Spina bifida

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Overview
Spina bifida (Latin: "split spine") is a developmental birth defect involving the neural tube: incomplete closure of the embryonic neural tube results in an incompletely formed spinal cord. In addition, the vertebrae overlying the open portion of the spinal cord do not fully form and remain unfused and open. This allows the abnormal portion of the spinal cord to stick out through the opening in the bones. There may or may not be a fluid filled sac surrounding the open spinal cord. Other neural tube defects include anencephaly, a condition in which the portion of the neural tube which will become the cerebrum does not close, and encephalocele, which results when other parts of the brain remain unfused.

Spina bifida malformations fall into three categories: spina bifida occulta, spina bifida cystica (myelomeningocele), and meningocele. The most common location of the malformations is the lumbar and sacral areas of the spinal cord. Myelomeningocele is the most significant form and is that which leads to disability in most affected individuals. The terms spina bifida and myelomeningocele are usually used interchangeably.

Spina bifida can be surgically closed after birth, but this does not restore normal function to the affected part of the spinal cord and an individual with this condition will have dysfunction of the spinal cord and associated nerves from the point of the open defect and below. Intrauterine surgery for spina bifida has also been performed and the safety and efficacy of this procedure is currently being investigated. The incidence of spina bifida can be decreased up to 70 percent when daily folic acid supplements are taken prior to conception.

Signs and symptoms
These vary with the extent of the spinal defect and differ between the subtypes described below.

The most common location of the malformations is the lumbar and sacral areas of the spinal cord. The lumbar nerves control the muscles in the hip, leg, knee and foot, and help to keep the body erect. The sacral nerves control some of the muscles in the feet, bowel and urinary bladder, and the ability to have an erection. Some degree of impairment can be expected in these areas, resulting in varying degrees of paralysis, absence of skin sensation, and poor or absent bowel and/or bladder control, curvature of the spine (scoliosis), depending on the severity and location of the lesion damage on the spine. Although these individuals are rarely mentally retarded, in most cases there are cognitive problems.

Tethered Spinal Cord syndrome, with symptoms such as lower body pain, leg weakness, incontinence, scoliosis, numbness, is a common problem associated with spina bifida. Indeed, all spina bifida myelomeningocele patients have Tethered Cord on imaging studies such as Magnetic resonance imaging, but not all will develop symptoms. A tethered cord is thought to result from scar tissue which forms following the initial surgery to close the open defect. Symptoms caused by a tethered cord are rare in infancy and early childhood. Once symptoms develop it is important to make the diagnosis early, before permanent damage is done to the spinal cord and nerves.

According to the Spina Bifida Association of America (SBAA), over 73 percent of people with spina bifida develop an allergy to latex, ranging from mild to life-threatening. The common use of latex in medical facilities makes this a particularly serious concern. The most common approach is to try to avoid development of the allergy by avoiding contact with latex containing products such as examination gloves, catheters, and many of the products used by dentists.

Spina bifida occulta
Occulta is Latin for "hidden." This is one of the mildest forms of spina bifida although the degree of disability can vary depending upon the location of the lesion.

In occulta there is no opening of the back, but the outer part of some of the vertebrae are not completely closed. The split in the vertebrae is so small that the spinal cord does not protrude. The skin at the site of the lesion may be normal, or it may have some hair growing from it; there may be a dimple in the skin, a lipoma, a dermal sinus or a birthmark.

Many people with the mildest form of this type of spina bifida do not even know they have it (sufferers of spina bifida occulta have been noted to have achieved careers as demanding as the British Special Air Service — even with tethering) or symptoms do not appear until later in life. More recent studies not included in the review support the negative findings.

However, other studies suggest spina bifida occulta is not always harmless. One study found that among patients with back pain, severity is worse if spina bifida occulta is present.

