A 62-year-old woman is referred to dermatology for evaluation of a pruritic rash that has been present on both soles for more than two years.

Attempts to treat it with different OTC topical creams (eg, clotrimazole, miconazole), prescription antifungal creams (oxiconazole and nystatin), and even a combination cream (clotrimazole/betamethasone) have been unsuccessful. The last eased her symptoms but yielded no lasting relief.

Her primary care provider had prescribed a month-long course of oral terbinafine (250 mg/d). When that did not help, a two-week course of oral antibiotic (cephalexin 500 mg qid) was tried. Neither had any salutary effect, so the patient was referred first to a podiatrist, who quickly sent her to dermatology.

Complaining bitterly of itching on these areas, she explains that the condition began with tiny pustules, then tiny blisters that gradually covered both insteps. She denies having any such rash elsewhere (eg, on elbows or knees or in the scalp). There is no known family history of skin problems, and no personal history of arthritis.

Just prior to the onset of this problem, she had received a diagnosis of bipolar affective disorder, which forced her to leave her job and start taking lithium. Although the lithium has helped, she remains unable to work.

On examination, the skin of both insteps is covered with discrete and confluent papules and tiny pustules on an erythematous and hyperpigmented (brown), sharply demarcated, and highly symmetrical base. As the patient said, her skin, nails, and scalp elsewhere are free of any such lesions. Given her Hispanic ancestry, her skin is phototype IV/VI.

Punch biopsy is performed and shows almost no spongiosis or edema, relatively tortuous capillary loops, and collections of neutrophils above foci of parakeratosis. Special stain for fungal elements fails to identify any sign of that family of organisms.

ANSWER
The correct answer is a form of psoriasis (choice “a”)—in this case, pustular psoriasis, which is seen mostly on hands and feet.

As in many such cases, biopsy was necessary; it successfully ruled out several items in the differential, in particular contact dermatitis (choice “b”). Ringworm and athlete’s foot (choice “c” and “d,” respectively) are archaic lay terms for fungal infection, which was not only ruled out by its absence in the biopsy specimen, but was unlikely given the lack of response to multiple antifungal medications.

DISCUSSION
Palmoplantar pustulosis is the term most often used to describe a fairly common form of psoriasis typified by this case. Many patients are genetically predisposed to psoriasis, but they may require a trigger to set it off, such as strep infection or occasionally, medication. Notable among the latter are the b-blockers and lithium. Stress is often involved as well.

The bilateral symmetrical involvement of both insteps is highly suggestive of this diag­nosis, which often also affects ­either peripheral or central palms. A secondary form of ­neurodermatitis (itch–scratch–itch cycle) can follow, complicating the picture and perpetuating the problem.

Biopsy was deemed necessary to clarify the diagnosis and gain confidence in the therapeutic process, which can be difficult with this problem.

The patient was urged to consult her psychiatrist about finding a substitute for her lithium. At the time of this description, she is currently applying clobetasol cream under occlusion for three weeks, at the conclusion of which she will be re-evaluated. Many such patients go on to require the use of other medications, such as methotrexate or acetretin.