Melanotic Macules

The term melanotic macule is used to refer to benign flat pigmented lesions, which are histopathologically characterized by melanin pigment deposited in basilar keratinocytes without or at times with a slight increase in the density of solitary units of junctional melanocytes. Cutaneous and mucosal melanotic macules are often labeled lentigo . The conjunctival equivalent has historically been termed acquired melanosis . In contrast to melanocytic nevi, there are no junctional nests in melanotic macules, lentigines, or benign acquired melanosis. The lesions differ from melanoma in situ clinically by a tendency toward small size and sharp circumscription and histopathologically by a low density of melanocytes. Melanotic macules are a common clinical presentation. They are benign and do not need to be treated. However, they may be biopsied when they display irregular features, are new, or are changing, to exclude melanoma in situ.

Solar Lentigo

Clinical Features

Solar lentigo manifests as a discrete pigmented macule or patch on sun-exposed skin of middle-aged or elderly individuals ( Box 1.1 ). Its color may range from light tan to dark brown. A lesion's color may be uniform or heterogeneous. The size of a solar lentigo may be small (2 mm) or large (>1 cm). Its peripheral borders may be regular or ill-defined ( Figs. 1.1 and 1.2 ). Multiple lesions of solar lentigo may coexist in the same anatomic region. They may collide with each other or with seborrheic or actinic keratoses, become confluent, and appear clinically complex. Clinical atypical lesions, which stand out from the background, prompt concerns for lentigo maligna (melanoma in situ) and are usually biopsied to assess for or exclude melanoma. Solar lentigines become more frequent and/or larger in area with age. Lesions of solar lentigo can also be found in young patients with sun-damaged skin and are common in children with xeroderma pigmentosum.

Box 1.1

Solar Lentigo

Clinical Findings

• Usually adults

• Men > women

• Predominantly whites

• Pigmented macule or patch

• Often in sun-exposed areas

Histopathology

• Basal layer hyperpigmentation

• Normal density of melanocytes or solar melanocyte hyperplasia

• No cytologic atypia

• May be inflamed

• May be associated with melanophages

Differential Diagnosis

• Pigmented actinic keratosis

• Macular pigmented seborrheic keratosis

• Lichenoid keratosis

• Lentigo simplex

• Melanoma in situ

A variant of solar lentigo clinically characterized by presentation as small dark macules, especially on the upper back, has been referred to as “ink spot” lentigo or reticulated melanotic macule.

Histopathologic Findings

Solar lentigo is histopathologically characterized by hypermelanization of basilar keratinocytes ( Figs. 1.3–1.5 ). Often there is associated slight epidermal hyperplasia resulting in elongated, club-shaped rete ridges. As rete ridges become longer, they may form a reticulate pattern. Lesions of solar lentigo may be associated with or develop into a seborrheic keratosis ( Fig. 1.6 ). Pigmentation of keratinocytes is then typically most pronounced at the tip of rete ridges. In some lesions the rete ridges may be effaced, especially when located on the face at acral sites, and/or the epidermis may be atrophic. The stratum corneum of a solar lentigo may be normal or thickened. A solar lentigo may or may not be associated with a slight increase in the density of cytologically bland solitary units of melanocytes. Some lesions are inflamed, often in the pattern of a lichenoid lymphocytic dermatitis, with variable numbers of melanophages (see Figs. 1.4 and 1.5 ). Some of these lesions may also be termed lichenoid keratosis . In a later stage of an inflamed lentigo, there may be no more lymphocytes. Scattered melanophages may be the only evidence to indicate a prior inflammatory reaction.

Lesions of ink spot lentigo are histopathologically characterized by small diameter (“narrow”) and intense hyperpigmentation of basilar keratinocytes ( Fig. 1.7 ). The pigmentation is usually accentuated at tips of rete ridges.

Differential Diagnosis

Solar lentigo may be confused with a lentigo simplex, macular seborrheic keratosis, or pigmented actinic keratosis, but the most important diagnostic issue is its distinction from lentigo maligna (melanoma in situ). The latter may evolve in association with or simulate the appearance of a solar lentigo.

The key features for the distinction of melanoma in situ from solar lentigo with melanocyte hyperplasia are the density of junctional melanocytes, their growth pattern, and cytology. In melanoma in situ, there is a proliferation of solitary units of melanocytes at the dermoepidermal junction, often also focally above it, and extending into adnexal structures. Melanocytes of melanoma in situ may be cytologically bland, but not uncommonly at least a few nuclei are enlarged and/or hyperchromatic. Melanocyte nests or multinucleated melanocytes are not a feature of solar lentigo.