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Linear sebaceous naevus syndrome
This syndrome is a rare condition affecting the skin, brain, and other body systems. It typically presents with a specific type of birthmark, a sebaceous naevus, which can be associated with neurological, eye, and skeletal issues. The severity can vary significantly among individuals.
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Overview
Linear sebaceous naevus syndrome, sometimes referred to simply as sebaceous naevus syndrome, is a rare neurocutaneous condition. This means it affects both the skin and the nervous system, although other body systems can also be involved. The hallmark of the syndrome is the presence of a sebaceous naevus, which is a type of birthmark made of overactive sebaceous (oil) glands [PMID:33671239]. These skin lesions are typically present from birth or develop in early childhood and often appear as yellowish, waxy, or bumpy patches, most commonly on the scalp or face.
While the skin lesion is the defining feature, the syndrome can also include a wide range of other symptoms affecting the brain, eyes, skeleton, and sometimes other internal organs. The collection and severity of these additional symptoms vary greatly between individuals, making it a highly variable condition. Because it is a rare condition, its exact prevalence in the UK population is not well established.
Symptoms & clinical features
The primary symptom of linear sebaceous naevus syndrome is the sebaceous naevus itself. These are typically linear (following a line) or patchy, yellow-orange to tan lesions that can be smooth or bumpy. They are usually found on the scalp or face, though they can appear elsewhere on the body. Over time, particularly around puberty, these lesions may become thicker or more warty in appearance due to hormonal changes.
Beyond the skin, individuals with linear sebaceous naevus syndrome may experience a variety of other symptoms. Neurological involvement is common and can include developmental delay, learning difficulties, seizures, and structural abnormalities of the brain [PMID:30138927]. Eye abnormalities, such as coloboma (a gap in part of the eye), clouding of the cornea, or other visual impairments, can also occur. Skeletal issues, such as bone deformities, curvature of the spine (scoliosis), or differences in limb length, are sometimes observed. Less commonly, heart, kidney, or dental anomalies may be present.
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Affected organs
The most consistently affected organ in linear sebaceous naevus syndrome is the skin, where the characteristic sebaceous naevus develops. This birthmark is typically composed of mature or immature sebaceous glands, hair follicles, and sweat glands, which can proliferate.
After the skin, the brain and eyes are frequently affected. Brain involvement can range from mild developmental differences to more significant neurological challenges like epilepsy and structural brain anomalies. Eye abnormalities can impact vision. The skeletal system is another commonly involved area, with various bone and cartilage abnormalities reported. Other internal organs like the heart and kidneys are less commonly affected, but their involvement can contribute to the overall clinical picture.
Risks & severity
The severity of linear sebaceous naevus syndrome is highly variable. Some individuals may only have the skin lesion with no other health problems, while others can have significant neurological impairment, visual difficulties, and skeletal issues. The presence and severity of associated features do not always correlate with the size or location of the sebaceous naevus.
There is a small risk that the sebaceous naevus can develop into other skin growths, both benign (non-cancerous) and, very rarely, malignant (cancerous) tumours, especially later in life [PMID:33671239]. Therefore, regular monitoring of the skin lesion is generally recommended. The specific risks and long-term outlook for an individual will depend on the range of systems affected and the nature of their symptoms.
Genetic causes
Linear sebaceous naevus syndrome is caused by genetic changes (pathogenic variants) in genes such as HRAS and KRAS. These genes are proto-oncogenes, meaning they play a crucial role in controlling cell growth, division, and communication within the body. Specifically, the HRAS and KRAS genes produce proteins that act as molecular switches in important cell signalling pathways, like the RAS/MAPK pathway. These pathways are vital for regulating fundamental cellular processes.
When a pathogenic variant occurs in HRAS or KRAS, it can lead to uncontrolled cell signalling in the affected cells. This overactivity contributes to the abnormal cell growth seen in the sebaceous naevus and can also impact the development of other tissues and organs in the body. Because the condition is typically mosaic, the genetic change is not present in every cell of the body but only in a subset of cells, often in a specific pattern, which explains why the symptoms can be localised and vary significantly.
- HRAS HRas proto-oncogene, GTPaseThe HRAS gene encodes the H-Ras protein, a key regulator of cell growth, division, and differentiation, involved in signal transduction pathways that influence cellular behaviour.
- KRAS KRas proto-oncogene, GTPaseThe KRAS gene encodes the K-Ras protein, a vital cellular switch involved in relaying signals that regulate cell growth, division, and differentiation, with mutations frequently implicated in cancer development.
Inheritance pattern
Linear sebaceous naevus syndrome is typically caused by a somatic mosaic genetic change. This means the genetic alteration occurs after conception, during the early stages of development, and is therefore only present in some cells of the body, rather than in every cell. It is not usually inherited from a parent in the same way as conditions with a germline (present in egg or sperm) inheritance pattern.
Because the genetic change is somatic and mosaic, it is generally not passed on to future children. This also means that parents of an affected child are typically not at an increased risk of having another child with the syndrome. The occurrence is usually sporadic, meaning it arises by chance.
