By Dr. Namrata Todurkar
The creation of a normal human body in the mother’s womb is nothing short of a miracle. While the genesis of organs and its complex functions occur so meticulously in the growing embryo, nature can do some mistakes. Among the various organ systems that can be affected by such formative defects, the nervous system is probably the most important one. More so because the proper functioning of other systems is dependent on the intactness of the brain and spinal cord. For example, if the brain stem is being compressed, important bodily function like breathing is hampered and can endanger life. Or, if the spinal cord is injured at a certain level, the part of the body supplied by the portion below the level of the injury might be paralyzed. Spina bifida is one such birth defect that affects the nervous system (spinal cord) and is frequently accompanied by a condition called hydrocephalus (accumulation of excess cerebrospinal fluid in the brain).
Spina Bifida:
Spinal bifida, as the name suggests is a defect that resembles something that is split open. It falls into the category of Neural tube defects (NTDs), and originates during embryonic development when the neural tube(the precursor of the central nervous system) fails to close completely. The two most common NTDs are spina bifida (exposed spinal cord tissue) and anencephaly (exposed brain tissue). Neural tube defects are multifactorial, with contributions from both genetic and environmental factors. The genetic basis is not yet well understood, but several nongenetic risk factors have been identified.
NTDs are among the most common birth defects with a prevalence between 0.5 to more than 10 per 1,000 pregnancies. This difference in prevalence worldwide likely reflects differing contributions from risk factors such as nutritional status, prevalence of obesity and diabetes, usage of folic acid supplementation and/or fortification, the presence of environmental toxins, and differing genetic predisposition among different ethnic groups.
A baby’s neural tube normally starts out as a tiny, flat ribbon that turns into a tube by the end of the first month of pregnancy. If the tube doesn’t close completely, it can result in a Neural tube defect. Failure of the developing neural tube to fuse at the uppermost part results in anencephaly, and the similar issue in the lowermost segment leads to spina bifida. They are collectively called as Open neural tube defects.
Abnormal development of the skull or vertebrae may also allow herniation of the closed neural tube through the affected bony region, as in encephalocele or meningocele, respectively. The neural tissues may be contained within a meninges-covered sac that protrudes through the open vertebrae (myelomeningocele) or exposed directly to the amniotic fluid (myelocele).
Anencephaly is one of the most severe NTDs. As mentioned earlier, anencephaly is caused when the upper part of the neural tube that forms the brain doesn’t close completely. Babies with this condition will be missing major parts of the brain, skull and scalp. They do not survive long after birth beyond a few hours.
There is also a less well-defined group of closed spinal NTDs in which the underlying spinal cord tissue is abnormal and the vertebral column bones are malformed, but are covered by skin. Subtle clues like a swelling (lipoma), tuft of hair, skin tag or a pit may be present at the bottom of the spine, which may indicate an underlying spina bifida occulta. Babies with spina bifida may have paralyzed legs and problems controlling their bladder and bowel (going to the bathroom). Milder forms of spina bifida may however cause fewer problems for children and might go unnoticed for many years.
Families at high-risk for NTDs:
History of having a baby with an NTD: There is a 2- to 3-percent chance of having a baby with an NTD in another pregnancy.
If one of the parents has a NTD
If there is a family history of NTD
Intake of certain anti-seizure medicines during pregnancy: It is recommended to discuss with a health care provider before planning pregnancy about how the seizure medicine may affect the fetus.
Obesity: A body mass index (BMI) of 30 or higher.
Presence of diabetes: Uncontrolled Diabetics have an increased risk for having a baby with NTD.
Intake of folic acid before and during early pregnancy can help prevent NTDs. Folic acid is a B vitamin that every cell needs for normal growth and development. Studies show that taking folic acid before and during early pregnancy (NTDs happen in the first month of pregnancy) can help reduce the risk of having another baby with NTD by about 70 percent.
In case of anencephaly and open spina bifida, the neural tissue which is exposed to the amniotic fluid degenerates. It often results in further neurological damage. These considerations led to the development of in utero fetal surgery for spina bifida, which may improve neurological outcomes compared with repair after birth. Associated conditions include hydrocephalus (accumulation of excessive fluid within the brain), Chiari malformation type II (presence of myelomeningocele with a type of brain malformation in which both the cerebellum and brain stem tissue extend into the foramen magnum -the hole at the skull base for passing of the spinal cord), vertebral abnormalities as well as genitourinary and gastrointestinal disorders.
NTDs can be diagnosed during pregnancy by ultrasound and by certain biomarkers. Babies born with open spina bifida usually survive with appropriate medical care but have neurological impairment, the severity of which depends on the level of the affected area. Medical care for babies born with open spina bifida usually involves surgery to close and cover the defect. Most of the serious issues occur in the first year of life, mostly due to kidney related and respiratory problems associated with spina bifida. Multiple surgeries are commonly required to prevent sticking of the spinal cord to the bones around it, treat hydrocephalus, and treat orthopedic and urological problems.
Hydrocephalus:
Hydrocephalus affects the majority of patients with Spinal Open Neural Tube Defect (SONTD) who have Myelomeningocele and Chiari malformation type II. Exact incidence of hydrocephalus in SONTD is not known. In a significant proportion of patients, hydrocephalus is absent at birth but develops in the first few weeks or months of life. A variety of factors are implicated as the cause of hydrocephalus in these children. Hydrocephalus usually develops due to impaction of the brain contents on the opening at the bottom of the skull (foramen magnum), leading to occlusion of the outlets of the cerebrospinal fluid (CSF) and resulting in progressive collection of CSF. Apart from SONTD, meningitis with or without ventriculitis is one of the most common causes of hydrocephalus in the neonatal period.
The predominant treatment of hydrocephalus is CSF diversion, most commonly into the abdomen (peritoneal cavity) by means of ventriculoperitoneal shunt. However, these shunts may occasionally develop issue like mechanical obstruction, excessive CSF drainage, and infection. The long-term prognosis of patients with SONTD-associated hydrocephalus is not well known. Children with SONTD routinely undergo MRI examination of brain and neck (craniocervical junction) to check the size of the ventricle, and the presence of Chiari malformation type II.
A 15 year database search of 82 patients with confirmed diagnosis of SONTD between the year 1995 and 2010 revealed that Chiari Malformation type II was the main cause of hydrocephalus in 71 (86.6%) patients. Fifty seven of them received treatment for active hydrocephalus. As many as 10-20% of children with SONTD and CM II may not develop hydrocephalus and do not require shunt placement. In the present series, 19.5% (16/82) did not develop hydrocephalus during follow up period, which ranged between 1 and 12 years.
Conclusions
Spina bifida may be apparent or occult, and the resulting neurological impairments depend on the severity and the level of the spinal cord affected. Multidisciplinary care and long term follow up lead to promising outcomes. Hydrocephalus is commonly seen in patients with SONTD especially those having Chiari malformation II, and such patients are likely to remain shunt dependent. It may be advisable to delay treatment in the absence of active hydrocephalus as patients with normal size ventricles or slightly enlarged ventricles may not develop hydrocephalus. The multidisciplinary management of patients with SONTD has resulted in significant improvement in survival and functionality, achieved through regular follow up and timely interventions.
Dr. Namrata Todurkar, MBBS, MD (Pediatrics), DNB (Pediatrics). Fellowship in Neonatology from National Neonatology Forum India. Fellow in Neonatal-Perinatal Medicine in University of British Columbia. Areas of interest: Neonatal nutrition, Fluid and Electrolyte Management, Inborn Errors, Neurodevelopmental followup of preterm infants. Dr. Todurkar is a volunteer blogger at CPBF.