Embryology and congenital

Question 1

From which germ layer does the nervous system form?

What else forms from this layer?

Answer 1

Ectoderm

Ectoderm also gives rise to epidermis

Question 2

When does the Central nervous system appear?

(including the stage of embryonic development)

Answer 2

Beginning of 3rd week - during Gastrulation

Question 3

Describe how the primitive CNS forms during the 3rd week of development

Answer 3

1. Thickening of Ectoderm anterior to the Primitive node/streak - forming Neural plate
2. Edges of Neural plate thicken and move upwards - forming the Neural folds

Question 4

hehe xox

Answer 4

Question 5

Describe what happens after the neural folds have formed

Answer 5

At 22 days:

• Neural folds migrate towards each other and fuse at midline forming the NEURAL TUBE
  
  • tube initially remains open at both ends

Question 6

What name is given to the openings at each end of the neural tube?

Which closes first?

Answer 6

Anterior (cranial/rostral) neuropore:

• Closes first - 25 days (somite stage)

Posterior (caudal) neuropore:

• Closes - 27 days

Question 7

Failure of the neural tube to close causes what conditions?

Answer 7

• Anencephaly (+ variations)
• Encephalocoele
• Spina bifida

Question 8

What is anencephaly?

Answer 8

Absence of a major portion of the brain, skull, and scalp

Failure of Anterior neuropore closure

Incompatible with life

Question 9

What is Exencephaly?

Answer 9

Condition in which the embryo’s brain is located outside of the skull

This condition is usually found in embryos as an early stage of anencephaly

It is hence also caused by failure of the anterior neuropore to close

Question 10

What is encephalocoele?

Answer 10

Condition caused by failure of the posterior neuropore (rostral neural tube) to close

Characterised by the Herniation of cerebral tissue through a defect in the skull. This forms large, sac-like protrusions

Question 11

Describe how spina bifida arises

Answer 11

Defective closure of the caudal (anterior) neural tube - such that the vertebral arches do not fuse

This basically leaves a section of the spinal canal open at the back

Question 12

What are the types of Spina bifida?

Answer 12

Spina bifida occulta

Spina bifida cystica:

• SB w/ meningocele
• SB w/ meningomyelocele
• Myeloschisis

Question 13

What are the main features of SB occulta?

Answer 13

Most minor form of SB

Failure of embryonic halves of vertebral arch to grow normally and fuse - usually L5, L6

Usually no clinical symptoms - however, may have a dimple with tuft of hair at site of problem

Question 14

Spina bifida cystica encompasses 3 types of spina bifida.

What characterises this class of Spina bifida?

Answer 14

Protrusion of spinal cord and/or meninges through the defect in the vertebral arches

Question 15

What is spina bifida w/ meningocele?

Answer 15

Rarest form

Protrusion of meninges and CSF outwards - but Spinal cord remains where it should be

Question 16

What is Spina bifida w/ meningomyelocele (MMC)?

Answer 16

Protrusion of sac - containing Spinal cord and/or nerve roots

This form of SB involves neurological deficits - loss of sensation and muscle paralysis

The area affected depends where the lesion is located

Question 17

What other condition is associated with SB w/ meningomyelocele (MMC) and how?

Answer 17

Hydrocephalus

Hydrocephalus can develop in babies with SB w/ meningomyelocele

Question 18

What is myeloschisis?

Answer 18

The most severe form of SB

Failure of neural folds to fuse causes the spinal cord to be open in the affected area

Question 19

Describe how spina bifida is prevented

Answer 19

Folic acid supplements - 400 µg/day –> 50-70% decrease in risk

Question 20

Describe how prenatal diagnosis of spina bifida is achieved

Answer 20

Maternal blood screening:

• Elevated α-fetoprotein (AFP) in serum - best detected at 16-20 weeks

Amniocentesis:

• Elevated α-fetoprotein (AFP) in amniotic fluid

Ultrasound:

• Spina bifida after 16-20 weeks
• Anencephaly after 12 weeks

Question 21

What are the risk factors for spina bifida etc?

Answer 21

Genetic predisposition

Nutritional

• (e.g. too little folate, too much vitamin A)

Environmental

• (e.g. hyperthermia; taking certain drugs – e.g. sodium valproate)