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Describe the neural tube defects with their causes, diagnosis and prevention.

Anatomy · 1st Professional — Pre-Clinical

Neural tube defects arise from the failure of the neural folds to fuse and of the neuropores to close by the end of the fourth week.

Defect	Site	Features
Anencephaly	Failure of the anterior neuropore	Absent cranial vault and cerebral hemispheres; incompatible with life; associated with polyhydramnios because the fetus cannot swallow
Encephalocele	Defect in the skull, usually occipital	Herniation of meninges with or without brain
Spina bifida occulta	Failure of the vertebral arches to fuse; the cord is normal	Commonest, about 10% of people; usually asymptomatic; marked by a tuft of hair, a dimple or a naevus
Meningocele	Meninges herniate through the defect	Neural tissue normal; usually no neurological deficit
Meningomyelocele	Meninges and spinal cord or roots herniate	Paraplegia, sensory loss, bladder and bowel involvement, club foot; commonly with hydrocephalus and Arnold–Chiari malformation
Rachischisis	The neural tube remains completely open	The most severe form

Maternal serum alpha-fetoprotein is raised in open defects, measured at 16–18 weeks
Amniotic fluid AFP and acetylcholinesterase confirm it
Ultrasound at 18–20 weeks shows the defect; the "lemon" and "banana" signs of the skull and cerebellum are indirect markers
AFP is not raised in closed defects such as spina bifida occulta or a skin-covered meningocele

The neural tube closes by day 27, often before the woman knows she is pregnant — which is why supplementation after a positive test is too late
Food fortification has reduced the incidence substantially where it has been implemented

Risk factors — folate deficiency, a previously affected child (recurrence about 4%), maternal diabetes, obesity, sodium valproate and carbamazepine, and hyperthermia in early pregnancy
Meningomyelocele is closed surgically within 48 hours to prevent infection; a shunt is usually needed for the associated hydrocephalus
Fetal surgery before 26 weeks improves motor outcome and reduces the need for shunting, though it carries maternal risk
Lifelong care is needed for bladder and bowel management, orthopaedic deformity and pressure sores; latex allergy is unusually common in these children
Tethered cord syndrome may present later with deteriorating gait or bladder function as the child grows

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