NEURAL TUBE DEFECTS
INTRODUCTION
- Deformities involving CNS covering
- Vary in severity – from the simples Spina Bifida Aperta (incomplete fusion of vertebral arches)
- Extreme Forms; Anencephaly and Raquischisis where wide openings in cranial and vertebrae bones also absence of brain, nerves and meninges
- Most malformation especially NTD’S occur early embryogenesis – are likely aberrant expression of yet undefined developmental gene.
- CNS develops in a precise embriologic sequence; therefore interruption of one area affects remaining development.
- NTD’S – mainly embryologic induction disorder failure to properly form mesoderm-neuroectoderm.
- Primary defect = failure of Neural tube to close affecting neural-cutaneus ectodermal structures.
- Inciting events traced to 17-30 days gestation.
- Primary defect – failure of neural folds to fuse in midline and form neural tube (Neuroectoderm)
- Also maldevelopment of mesoderm (forms skeletal and muscular structures that cover neural structures)
- Defects can be open (communicateatmosphere) or closed (skin covered).
- They can be ventral or dorsal
EMBRYOLOGY:
- 2 processes form the C.N.S.
- Neurulation – formation o neural structures into a tube thereby forming brain – spinal cord
- Neurulation – formation lower spinal cord lumbar and sacral elements.
- First Neural plate at 17 days, then neuronal fold at 21 days and fusion of n. folds at 23 days.
- Any disruption in this 3 stages = Craniorachischisi (most severe form NTD)
- By 23-26 days closure Ant. Neuropore Failure = Anencephaly.
- By 26-30 days closure of caudal neuropore Failure = Myelomeningocele.
- Notocord = (precursor of skeletal axis).
- Somites = Derived from mesoderm
ETIOLOGY:
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The exact causes of NTDs are not known. Many different factors, including genetics, nutrition, and environmental factors, are known to play a role.
- Teratogens-(hyperthermia, sulphas, antihistaminic, nutrition deficiencies and anticonvulsants use)
- Most strongly tied = carbamazepine, valproic acid (folate antagon)
- Folate deficiency
- women who are obese, have poorly controlled diabetes are at greater risk
- NTDs and miscarriage are more common among fetuses of women who experience high temperatures (such as using a hot tub or sauna or having a fever) during the first 4 to 6 weeks of pregnancy
ANENCEPHALY:
- One of the most common – no survival possible.
- Absence of calvarium, posterior bone elements and deficiency in procencephalus.
- The pituitary gland is often absent or hypoplastic
- Facial presentation,Shoulder dystocia,Prematurity, Increased alpha-fetoprotein & Polyhydramnios
- Marked diminution of the size of the adrenal glands probably secondary to the absence of the pituitary gland.
- About 70% of anencephalic fetuses are females
SPINA BIFIDA OCCULTA:
- Mildest form, one or more vertebrae (usually L5 and S1) are not closed but no protrusion of spinal cord.
- Could be associated with Syrinomelia or tethered cord.
- Most common birth defect in north India is Neural tube defects (Spina bifida)
- Banana sign seen in the fetal brain
- 17% of population – most asymptomatic – just skin dimple with hair or discoloration over lumbar area.
- Outer part of vertebrae not completely joined.
- Spinal cord and covering (meninges) usually undamaged.
- Hair often at site of defect
MENINGOCELE:
- Common in Lumbosacral region
- Meninges protrude trough a small opening – defect closed.
- Outer part of vertebrae split.
- Widened neural foramen(MRI/CT )
- Meninges damaged and displaced through opening
MYELOMENINGOCELE :
- Outer part of vertebrae split. Spinal cord and meninges damaged and displaced through opening. Usually hydrocephalus
- Saccular protrusion containing neural placode and CSF.
- Surface covered by arachnoid but no dura nor skin.
- The sac appears yellow with fragile vessels.
- Nerve roots pass into the sac.
- Spine tethered by bony defect
LARGE OCCIPITAL ENCEPHALOCELE:
- Skin covered sac representing close NTD.
- Often cranium bifidum – represent a failure of a neuropore.
- Needed to have V-P shunt to controlled associated hydrocephalus.
- At 5 years moderate developmental delayed.
DIAGNOSIS:
In the first half of pregnancy:
- Neural tube defect is best detected by Amniocentesis
- Diagnosis made by elevated alpha-fetoprotein in amniotic fluid.
- Diagnosis is confirmed by sonography:
- Spina bifida :Ventriculomegaly & Obliteration of cisterna magna
Around 10 week
- Absence of cranial vault
- Angiomatous, brain tissue.
- Accurate ancephaly diagnosis
- ↑MSAFP :Open neural tube defects(15–20 weeks)
- Neural tube defects are associated with high levels of Acetylcholinesterase.
In the latter half of pregnancy
- Diagnosis is difficult especially when associated with hydramnios.
- Inability to locate the fetal head on abdominal palpation arouses the suspicion.
- Confirmation is done by sonography
COMPLICATIONS:
- Hydramnios (70%),
- Malpresentation—face or breech
- Premature labor, especially when associated with hydramnios,
- Tendency of postmaturity,
- Shoulder dystocia,
- Obstructed labor if the head and shoulders try to engage together because of short neck.
MANAGEMENT:
If confirmed before 20 weeks: Termination of pregnancy is to be done.
- If diagnosed in late pregnancy:Termination is to be done.
- Uterus is most often refractory to oxytocin as a result of insufficient production of its precursor cortisol from fetal adrenals.
- Use of prostaglandin vaginal gel (PGE2) h
- During labor, there is tendency of delay.
- Shoulder dystocia should be managed by cleidotomy.
PREVENTION:
- Folic acid supplementation beginning 1 month before conception to about 12 weeks of pregnancy has reduced the incidence of NTD significantly (85%).
- A dose of 4 mg daily is recommended.
- Risk of recurrence is about 2% in subsequent pregnancy.
Exam Important
- Pre-conceptional intake of Folate results in decrease in incidence of neural tube defect
- Myelomeningocele, Anencephaly & Encephalocele are neural tube defect
- ↑Acetylcholinesterase is a marker for neural tube defects
- Acetylcholinesterase is the Most useful maternal serum test used for distinguishing open neural tube defects and ventral wall defect in a fetus
- Sodium valproate causes open neural tube defect
- Neural tube defects are Multifactorial
- Facial presentation, Increased alpha-fetoprotein & Polyhydramnios are seen in anencephaly
- Anencephaly is best diagnosed using USG
- Most common presentation in anencephaly is Face
- Banana sign seen in the fetal brain Spina bifida
- Most common birth defect in north India is Neural tube defects (Spina bifida)
- Widened neural foramen can be seen in MRI/CT appearance of lateral meningocele
- Sonographic finding of Spina bifida Ventriculomegaly & Obliteration of cisterna magna
- Anencephaly can be diagnosed in first trimester by ultrasound
- Neural tube defect is best detected by Amniocentesis
- Commonest site of meningocele is Lumbosacral
- Accurate diagnosis of anencephaly on ultrasound can be done at 10 weeks of gestation
- Shoulder dystocia is seen predominantly in Anencephaly
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