The patients in this manuscript have given written informed consent to publication of their case details.

Lineal (or nevoid) sebaceous hyperplasia (LSH) is a lesser-known entity clinically and histologically different from nevus sebaceous (NS). Although both represent sebaceous hamartomas with common clinical and molecular features, some relevant differences in their clinical presentation and behavior must be stressed.

A 39-year-old woman consulted about the growth of a pinkish lesion on the helix of her left ear, which she had noticed during the previous months. The lesion had appeared among multiple pearly skin-colored papules that the patient recalls since adolescence (Fig. 1). Her personal and family history was unremarkable.

Numerous whitish, nonconfluent, smooth papules were seen forming a lineal plaque along the left helix. Among them, a greater pink, mildly keratotic formation was noted.

A shave of the greater lesion and a punch biopsy of one of the remaining papules were concurrently performed. Histopathological examination (Fig. 2) showed an overlapping picture in both with mild epidermal hyperplasia and the presence of small but mature sebaceous glands, some of them draining directly into the epidermal surface. Some small rudimentary hair follicles were present. Apocrine glands were not noted. Immunohistochemical staining did not evidence loss of mismatch proteins (MLH1, MSH2, MSH6, and PMS2). No mutations in BRAF were detected (COBAS 4800, Roche).

These findings, within this clinical context, allowed the diagnosis of LSH. Expectant management was decided.

LSH has been scarcely reported in literature. It presents as multiple, lineally arranged and nonconfluent, smooth skin-colored to yellowish millimetric papules, some of which can be umbilicated. It is usually acquired, more often during the first 2 decades of life. LSH has been described affecting the face (with a preference for its lateral aspects and the periauricular region), chest, and genitalia. This lesion must be distinguished from solitary sebaceous hyperplasia, NS, and striated and juxtaclavicular beaded lines. The clinical picture of NS is remarkably different, usually appearing at birth as velvety yellowish plaques located on the scalp, but it can also affect the face.

Histologically, it is defined by an increase of mature sebaceous glands draining to the skin surface through common central ducts. Sometimes, a peripheral row of undifferentiated sebocytes can be observed. Epidermal hyperplasia, if present, is usually mild and lacks the verrucous aspect of NS, which is the most relevant differential diagnosis. Although a more typical feature of NS, small immature hair follicles have also been reported in some instances of LSH, including the original description of this entity by Fernández and Torres. Apocrine glands are lacking.

Both LSH and NS represent sebaceous hamartomas and share some clinical and histological similarities. However, it must be underlined that while multiple adnexal benign and even malignant neoplasias (ie, basal cell carcinoma) are known to arise within NS, no secondary neoplasms have been described in LSH to date. A greater differentiation potential of NS may account for this, the latter usually showing less mature sebaceous glands and a more conspicuous follicular and even apocrine component.

Despite their different behavior, mutations in the Ras/Raf/mitogen-activated protein kinases pathway seem to underlie the genesis of both LSH and NS. NS are known to harbor postzygotic mutations of HRAS and KRAS in 95% and 5% of cases, respectively. While pathophysiological events behind LSH have not been studied as such, mutations in HRAS, KRAS, and epidermal growth factor receptor have been demonstrated in up to 60% of sporadic sebaceous hyperplasia. These findings support the hypothesis of LSH being a mosaicism of postzygotic mutations in this pathway.

We present an instructive case of the first baffling sebaceous lesions, which were later shown to be concordant with LSH. Unlike NS, it usually appears later in childhood or adolescence and is made up by mature sebaceous glands. These traits may go in line with a greater degree of cellular differentiation and thus reduced potential to give rise to secondary neoplasms.