with lissencephaly may present with seizures. The brain of a patient
with lissencephaly shows a smooth cortical surface. Pathological examination
of the brain surface in patients with lissencephaly may show agyria
(absence of gyri) or pachygyria (few broad and flat gyri) or polymicroglia.
Lissencephaly are classified into types I, II, III, and IV, based on
the morphology of the brain and associated brain anomalies.
The only common finding among the different types of lissencephaly is
that the brain surface looks smooth. Lissencephaly
is diagnosed by CT or MRI of the brain. Magnetic resonance imaging is
the study of choice.
type I results from a complete arrest of cortical neuronal migration
between 12- and 16-weeks gestation (Figure 47.1 [A]). The MRI of the
brain has a figure-8 appearance on axial images (Figure 47.1 [A]). This
appearance results from the smooth brain surface, large and vertically
placed Sylvian fissure, hypoplastic operculum, and enlarged ventricles.
The MRI of the brain also shows that the cerebral cortex has 2 bands
of cortical gray matter (Figure 47.1 [A]), an outer layer
that is thin and an inner layer that is thick. The outer and inner layers
are separated by a zone of white matter. Cerebral anomalies often associated
with lyssencephaly type I are hypoplasia of the corpus callosum, colpocephaly
(enlargement of the occipital horns of the lateral ventricles), and
The cerebellum and third and fourth ventricles are normal. Neonates
with lissencephaly type I do not have ocular or muscle abnormalities.
type I may occur as: (1) isolated lissencephaly syndrome (no specific
dysmorphysm); (2) Miller-Dieker syndrome (specific dysmorphysm and deletion
of the distal part of the short arm of chromosome 17); and (3) Norman-Robert
syndrome (dysmorphic features but no chromosome 17 abnormality). Miller-Dieker
syndrome is probably the most common.
Neonates with Miller-Dieker
syndrome have characteristic facial features: microcephaly with bitemporal
narrowing, vertical ridging and furrowing in the central forehead (especially
when crying), small nose with antiverted nostrils, upslanting palpebral
fissures, protuberant upper lip, thin vermilion border of upper lip,
and micrognathia (Figure 47.1 [B]).
A deletion in the p13 region
of chromosome 17 is present in patients with Miller-Dieker syndrome.
Parents of patients with Miller-Dieker syndrome should undergo genetic
evaluation to determine whether they are carriers of a balance translocation
of the terminal fragment of chromosome 17 onto a chromosome in the 13-15