What is Spina Bifida?
Spina bifida is a condition resulting from the malformation of the brain, skull, spinal cord, and spine to varying degrees that occurs within the first month after conception (pregnancy).
Causes of Neural Tube Defects:
The embryonic tissues which will later specialize to become the central nervous system (brain and spinal cord), the skull, and the spine start out as a flat plate that rolls into a tube (neural tube). Defects in the neural tube may be due to:
- Genetic inheritance from the parents.
- Malnutrition especially the deficiency of the B vitamin, folic acid.
These have been implicated as the cause of disruption of the normal formation process leading to an incomplete neural tube.
Sequelae of Neural Tube Defects:
- Defects in the neural tube can cause the brain to form outside the skull - encephalocele.
- It may lead to abnormal structure of the brain and skull leading to compression of the lower brain - Arnold-Chiari II malformation.
- It may lead to abnormal structure of the cerebrospinal fluid circulation causing ventricular enlargement - hydrocephalus.
- It may also lead to defects in the spine and spinal cord - Defects in the spine and spinal cord can range from an asymptomatic opening in the lower lumbar spine (spina bifida occulta) to more severe forms including defect in the bone and covering of the nerves or dura (meningocele) or most severe, a defect in the bone, dura, and an incompletely formed spinal cord exposed to the enviroment (myelomeningocele). The thin layer of tissue present over the spinal cord and cerebrospinal fluid containing space around it in meningocele and myelomeningocele usually ruptures, if left untreated leading to meningitis (bacterial infection around the central nervous system) which can lead to more severe disability or death.
Usually these defects are in the posterior midline.
Importance of Folic Acid Intake in Women:
Neural tube defects occur with an incidence of between 0.1 to 1 per thousand births in developed countries but are significantly higher in undeveloped countries. The intake of 400 micrograms of folic acid in the daily food intake of women of child bearing age regularly before they become pregnant significantly reduces but does not eliminate the possilibility of occurrence.
Treatment for Neural Tube Defects:
- Surgery for meningocoel and myelomeningocoel closes the potential for exposure of the spinal cord to the enviroment and usually protects it from further damage. Unfortunately, because the nerves are not well formed there will still occur in the case of myelomeningocele permanent disability and dysfunction including loss of bladder, bowel, and sexual voluntary function as well as significant weakness or paralysis of at least the lower leg and foot. Despite this impairment, many children can learn to ambulate with bracing and crutch or cane devices.
- The obstruction of cerebrospinal fluid flow (hydrocephalus) leads to increasing pressure on the brain which lowers the blood supply to the neurons (nerve cells) and directly impairs their development. Many times the process accelerates after surgery for the myelomeningocele because its escape outside the body which can lead to infection is prevented. In the first two years of life the head may abnormally enlarge (macrocephaly) as the plates that form the skull are not yet fused. Sometimes the hydrocephalus delays its progression until the child is older and skull plates have already closed which can lead to rapid onset of blindness, confusion, coma, and then death.
- It is now well known that surgery for hydrocephalus in this situation results in most children functioning within the realm of normal intelligence while no treatment in the face of progressive hydrocephalus certainly will result in permanent loss of intellectual function, coordination, vision, and even death. The most common treatment for hydrocephalus is the placement of a ventricular peritoneal shunt (a tube and valve system that controls pressure or flow) which creates a detour of the cerebrospinal fluid from the brain to the peritoneal space (the space between the abdominal organs) where it is absorbed.
- Newer alternatives involve placing a surgical telescope in the brain and attempting to open channel to restore flow which may also include destroying the choroid plexus (tissue which produces the cerebrospinal fluid). The advantage of these newer procedures over the traditional shunt surgery is controversial. Those who do undergo this type of procedure may not respond and still require a ventricular peritoneal shunt. Unfortunately ventricular peritoneal shunts may suddenly stop functioning or become infected needing repeat surgery. For reasons not always clear a subset of children end up having multiple operations especially in the first few years of life.
- Children with myelomeningocele can develop choking and upper extremity weakness or spasticity (stiffness caused by abnormal spasm of the muscles) from an Arnold-Chiari malformation. This may require removing the skull and sometimes the posterior part of the upper cervical vertebrae to remove pressure on the brain.
Tethered Spinal Cord Syndrome:
- Occasionally at or near adolescense, a child with a history of myelomeningocoel surgery will start to lose what lower extremity function they previously had. It is theorized that a tethering of the spinal cord (restriction of the movement of the spinal cord up the spinal canal which normally occurs with growth) causing mechanical disruption and/or decreased blood supply which can lead to further dysfunction.
- This can be difficult to assess as MRI (Magnetic Resonance Imaging) which is used to evaluate the spinal cord, most often shows the spinal cord to be below its normal position and be tethered even in those with stable function.
- Sometimes abnormal curvatures of the spine from the defects and muscle weakness threaten respiratory function or nerve function and require reconstructive spine surgery.
- The treatment of tethered cord syndrome for child with a history of myelomeningocoel surgery can be a difficult decision. Often, anxious parents feel desperation in wanting more function and may give a misleading history of loss of function. Some have advocated this type of surgery as a way to restore control of bladder or bowel function but this loss of function is due to the original incomplete formation of the nerves rather than a later tethering in most cases.
In the past, surgery was often delayed for sometime for spina bifida and the associated hydrocephalus with the idea that these children would be sentenced to live poor quality lives. But this has been proven wrong. Today, most experts advocate surgery as early as possible to reduce the risk of infection and further injury. Attempts at surgery while the child is in the mother's womb are done in a few centers but the benefit of this procedure which puts both mother and child at risk over early intervention after birth are not yet clear.
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