Nevus Sebaceus

Etiology

Previously, it was thought that secondary carcinoma was a relatively common phenomenon to occur in nevus sebaceus. However, studies have since suggested that most secondary neoplasms that develop in adulthood have benign origins. Many speculate that the supposed malignant growths that arose in nevus sebaceus were previously misdiagnosed as basal cell carcinoma but were likely trichoblastomas, which can share a very similar histological appearance. The actual incidence of basal cell carcinoma in nevus sebaceus is less than 1%. Trichoblastoma is the most frequently occurring tumor in nevus sebaceus, followed by syringocystadenoma papilliferum. Other common secondary neoplasms include tricholemmoma, sebaceous adenoma, desmoplastic tricholemmoma, apocrine adenoma, and poroma.[4][5][6][7]

Epidemiology

Approximately 0.3% of newborns are affected by nevus sebaceus, and the incidence is equal between male and female patients. The disorder affects all races and ethnicities. Usually, nevus sebaceus appears as a solitary lesion at birth. Sometimes, it may not be identified until after puberty once its classical wart-like appearance has fully developed.

Pathophysiology

Nevus sebaceus is not an inherited skin lesion. It carries postzygotic somatic mutations of the Ras protein family, most commonly HRas. These Ras mutations are also present in commonly occurring secondary tumors of nevus sebaceus such as trichoblastomas.

Histopathology

In infancy or early childhood, nevus sebaceus histologically features immature and abnormally formed pilosebaceous units. At this stage of development, epidermal changes may reveal some acanthosis and mild papillomatosis. During puberty, the lesions enlarge clinically as well as microscopically. The sebaceous glands become much more prominent and are located unusually high in the dermis. The number of sebaceous lobules and malformed ducts increases. The hair follicles are typically immature vellus hairs rather than terminal hairs. At this growth phase, the epidermis becomes more papillated and acanthotic. Some lesions may also demonstrate ectopic apocrine glands or eccrine hyperplasia.[8]

History and Physical

The lesions either appear at birth or shortly afterward. They initially present as a solitary smooth, yellow, well-circumscribed plaque with an oval or linear configuration. When multiple, lesions often arise along the lines of Blaschko. Nevus sebaceus is typically asymptomatic. In the scalp, they are classically associated with overlying partial or total alopecia. At puberty, nevus sebaceus becomes more prominent. It takes on a verrucous or mamillated appearance secondary to the hormonal changes of the child. This evolution mirrors the histologic differentiation of normal sebaceous glands of the skin. The patient or their parents may present to the dermatology clinic for evaluation and potential removal due to its clinical appearance.

Although additional extracutaneous findings would be exceedingly rare, a complete physical should be conducted looking for features such as a coloboma, corneal clouding, strabismus, spinal curvature, hip dislocation, or limb deformities. Consultation with an ophthalmologist or orthopedist should be requested; if any of these extracutaneous findings are present, a neurologist should also be consulted as the neurologic sequelae (such as seizures) may not be evident on physical examination.

Evaluation

Nevus sebaceus is a clinical diagnosis; additional workup is typically unnecessary. The exception would be a child with nevus sebaceus syndrome that presents with a large or multifocal lesion that may be associated with extracutaneous involvement of the ocular, skeletal, or central nervous systems. In this case, referral to neurology and other specialists may be indicated if other physical anomalies are present.

Treatment / Management

The management of nevus sebaceus remains a controversial topic. Because nevus sebaceus was previously thought to indicate a high risk for developing secondary carcinomas later in life, prophylactic excision was an acceptable treatment. However, research suggests the risk for developing secondary carcinomas is low, while the risk of developing secondary benign neoplasms is relatively high. Most secondary tumors arising in a nevus sebaceus occur in adults 40 years or older. Surgical excision is considered an acceptable treatment for nevus sebaceus as early as late childhood, once the risk from general anesthesia lessens or the patient is mature enough to tolerate the procedure with local anesthesia. Other factors to consider prior to surgery include the size and location of the lesion. Superficial procedures such as shave removal, dermabrasion, or laser resurfacing are typically unsuccessful as these do not remove the entire lesion.[5][9][10][11](B2)

Differential Diagnosis

The differential diagnosis of nevus sebaceus depends on the stage of development. In early infancy, the differential diagnosis may include aplasia cutis congenita or mastocytoma. Other differential diagnoses in later stages of development include congenital nevus, epidermal nevus, seborrheic keratosis, verruca, and juvenile xanthogranuloma.

Prognosis

Generally, nevus sebaceus will follow a benign course. Rarely, the presence of a secondary malignancy or sequelae of the nevus sebaceus syndrome may portend a variable prognosis.

Complications

The risk of malignancy is minimal and almost nonexistent in children. Besides the development of a basal cell carcinoma, other secondary malignant tumors may very rarely occur and include sebaceous carcinoma, squamous cell carcinoma, microcystic adnexal carcinoma, and melanoma.