What is spinal dysraphism?
Spinal dysraphism is a condition in which the spine does not form properly in a foetus, causing a gap in the spine.
The spinal cord is the inner column of nerve tissue running along the length of the spine required for communication between the brain and the lower body.
We know that dysraphisms are rooted in neural tube defects.
The neural tube develops into the nervous system, but when the lower part of the foetus’ neural tube does not fully close up, a dysraphism is created. This means that a section of the spine and the associated nerves are left exposed in the back. This failure of closure is influenced by the lack of a molecule called folic acid, or Vitamin B9, which is why it is so vital for women to take it during pregnancy. If they do, the risk of their children developing a dysraphism can be reduced by up to 70%.
However, since this is not a complete reduction, this does mean there are other, more complex factors at play in the formation of a dysraphism, such as genetics. In essence, sometimes a dysraphism will occur even when taking folic acid.
Whatever the cause, the dysraphism can cause damage to the spinal cord and surrounding nerves as the normal layers of protection have been malformed or damaged. As a result, we see patients suffer from neurological problems as well as issues with lower body movement, limb weakness and sometimes cognitive impairment.
What are the various types of spinal dysraphism?
Much of what you might hear about spinal dysraphisms will be about spina bifida. These are a set of three conditions, (spina bifida occulta, meningocele and myelomeningocele) which increase in severity due to the degree of exposure of the spinal cord. However, spina bifida as a term is not the best way to categorise dysraphisms.
Neurologists, neuroradiologists and neurosurgeons prefer to categorise them as open or closed.
Open dysraphisms are when the spinal cord itself, encased in a sac of spinal fluid, is exposed to the air.
A closed dysraphism is where the cord-containing sac is covered by tissue such as fat and skin, even though there is still greater than normal exposure due to the neural tube not closing properly.
Since the skin and nervous system are formed from similar cell lines in the embryo, closed dysraphisms are also often accompanied with skin changes.
These range from (graphic) – simple hair tufts over the region of the dysraphism (as found in the least severe spina bifida) to dermal sinuses, neurenteric cysts and even in very rare cases, a small fleshy branch of tissue colloquially known as a human tail.
The most common open dysraphism is the myelomeningocele, occurring in about 98% of open dysraphism cases. Here, the aforementioned sac – filled with spinal cord and nerves – is completely exposed to the air through the neural tube opening; the other two percent have lesser degrees of exposure.
By contrast it is difficult to say what the most common closed dysraphism is. These would previously have come under the term spina bifida occulta – which essentially just means a non-symptomatic fusion defect that is covered by skin and other tissue – but the range of pathology this encompasses merits deeper exploration.
The best way to think about these is by dividing conditions into those which have subcutaneous (beneath the skin) masses and which do not. The most benign of these are present in around 15% of the global population, with more developed countries having fewer cases than developing countries. This tends to be because of better access to folic acid, though in some countries such as France, there is poor adherence (ref). In general, people of European descent will have a greater chance of developing closed spinal dysraphisms than those from other areas of the world.
Lipomas in particular are a very frequent presentation to paediatric neurosurgical clinics. These are lumps of fat that have either infiltrated the spinal canal and interacted with the neural tissue (lipomyelocele), or have done so when that neural tissue is outside the canal (lipomeningomyelocele). They can be located anywhere along the spine, but cluster around the lumbosacral region at the base of the vertebral column, just above the crease of the buttocks. Depending on their relationship with the end of the spinal cord – the conus medullaris, which may run further down in the back than normal – they may be classified as simple or complex.
Meningoceles are essentially when the covering of the spinal cord protrudes through a gap in the vertebrae and has contact with the air. They are the rarest form presents with small sacs of spinal fluid, but these do not contain nerve tissue. Therefore the symptoms are less severe than in myelomeningocele although some patients suffer from minor disability.
When it comes to closed dysraphisms without a mass beneath the skin, the following are of note:
Posterior Spina Bifida is when there is an isolated deficit in the posterior neural arch, a bit of the vertebrae just behind the spinal canal.
Diastematomyelia is when the cord is split into two, vertically, usually in the lumbar (lower spine) area. There may be accompanying spinal curvature (scoliosis). Interestingly, we find patients generally present with more severe symptoms when there is only one dural (protective) sleeve covering the two strands of cord.