Diagnosis & testing
Diagnosis of linear sebaceous naevus syndrome is primarily based on clinical examination, recognising the characteristic sebaceous naevus and assessing for any associated symptoms affecting other body systems. Imaging studies, such as MRI scans of the brain, may be performed if neurological symptoms are present, or X-rays for skeletal concerns.
Genetic testing can confirm the diagnosis by identifying pathogenic variants in genes like HRAS or KRAS. This testing is often performed on a tissue sample from the sebaceous naevus itself, as the genetic change is mosaic and may not be detectable in a blood sample [PMID:30138927]. Referral to a clinical geneticist or a dermatologist with expertise in genetic skin conditions within the NHS would typically initiate the diagnostic pathway. Genetic testing for conditions like this may be accessed through the NHS Genomic Medicine Service, often guided by specific R-codes for rare developmental disorders or neurocutaneous conditions.
Management & lifestyle
Management of linear sebaceous naevus syndrome is individualised, focusing on addressing the specific symptoms present. Regular surveillance of the sebaceous naevus by a dermatologist is important to monitor for any changes, and surgical removal might be considered, particularly if there are concerns about cosmetic appearance, growth, or potential for complications. However, surgical intervention is not always necessary or recommended.
If neurological symptoms such as seizures or developmental delay are present, a neurologist and developmental paediatrician would be involved to provide appropriate care and support. Eye specialists (ophthalmologists) would manage any visual impairments. Other specialists, such as orthopaedic surgeons for skeletal issues or speech and language therapists for developmental support, may also be part of the care team. Management within the NHS would involve a multidisciplinary approach, often coordinated through a paediatrician or clinical geneticist, ensuring access to relevant specialists and support services. Genetic counsellors can provide essential information and support to families regarding the genetic aspects and implications of the syndrome.
UK care pathway
In the UK, individuals suspected of having a rare genetic condition like linear sebaceous naevus syndrome would typically be referred by their GP or another specialist (e.g., dermatologist, paediatrician) to a Clinical Genetics service. This service forms part of the NHS Genomic Medicine Service (GMS).
Clinical geneticists can provide a diagnosis, explain the genetic basis of the condition, and coordinate further investigations and specialist referrals. They can also access genomic testing, which may be covered by an NHS R-code (a code used to define which genomic tests are available for specific conditions). Genetic counsellors, working within Clinical Genetics teams, offer crucial support, information, and help families understand the implications of the diagnosis.
Frequently asked questions
Is linear sebaceous naevus syndrome always severe?
No, the severity varies widely. Some individuals may only have the skin lesion, while others may experience a range of symptoms affecting the brain, eyes, or bones. The extent of involvement is highly individual.
Can the sebaceous naevus be removed?
Yes, surgical removal of the sebaceous naevus may be considered, especially if there are cosmetic concerns, significant growth, or a very rare suspicion of malignant change. This decision would be made in consultation with a dermatologist or plastic surgeon.
Will my children inherit linear sebaceous naevus syndrome?
Linear sebaceous naevus syndrome is typically caused by a genetic change that happens by chance after conception, meaning it is not usually inherited from a parent. Therefore, the risk of passing it on to future children is generally very low.
What kind of doctors treat this condition?
Management often involves a team of specialists, including dermatologists for skin care, neurologists for brain-related symptoms, ophthalmologists for eye issues, and orthopaedic surgeons for skeletal concerns. A clinical geneticist or paediatrician often coordinates this care.
Is there a cure for linear sebaceous naevus syndrome?
There isn't a single 'cure' for the syndrome itself, but treatments are available to manage the specific symptoms. This can include monitoring skin lesions, anti-seizure medications for epilepsy, and various therapies for developmental support.
References
- Hosalkar HS, Jones DH, Offiah A. Linear sebaceous naevus syndrome and resistant rickets. The Journal of bone and joint surgery. British volume. 2003. PMID: 12793567
- Guhan B, Duncan RD. Linear sebaceous naevus syndrome and resistant rickets. The Journal of bone and joint surgery. British volume. 2004. PMID: 14765888
- Park JM, Kim DS, Kim J. Epibulbar complex choristoma and hemimegalencephaly in linear sebaceous naevus syndrome. Clinical and experimental dermatology. 2009. PMID: 19583735
- Insler MS, Davlin L. Ocular findings in linear sebaceous naevus syndrome. The British journal of ophthalmology. 1987. PMID: 3580338
- Levin S, Robinson RO, Aicardi J. Computed tomography appearances in the linear sebaceous naevus syndrome. Neuroradiology. 1984. PMID: 6504316
- O'Neill EM. Linear sebaceous naevus syndrome with oncogenic rickets and diffuse pulmonary angiomatosis. Journal of the Royal Society of Medicine. 1993. PMID: 8459389
- Seawright AA, Sullivan TJ, Pelekanos JT. Coexistent orbital and cerebellar venous anomalies in linear sebaceous naevus syndrome. Australian and New Zealand journal of ophthalmology. 1996. PMID: 8985552