Spina bifida cystica (myelomeningocele)
In this, the most serious form, the unfused portion of the spinal column allows the spinal cord to protrude through an opening in the overlying vertebrae. The meningeal membranes that cover the spinal cord may or may not form a sac enclosing the spinal elements. Superficially, the cyst may resemble an unrelated defect, sacrococcygeal teratoma. Spina bifida with myeloschisis is the most severe form of spina bifida cystica. In this defect, the neural folds fail to meet and fuse leaving the spinal cord open and the involved area represented by a flattened, plate-like mass of nervous tissue with no overlying skin or membrane. The unfused elements of the spinal cord can be surgically closed along with the overlying muscle and skin shortly after birth (see treatment section below).

The incompletely closed portion of the spinal cord and the nerves which originate at that level of the cord are damaged or not properly developed. As a result, there is usually some degree of paralysis and loss of sensation below the level of the spinal cord defect. Thus, the higher the level of the defect the more severe the associated nerve dysfunction and resultant paralysis. People may have ambulatory problems, loss of sensation, deformities of the hips, knees or feet and loss of muscle tone. Depending on the location of the lesion, intense pain may occur originating in the lower back, and continuing down the leg to the back of the knee.

Most children and adults with this condition experience problems with bowel and bladder control since the nerves which control these functions originate at the lowest part of the spinal cord. This may result in incontinence from neurogenic bladder.

Many individuals with spina bifida will have an associated abnormality of the cerebellum, called the Arnold Chiari II malformation. In affected individuals the back portion of the brain is displaced from the back of the skull down into the upper neck. In approximately 90 percent of the people with myelomeningocele, or hydrocephalus will also occur because the displaced cerebellum interferes with the normal flow of cerebrospinal fluid.

The myelomeningocele (or perhaps the scarring due to surgery) tethers the spinal cord to the enveloping vertebra. In some individuals this causes significant traction on the spinal cord and can lead to a worsening of the paralysis, scoliosis, back pain, or worsening bowel and/or bladder function.

Meningocele
The least common form of spina bifida is a posterior meningocele (or meningeal cyst).

In a posterior meningocele, the outer faces of some vertebrae are open (unfused) and the meninges are damaged and pushed out through the opening, appearing as a sac or cyst which contains cerebrospinal fluid. The spinal cord and nerves are not involved and their function is normal.

In an anterior meningocele, the inner faces of vertebrae are affected and the cyst protrudes into the retroperitoneum or the presacral space.

Apart from spina bifida, causes of meningocele include teratoma and other tumors of the sacrococcyx and of the presacral space, and Currarino syndrome. Usually a meningocele has no negative long-term effects, although there are reports of tethered cord.

Pathophysiology
Spina bifida is caused by the failure of the neural tube to close during the first month of embryonic development (often before the mother knows she is pregnant).

Normally the closure of the neural tube occurs around 28 days after fertilization. However, if something interferes and the tube fails to close properly, a neural tube defect will occur. Medications such as some anticonvulsants, diabetes, having a relative with spina bifida, obesity, and an increased body temperature from fever or external sources such as hot tubs and electric blankets can increase the chances a woman will conceive a baby with a spina bifida. However, most women who give birth to babies with spina bifida have none of these risk factors, and so in spite of much research, it is still unknown what causes the majority of cases.

The varying prevalence of spina bifida in different human populations and extensive evidence from mouse strains with spina bifida suggests a genetic basis for the condition. As with other human diseases such as cancer, hypertension and atherosclerosis (coronary artery disease), spina bifida likely results from the interaction of multiple genes and environmental factors.

Research has shown that lack of folic acid (folate) is a contributing factor in the pathogenesis of neural tube defects, including spina bifida. Supplementation of the mother's diet with folate can reduce the incidence of neural tube defects by about 70 percent, and can also decrease the severity of these defects when they occur. As yet it is unknown how or why folic acid has this effect.

Spina bifida does not follow direct patterns of heredity like muscular dystrophy or haemophilia. Studies show that a woman who has had one child with a neural tube defects such as spina bifida, have about a three percent risk to have another child with a neural tube defect. This risk can be reduced to about one percent if the woman takes high doses (4 mg/day) of folic acid before and during pregnancy. For the general population, low-dose folic acid supplements are advised (0.4 mg/day).