Tight Filum Terminale occurs when the fibrous cord (or filum terminale) supporting the end of the spinal cord by attaching to the coccyx, or tailbone, becomes too tight. This pulls the spinal cord down and stretches it.
Filum Lipomas have a different background to lipomas further up the spine. They infiltrate the filum terminale as a result of an accident in embyrological development. Most of the time they are asymptomatic but, as we’ll talk about below, they can be associated with dysraphisms as well as tethering of the spinal cord.
Symptoms
Many spinal dysraphisms don’t present with any symptoms, especially closed ones. However, with more drastic anatomical abnormalities, especially in open dysraphisms, symptoms are invariably worse. Some paralysis and loss of sensation usually accompany the dysraphism, with the affected area of the spinal cord and below struggling to relay messages to and from the brain. Bowel and urinary issues are also very common.
A tethered cord is when the conus medullaris is limited or ‘tethered’ in its movement, and so is unable to move freely in the spinal fluid. The conus may also be lower than average. It might occur for numerous reasons, such as a tight filum or a filum lipoma.
Tethered cord syndrome can result in problems with lower body movement such as walking as well as back pain, scoliosis, motor or sensory deficits. We talk more specifically about the causes of this in our tethered cord article.
Link to tethered cord blog
A Disclaimer:
We should point out that spinal cord malformations may arise in a number of combinations not covered here in detail. For instance, they are quite common in genetic syndromes such as when you get triples of a certain chromosome (Edwards Syndrome, 18 and Patau Syndrome, 13), or when there are multiple malformations present in other organ systems (e.g. VACTERL). The symptoms a child might present with are basically conserved across all these malformations however.
What are the risk factors?
While we do not fully understand the cause of dysraphisms, genetic and environmental factors appear to play a role.
We find a higher risk in families with a history of the condition.
Evidence also suggests the following can increase the risk:
- Poorly controlled diabetes or obesity in the mother
- Low folate levels in early pregnancy
- Use of some medications such as sodium valproate or phenytoin (commonly used for management of seizures in epilepsy)
How can we make a diagnosis?
Imaging helps diagnose dysraphism.
High-resolution ultrasound is effective in diagnosing myelomeningocele before birth, but an MRI scan (magnetic resonance imaging) would be more useful in diagnosing a tethered cord or diastematomyelia after birth.
Imaging can also help monitor a child’s head growth, as excess fluid in the brain (hydrocephalus) often presents alongside a spinal dysraphism.
Link to hydrocephalus blog
After birth, pigmentation of the skin or patches of hair on the back can help identify the condition.
Moreover, SBO and diastematomyelia can be diagnosed when they present with scoliosis, leg weakness or incontinence. However, they can also go undiagnosed and not require treatment well into adulthood in milder cases.
With open dysraphisms, the presence of alpha-fetoprotein (AFP) in the mother’s amniotic fluid can also support a diagnosis.
What treatments are available?
Surgery
We favour surgery for both spinal dysraphism types if the patient is symptomatic. Such cases include those with skin abnormalities and in those whose abnormalities cause changes in the flow of cerebrospinal fluid within the spinal cord, leading to a build-up in the skull known as hydrocephalus.
For open dysraphisms, ‘back closure’ surgery is often performed, either before or after birth.
This hopes to limit further neurological damage to the baby; as we observe with fetal surgery for myelomeningocele.
Holistic management
Physiotherapy and occupational therapy are widely used to support patients to improve mobility and manage daily life.
We know lots of patients benefit from the use of wheelchairs and walking aids.
While treatment for bowel or urinary issues is also common.
Many parents report great benefit from psychosocial or pastoral support throughout the patient journey.
References
https://www.cdc.gov/ncbddd/spinabifida/facts.html
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3208922/
https://radiopaedia.org/articles/spinal-dysraphism-3?lang=gb
https://www.chop.edu/conditions-diseases/spina-bifida
https://www.shinecharity.org.uk/spina-bifida/spina-bifida
https://www.columbiaspine.org/condition/spina-bifida-occulta/
de la Fournière B, Dhombres F, Maurice P, et al. Prevention of Neural Tube Defects by Folic Acid Supplementation: A National Population-Based Study. Nutrients. 2020;12(10):3170. Published 2020 Oct 16. doi:10.3390/nu12103170
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Sidi
Brainbook Editorial Officer
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