HISTORY
A 61-year-old man is urgently referred to dermatology by his primary care provider, at the request of the patient’s family. During a recent visit, family members happened to see his exposed back, which is covered in lesions. The patient claims the lesions have “been there for years,” growing in size and number. However, his family was greatly alarmed and insisted he seek immediate evaluation.
The patient denies any symptoms associated with the lesions, but does volunteer that they are very much like those his father had as an older man.
DISCUSSION
These are, of course, benign seborrheic keratoses—arguably the single most common problem seen in dermatology offices and the source of much consternation, often to everyone except the patient. Dark, sometimes large, often numerous, they look worrisome. But what are they, exactly?
Seborrheic keratoses (SKs) are benign epidermal excrescences that exhibit a variety of colors and finishes. Varying greatly in size, the typical SK is about a centimeter in diameter, papular to nodular, and tan to grayish brown, with a rough, friable surface. Though little (if any) evidence proves that sun causes them, they tend to appear on the sun-exposed skin of patients ages 50 and older. However, I’ve seen them on patients in their 30s. SKs larger than 10 cm are not unusual, especially on the mid-trunk.
Not everyone develops as many lesions as this patient, but multiple SKs are the rule rather than the exception. Many patients mislabel them as “age spots,” “liver spots,” or “barnacles.” But just as many know that development of SKs is probably hereditary in nature; patients often recall seeing them on their parents or grandparents. Hardly anyone is pleased when they appear, though they are an inevitable consequence of aging for most of us.
The provider’s role, of course, is to distinguish benign from malignant, and that task isn’t always easy. But there are ways to make sure an SK is just that, and not something dangerous.
First of all, anytime you see a multitude of similar lesions that have been present, unchanged, for extended periods on older patients, it’s likely they’re benign.
Next, the “stuck-on” (epidermal) nature of SKs is a very reassuring sign of benignancy. SKs are rough, warty, and dry and clearly develop on the surface of the skin. Their appearance is in sharp contrast to the intradermal nature of, say, melanoma, which may have a raised portion (though at least 80% of melanomas are essentially flat) but cannot be scraped off. Patients will often comment that they’re able to peel SKs off with minor trauma, something one cannot do to a malignant lesion.
Third, on close inspection of SKs, tiny comedone-like pores can be seen scattered over their surface, a feature accentuated by application of liquid nitrogen (which is thus diagnostic as well as therapeutic).
Finally, the application of liquid nitrogen also highlights another indication of the benignancy of SKs: Since SKs have almost no superficial vasculature, they are relatively cool, and when treated with liquid nitrogen, the white color that results persists for several seconds longer than in normal well-vascularized skin or in most melanomas.
This may sound like trivia to the unwary. Unfortunately, there is no rule that says a melanoma cannot develop in the midst of numerous SKs, or even concurrent with an SK. So it does pay to “stay awake” while freezing SKs.
SKs are common on the trunk, arms (especially the triceps), face, scalp, and legs. On the latter, which have fewer oil glands, SKs tend to be drier and whiter. On the face, where sebum is relatively abundant, SKs can be quite dark and shiny. SKs are also relatively common on areas that never see the sun, such as genitals, but they spare the palms and soles, preferring the company of hair follicles.
Biopsy is often required to rule out the other items in the differential, such as melanoma, pigmented basal cell carcinoma, Bowen’s disease, angiokeratoma, and warts.
TAKE-HOME LEARNING POINTS
1. Seborrheic keratoses (SKs) are, by far, the most common benign skin lesions seen in dermatology.
2. The fact that they appear in large numbers speaks loudly to their benignancy, other things being equal (older patient, sun-exposed skin, unchanging nature).
3. The color of SKs can vary a great deal—from orange to brown, gray, or tan to black—in part related to their location.
4. Liquid nitrogen treatment is not only highly effective in treating SKs, but serves as a useful diagnostic tool in two ways: First, it highlights the surface pore-like structures, called horn pseudocysts, which are pathognomic for SKs. Second, the frozen SK stays white considerably longer than relatively well-vascularized normal skin or melanoma, which thaw almost instantly.