Embryology · Week 3-4

Neural Tube Defects: when the neuropore won't close

Two ends, two failures. Rostral fails: anencephaly. Caudal fails: spina bifida spectrum. Skin on top or skin off the top decides whether the AFP screen catches it.

An 18-week pregnant patient comes in for routine second-trimester screening. MSAFP is 2.7 multiples of the median. Anatomic ultrasound shows a lumbar lesion with herniation of the spinal cord and meninges, no skin covering. Lemon sign and banana sign are seen on the head.

What is the most likely diagnosis?

Spina bifida occulta

Meningocele

Myelomeningocele

Encephalocele

Anencephaly

Open caudal lesion + cord and meninges herniated + elevated MSAFP = myelomeningocele.

The three pieces stack. Open (no skin) explains the elevated MSAFP because fetal protein leaks freely into the amniotic fluid and then the maternal serum. Cord in the sac (not just meninges) is what makes it myelomeningocele rather than the simpler meningocele. Lumbar plus lemon plus banana on the head means the brain is being tugged caudally: classic Arnold-Chiari II that travels with myelomeningocele.

Spina bifida occulta is closed (skin intact, MSAFP normal). Meningocele has meninges only without cord involvement. Encephalocele is rostral, not lumbar. Anencephaly has no brain or skull cap.

Arnold-Chiari II pairing › · CNS embryology / neurulation · Antiepileptic teratogens

Open vs Closed: the four-quadrant map

One axis is location (rostral = head, caudal = tail). The other axis is skin (open = neural tissue exposed, closed = skin covered). The skin column is the AFP column. Tap any cell.

Open · AFP up

Closed · AFP normal

Rostral (head)

Caudal (spine)

Open / Rostral

AnencephalyNo brain, no skull cap. Frog-eye facies.

Rostral neuropore fails. No forebrain develops. Fatal in utero or shortly after birth.

MSAFP ↑↑↑

Closed / Rostral

EncephaloceleSkin-covered skull defect, brain herniates out.

Most often occipital. Skin intact, so MSAFP is normal. Ultrasound catches it.

MSAFP normal

Open / Caudal

Spina bifida (open)Three flavors, increasing severity.

Meningocele · meninges out, no cord. Relatively mild.

Myelomeningocele · meninges + cord. Most severe common form. Deficits below the lesion.

Rachischisis · cord exposed, no covering at all. Catastrophic.

MSAFP ↑↑

Closed / Caudal

Spina bifida occultaVertebral arch defect, skin intact.

Skin markers over the spine: tuft of hair, dimple, lipoma, hyperpigmented patch. Usually asymptomatic; may have tethered cord. MSAFP normal, so prenatal screen misses it.

MSAFP normal

Open · neural tissue exposed · AFP leaks into amniotic fluid Closed · skin covers the defect · AFP stays normal

Spina bifida (illustrated) · tap to expand

Open lumbar lesion · tap to expand

Anencephaly · tap to expand

Tap the spine: where the defect lands

The caudal neuropore can fail at any level. Tap a region: severity ramps up as the lesion goes from closed-and-tiny (occulta) to wide-open-with-cord-out (rachischisis).

Lumbar Myelomeningocele Severe

Most common location for myelomeningocele. Meninges + spinal cord herniate through an open vertebral defect with no skin covering. Motor and sensory deficits start below the level of the lesion. Pairs with Arnold-Chiari II about 90% of the time. MSAFP is high; ultrasound shows lemon (frontal bone scalloping) and banana (cerebellum pulled caudally).

AFP screening: what makes it go up, what makes it go down

Maternal serum alpha-fetoprotein is drawn in the second trimester (16-20 weeks). Each scenario shifts the bar. Tap a scenario to highlight.

Normal pregnancy: MSAFP sits around 1.0 multiples of the median. The baseline. Everything below is read off this.

Risk factors: what blocks the neuropore from closing

Folate runs through almost all of them. The teratogens either antagonize folate directly or steal it through enzyme induction.

Top driver

Folate deficiency

Folate provides one-carbon units for DNA synthesis. Neural tube cells are dividing fast at week 3-4. No folate, no closure. Preconception supplementation is the only proven prevention.

Antimetabolite

Methotrexate

Blocks dihydrofolate reductase. Pulls folate out of circulation entirely. Contraindicated in pregnancy. Used as an abortifacient for ectopics for this exact reason.

Antiepileptic

Valproate

The worst NTD offender among antiepileptics. Interferes with folate metabolism and inhibits methionine synthase. NTD risk roughly 1 to 2 percent on monotherapy.

Antiepileptic

Phenytoin

Induces hepatic CYP enzymes that burn through folate stores faster. Adds cleft lip/palate and fetal hydantoin syndrome to the NTD risk list.

Antiepileptic

Carbamazepine

Same folate-depleting mechanism as phenytoin via CYP induction. Spina bifida risk roughly 1 percent. Counsel on switching agents before pregnancy.

Maternal

Maternal diabetes

Hyperglycemia at conception is teratogenic across body plans. NTDs, caudal regression, cardiac defects. Pre-pregnancy A1c is the lever.

Maternal

Maternal obesity

Independent NTD risk roughly doubled at BMI above 30. Mechanism includes lower folate bioavailability and undiagnosed glucose intolerance.

Maternal

Hyperthermia (week 3-4)

Early-pregnancy fever or hot tub exposure raises NTD risk. The neuropore is heat-sensitive during the window it should be fusing. Avoid saunas and untreated fevers in the first trimester.

Prevention · the only proven move

Folate before conception, not after the test turns positive

By the time most patients know they're pregnant, the neuropore has already closed (or failed to close). Folate has to be on board before week 3-4. Start it at the start of any reproductive plan.

0.4 mg/day All reproductive-age patients planning or capable of pregnancy

4 mg/day Prior pregnancy affected by NTD, or on folate-antagonist antiepileptic

Keep going

These two pages stack on this one.

Pairs with this Arnold-Chiari II Cerebellar tonsils dragged through the foramen magnum, paired with myelomeningocele about 90% of the time. Banana sign and lemon sign on ultrasound. Backstory CNS embryology · neurulation How the neural plate folds into the neural tube week 3-4. The thing that breaks when an NTD happens. Teratogens Antiepileptic teratogenicity Valproate, phenytoin, carbamazepine and how each one steals folate from the developing neural tube.

Test yourself

Six vignettes. No timer. Explanations show the clue, the chain, and the high-yield discriminator.

Before you reveal an answer: elevated MSAFP, correct dates, singleton pregnancy, and a nondiagnostic ultrasound. What has to prove neural tissue is leaking?

Amniotic acetylcholinesterase is the specific neural-tissue leak signal when ultrasound has not shown the defect.