ANSWER
The correct answer is lichen planus (choice “d”), an inflammatory condition marked by pathognomic histologic changes such as those described. Hardly on the tip of most primary care providers’ tongues, lichen planus is nonetheless quite commonly occuring.
Tinea pedis (choice “a”) certainly belongs in the differential, but it would be unusual in a woman of this age. The biopsy effectively ruled it out, as it did psoriasis (choice “b”) and contact dermatitis (choice “c”).
DISCUSSION
Lichen planus (LP) is the prototypical lichenoid interface dermatitis; an inflammatory infiltrate erases the normally well-defined dermoepidermal junction, replacing it with a jagged “sawtooth” band of lymphocytes. In the process, the cells it kills off (keratinocytes) collect at this interface and are incorporated as necrotic keratinocytes into the papillary dermis.
Classically, the recognition of LP is taught with “The Ps.” LP is said to be:
• Papular
• Planar
• Purple
• Polygonal
• Pruritic
• Plaquish
• Penile
• Puzzling
The word “puzzling” may sound nebulous, but it is actually quite useful for dermatology providers. It comes into play whenever a condition stumps us—as in, “What in the world is this?” This puzzlement causes us to at least consider LP.
LP is often papular, and these papules often have flat (planar) tops. The purple color is more striking in lighter-skinned patients. (Almost all inflammatory conditions are more difficult to diagnose in those with darker skin.) More typically, LP lesions are multi-angular or polygonal, often plaquish, and almost invariably pruritic.
LP is common on the legs and/or trunk, favoring the sacral area. It can affect the scalp (being in the differential for hair loss), can cause nail dystrophy, and is relatively common in the mouth, where it presents as a lacy, reticular, slightly erosive process, usually affecting the buccal mucosae. It may be found on the penis, where it is usually confined to the distal shaft and proximal coronal areas.
Like many otherwise benign conditions, LP can present with bullae. In anterior tibial areas, especially on darker-skinned persons, it can be remarkably hypertrophic.
The cause is usually unknown, although certain drugs—especially the antimalarials, gold salts, and penicillamine—have been known to cause LP-like eruptions. It’s been my observation that flares of LP are prompted by stress (certainly present in this patient’s case). LP may have a connection to hepatitis, although no convincing connection has been established.
TREATMENT
This patient’s condition was treated with topical class I corticosteroid ointment (halobetasol). For her, having a certain diagnosis was almost as important as successful treatment.