Understanding Neural Tube Defects: Causes, Types, and Detection

Pregnancy is a journey filled with hope, but it also comes with new vocabulary and medical concepts to learn. One of the most critical areas of prenatal health involves the neural tube, the embryonic structure that eventually develops into the baby’s brain and spinal cord.

What is a Neural Tube Defect?

To understand the definition of neural tube defect, we must look at the first month of pregnancy. By the 28th day after conception, the neural tube is supposed to fold and close. If it remains open, a neural defect occurs.

A neuro tubular defect can occur anywhere along the spine or head. These are often categorized as an open neural tube defect (where the brain or spinal cord is exposed) or a closed defect (covered by skin).

Common Examples of Neural Tube Defects

There is a wide list of neural tube defects, ranging in severity:

Anencephaly: A serious condition where parts of the brain and skull do not form.
Encephalocele: A sac-like protrusion of the brain through an opening in the skull.
Spina Bifida: The most common tube defect, where the spinal column doesn’t close completely.
Craniorachischisis: A severe defect involving both the skull and the spine.
Iniencephaly: A rare defect where the head is bent severely backward.
Acrania anencephaly: A condition where the flat bones of the skull are completely absent.

Deep Dive: Anencephaly

Anencephaly is one of the most severe neural tube defects examples. The anencephaly medical definition describes a condition where a baby is born without parts of the brain and skull.

The Anencephaly Fetus: What Happens?

In a fetus with anencephaly, the anencephaly brain often lacks the forebrain and the cerebrum (the thinking and coordinating part of the brain). While there are different anencephaly types regarding the severity of tissue loss, the prognosis is heartbreakingly poor.

Anencephaly Signs and Symptoms

Many parents ask about anencephaly symptoms during pregnancy or symptoms of neural tube defects in pregnancy. It is important to note that the mother usually experiences no physical symptoms. There are no warning signs the mother can feel. The signs of neural tube defects in pregnancy are almost exclusively found through clinical testing and imaging.

Deep Dive: Encephalocele

Encephalocele definition refers to a sac-like protrusion of the brain and the membranes that cover it through an opening in the skull.

Encephalocele Types

The classification depends on the location of the defect:

Sincipital encephalocele: Located at the front of the head.
Frontal encephalocele: Specifically protruding between the forehead and nose.
Small encephalocele: These may contain only meninges and spinal fluid, whereas larger ones contain brain tissue (brain encephalocele).

While anencephaly and encephalocele are distinct, they are related errors in neural tube closure. Rarely, a case may present as anencephaly with encephalocele.

Etiology: Causes and Risk Factors

The etiology of neural tube defects is complex and multifactorial. The cause of neural tube defect issues is often a mix of genetics and environment.

The Role of Folate

The most significant factor in neural tube defects pathophysiology is nutrition.

Folate and neural tube defects: Low levels of folic acid (Vitamin B9) before and during early pregnancy significantly increase the risk.
Methylfolate neural tube defects: Some women have a gene mutation (MTHFR) that makes it hard to process synthetic folic acid; they may require methylfolate to prevent defects.

Other risk factors include uncontrolled diabetes and obesity, which increases risk of congenital heart defects and neural tube defects. The specific anencephaly cause or causes of encephalocele are generally the same as other NTDs.

Detection: Screening and Diagnosis

Modern medicine allows for early neural tube defect detection. If you are pregnant, your doctor will offer an NTD test.

The Blood Test

The NTD blood test (often part of the “quad screen”) measures Alpha-fetoprotein (AFP).

Neural tube defect screening positive: This means AFP levels are abnormal. It does not diagnose a defect but indicates a higher risk, requiring further testing (usually an ultrasound).
Open NTD risk assessment: This calculation helps decide if further invasive testing like amniocentesis is needed.

Ultrasound Detection

An NTD ultrasound is the primary way to confirm a diagnosis.

Anencephaly Detection Timeline:

Anencephaly 8 week ultrasound / 9 weeks: It is very difficult to diagnose this early, though a skilled technician might suspect acrania.
Anencephaly ultrasound 10 weeks: The absence of the cranial vault becomes more visible.
Anencephaly 12 week ultrasound: This is a common time for diagnosis. Anencephaly on ultrasound at this stage shows the characteristic “Mickey Mouse” absence of the cranium.
Anencephaly ultrasound 20 weeks: During the anatomy scan, the diagnosis is confirmed with certainty.

Similarly, iniencephaly ultrasound imaging will show the head retroflexion, and a brain encephalocele will appear as a sac-like structure on the skull.

Moving Forward

Receiving a diagnosis of a neuro defect or tube defects pregnancy is incredibly difficult. Whether referencing the anencephaly NHS guidelines in the UK or guidelines in the US, the medical advice focuses on prevention through folate and early neural defect tube screening.

If you have a history of ntd in pregnancy, consult a genetic counselor. They can help you understand the specific neural tube defects meaning for your family planning and help manage ntd screening for future pregnancies.

Disclaimer: This information regarding neural defects test, open neural tube defect test, and medical definitions is for educational purposes. Always consult an OB-GYN for ntd pregnancy test advice and interpretation.