Clark nevi are the most common nevi in man and, moreover, are regarded by many authors as the most relevant precursor lesions of melanoma. Clinical, dermoscopic, and histopathologic variants of Clark nevi are protean, and the differentiation of Clark nevi from melanoma in situ and early invasive melanomas is the major challenge in the realm of pigmented skin lesions.
Clark nevi have been eponymically named after Wallace H. Clark, Jr., who, in 1978, first drew attention to this particular type of nevus by studying numerous melanocytic nevi in patients with concomitant melanomas [Clark 1978].

Clinical features

They are flat to elevated or even slightly papillated pigmented lesions characterized by various shades of brown coloration, and situated on the trunk and extremities. They are usually called just common junctional nevi or common compound nevi. Although Clark nevi are found mostly in skin that has been exposed to sunlight, they may be seen also on the buttocks, the volar surfaces and other covered parts such as genitalia and soles. It is fair to say, however, that Clark and coworkers originally meant that this particular type of nevus, called by them dysplastic nevus, actually represents a distinctive precursor lesion of melanoma with special implications on the management and treatment of patients bearing these nevi.

Ackerman challenged this concept in a series of articles concluding that there is no unanimity among pathologists, dermatologists, surgeons and other physicians about what this term means and about the reproducibility of the correlation between clinical and histopathologic features [Ackerman 1994]. Indeed, in the scientific community there is no agreement on the nature of Clark nevi, on what criteria are necessary for diagnosis, both clinically and histopathologically, and on the number of lesions which are needed to have a markedly increased risk to develop melanoma.

We basically agree with Ackerman that Clark nevi are nothing but common ‘flat’ acquired melanocytic nevi, so frequently found on the trunk and extremities of white Caucasians. In our estimation, the real challenge is to recognize within the many variations of Clark nevi those actually being melanoma in situ or early invasive melanoma. To this end, dermoscopy is by consensus essential, and we will describe those variants of Clark nevi that need to be excised or, at least, followed-up closely by using digital equipment. This goal is hampered by the fact that a ‘gray zone’ between Clark nevi and melanomas exists and that in a certain number of cases this distinction cannot be made even when using all available technologies.

Dermoscopic features

Based on a morphologic study of about 450 Clark nevi in nine patients, we classify Clark nevi dermoscopically into three types, namely, reticular, globular, and homogeneous [unpublished data]. Frequently, combinations of these types are found, the combination of reticular and globular types being the most common one.

The reticular type, by far the most common one, is characterized by a more or less prominent pigment network with thin lines and regular meshes. The pigment network is usually evenly distributed throughout the lesion and fades out at the periphery.

The globular type is characterized by a dotted and/or globular pattern composed of numerous dots/globules of variable size and shape (oval, round or rectangular) more or less evenly distributed throughout the lesion. As already mentioned, the combination of the globular and reticular types is rather common. An interesting morphologic presentation of this combined pattern is a more or less annular arrangement of dots/globules at the periphery of an otherwise typical reticulated Clark nevus.

The least frequent of the three major patterns of Clark nevi is the homogeneous one, characterized by a diffuse pigmentation of various shades of brownish coloration with only isolated reticular and/or globular areas.

Besides the three dermoscopic archetypes of Clark nevi, a number of rather characteristic dermoscopic variants have been noted and are, at least as we perceive it, basically due to a specific distribution of hypopigmentation or hyperpigmentation throughout the lesion, namely, central, multifocal, or peripheral. In this context four rather distinctive subtypes are described and illustrated as follows.

Clark nevus with central hypopigmentation: This is a variant of the reticular type with a more or less centrally situated hypopigmented area almost devoid of other dermoscopic features displaying an annular appearance.
Clark nevus with central hyperpigmentation: This type, also called hypermelanotic nevus, represents a distinctive variant composed of a more or less broad rim of prominent pigment network lines at the periphery and a central, diffuse, irregularly outlined black hyperpigmentation, also called black lamella.
Clark nevus with multifocal hypo/hyperpigmentation: Basically, this type is just a variation on the theme of the reticular pattern with a multifocal hypopigmentation due to several, small, isolated hypopigmented areas, thus leading to an uneven distribution of the pigment network. Another variant of this type is characterized by multifocal zones of prominent, dark-brown to black pigmented network structures in a patchy distribution.
Clark nevus with peripheral hyperpigmentation: In our estimation, this type of Clark nevus is of the uppermost significance, because this group commonly encompasses melanoma in situ or even early invasive melanoma. Dermoscopically, this type has a reticular pattern with a prominent hyperpigmented, and sometimes also atypical pigment network. Certainly, this type of Clark nevus has to be excised.

Besides the three major patterns and the above mentioned modifications based on the distribution of hypopigmented and hyperpigmented areas, additional dermoscopic criteria may be occasionally found in Clark nevi, such as streaks and blue areas, to name but a few. Very rare milia-like cysts and comedo-like openings can be observed in the compound and dermal types of Clark nevi. According to Kreusch and Koch [Hautarzt 1996] a delicate vascular pattern characterized by the presence of comma and dotted vessels is rather common in Clark nevi.