Lichen planus most often manifests in the fourth through sixth decade of life as flat-topped itchy pink-to-purple polygonal papules to plaques. Lesions range from 2 to 10 mm and favor the volar wrists, shins, and lower back, although they may be widespread. Oral lesions manifesting as ulcers or white lacy patches in the buccal mucosa are common and may be a clue to the diagnosis. Unlike more generalized contact dermatitis, lichen planus lesions are discrete.
Psoriasis manifests as well-demarcated scaly plaques distributed symmetrically over extensor surfaces. The plaques commonly are found on the elbows, knees, and scalp. When psoriasis manifests in a very limited form (as just a single plaque or limited number of plaques), it can be hard to confidently exclude other etiologies. In these circumstances, look for psoriasis signs in more unique locations (eg, pitting in the nails or plaques on the scalp or in the gluteal cleft). Adding those findings to an otherwise solitary plaque significantly adds to diagnostic certainty.
Diagnosis entails getting the shape of things
Diagnosis is based on history of exposure to irritating or allergic substances, as well as a clinical exam. Skin examination of contact dermatitis can vary based on how long it has been present: Acute manifestations include erythema, oozing, scale, vesicles, and bullae, while chronic contact dermatitis tends to demonstrate lichenification and scale.1
Distinctive findings. The most distinctive physical exam findings in patients with contact dermatitis are often shape and distribution of the rash, which reflect points of contact with the offending agent. This clue helped to elucidate the diagnosis in our patient: his rash was perfectly demarcated within the precise area where the patch was applied daily.
Irritant vs allergic. Patch testing can be performed to differentiate irritant vs allergic contact dermatitis.1 Irritant contact dermatitis usually is apparent when removing a patch and will resolve over a day, whereas allergic contact dermatitis forms over time and the skin rash is most prominent several days after the patch has been removed.1
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