Seborrheic keratoses are benign epithelial lesions that can appear on any part of the body except for the mucous membranes, palms, and soles. The lesions are quite prevalent in people older than 30 years. The etiology of seborrheic keratoses remains unclear. Ultraviolet light exposure may be responsible for the development of some seborrheic keratoses because they appear to evolve from solar lentigines; however, many develop in areas of the skin naturally protected from ultraviolet light exposure, such as the inframammary (intertriginous) areas. Clinically, early seborrheic keratoses are light- to dark brown oval macules with sharply demarcated borders (solar lentigo). As the lesions progress, they transform into plaques with a waxy or stuck-on appearance.
The surfaces of these lesions have a warty and keratotic appearance. Often, the lesions have follicular plugs scattered over their surfaces. The size of the lesions varies from a few millimeters to a few centimeters. Histologically, there are several distinct forms of seborrheic keratoses. In general, the lesions are characterized by papillomatous epidermal hyperplasia of uniform and monotonous keratinocytes and the presence of pseudocysts. The diagnosis of most seborrheic keratoses is straightforward. However, some seborrheic keratoses, especially the deeply pigmented variant, can simulate malignant melanomas. Thin, early lesions have moth-eaten borders and fingerprint-like structures as described for solar lentigines. Thicker pigmented seborrheic keratoses have the typical dermoscopy features as follows :
They are white-to-yellow, round structures that appear very bright when contrasted with their dark brown or black surroundings. The presence of multiple milia-like cysts in pigmented seborrheic keratoses conjures up an image of “stars in the sky.” Milia-like cysts can also be seen in non-pigmented seborrheic keratosis and other lesions such as basal cell carcinoma and melanocytic lesions including congenital nevi and melanoma. However, if the lesion is non-melanocytic and is not a basal cell carcinoma then the presence of milia-like cyst is diagnostic of seborrheic keratoses specially if more then three are seen. It is interesting to note that the quality of milia-like cysts appear somewhat different in seborrheic keratosis and melanocytic lesions. In melanoma and congenital nevi the cysts appear “starry”, which is defined as small, bright and sharp. In seborrheic keratosis they appear “cloudy”, defined as larger and hazier in appearance. Histologically, the cysts correspond to intraepidermal, keratin-filled cysts. It is important to be aware that milia-like cysts are more conspicuous with non-polarized dermoscopy and are often difficult to visualize with polarized dermoscopy.
They are round to ovoid craters that have black or brown comedo like plugs. Histologically, they correlate with keratin-filled invaginations of the skin surface.
Fissures and ridges:
Fissures (sulci) are comedo-like openings, which are not round but rather linear and appear as dark brown to black linear to curvilinear structures within the lesion. The presence of numerous fissures and ridges can result in the formation of network-like structures or result in a cerebriform pattern. Histologically, they represent deep invaginations of the epidermis, filled with keratin.
Interlacing fissures and ridges can create an appearance of network-like structures. The quality of the grid of network-like structures in a seborrheic keratosis differs from the network grid seen in melanocytic nevi by being significantly larger. However, at times the network-like structure in a seborrheic keratosis can look very similar to that of a nevus. Clinical examination of the lesion via side lighting can make the ridges more evident, thereby making it easier to differentiate network-like ridges in seborrheic keratosis and pigment network in melanocytic lesions.
Multiple fissures (sulci) and ridges (gyri) may produce a cerebriform pattern, where the structures resemble sulci and gyri of the brain (brain-like appearance). These features are generally associated with an acanthotic seborrheic keratosis.
Fat fingers are linear and wide dermoscopic structures corresponding to ridges. They often appear as short sausage-shaped structures. Their colors vary from tan/brown, blue to hypopigmented. They are named fat-fingers because their shapes can resemble a straight finger (linear), bent finger (curvi-linear), or finger tip (oval–circular).
Sharply demarcated borders:
As known from clinical examination, seborrheic keratosis often have sharply demarcated borders.
Typical hairpin blood vessels :
Some seborrheic keratoses are associated with hairpin vessels. These hairpin vessels can appear as perfect “U”-shaped vessels of as “U”-shaped vessels that are twisted upon themselves. Typical hairpin blood vessels have a whitish halo around the blood vessel corresponding to the surrounding keratin. It is important to note that some melanomas can have hairpin vessels, but these vessels generally do not have the surrounding white halo but rather have a pink halo. Similar hairpin vessels on a pink background can be seen in irritated seborrheic keratosis.
This is a dynamic test that can only be performed with a contact dermoscopy device. The test is performed as follows: once the contact plate of the dermoscope is firmly pressed down (pressure in vertical direction) against the lesion, it is then moved slightly back and forth in the horizontal plane (parallel to the skin surface). Seborrheic keratosis will appear to stick to the glass plate and move en bloc with the movement of the dermoscopic face plate. In other words, they will slide back and forth. In contrast, nevi will not move en bloc but, rather roll back and forth. In other words, intradermal nevi will wobble. Knowledge of the above-described dermoscopic features and patterns seen in seborrheic keratosis will often prove valuable in differentiating seborrheic keratoses from other lesions, including melanoma. However, irritated or traumatized seborrheic keratoses can mimic melanoma or squamous cell carcinoma. In these cases the history of trauma or the presence of typical criteria for seborrheic keratoses in another part of the lesion might be comforting. However, it is important to remember that skin cancer can develop within a seborrheic keratosis, and thus a biopsy is justified for atypical appearing seborrheic keratosis.
- An Atlas of Dermoscopy, Second Edition. Marghoob A. et al. CRC Press; 2012.
- Braun et al.: Dermoscopy of pigmented seborrheic keratosis: a morphological study. Arch Dermatol 2002;138:1556-60. PMID: 12472342.