The neural tube represents a foundational structure that develops very early in a pregnancy, eventually forming the brain and spinal cord of a developing embryo. An open neural tube defect arises when this tube fails to fully close during the initial stages of gestation, typically within the first month. This incomplete closure results in a portion of the delicate nervous system remaining exposed, which can lead to a range of health considerations.
Types of Open Neural Tube Defects
Two significant open neural tube defects are anencephaly and myelomeningocele, each affecting different parts of the developing nervous system with distinct outcomes. Anencephaly occurs when the upper part of the neural tube does not close completely, leading to the failure of the cerebrum and cerebellum, which are major parts of the brain, to form. The skull bones also do not fully develop over the absent brain tissue, leaving the brainstem and other rudimentary structures exposed. This condition is fatal; infants are typically stillborn or survive only briefly after birth.
Myelomeningocele, the most common and severe form of spina bifida, involves the incomplete closure of the neural tube along the spine. In this defect, the spinal canal remains open along several vertebrae in the back, allowing the spinal cord and its protective membranes (meninges) to protrude through the opening. This protrusion forms a sac on the baby’s back, which can be covered by skin or exposed. The exposed spinal cord and nerves are susceptible to damage, leading to varying degrees of paralysis, bladder and bowel control issues, and hydrocephalus, a buildup of fluid in the brain.
Causes and Prevention
The development of open neural tube defects is influenced by a combination of genetic and environmental factors, with the nutrient folic acid playing a significant preventative role. Folic acid, a B vitamin, is crucial for rapid cell division and the proper formation of the neural tube during early embryonic development. Women of childbearing age are recommended to consume 400 micrograms (mcg) of folic acid daily, ideally starting at least one month before conception and continuing through the first trimester of pregnancy. This consistent intake can reduce the risk of these defects by up to 70%.
Other factors can also increase the risk of these defects. Poorly controlled maternal diabetes before and during early pregnancy increases the risk of neural tube defects. Maternal obesity is another risk factor. Certain medications, such as some anti-seizure drugs (e.g., valproic acid), can also interfere with neural tube development, requiring careful management for women planning pregnancy.
Prenatal Diagnosis
Prenatal diagnosis of open neural tube defects involves a sequence of tests during pregnancy. The process begins with a maternal serum alpha-fetoprotein (AFP) test, usually between 15 and 20 weeks of gestation. Elevated AFP levels in the mother’s blood can indicate a potential open neural tube defect, as AFP, a fetal protein, leaks into the bloodstream from an open defect. A high AFP result prompts further investigation, as it is a screening test and not a definitive diagnosis.
If the AFP screening suggests a risk, a high-resolution fetal ultrasound is the next step to visually inspect the fetal spine and head. This imaging can identify abnormalities like a “lemon sign” or “banana sign” in the brain, or a visible sac on the fetal back, associated with anencephaly or myelomeningocele. This non-invasive ultrasound provides detailed images that can confirm or rule out the presence of a defect. If the ultrasound findings are inconclusive or further confirmation is needed, amniocentesis may be offered. This procedure collects a small sample of amniotic fluid, tested for AFP and acetylcholinesterase, which are typically elevated in the presence of an open neural tube defect.
Treatment and Management
Treatment for open neural tube defects depends on the type and severity, ranging from surgical intervention to palliative care. For myelomeningocele, fetal surgery before birth is an option, performed between 19 and 26 weeks of gestation. This procedure involves opening the uterus to close the defect on the baby’s spine, protecting the exposed spinal cord from further damage by amniotic fluid and uterine contractions. Studies suggest that fetal surgery can reduce the need for shunts to treat hydrocephalus and improve motor outcomes for some children compared to postnatal repair.
Following birth, postnatal surgical repair for myelomeningocele occurs within the first 24 to 48 hours to prevent infection and further nerve damage. This surgery involves closing the skin and muscle layers over the exposed spinal cord. This initial surgery is only the beginning of lifelong management for individuals with myelomeningocele. These children require ongoing care from a multidisciplinary team of specialists, including neurosurgeons, urologists, orthopedic specialists, physical therapists, and occupational therapists. This approach addresses complications such as hydrocephalus, bladder and bowel dysfunction, orthopedic issues (e.g., clubfoot or scoliosis), and mobility challenges.
Anencephaly has no treatment, and the prognosis is poor. Infants with anencephaly are typically stillborn or survive for only a few hours or days, receiving palliative care for comfort. The focus for families facing anencephaly is on supportive and emotional care, as the condition is incompatible with long-term survival.