Embryology and Congenital Malformations Flashcards
The nervous system develops from
the embryonic ectoderm
At what week of development does the CNS appear?
At the beginning of the third week
How are the neural plate and neural folds formed?
Thickening of ectoderm anterior to primitive node forms the neural plate, edges thicken and move upwards to form neural olds
Neural folds migrate towards each other and fuse at the midline to form
the neural tube
When does the anterior neuropore close?
18-20 somite stage, around 25 days
When does the posterior neuropore close?
Around day 27
The closure of the neural tube is essential for
normal development of and function
Closure of the neural tube is completed by
the 4th week of development
What are the most common defects which result from failure of the neural tube to close?
Anencephaly
Encephalocoele
Spina bifida
Prevalence of Anencephaly
1:1500 births
Is anencephaly more common in males or females?
Females
Features of anencephaly
Failure of anterior neuropore to close
Skull fails to form
Brain tissue degenerates
Incompatible with life
What is craniorachischisis?
Failure of anterior neuropore and rostral neural tube to close
Prevalence of Encephalocoele
1:4000 births
Features of encephalocoele
Herniation of cerebral tissue through a defect in the skull
Failure in closure of rostral neural tube
Most frequent in occipital region
Variable degree of neurological deficits
Features of spina bifida
Defective closure of the caudal neural tube
Affects tissues overlying spinal cord
Non-fusion of vertebral arches
Neural tissue may or may not be affected
Severity ranges from minor abnormalities to major clinical symptoms
Features of spina bifida occulta
Most minor form
Failure of embryonic halves of vertebral arch to grow normally and fuse
Occurs in L5 and L6 vertebrae of otherwise healthy people
Usually no clinical symptoms
May result in dimple with small tuft of hair
Features of spina bifida cystica
Protrusion of the spinal cord and/or the meninges through the defect in the vertebral arches
Features of spina bifida with meningocele
Rarest form
Protrusion of meninges and CSF
Features of spina bifida with meningomyelocele
Nerve roots and/or spinal cord included in the sac
Neurological deficits - loss of sensation and muscle paralysis
Area affected determined by level of lesion
Often associated with hydrocephalus
What is the most severe form of spina bifida with meningomyelocele?
Myeloschisis - spinal cord in affected area is open due to failure of neural folds to fuse
What is hydrocephalus?
Accumulation of CSF which results in an enlarged brain and cranium
How does a blocked aqueduct cause hydrocephalus?
Prevents CSF from lateral and third ventricles from passing into fourth ventricle so can’t drain properly
Causes of hydrocephalus
Genetic
Prenatal viral infection or intraventricular haemorrhage
Spina bifida cystica
Folic acid supplements decrease the risk of spina bifida by what percentage?
50-70%
How is spina bifida diagnosed prenatally?
Maternal blood screening - high levels of alpha-fetoprotein in serum
Amniocentesis
Ultrasound
Risk factors for spina bifida
Genetic predisposition
Nutritional - too little folate, too much vitamin A
Environmental - hyperthermia, sodium valproate
What is genetic counselling?
Process by which patients/relatives at risk of an inherited disorder are advised of the outcomes and nature of the disorder, probability of transmitting it and the options open to them
