Glossary in Congenital Malformations
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#Lissencephaly: "smooth brain", the brain has very few
or no gyri. Lissencephaly has the same meaning as pachygyria. The morphology of
lissencephaly came in many different forms. The distribution of the
lissencephaic or pachygyric parts is also an important clue for diagnosis. The
better-known ones are:
Classic
lissencephaly (type I or Bielschowski type): The
classic example is Miller-Dieker syndrome and X-linked lissencephaly. In this
form, the brain is small and only has primary and sometimes a few secondary gyri.
Areas of microgyria are occasionally visable. The cerebral cortex is abnormally
thick but the white matter appears only as a narrow ribbon along the ventricle
where numerous gray matter heterotopias are found. Cerebral vessels in the
absence of sulci are tortuous and can be diagnosed with angiogram. Typically,
the cortex consists of four layers with the neurons oriented reversely (dendritic
process pointing downward). One of the hypotheses for the formation of the four
layered cortex is that the migration pattern is an outside-in rather than the
normal inside-out pattern. This explains why large pyramidal cells that belong
to the deeper layer (layer V and VI) adpoted a superficial location.
Cajal-Retzius cells may not be found in the molecular layer. Migration defects (ectopias)
of the olivary nuclei in the medulla is also common.
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Layer I:
a superficial, cell sparse layer that corresponds to the molecular layer of the
normal brain.
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Layer II:
a narrow cell-rich layer in which large pyramidal cells that should normally be
located in the deeper layers are present.
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Layer III:
a thin layer of white matter.
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Layer IV:
a thick band of ectopic neurons extends almost to the ventricular wall.
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The
migration defects may be secondary to disturbance of the protein Relin (Relin
mouse as an animal model). Relin provide a stop signal for the migrating cells.
Cobble stone type lissencephaly (type II): Walker-Warburg syndrome is the classical example.
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GROSS PATHOLOGY:
There is often fusion of frontal poles. The crotex is smooth, although areas or
unlayered microgyria may be present. The meninges are thick and have a mioky
appeaance due to massive mesenchymal proliferation, especially around the brain
stem. The cerebellum is small and lacks a vermis; histologically dysplastic. The
brain stem is hypoplastic. The
pyramidal tracts are usually absent and hydrocephalus is seen in 75% of cases.
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Histology:
Microscopically, there is complete disruption of cortical architecture, the
cortical plate consisting of unlayered, poorly oriented cells separated by
trabeculae of gliomesenchymal tissue in continuity with that of the meninges.
This generates a nodular distribution of cortical neurons. The leptomeninges
show a remarkable mesodermal proliferation with extensive glioneuronal
heterotopia, obliterating the subarachnoid architecture and fusing with the
cerebral cortex. The cerebellum is completely disorganized. Muscle involvement
is probably an integral part of this type of lissencephaly.
#Localized cortical migration disorders: Characterized by focal abnormal areas usually of
polymicrogyria that may affect any part of the cortex and are often responsible
for focal epilepsy. Lesions may be symmetrical and bilateral. The reported
entites include:
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Bifrontal
dysgenesis,
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Bioccipital
dysgenesis
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Bilateral
perisylvian syndrome (developmental Foix-Chavany Marie syndrome)
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Posterior
cortical dysgenesis
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Unilateral
opercular dysplasia
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Nodular
cortical dysplasia (brain warts)
·
Glioneuronal
heterotopia
NeuroLearn NeuroHelp Malformations General Syndromes For Comment: KarMing-Fung@ouhsc.edu