Nevus sebaceous is a common congenital skin lesion typically present at birth or appearing shortly thereafter in infancy. It is classified as an organoid nevus, representing a localized overgrowth of normal, mature skin components. This benign malformation is known as a hamartoma.
Defining Nevus Sebaceous and Clinical Appearance
Nevus sebaceous is a cutaneous hamartoma, a benign growth composed of an abnormal mixture of sebaceous glands, hair follicles, and epidermal tissue. It most frequently appears on the scalp, often creating a well-defined area of hair loss, or on the face. It commonly presents as a solitary, oval, or linear plaque.
The clinical presentation changes with age. Its most characteristic form is a slightly raised, waxy, yellow-to-orange plaque. When fully developed, the surface often becomes thickened and bumpy, adopting a verrucous or cobblestone-like texture. The full name, Nevus Sebaceous of Jadassohn, references the dermatologist who first described the condition in 1895.
Understanding the Cause and Developmental Progression
Nevus sebaceous is not hereditary but results from a spontaneous genetic change occurring after conception. This is a post-zygotic somatic mutation, meaning the error happens in a single cell after fertilization. This results in a mosaic pattern where only localized cells carry the mutation.
The mutations most frequently involve the HRAS or KRAS genes, which are proto-oncogenes that regulate cell growth and division. These activating mutations drive the localized overgrowth of skin components. The nevus progresses through three distinct life phases corresponding to hormonal changes.
In the infantile stage, the lesion is typically flat, smooth, and subtle, with underdeveloped sebaceous glands. During childhood, the lesion remains stable. The most significant changes occur during puberty or adolescence, stimulated by rising androgen hormone levels. This hormonal influence causes the sebaceous glands to mature and become hyperplastic, leading to the characteristic thickened, verrucous, and nodular appearance.
Potential for Secondary Growths and Associated Risks
The primary medical concern is the possibility of secondary tumors developing within the lesion, typically after puberty or in adulthood. This risk is related to the underlying genetic mutation, which predisposes the tissue to abnormal proliferation. Studies suggest that up to 30% of lesions may develop a secondary neoplasm over a person’s lifetime.
The vast majority of these secondary growths are benign, meaning they are non-cancerous and slow-growing. The two most common benign tumors are trichoblastoma and syringocystadenoma papilliferum, arising from the hair follicle and sweat gland components, respectively. These growths may present as new bumps or nodules within the existing plaque.
Historically, there was concern regarding the development of malignant tumors, particularly basal cell carcinoma (BCC). While malignant transformation is possible, it is rare, with an incidence now estimated to be less than 1% of all cases. Signs that might indicate a secondary malignant change include rapid growth, bleeding, or ulceration within the lesion.
Diagnosis and Management Strategies
Diagnosis is usually made clinically based on the characteristic appearance and location, especially on the scalp or face of an infant. If the clinical picture is uncertain, a biopsy can confirm the diagnosis through histopathology. Microscopic examination reveals an overgrowth of adnexal structures like sebaceous glands and underdeveloped hair follicles.
Management centers on the decision between observation and prophylactic excision. Excision, which is surgical removal before puberty, was historically recommended to eliminate the risk of secondary tumor formation. Newer studies, recognizing the low risk of malignancy and the benign nature of secondary growths, suggest that observation is an acceptable alternative for many patients.
Surgical removal is typically performed if there is a significant cosmetic concern, if the lesion is large, or if changes suggest a secondary neoplasm is developing. The procedure involves a simple full-thickness skin excision. If surgery is chosen, many clinicians recommend delaying it until adolescence, when general anesthesia risks are lower.