Developmental Abnormalities of the CNS

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Developmental abnormalities of the CNS are common, affecting 1% of newborns.
They can be divided into two main groups: primary developmental abnormalities,
which are the direct result of a genetic abnormality, and secondary developmental
abnormalities, which are due to disruption of development by an intrauterine
disease process such as infection or ischaemia, or toxic factors.
  
The majority of malformations of the nervous system have no ascertainable cause
  
Establishing the cause of a developmental abnormality of the nervous system is
important from the perspective of counselling for parents.
Unfortunately, 60% of developmental abnormalities of the nervous system have no
identifiable causative factor.
Of the remainder, 20% are mixed environmental and genetic, 5% are due to single
gene defects, 5% are due to chromosomal abnormalities, and 10% are due to exogenous
factors such as infection, toxins, or poor nutrition. 

  
Neural-tube defects

Neural-tube defects are the most common form of developmental abnormality of the CNS
Defects in the closure of the neural tube are the most common cause of congenital
malformation of the nervous system.
Such defects may affect either the cranial or the spinal closure of the neural tube,
and may be an open defect or one closed by meninges and skin.
  
ANENCEPHALY

The most severe and common form of cranial neural-tube defect is anencephaly.
The cranial vault is generally not formed, although the face and eyes
are usually well developed.
The brain is replaced by a disc of abnormally developed neural tissue.

A less severe cranial neural-tube defect is development of an encephalocele,}
in which a defect in the bone of the skull is associated with cystic outpouching
of meninges, which may contain brain.
  
Neural-tube defects of the spinal cord are most common in the lumbar region.
The severity of the defect is variable.

The main defects include:
  
   Spina bifida occulta. Abnormal development of the bony arch
    of the spinal column. Meninges and cord normal.
    There may be an associated sinus track to theskin surface, or subcutaneous lipoma.
  
   Meningocele. Abnormal development of the bony arch of the spinal cord, with cystic
    outpouching of meninges covered by skin.
    Spinal cord may be normally or abnormally formed.
  
   Meningomyelocele. Abnormal development of the bony
    arch of the spinal cord, with cystic  outpouching of meninges including nerve
    roots and incorporating abnormally developed spinal cord.
  
   Myelocele. Abnormal development of the bony arch of the
    spinal cord, with exposure of abnormally developed spinal cord.
  
Patients with spinal neural-tube defects commonly have paraplegia with urinary and
faecal incontinence.
Without surgical correction, they develop spinal and limb deformities.
A major complication is recurrent urinary tract infection leading to chronic
pyelonephritis and renal failure.
  
Diagnosis can be made in utero by ultrasound scanning or by detecting
raised levels of a fetoprotein (AFP) in serum or amniotic fluid.

  
The Arnold-Chiari malformation type II

is the second most common developmental abnormality of the CNS.
  
In Arnold-Chiari Type II malformations there is herniation of the brain stem and
the lower part of the cerebellum into the foramen magnum, blocking drainage of CSF
and causing hydrocephalus.
This abnormality is nearly always associated with developmentmof a lumbar
meningomyelocele.
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