Neural tube defects

Overview of neural tube defects

Diseases of the peripheral and central nervous systems are becoming major threats to healthy living, especially after a certain age. One in every six people suffers from different types of neurological defects in world, which is equal to about 1 billion people across the globe

Neural tube defects belong to this category and are affecting millions all over the world. These conditions are usually congenital and arise from genetic defects in many cases. People having a familial history of these disorders are 5-10 times more likely to develop neural tube defects than the rest.

With the major technological advancement in the medical field, the occurrence of these diseases can be detected in the pregnancy period itself. Therefore, matters like continuation of the pregnancy and the associated risks should be consulted with a doctor.

What are neural tube defects?

Neural tube defects (NTD) refer to a set of conditions in which an opening in the brain or the spinal cord remains exposed from the very early stages of foetal development. Normally in the third week of pregnancy, which is known as the gastrulation period, certain specialised cells on the dorsal side of the embryo undergo alterations in shape and size and form the neural tube. Defects develop when the neural tube does not close completely. According to the statistics of the National Center on Birth Defects and Developmental Disabilities neural tube defects affect over 3,00,000 newborns annually, all over the world.

Two main types of NTDs have been identified so far-

Open NTDs and Closed NTDs. In Open NTDs, the brain or the spinal cord are exposed due to a defective formation of the skull or the vertebrae during foetal development. The commonly observed types of Open NTDs are- Encephaloceles, Hydranencephaly, Anencephaly, Iniencephaly, Schizencephaly and Spina Bifida.

Closed NTDs are relatively rare. They occur when the spinal defects are loosely covered by skin. The commonly studied categories of Closed NTDs are- Lipomyelomeningocele, Lipomeningocele and Tethered Cord Syndrome. All these diseases will be discussed below in separate sections

Anencephaly

It is a very serious form of Neural Tube Defect. Here, an essential portion of the brain, skull and the scalp that occurs during the foetal development is absent. Children having this abnormality lack the telencephalon. This is actually the largest part of the brain that mainly comprises of the cerebral lobes and also the neocortex, that is controls the cognitive functions. Even the bone, meninges and the skin, all are absent. The residual part of the brain is loosely covered by a very thin layer of membrane.

Children born with Anencephaly do not have audio-visual functions and pain sensation. The most basic reflex actions such as breathing and response to touch or sound may be present.

What are the causes of Anencephaly?

The deficiency of Folic acid in the mother’s body can lead to Anencephaly in their children. Studies on the genetic linkage of the disease have shown that deficiencies of the Transcription factor TEAD2 can also contribute to the disease. Women having a pregnancy with Anencephaly have a 3% risk of having another pregnancy with the same disease.

What are the different types of Anencephaly?

Detailed diagnoses and a myriad of case studies have identified three principal types of Anencephaly so far. These are-

• Meroanencephaly: It is characterised by poorly formed cranial bones, a median cranial defect and an abnormal cranial protrusion called area
• Holo anencephaly: It is the most commonly observed form where the brain is completely absent.
• Craniorachischisis: This is the most severe form where bony defects in the spine and exposed neural tissues are observed.

Spina Bifida

It is a congenital defect characterised by incomplete closure of the backbone and the membrane surrounding the spinal cord. The three main types of Spina Bifida are- Spina Bifida Occulta, Meningocele and Myelomeningocele.

In the Spina Bifida Occulta, the exteriors of certain portions of the vertebra are not completely closed. In Meningocele, the meninges in trapped (herniated) between the vertebrae. In Myelomeningocele, the spinal cord can protrude through an opening due to the presence of an unfused portion of the spinal column.

What are the main identifiable signs?

The major signs and symptoms are-

• Weakness in legs
• Pseudoparalysis
• Deformities like club-foot, scoliosis and hip-dislocation
• Bladder problems like incontinence and urinary tract infections
• Bowel incontinence
• Skin irritations
• Pressure sores
• Abnormal eye movements 

Encephaloceles

In this type of NTD, the brain protrudes through the skull which is loosely covered by thin membranes. A deep groove is often found on the middle and upper part of the skull or at the back of the skull.

This disease is mainly attributable to harmful substances like- teratogens, arsenic and trypan blue (a type of stain). In many cases it occurs due to the deficiency of folic acid in the body.

The disease has the following subtypes-

• Nasofrontal
• Nasoethmoidal
• Naso-orbital

Hydranencephaly

It is a severe form of NTD in which the cerebral hemispheres (lobes) are partly or wholly absent and the remaining portion of the cranial cavity is filled with Cerebrospinal fluid (CSF).

Iniencephaly

In this type of NTD, there are three main conditions-

• A defective occipital bone
• Spina Bifida of the cervical vertebra
• Retroflexion (backward bending) of the head on the cervical spine

The two main types of this disease are- iniencephaly apertus and iniencephaly clauses.

Diagnosis and treatment

The following diagnostic tests can detect the occurrence of Neural Tube Defects even in the foetal stage-

• Ultrasound examination
• Maternal serum Alpha-fetoprotein (AFP screening)
• MRI and CT scans

Till date there are no standard treatments for neural tube defects. The newborns having facial or developmental defects can be cured to some extent by performing neuro-surgeries. Folic acid tablets or dietary sources rich in Folic Acid are recommended in certain cases. In some rare cases, the doctors recommend anticonvulsants and physical therapies to prevent contractures.

Research works are being conducted in the popular global organisations to devise effective means of treatment for the NTD patients. Progress has been made in the fields of stem cell therapy which has given the doctors hopes of curing several neurological congenital disorders in future.

References