Case 1 History
A 45-year-old male with a history of hepatitis C presents with multiple purple, pruritic, polygonal flat-topped papules distributed symmetrically on the wrists and thighs. Close examination reveals a surface network of delicate thin white lines.
Microscopic Findings
There is a dense bandlike infiltrate of lymphocytes that permeates and obscures the dermal–epidermal junction (DEJ). The epidermis shows irregular, jagged rete ridges with a serrated configuration subjacent to wedge-shaped hypergranulosis and compact orthohyperkeratosis. The stratum spinosum seems to merge with the perijunctional infiltrate, reflecting loss of the stratum basale ( Fig. 2.1 ).
Diagnosis
Lichen Planus
Clinical Presentation
Lichen planus (LP) typically presents as a multiple, purple, pruritic, polygonal flat-topped papules. The papules are often surmounted by a network of delicate thin white lines known as Wickham stria. Associated pruritus is the rule. The most common distribution involves the extremities, particularly the wrists and thighs; the genital region, especially the penis; and the oral mucosa of adults. Spontaneous resolution often occurs within 1 year. LP may show an association with hepatitis C infection in some individuals, and other associations have been described. The clinical morphology is diverse, with LP presenting in an annular, atrophic, or hypertrophic fashion. LP can also involve the oral cavity, with or without concomitant cutaneous involvement.
Histopathology
LP is the prototype of lichenoid dermatitis. Sections show a dense bandlike infiltrate of lymphocytes that permeates and obscures the DEJ with accompanying interface changes, including vacuolar degeneration and necrosis of basal keratinocytes (historically termed colloid bodies). The epidermal undersurface converts to a serrated rete ridge configuration, which is coupled with hypergranulosis and orthohyperkeratosis. Because of chronic vacuolar change, the stratum spinosum may merge directly with the subjacent infiltrate, which reflects loss of the basal epidermis. Tiny perijunctional clefts, historically known as Max-Joseph spaces, can also be identified along the junction. The infiltrate consists mostly of small mature lymphocytes, but rare plasma cells and eosinophils can be found, particularly in oral involvement or in the hypertrophic subtype of LP. Direct immunofluorescence is uncommonly used as a diagnostic tool in this context but does highlight fibrinogen deposited linearly along the junction. In atrophic LP, the rete ridge pattern is muted or effaced. In the hypertrophic subtype of LP, associated acanthosis is prominent.
Differential Diagnosis
The primary differential diagnostic considerations include lichenoid drug eruption, lichen planus–like keratosis (LPLK), and secondary syphilis ( Table 2.1 ).
| Lichen Planus | Lichenoid Drug Eruption | Lichen Planus–Like Keratosis | Secondary Syphilis | |
|---|---|---|---|---|
| Pathophysiology | Idiopathic; sometimes associated with hepatitis C | Triggered by a drug epitope, presumably expressed by keratinocytes | Cytotoxic attack directed against a lentigo or keratosis | Treponema pallidum (spirochete) infection |
| Parakeratosis | Uncommon | Common | Variable | Variable |
| Location of necrotic keratinocytes in the epidermis | Perijunctional | All layers | All layers | Uncommon |
| Infiltrate | Lymphohistiocytic | Lymphohistiocytic with eosinophils | Lymphohistiocytic | Lymphohistiocytic with plasma cells |
Lichenoid Drug Eruption
Although eosinophils are rare in conjunction with conventional LP, eosinophils may represent a more significant component of the infiltrate in a lichenoid drug reaction. In conventional LP, epidermal turnover is slow, and an orthokeratotic surface is expected. By contrast, in a lichenoid drug reaction, epidermopoiesis may be accelerated, and surface parakeratosis may become prominent. By similar logic, whereas necrotic keratinocytes may be found in outer epidermal layers in a lichenoid drug reaction, necrotic keratinocytes tend to be junctional or subjunctional in conventional LP. Rendering a specific diagnosis is often not possible based solely on histopathologic findings. Clinicopathologic correlation is essential.
Clinical Presentation
Erythematous flat-topped papules with associated scale constitute a typical presentation. The emergence of the eruption often coincides with the initiation of a new medication, such as a beta-blocker, gold, captopril, penicillamine, or anti–PD-1 (programmed cell death protein 1) therapy. The eruption may persist for several weeks after discontinuation of the offending drug. Postinflammatory dyspigmentation may persist after resolution.
Histopathology
Lichenoid drug eruptions show a bandlike infiltrate of lymphocytes or a mixed infiltrate along the DEJ. There may be jagged rete ridges and hypergranulosis, but the epidermal configuration shows greater variation in comparison with LP. Modest spongiosis may be found, and surface hyperkeratosis may include parakeratosis ( Fig. 2.2 ). As is typical of any form of dermatitis, the infiltrate is composed largely of T cells and does not exhibit a distinctive or unique immunophenotype.
Lichen Planus–Like Keratosis
LPLK shows histopathologic features that can be essentially identical to LP in many instances. However, the spectrum of findings is broad. It is generally thought that LPLK represents a localized inflammatory reaction triggered by an actinic (solar) lentigo or a macular seborrheic keratosis.
Clinical Presentation
LPLK, also known as benign lichenoid keratosis, presents as a solitary erythematous slightly raised scaly papule or plaque in an adult. Clinically, the presentation mimics superficial basal cell carcinoma.
Histopathology
Sections show a pattern that is sometimes indistinguishable from LP. LPLK includes a bandlike infiltrate of lymphocytes that obscures the junction ( Fig. 2.3 ). The infiltrate may be mixed and often includes melanophages. The epithelium ranges from atrophic to acanthotic, and in some instances, remnants of preexistent lentigo or seborrheic keratosis may be detectable. There may be associated spongiosis, parakeratosis, and lymphocyte exocytosis. An involutional LPLK is commonly accompanied by perijunctional fibrosis and melanophages.
