II. REGIONAL REVIEW OF DEVELOPMENTAL DISORDERS
The important and common developmental disorders will be reviewed first,
and the anomalies at different levels of the nervous system (spinal cord,
brainstem, cerebellum, and cerebral hemispheres) will be discussed. In addition
to regional defects, hydrocephalus is an important problem and will be discussed
in the last section.
A. Defects in closure of the neural tube.
Recall that the neural tube forms gradually from the neural plate by "zippering"
in caudal and cephalad directions from the cervical region. The last portions
of the neural tube to close are the anterior and posterior neuropores. The
latter closes at about the 26th day of gestation. Midline defects,
which are directly related to the lack of closure of the neural tube, are
among the most common of central nervous system malformations. These defects,
called dysrhaphic states, are probably related to both genetic and environmental
causes. Since amniotic fluid contains increased alpha-fetoprotein content
with "open" neural tube closure defects, prenatal diagnosis is
possible.
- 1. Anencephaly - absence of the brain or of all parts except
the basal ganglia, brainstem and cerebellum. The skull and meninges are
often absent or defective. This defect is possibly related to failure of
closure of the anterior neuropore. Some affected infants live a few months.
There is a recurrence rate of 1-3% within a family. Some evidence exists
for a recessive gene in families with multiple affected siblings (recurrence
risk with 2 affected is 10%). There is an x-linked form of anencephaly.
There are ethnic and regional differences in the incidence (e.g., more
common in U.K. than U.S.)
- 2. Other midline defects.
- a. Spina bifida - midline skeletal defect in spine
- b. Meningocele - sac containing meninges CSF protrudes through
skeletal defect(rare)
- c. Meningomyelocele - meninges and spinal cord protrude through
overlying defect in the vertebral column. Most meningomyeloceles are found
in a lumbosacral location. The majority of patients with meningomyeloceles
also have hydrocephalus and Arnold-Chiari malformation.
- d. Encephalocele - meninges and brain tissue protrude through
a skull defect.
- e. Congenital dermal sinus - less serious but more common mid-line
defect. Defects range from dimpling of skin over lumbosacral area to sinus
tracts in this region.