Seborrheic keratosis is a benign, exceedingly common epithelial skin neoplasm and most individuals will develop one or even numerous of them during their lifetime.

Clinical features

Seborrheic keratoses appear on any body site except palms and soles, but are most frequent on the face and the trunk. They usually begin as flat, sharply demarcated, brown macules and usually evolve within a solar lentigo. Later on, seborrheic keratosis may become polypoidal with an uneven, papillated surface. From a clinical standpoint, seborrheic keratoses typically have a “stuck on” appearance with a verrucous and dull surface. Their coloration varies from dirty yellowish to opaque brownish-black. Follicular prominence is one of the clinical hallmarks. Although the clinical diagnosis of most seborrheic keratoses can be made easily at a glance, in some instances, due to its many clinical facets even experienced clinicians may have diagnostic problems (See Table).

Dermoscopic features

The dermoscopic features of each of the three major types of seborrheic keratosis, namely, acanthotic, reticulated and verrucous, are different, albeit rather distinctive.

Acanthotic type

The dermoscopic hallmark of acanthotic seborrheic keratosis are few to numerous milia-like cysts and several comedo-like openings, the latter sometimes resembling the so-called irregular crypts. The background coloration varies from an opaque light-brown to dark-brown or even black pigmentation. In about 50% of acanthotic seborrheic keratoses, a vascular pattern exhibiting hairpin vessels and dotted vessels can be appreciated. In lesions showing papillations upon clinical examination, exophytic papillary structures are observed dermoscopically. As a rule, in the acanthotic type of seborrheic keratosis there is no pigment network, but small foci of a fine, delicate pigment network may be evident at the periphery.

Another morphologic finding that is sometimes observed in evolving acanthotic seborrheic keratosis is a global pattern resembling the surface of the brain, and the underlying dermoscopic structures are therefore called gyri and sulci. The dark-brownish gyri as well as irregular crypts and comedo-like openings are basically nothing but keratin plugs within variously shaped indentations of a more of less acanthotic seborrheic keratosis. The yellowish to light-brownish lines between gyri are called sulci or fissures and are arranged reciprocal to the gyri, thus giving rise to the peculiar ‘brain-like’ appearance.
Two variation on the theme of acanthotic seborrhec keratosis may aggravate the dermoscopic diagnosis:

1. The melanoacanthoma variant with a pronounced black pigmentation camouflaging the pathognomonic local features;

2. irritation with variously sized and shaped scale-crusts masking the diagnostic features

Reticulated type

The reticulated type of seborrheic keratosis is characterized by a reticulated pattern that on the face is combined with the site-specific pseudonetwork. This frequently causes diagnostic difficulties in the differentation from melanoma in situ on severely sun-damaged skin (lentigo maligna).

Verrucous type

The verrucous type of seborrheic keratosis basicallt has an unspecific dermoscopic pattern. Because of the exaggerated orthohyperkeratosis, local features are not visible and therefore the dermoscopic examination only reveals withish to yellowish horn masses.