Neural tube defects—Spina bifida, Anencephaly and Encephalocele
The neural tube is formed by a layer of cells in the growing embryo folding over and joining together during the sixth week of pregnancy. The neural tube goes on to develop into your baby's brain and spinal cord. When the neural tube does not fuse together properly a problem with the development of the spine or brain occurs. This is known as a neural tube defect.
The two most common types of neural tube defects are spina bifida and anencephaly, affecting approximately one in every 500 pregnancies in Australia.
We do not know exactly why neural tube defects occur. However we do know that the incidence is higher in some ethnic groups than others. We also know that dietary supplementation with folic acid can help to reduce the chance of a baby having a neural tube defect.
The name spina bifida means split or divided spine and is used to describe a neural tube defect that affects the spine. If the neural tube fails to close in an area that goes on to develop into the spine the protective bones surrounding the spinal cord (vertebra) are unable to fuse together properly. As a result the spinal cord and its protective sac (meninges) may be pushed through the open part of the spine.
The symptoms associated with spina bifida vary depending on where in the spine the opening has occurred and how much of the spinal cord and/or protective sac are pushed through this opening. Generally the condition is more severe if the opening is above the base of the spine and if the spinal cord is pushed through the opening.
There are three different types of spina bifida which are defined according to the location and severity of the abnormality:
Occulta is the mildest form and refers to when the outer vertebrae are not completely joined but the spinal cord and protective covering are unaffected. The only visible sign maybe a dimple, tuft of hair or small swelling at the site of the abnormality. There are usually no problems associated with this form of spina bifida.
Meningocele is when the outer parts of the vertebrae fail to fuse and a cyst or fluid filled sac is pushed through the divide in the vertebrae. The sac contains the membranes that protect the spinal cord but no spinal nerves. The spinal cord itself is normal and not affected. The defect can be surgically closed after birth.
Myelomeningocele is severest form of spina bifida. In these cases both the spinal cord and the protective covering are pushed through the opening in the vertebrae. Affected babies will have some degree of leg paralysis, after surgery, as well as bowel, bladder and other complications.
Immediately following your baby's birth the neonatal staff will:
- wrap the opening with sterile damp dressings and may cover it with plastic wrap
- place a drip into your baby's arm to give fluids and antibiotics
- place your baby in an incubator so that they do not get cold.
They will then transfer your baby to the Neonatal Critical Care Unit.
Surgery to replace the spinal cord and exposed nerves back into the spinal canal is usually performed by the third day after birth.
Complications and treatment options
The complications associated with myelomeningocele tend to vary according to the location and severity of the defect. Defects higher up the spine produce a greater risk of paralysis and other debilitating complications.
The degree of paralysis of the lower body and legs. Prompt surgery helps prevent further nerve damage and infection. However, nerve damage that has already occurred cannot be repaired. If the defect is in the lower spine many children are able to walk with the aid of leg braces and crutches. However if it is higher up the spine most children require a wheelchair.
Hydrocephalus (build-up of fluid in and around the brain) affects 70 to 90 per cent of children with myelomeningocele. To prevent brain damage and over enlargement of the head, doctors usually treat this early by inserting a tube called a shunt into your baby's brain to divert the excess fluid.
Chiari II malformation affects nearly all children. The lower part of the brain is located further down than normal and is partly displaced into the upper part of the spinal canal. This can increase the risk of hydrocephalus but usually causes no other problems. In a small number of children it does cause problems with breathing and swallowing and surgery may be needed to relieve the pressure on the brain.
Urinary problems such as incontinence, difficulty emptying the bladder and repeated urinary tract infections are common complications due to damage to the nerves. Catheterisation (insertion of a tube to empty the bladder) several times a day is one of the treatment options.
Bowel problems such as incontinence and constipation are common. Treatment options include laxatives, enemas and surgery if needed.
Hip, knee and foot deformities (e.g. club feet) are relatively common and can be further complicated by unbalanced muscle action around these joints.
Learning difficulties affect many children with myelomeningocele even though they usually have intelligence in the normal range.
Scoliosis where the spine bends into an S shape is common in children when the defect is above the second lumbar vertebra. Treatment options include surgery and the child wearing a cast to correct the deformity.
Cord tethering where the spinal cord sticks to the area where the original defect was and becomes stretched. If this occurs and causes problems surgery may be needed to release the spinal cord.
Ninety two to 95 per cent of cases can be accurately diagnosed with ultrasound at the 18 to 20 week ultrasound scan. It is uncommon for spina bifida to be detected earlier than this.
When the neural tube doesn't close at the head a condition called anencephaly usually results. This is when the top part of the brain, skull and scalp are partially or totally missing. This abnormality can be identified from 13 week gestation. Sadly these babies usually die at, or soon after, birth.
When part of the brain and/or surrounding membrane are pushed through an opening in the skull it is called an encephalocele. This abnormality is much rarer than the other neural tube defects and affects approximately one in every 10,000 live births.
The protruding sac ranges in size and may or may not contain brain tissue. The defect is usually treatable with surgery provided other severe abnormalities are not present and the defect is not too large. This lesion is usually detected part of the routine 18 to 20 week scan.
If a woman has had a baby with a neural tube defect her chance of having another affected baby is about one in 20 or five per cent. In most women it is possible to reduce the risk of having a baby with a neural tube defect by taking folic acid at least one month prior to conception and continuing to do so during the first few months of pregnancy. A doctor will be able to recommend the appropriate dose as this varies according different factors.
If you would like more information we recommend you talk to your doctor. Alternatively the following websites may be of assistance:
Centre for Maternal Fetal Medicine Level 7
Mater Mothers' Hospital
South Brisbane Qld 4101
Phone 07 3163 1896
Fax 07 3163 1890
Mater acknowledges consumer consultation in the development of this patient information.
Last modified 09/11/2015.