Neural Tube Defects
On this page
Direct answer
Neural tube defects are failures of neural tube closure in the early weeks of pregnancy, with spina bifida, anencephaly and encephalocele as the main types. Periconceptional folic acid at the standard dose of 0.4 mg daily reduces their risk substantially. Maternal serum alpha-fetoprotein and detailed anomaly scanning help detect affected pregnancies before birth.
What you must remember
- Myelomeningocele: the commonest open spina bifida, with neural tissue in a sac on the back, causing limb weakness and bladder and bowel dysfunction.
- Spina bifida occulta: a closed vertebral arch defect with intact skin, usually without neurological deficit.
- Anencephaly: absence of most of the cranial vault and brain, which is incompatible with survival.
- Encephalocele: brain and meninges herniating through a skull defect, often in the occipital region.
- Folic acid prevention: 0.4 mg daily from before conception through early pregnancy, with a higher dose for women with a previous affected pregnancy.
- Arnold-Chiari type II: hindbrain herniation seen with myelomeningocele, often with hydrocephalus.
- Prenatal markers: raised maternal serum alpha-fetoprotein, and the lemon and banana signs on ultrasound.
Comparing the main neural tube defects
Neural tube defects range from a silent bony defect to a lethal brain malformation, so the table compares the main types on three features. Each type carries its own counselling and prognosis.
| Defect | Basic feature | Neurological outcome | Key association |
|---|---|---|---|
| Spina bifida occulta | Vertebral arch defect with intact skin | Usually none | Dimple, hair tuft or lipoma over the spine |
| Meningocele | Meninges protrude without neural tissue | Usually mild | Less frequent hydrocephalus |
| Myelomeningocele | Neural tissue lies in an open sac | Paraplegia, bladder and bowel dysfunction | Chiari II malformation and hydrocephalus |
| Anencephaly | Absent cranial vault and most of the brain | Incompatible with survival | Polyhydramnios |
| Encephalocele | Brain and meninges herniate through the skull | Variable | Often occipital |
Myelomeningocele causes the most disability among survivors, and Chiari II malformation contributes to its hydrocephalus and brainstem symptoms. Recurrence risk rises after an affected pregnancy, so the folic acid dose changes in later pregnancies.
Where the exam tries to catch you
Neural tube defect questions often hinge on timing and dose rather than anatomy.
- Trap: starting folic acid after conception as prevention Correct: prevention works only when folic acid starts before conception, because neural tube closure occurs in the first weeks.
- Trap: giving 0.4 mg folic acid to a woman with a previous affected pregnancy Correct: a previous neural tube defect warrants a higher dose, commonly around 4 mg daily under specialist advice.
- Trap: assuming spina bifida occulta always causes paralysis Correct: occult defects are usually silent, and a skin marker or tethered cord is the only clue.
- Trap: reading raised maternal serum alpha-fetoprotein as a confirmed diagnosis Correct: it is a screening marker, and detailed ultrasound or amniotic fluid testing is needed to confirm.
Frequently asked questions
What dose of folic acid prevents neural tube defects?
The standard periconceptional folic acid dose for preventing neural tube defects is 0.4 mg daily, started before conception and continued through early pregnancy. Women with a previous neural tube defect-affected pregnancy usually receive a higher dose, around 4 mg daily, under specialist advice.
Which ultrasound signs suggest spina bifida before birth?
The lemon sign and the banana sign are prenatal ultrasound signs suggesting spina bifida, reflecting changes in the skull and posterior fossa. A detailed anomaly scan together with raised maternal serum alpha-fetoprotein helps flag affected pregnancies for further assessment.
What is the Arnold-Chiari type II malformation?
Arnold-Chiari type II malformation is hindbrain herniation with downward displacement of the cerebellar vermis, medulla and fourth ventricle through the foramen magnum, and it is seen with myelomeningocele. It is commonly associated with hydrocephalus and brainstem dysfunction that may need neurosurgical management.
What is the difference between spina bifida occulta and myelomeningocele?
Spina bifida occulta is a closed defect of the vertebral arch with intact skin and usually no neurological deficit. Myelomeningocele is an open defect in which neural tissue sits in a sac on the back and typically causes paralysis and bladder and bowel dysfunction.
Why is anencephaly associated with polyhydramnios?
Anencephaly is associated with polyhydramnios because impaired fetal swallowing disturbs amniotic fluid balance. Polyhydramnios is therefore a useful prenatal clue, and anencephaly is incompatible with long-term survival.