Solar lentigines are sharply circumscribed, uniformly pigmented macules that are located predominantly on the sun-exposed areas of the skin, such as the dorsum of the hands, the shoulders, and the scalp. Lentigines are a result of hyperplasia of keratinocytes and melanocytes, with increased accumulation of melanin in the keratinocytes. They are induced by ultraviolet light exposure.
Unlike freckles, solar lentigines persist indefinitely. Nearly 90% of Caucasians over the age of 60 years have these lesions. Due to the increased prevalence of lentigines in the elderly, these lesions are sometimes referred to as “lentigo senilis”. However, younger individuals who tend to burn after ultraviolet exposure can also develop lentigines after acute or prolonged ultraviolet light exposure. Clinically, solar lentigines may be oval, round, or irregular in shape and can vary from a few millimeters to a few centimeters in diameter. Most lesions have a uniform light brown color; however, there are instances when they vary from dark brown to black. One variant of solar lentigo, “ink-spot” lentigo, has a jet-black color. Actinic purpura or other signs of solar damage can frequently be found in the skin surrounding solar lentigines. Solar lentigines are benign lesions that can evolve to a pigmented seborrheic keratosis. Histologically, it is characterized by club-shaped rete ridges with small nub-like extensions. In addition, there is an increased number of melanocytes and increased pigmentation in the basal keratinocytes. Although most solar lentigines are easily recognized on clinical examination, some lesions pose diagnostic challenges because their clinical appearance resembles that of melanoma. Dermoscopy can be helpful in correctly differentiating a solar lentigo from melanoma. The key dermoscopic features of solar lentigines are as follows:
The presence of a sharply demarcated and irregularly curved border is characteristic of solar lentigines. Often, portions of the border are scalloped, giving a motheaten appearance
Homogenous light brown pigmentation
Many lesions have no structures or networks, only containing light brown and structureless areas; the term “jelly sign” had been proposed to describe the pigment quality of these lesions. The pigment appears as if jelly had been smeared on the skin surface.
Pigment network (Network like structures)
There may be an area of faint, reticulation. This correlates with the presence of melanocytes and melanin-filled keratinocytes in the rete ridges.
They are areas consisting of fine parallel running lines of light brown to dark brown colors. They resemble the dermatoglyphics of a human fingerprint.
Lentigines located on the scalp and face share features of pigmented melanocytic lesions in this special location revealing a pseudonetwork pattern. This is created when a diffusely pigmented area is interrupted by nonpigmented adnexal openings .
Symmetric brown follicular pigmentation
In a solar lentigo the pigment around hair follicles is distributed in a symmetric fashion creating small brown circles. The pigment is usually light brown in color and similar to the color of the rest of the lesion. While the pigment is usually distributed symmetrically around the follicle, some follicles may appear asymmetrically pigmented. These asymmetrically pigmented follicles appear as brown crescent shaped structures. However, these asymmetric follicles will also have a brown color like the rest of the lesion. If the color of the pigment around the follicle, whether symmetric or asymmetric, is of a grayish hue or differs from the rest of the lesion then melanoma needs to enter the differential diagnosis.
Ink-spot lentigines have their own distinct dermoscopic pattern.These lesions have a very prominent blackpigmented network, which has an almost three-dimensional quality under dermoscopy. The network lines can be either thin or thick in width, and the network ends abruptly at the edge of the lesion