Neural Tube Defects Flashcards
Screening for NTDs?
Increased alpha-fetoprotein in maternal serum and amniotic fluid in 90% of cases.
Ultrasound can often see damage- from 12th week for anencephaly. Spina bifida from 16-20 weeks (lemon sign)
What are neural tube defects?
Also known as dysraphisms, are defects of neural tube closure at 28 days gestation. Occurs in 2-3 per 100 births.
Can be case of reopening too.
Genetic and environmental causes of NTDs.
Genetic- genetic mutations in genes producing certain signalling molecules.
Environmental- external toxic teratogens. No folic acid
What is spina bifida?
NTD in lumbar region of spine. Incomplete fusion of posterior arches of lumbar vertebrae. Normally, during 5th week the mesenchymal vertebrae surrounding notochord give rise to neural arches that fuse at midline to form spine.
Types of spina bifida
Spina bifida oculta
Spina bifida aperta (myeloschisis)
Meningocele
Myelomeningocele
Spina bifida oculta
Mildest form. Only vertebral arches affected.
Overlying skin characterised by adipose nodule, dimple and tuft of hair.
Can be harmless or cause back pain.
Soina bifida aperta (myeloschisis)
Rarest and most severe SB.
Complete failure of fusion of caudal end of plates. Flat, plate like mass of nervous tissue with no overlying membrane. Nerves and tissue exposed, so baby more prone to infections like meningitis.
Proturding part of spinal cord have nerves that are damaged or not developed, so usually there is paralysis and loss of sensation.
Meningocele
Least common type.
Cystic lesion containing CSF protrudes at the back along with soinak membranes, but the spinal cord does NOT protrude.
No associated hydrocephalus and neurological exam usually normal.
What is Meningomyelocele
More severe than meningocele. Most common type -90%.
The protruding sac contains some spinal cord matter. Lubosacral in 80% of cases. May leak CSF.
Treatments of spina bifida
No treatment, mainly try to prevent infections.
Closure of myelomeningocele by surgery immediately after birth if csf leakage present, within 24-48 hours if no csf leakage.
Symptoms of meningomyelocele
Neurological damage leading to bowel and bladder incontinence.
Paralysis/paraplegia - no sphincter control, musculature of lower limbs undergo neurogenic atrophy
Meningitis common
Contracted bladder - UTI, renal failure
Muscular imbalance and spinal deformities.
Decrease in BMD and latex allergy
Increase obesity risk
Hydrocephalus in 90% of cases. Because of arnold chiari 2 malform
Types of cranial dysraphisms
Anencephaly
Encephalocele
Craniorachisis
Anencephaly
When anterior neuropores don’t close. Anterior end malformation where the cerebral hemispheres are absent as well as cavalrium. Only present structures are brain stem, cerebrovasculosa, choroid plexus.
Most end in spontanous abortion/ terminated. If born, initial neuro exam may be normal if brainstem still intact.
Encephalocele
Sac like protrusions on the brain.
Usually in the occipital region of post fossa. Brain tissue protrudes the skull.
Craniorachischisis
Entire length of neural tube opens onto head and neck. Rare. Most severe. Presents anencephaly and total spina bifida.
Arnold Chiari 2
Downwards displacement of cerebellar vermis or tonsils through foramen magnum to overlap spinal cord.
Hydrocephalus - 4th ventricle elongated and midbrain distorted causing palsies from involvement of lower cranial nerves. Can also be caused by aqueduct stenosis.
90% of time associated with meningomyelocele
Arnold chiari types
Chiari 1 - congenital but can be acquired. downward displacement of lower cerebellum. Rarely causes symptoms, but may lead to headaches and neck pain.
Chiari 3 - associated w occipital encephalocele.
Chiari 4 - lack of cerebellar development where cerebellum and BS lie in post fossa. Hypoplasia.
Causes of hydrocephalus
Excess CSF production
Obstruction of flow in ventricles or subarachnoid space
Decrease in reabsorbtion via arachnoid granulations
Types of hydrocephalus
Communicating - impaired CSF reabsorbtion in arachnoid granulations, obstruction of flow in subarachnoid space, excess CSF production
Obstructive/ non communicating - obstruction of flow in ventricular system
Signs and symptoms of hydrocephalus
Headache, nausea, vomiting, cognitive impairment, decrease level of conciousness, decreased vision, sixth nerve palsies
Treatment of hydrocephalus
- External ventricular drain drains liquid out of lateral ventricles.
- Ventriculoperitoneal shunt is more permanent option, one way valve to drain from lateral ventricle out of skull, under skin to peritoneal cavity of abdomen.
Syringomyelia
Uncommon condition where cavity develops in spinal cord.
Reasons;
Maldevelopment of cord and failure of fusion of ventral and dorsal segments
Disorders of csf flow
Tissue destruction following SC damage
Associated w arnold chiari 1 and hydrocephalus
Epidemiology of NTDs
50% of NTDS are aborted
1/1000 nerbordns have SB worldwide
Lemon sign
Only present up to 24 weeks
Frontal bones on ultrasound appear flattened or inwardly scalloped. Gives skull a shape similar to a lemon.
Decrease in intraspinal pressure from spina bifida causes the brain to move downards, decreasing the intercranial pressure.
Frontal bones most vulnerable to decreased ICP. As fetus matures, the frontal bones get stronger and lemon sign disappears.
Alpha-fetoprotein
Produced by the foetal liver and excreted by foetal kidneys. Can be detected in maternal serum after 12 weeks as it leaks through the open neural tube.
Best detected 16-18 weeks of pregnancy. Less sensitive in women taking sodium valproate.
In utero foetal surgery
Opening womb and sewing up open NTD. earlier the better prognosis.
Risks of premature birth, brain haemorrhage, diabetes and infection
Cesarean delivery
If myelomeningocele has been identified then a cesarean delivery is recommended before the rupture of amniotic membrane to reduce the injury to exposed myelomeningocele sac, decreasing degree of paralysis.
Daily intake of folic acid
If you could be getting pregnant - 0.4mg of folic acid
If you are pregnant then 1mg for first 12 weeks.
How does folic acid decrease NTDs?
Folic acid is involved in synthesis of purines and thymines needed for DNA. needed for post transitional methylation of neural cytoskeleton.
So if not present then critical genes left unmethylated, leading to their improper expression, so failure of normal neural tube development.
Recent studies show that this also leads to misexoression of miRNAs.
Why is baby with spina bifida usually in breech position
Because only undergoes spontaneous leg movements due to leg not developed therefore cant kick into normal birthing position (head down).
