Congenital malformations of the nervous system

Birth defects of spinal cord

• Most of them are the neural tube defects (NTD)

– result from failure of fusion of one or more neural arches of the developing vertebrae during the 4th week.

– affect the tissues overlying the spinal cord (meninges, vertebral arches, muscles, skin).

– Spina bifida

• Spina bifida occulta

• Spina bifida cystica

Dermal sinus • Failure of the surface ectoderm (future skin) to detach from

the neuroectoderm and meninges.

• Dimple in the skin of the sacral region of the back.

Spina bifida occulta

• Failure of the fusion of neural arches in the median plane.

• Occurs in the L5 or S1 vertebra in %10 of the normal people.

• Most minor form - a small dimple with a tuft of hair.

• Usually no symptoms.

Spina bifida cystica • Protrusion of the spinal cord and/or meninges through defects in

the vertebral arches. 1. Spina bifida with meningocele (meningeal cyst contains

meninges). 2. Spina bifida with meningomyelocele (meningeal cyst contains

spinal cord). – common in lumbosacral region

Symptoms of spina bifida cystica include varying degrees of neurologic deficit depending on the position and extent of the lesion.

• Loss of sensation at corresponding dermatome

• Complete/partial skeletal muscle paralysis

• Sphincter paralysis (lumbosacral lesion)

3. Myeloschisis (rachischisis) – most severe type of spina bifida

– the neural folds fail to elevate and fuse

– spinal cord is represented as a flattened mass of neural tissue

– neural deficit caudal to lesion

Etiology of Neural Tube Defects

• Genetic, environmental, nutritional factors • Low maternal vitamin B12 levels • Valproic acid in first 4 weeks (meningomyelocele)

Folic acid (400µg/day)

Birth defects of brain • Because of the complexity of its embryologic history,

abnormal development of the brain is common (3/1000 births)

• Results from the defects in the cranium (NTD) • Birth defects of the brain can also be caused by

– alterations in the histogenesis of the nervous tissue. – developmental failures of the associated structures.

• Abnormal histogenesis of brain can result in mental and motor deficiency. – Exposure to certain viruses and high levels of radiation (8-

16 week) – Prenatal factors

• Maternal infection or thyroid disorder • Rh factor incompatibility • Genetic conditions

Cranium bifidum • Herniation of intracranial contents through a defect in the

cranium. • 1/2000 • Most common in the occipital region

meningocele meningoencephalocele meningohydroencephalocele

• Failure of the rostral neuropore to close during the 4th week

• Associated with – incomplete development of calvaria (acrania)

– partial absence of the brain

– exencephaly

– rachischisis

– facial abnormalities

DIOGNOSIS

• Serum alpha fetoprotein (AFP)

• Amniocentesis

• USG

• MRI

Meroencephaly (anencephaly)

stillborn fetus polyhydramnios

• 1/1000 birth • 2-4 times more

common in girls

Craniorachischisis

Microcephaly • Calvaria and brain are small, face is normal size • Ressults from the reduction in brain growth • Low pressure from the growing brain leads to the small size of the

neurocranium

Genetical factors Large amounts of ionizing

radiation Cytomegalovirus, Rubella virus

Toxoplasma gondii Maternal alcohol abuse

Etiology

Hydrocephalus • Enlargement of the head • There is excess CSF in the ventricular system • Result from

• Impaired circulation and absorption of CSF • Congenital aqueductal stenosis • Subarachnoid hemorrhage

• Increased production of CSF (choroid plexus adenoma)

Holoprosencephaly

• Defective formation/ failure of cleavage of the forebrain

• Facial abnormalities • 1:250 fetuses • Maternal diabetes, high doses

of alcohol during the 3rd week • Shh is a holoprosencephaly

related gene

References

1. The Developing Human: Clinically Oriented Embryology by Keith L. Moore, T. V. N. Persaud and Mark G. Torchia (2013). 9th ed. Elsevier Saunders, Philadelphia. ISBN: 978-0-8089-2444-9

2. Langman’s Medical Embryology by T.W. Sadler (2012). 12th ed. Lippincott Williams & Wilkins, Philadelphia. ISBN: 978-1-4511-4461-1

3. Human Embryology by Larsen WJ (2001). 3rd ed. Churchill Livingstone, Philadelphia. ISBN: 978-0-443-06583-5

4. Human Embryology and Developmental Biology by Bruce M. Carlson (2009). 4th ed. Mosby, Elsevier, Philadelphia. ISBN: 978-0-323-05385-3