CASE #1: Myrmecia

The young man’s painful “bump” was a myrmecia, commonly known as a “plantar wart.” This benign epidermal neoplasm is caused by the human papillomavirus (HPV) subtype 1 and is usually found on weight-bearing areas, such as the metatarsal head and heel. The lesions occur less frequently on the palms or subungual areas. Plantar warts begin as small, shiny papules and progress to deep endophytic, sharply defined, round lesions, with a rough keratotic surface, surrounded by a smooth collar of thickened horn. Because myrmecia grow deep into the plantar surface, they tend to be more painful than common warts. This condition is most often observed in patients aged 12-15. Those with one wart will usually have additional lesions on further inspection.

On physical examination, a crateriform plaque (endophytic wart) or stucco papule (exophytic wart) is seen. Either morphology will interrupt skin lines, an important distinguishing feature from the corn. Myrmecia lesions may be slightly erythematous and have a corrugated surface; they typically measure 2-5 mm in diameter. Warts also usually contain black puncta, which patients may refer to as the “root.” These discolorations are actually thrombosed capillaries caused by HPV infection of endothelial cells. Since warts are viral-induced, an autoinoculation or Koebner’s phenomenon may occur; that is, lesions that are scratched or otherwise traumatized may lead to a row of sister lesions. Similarly, warts that have been treated with destructive modalities, such as cryotherapy, occasionally develop into “ring” lesions around the initial site.

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The differential diagnosis for a wart is vast but most commonly includes the clavus (“corn”), molluscum contagiosum, stucco keratosis, actinic keratosis, and angiokeratoma.

The diagnosis of a plantar wart is nearly always made on physical examination alone. On biopsy, an epidermal papillomatous or a verrucous (jagged) papule is seen. Koilocytes (infected keratinocytes) may sometimes be observed as large cells with small pyknotic nuclei. Very rarely, warts may develop into squamous dysplasia and malignancy. Lesions that are wholly unresponsive to therapy and crust over frequently should be biopsied to rule out malignant deterioration.

Plantar warts are notoriously stubborn to treat, and patients often report using numerous OTC preparations before seeking medical attention. Many warts resolve spontaneously over several months, but few people have the patience to wait. A majority of therapies are destructive modalities. These include various acids (salicylates and trichloroacetic acid are the most commonly used), cryotherapy, cantharone, podophyllin, surgery, and carbon-dioxide laser. Zealous therapy results in scarring and should be avoided given the benign and self-limited nature of this condition. More novel and noninvasive therapies include application of duct tape, which has been shown to be more effective than cryotherapy. Such other therapies as retinoids and imiquimod have also shown utility in those lesions unresponsive to more traditional options.

Our patient was treated with three courses of cryotherapy combined with a home regimen of paring followed by topical application of salicylic acid as well as duct-tape occlusion nightly. Three months later, the patient’s smaller warts had resolved, and the larger wart had decreased significantly.