Flat warts (Verruca plana) are a distinct type of skin growth caused by specific, low-risk strains of the human papillomavirus (HPV). These growths are typically small, smooth, and only slightly raised above the surrounding skin surface. While flat warts are noncancerous and generally do not cause pain, they are often a source of cosmetic concern, particularly when they appear on visible areas. They are common in the general population, especially among younger individuals.
Identifying Flat Warts
Flat warts are characterized by their unique physical appearance, distinguishing them from other types of warts. They present as small, smooth papules that have a flattened top, which gives them their common name. These lesions generally measure only 1 to 5 millimeters across.
The color of flat warts often blends with the skin tone, appearing as flesh-colored, light brown, pinkish, or yellowish. They frequently occur in groups, sometimes numbering anywhere from a dozen to a hundred warts in one area. Their flat profile results from growing horizontally within the epidermis, instead of projecting outward like common warts.
These warts tend to appear in areas where the skin is frequently exposed or subject to minor trauma. Common locations include the face, forehead, neck, and the backs of the hands. In children, they are often found on the legs and knees, and in adults, they may appear in areas subject to shaving.
The Causes and Spread of Flat Warts
Flat warts are caused by infection with certain strains of the human papillomavirus (HPV), most commonly low-risk types 3, 10, 28, and 49. These strains are considered benign and are separate from the high-risk HPV types associated with certain cancers.
The virus is contagious and spreads through direct skin-to-skin contact with an infected person. Transmission can also occur indirectly through contact with contaminated surfaces or objects. HPV requires a break in the skin barrier to establish an infection, which is why minor cuts, scrapes, or skin trauma increase the risk of developing warts.
A person can also spread the warts from one area of their body to another through a process called autoinoculation. Activities that cause minor skin damage, such as scratching, picking, or shaving, can facilitate this spread, often leading to the warts appearing in a line along the trauma site. The incubation period for the virus can be lengthy, sometimes taking many months before a visible wart develops.
Options for Removal
Flat warts often resolve on their own, with approximately 78% disappearing within two years. However, many people seek treatment for cosmetic reasons or if the warts are spreading. The choice of treatment depends on the wart’s location, the number of lesions, and the patient’s age. Treatment requires persistence, and there is a possibility of recurrence or spread regardless of the method used.
Over-the-counter (OTC) treatments frequently involve topical medications containing salicylic acid. This acid works by slowly dissolving the layers of skin infected by the HPV virus. For facial warts, lower concentrations of salicylic acid or topical retinoids are recommended to minimize skin irritation.
Prescription topical treatments provide stronger options. These include tretinoin, a retinoid that promotes cell turnover and exfoliation. Imiquimod, an immune response modifier, stimulates the body’s immune system to attack the wart virus. Additionally, 5-Fluorouracil, a topical agent that blocks DNA synthesis in rapidly dividing cells, may be used for clearance.
In-office procedures are available for warts that are resistant to topical therapy or for extensive cases. Cryotherapy involves freezing the warts with liquid nitrogen to destroy the infected tissue, though this carries a risk of pigment changes, especially on the face. Electrocautery, which uses an electric needle or laser, is another destructive method reserved for certain cases.
Laser therapy and photodynamic therapy, which uses a light-activated medication, are also options for persistent lesions. Photodynamic therapy, in particular, shows high clearance rates for sensitive areas like the face.