Treatment
There is no cure for nerve damage due to spina bifida. To prevent further damage of the nervous tissue and to prevent infection, pediatric neurosurgeons operate to close the opening on the back. During the operation, the spinal cord and its nerve roots are put back inside the spine and covered with meninges. In addition, a shunt may be surgically installed to provide a continuous drain for the cerebrospinal fluid produced in the brain, as happens with hydrocephalus. Shunts most commonly drain into the abdomen.

Most individuals with myelomeningocele will need periodic evaluations by specialists including orthopedists to check on their bones and muscles, neurosurgeons to evaluate the brain and spinal cord and urologists for the kidneys and bladder. Such care is best begun immediately after birth. Most affected individuals will require braces, crutches, walkers or wheelchairs to maximize their mobility. The higher the level of the spina bifida defect the more severe the paralysis. Thus, those with low levels may need only short leg braces while those with higher levels do best with a wheelchair. Many will need to manage their urinary system with a program of catheterization. They will need to insert a tube into their bladders to drain urine, of which the intervals vary from person to person, and may need medications to improve their dryness. Most will also require some sort of bowel management program.

Epidemiology
Spina bifida is one of the most common birth defects, with an average worldwide incidence of 1-2 cases per 1000 births, but certain populations have a significantly greater risk.

In the United States, the average incidence is 0.7 per 1000 live births. The incidence is higher on the East Coast than on the West Coast, and higher in whites (1 case per 1000 live births) than in blacks (0.1-0.4 case per 1000 live births). Immigrants from Ireland have a higher incidence of spina bifida than do nonimmigrants.

The highest incidence rates worldwide were found in Ireland and Wales, where 3-4 cases of myelomeningocele per 1000 population have been reported during the 1970s, along with more than six cases of anencephaly (both live births and stillbirths) per 1000 population. The reported overall incidence of myelomeningocele in the British Isles is 2-3.5 cases per 1000 births. Since then, the rate has fallen dramatically with 0.15 per 1000 live births reported in 1998.

Parents of children with spina bifida have an increased risk of having a second child with a neural tube defect.

This condition is more likely to appear in females; the cause for this is unknown.

Prevention
There is no single cause of spina bifida nor any known way to prevent it entirely. However, dietary supplementation with folic acid has been shown to be helpful in preventing spina bifida (see above). Sources of folic acid include whole grains, fortified breakfast cereals, dried beans, leaf vegetables and fruits.

Folate fortification of enriched grain products has been mandatory in the United States since 1998. The FDA, PHAC and UK recommended amount of folic acid for women of childbearing age and women planning to become pregnant is at least 0.4 mg/day of folic acid from at least three months before conception, and continued for the first 12 weeks of pregnancy. Women who have already had a baby with spina bifida or other type of neural tube defect, or are taking anticonvulsant medication should take a higher dose of 4–5 mg/day.

Certain mutations in the gene VANGL1 are implicated as a risk factor for spina bifida: these mutations have been linked with spina bifida in some families with a history of spina bifida.

Clinical trial
Management of Myelomeningocele Study (MOMS) is a phase III clinical trial to evaluate the safety and efficacy of fetal surgery to close a myelomeningocele. This involves surgically opening the pregnant mother's abdomen and uterus to operate on the fetus. Fetal skin grafts are used to cover the exposed spinal cord, to protect it from further damage caused by prolonged exposure to amniotic fluid. The fetal surgery may decrease some of the damaging effects of the spina bifida, but at some risk to both the fetus and the pregnant woman.

Pregnancy screening
Neural tube defects can usually be detected during pregnancy by testing the mother's blood (AFP screening) or a detailed fetal ultrasound. Spina bifida may be associated with other malformations as in dysmorphic syndromes, often resulting in spontaneous miscarriage. However, in the majority of cases spina bifida is an isolated malformation.

Genetic counseling and further genetic testing, such as amniocentesis, may be offered during the pregnancy as some neural tube defects are associated with genetic disorders such as trisomy 18. Ultrasound screening for spina bifida is partly responsible for the decline in new cases, because many pregnancies are terminated out of fear that a newborn might have a poor future quality of life. With modern medical care, the quality of life of patients has greatly improved.