Linear nevus sebaceous syndrome (LNSS) is a condition characterized by the association of a large, linear sebaceous nevus (type of birthmark) with a broad range of abnormalities that may affect every organ system, including the central nervous system (CNS). The nevus usually is located on the face, scalp, or neck. The most common CNS abnormalities are intellectual disability, seizures, and hemimegalencephaly (abnormal enlargement of one side of the brain). Various other CNS abnormalities have been reported. Other signs and symptoms may include various eye abnormalities; skeletal (bone) deformities; heart defects; urogenital abnormalities; and an increased risk of cancer with age. LNSS is not inherited (it is sporadic). It can be caused by a somatic mutation in any of several genes. Mutations that cause LNSS occur after fertilization and are only present in some body cells (mosaicism). Treatment is directed towards the specific symptoms in each person.

The sebaceous nevus usually is located on the face, scalp, or neck. It can also be located on the arms, legs or trunk. While the nevus may be barely noticeable at birth, it typically becomes more pronounced with age (usually around puberty) and may appear scaly, warty or thickened. It typically does not cause any symptoms.

A variety of CNS abnormalities have been reported in people with LNSS. The most common are intellectual disability, seizures, and hemimegalencephaly (abnormal enlargement of one side of the brain). Other reported CNS abnormalities include:

• Dandy-Walker malformation
• abnormal formation of certain brain vessels
• agenesis of the corpus callosum (absence of nerve tissue that connects the two sides of the brain)
• defects of the folds of the brain, such as a lack of folds (agyria), small folds (microgyria) or thickened folds (pachygyria).

Other signs and symptoms of LNSS may include:

• ophthalmologic (eye) abnormalities such as "crossed eyes" (strabismus), retinal anomalies, coloboma, cataracts, or ocular hemangiomas
• skeletal (bone) abnormalities such as cranial fibrous dysplasia, skeletal hypoplasia (incomplete formation of bones), formation of bony structures, scoliosis, or vitamin D-resistant rickets or hypophosphatemia
• heart defects such as narrowing of the aorta (aortic coarctation)
• urogenital abnormalities such as a horseshoe kidney
• an increased risk of cancer with age

HRAS, KRAS and NRAS genes.Mutations that cause LNSS occur after fertilization (they are not inherited) and are only present in some body cells (mosaicism). These three genes belong to a class of genes called oncogenes, which when mutated, have the potential to cause normal cells to become cancerous. They play important roles in cell division, cell differentiation (the process by which cells learn to do specific "jobs") and the self-destruction of cells (apoptosis).

Somatic mutations in several genes, including HRAS and KRAS, are responsible for causing isolated nevus sebaceous. It is thought that the additional signs and symptoms in people with LNSS occur because the somatic mutation affects other tissues in addition to the skin. While it is unclear exactly how mutations cause the additional symptoms of LNSS, it may relate to disrupted regulation of cell growth and division.

Linear nevus sebaceous syndrome (LNSS) is not inherited. All cases reported have been sporadic, occurring by chance in people with no family history of the condition. While LNSS is caused by genetic mutations, these mutations occur after fertilization in the affected person. They are not present in a parent's egg or sperm.

along with abnormalities affecting other organ systems.

Because of the potential for multi-system involvement, anyone suspected of having LNSS should have a number of evaluations to determine the presence and extent of associated symptoms. These may include:

• imaging studies such as CT scan or MRI of the brain (some researchers think these should be avoided unless there are signs of central nervous system involvement)
• an electroencephalogram if epilepsy is present
• analysis of skin biopsies
• a complete ophthalmic (eye) exam
• a skeletal survey
• chests x-rays

Those who are concerned about cosmetic appearance regarding the nevus might consider surgery to remove the nevus. Surgery used to be recommended due to the risk of becoming cancerous (malignant); however, the risk of malignancy is now much less than previously believed. Surgical removal of the nevus may not always be possible due to its specific location.

Additional treatments depend on the specific abnormalities present and usually follow standard guidelines in the general population for these abnormalities. For example, epilepsy may be treated with anti-seizure medications.