Neural tube defects

Neural tube defect (NTD) refers to the incomplete closure of the neural tube in very early pregnancy. 

The neural tube comprises of a bundle of nerve sheath which closes to form brain at the anterior end and spinal cord at the posterior end. The closure should occur at around the 28^th day of conception failing which brain or spinal cord doesn't form properly.

Numerous types of neural tube defect are recognised including ^5-6:

• myelomeningocoele (spina bifida) (50%)
• anencephaly (40%)
• encephalocoele (5%)
• craniorachischisis
• tethered cord
• exencephaly 
• iniencephaly (rare)

Epidemiology

Neural tube defects affect approximately >1-11 out of 1000 pregnancies ^4-6. 

Pathology

Causes

Folic acid deficiency is one of the leading causes of anencephaly. By supplementing with folic acid in early pregnancy, this defect can be prevented. It is more prevalent in developing countries and in mothers of low socio-economic status.

Associations

• maternal diabetes
• maternal obesity ⁷
• anti-epileptic use in pregnancy ⁸

Markers

• elevated maternal serum alpha-fetoprotein (MSAFP) levels 
• elevated amniotic fluid acetylcholinesterase (AChE) levels: in an open neural tube defect ³

Radiographic features

These are different for each entity and best discussed under each subtype.

Diagnosis

Most neural tube defects can be diagnosed by one of the following tests:

• maternal serum alpha-fetoprotein (MSAFP): a screening test performed in the pregnant woman serum during 16-18 weeks of pregnancy (elevated)
• amniocentesis: invasive procedure, performed during 15 weeks of pregnancy
• antenatal ultrasound: allows detection of anencephaly/acrania at 12 weeks of pregnancy

Treatment and prognosis

Both the management and prognosis is heavily dependent on the type of neural tube defect. The risk for a subsequent pregnancy is thought to be ~5-10%.