains a dense, band-like infiltrate of lymphoid cells which may extend into the overlying epidermis in the pattern of a primary lichenoid reaction. Two
distinctive lesions may be included in the category of linear dermatitic nevus. One variant is not inflamed histologically, but shows the surface accumulation of a parakeratotic scale with sparing of the ostia of
skin appendages.
For those examples of linear, lichenoid lesions in which the superficial unit is hyperplastic and shows compact orthokeratosis, the differentiation between lichen striatus and lichen planus may be impossible.
Involvement of eccrine sweat glands is a feature favoring lichen striatus.
Lichen nitidus
is a lichenoid eruption in which discrete, tiny, uniform papules of 1-2 mm are grouped in patches, but do not coalesce. It is commonly seen in children. The trunk is a common site. Koebner phenomenon may occur. The lesions of lichen nitidus are discrete infiltrates of lymphocytes and histiocytes in the papillary dermis (3). The infiltrates form small, spherical aggregates which press upon the epidermis; they displace and distort neighboring rete ridges. Histiocytes and
lymphocytes are closely intermingled in the pattern of lymphohistiocytic granulomas. Many of the lymphocytes have the nuclear characteristics of transformed T lymphocytes. Multinucleated, histiocytic giant cells are
often a feature. Some of the patterns qualify as a senescent reaction. The histiocytes and giant cells commonly contain melanin granules. The epidermis over a papule shows effacement of rete ridges and lysis of the
basal epidermal unit. The superficial unit is often unaltered. As a consequence, the overlying epidermis may be thin (atrophic). There may be focal areas of parakeratosis. In occasional examples, the superficial
epidermal unit is hyperplastic and shows compact hyperkeratosis. The nodular infiltrates of lymphocytes and histiocytes are randomly spaced in the papillary dermis. The intervening areas are unaffected. For the
exceptional lesions with hyperplasia of the superficial unit and compact orthokeratosis, the differentiation of a papular variant of lichen planus from a lesion of lichen niditus may be difficult. In the setting of
a lichenoid reaction, the distribution lesions and the histiocytic qualities of the infiltrates in lichen nitidus might be cited as distinguishing features. Other clinical manifestations of lichen planus are usually
helpful, such as mucosal and flexural involvement, and Wickham's stria.
The prominent component of histiocytes in the infiltrates of lichen nitidus may be misinterpreted as an epithelioid granuloma; the infiltrates in lichen nitidus are lymphohistiocytic, rarely purely epithelioid. The
histiocytes are rounded, have distinct cell membranes, and are loosely aggregated. They closely intermingle with lymphocytes. These are not features of a classic epithelioid granuloma as seen in sarcoidosis,
leprosy, or some infectious granulomas.
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