KEN, often referred to as linear EN or linear verrucous EN or simply verrucous EN, are a common form of EN typically located on the trunk and extremities and very uncommonly to the head and neck area.¹,⁴,⁵ KEN are usually present at birth or occur during infancy as skin-colored or pink-to-hyperpigmented, Blaschko-linear streaks and swirls, which may be unilateral or bilateral, usually stopping abruptly at the dorsal and ventral midline. Lesions can be localized or widespread in distribution, and when they are widespread, symmetric, and bilateral, they have been referred to as systematized EN or ichthyosis hystrix. The lesions initially may be flat and subtle, but they tend to become more elevated, verrucous, and dark over time (Figure 12.3.1).⁵ Acral lesions often have a more warty appearance and if the nail matrix is affected, the nail may be dystrophic, whereas in the body folds, lesions are softer and less verrucous.¹,⁵ A special term has been used for localized lesions with a more warty appearance named nevus verrucosus.

NS is always present at birth; however, it may not be noticed until later in childhood or until after puberty, when hormonal influences to sebaceous glands trigger thickening and the development of a papillomatous surface. Most NS are salmon to yellow colored and have a characteristic smooth waxy surface, which can be complicated by eczematous reactions (Figure 12.3.2).¹² Most are located on the scalp or face, although they can be more widespread.⁵ If they are located on the scalp they are accompanied by alopecia. The size may vary from less than a centimeter to several centimeters in length, and a patient can rarely have multiple lesions.⁸ There are a few reports of unusually large and exophytic lesions, with histopathological picture consistent with nevus sebaceus.¹³,¹⁴

NC is a developmental anomaly of the pilosebaceous unit that typically presents at birth or during childhood. Clinically, it is characterized by a linear array or clusters of dilated follicles containing keratin, resembling comedones (Figure 12.3.3). Usually the face, trunk, and upper extremity are affected, but lesions may rarely be found on the distal extremities, the scalp, or genitalia.¹⁵,¹⁶ Lesions are usually unilateral and if large they follow Blaschko lines; however, bilateral lesions have been reported, as well as extensive involvement of half the body.¹⁵,¹⁷,¹⁸ NC may be complicated by superinfection with bacteria, chronic inflammation, and scarring.¹⁵,¹⁹

It usually presents in infancy or early childhood on the lower body as an intensely pruritic, unilateral array of linear erythematous and hyperkeratotic papules that often coalesce into plaques (Figure 12.3.4).⁴,⁷ The lesion is characterized by intense pruritus and the prognosis is variable, often waxing and waning over time.⁸ ILVEN is often mimicking psoriasis; however, it is resistant to standard psoriasis treatments and may be difficult to distinguish from linear (nevoid) psoriasis.⁵

PAON is found equally in boys and girls and the majority of cases present at birth, occasionally as neonatal erosions, or soon after.⁵,⁸,⁹,¹⁰,¹¹ Clinically, it presents as hyperkeratotic papules and plaques with comedo-like punctuate pits, often filled with keratin plugs.⁸ Lesions
are usually located on the palms or soles in a linear pattern, but several cases have been reported with widespread unilateral or bilateral lesions distributed along Blaschko lines.¹⁰,²⁰,²¹

PENS lesions appear at birth as small, scattered, hyperkeratotic papules; they are rather round, oval, comma shaped, or polygonal and do not follow the lines of Blaschko.⁶ They are found in many different areas of the body, but the palms and soles are spared.

BN mainly appears between the first and second decade of life, sometimes after sun exposure and is rarely described at birth²²,²³ (see also chapter 12.1.3). Lesions are typically located on the trunk or proximal upper extremities, but other parts of the body may also be involved, including the face²⁴,²⁵ and 12% of the lesions appear on the lower extremities.⁵,²⁶ Most are solitary and unilateral, although multiple and bilateral BN have been reported.²⁷,²⁸ BN may appear as a single irregular macular hyperpigmentation or as multiple hyperpigmented macules arranged in a checkerboard pattern.²⁹ Hypertrichosis within the lesion is common, but not universal.⁸ Three varieties of BN have been recognized by Parizzon et al³⁰ according to clinical presentation: melanotic (39%), hypertrichotic (19%), and mixed types (42%). The melanotic type is characterized by only hyperpigmentation, without hair or with only vellus hair and in some cases is difficult to differentiate from a large pale congenital nevus or a giant café au lait macule. The prevalence of BN has been estimated between 0.25% to 2.1%, and males are affected predominately, but the exact male to female ratio is unknown.⁸,³¹ When stroked or rubbed, BN become more elevated and appear more infiltrated (pseudo-Darier sign), secondary to piloerection of the hairs within the nevus.⁵

On dermoscopy features commonly recognized are the presence of a network, hair follicles, and vessels, with focal, skin furrow, and perifollicular hypopigmentation.²⁹