Polymicrogyria
From Holoprosencephaly
Polymicrogyria is a developmental malformation of the human brain characterized by an excessive number of small convolutions (gyri) on the surface of the brain. The brains surface normally has gyri which give it a wiggly appearance, but in PMG there are many small wiggles that can be difficult to see on an MRI or CT scan, even for an experienced radiologist. The diagnosis of PMG is merely descriptive and is not a disease in itself nor does it describe the underlying aetiology or cause of the brain malformation. There are frequent associations with other brain malformations, including abnormalities of the corpus callosum, white matter and cerebellum.
PMG is not lissencephaly, but is often included because of the similarity in brain scan appearance on poorer quality scans and because many of the symptoms are similar.
Most children with polymicrogyria, but not all, have some degree of global developmental disabilities or delays, seizures, feeding difficulties, respiratory problems, motor dysfunction and mental retardation. It is difficult to make a predictable prognosis for children with the diagnosis of PMG because each child is very unique in their presentation of this disorder.
PMG results from several different causes that are both genetic and non-genetic in nature as follows:
- a genetic disorder (sometimes inherited, but mainly sporadic);
- viral infections of the baby during the 2nd trimester (i.e. cytomegalovirus infection);
- insufficient blood supply to the baby's brain during the 2nd trimester (i.e. umbilical cord entanglement) including twin-to-twin transfusion syndrome;
- other causes which have not yet been identified.
Research is ongoing into PMG, both in identifying the different types and in the understanding of the underlying causes.
Diagnosis
With increased use of imaging techniques such as MRI or CT, polymicrogyria is becoming more widely diagnosed. However, polymicrogyria is often misdiagnosed as pachygyria or lissencephaly, even by experienced radiologists, since the differences between these conditions can be difficult to see on an MRI or CT scan.
Pathology
Polymicrogyria is a disorder of neuronal migration resulting in structurally abnormal cerebral hemispheres. The Latin roots of the name describe its salient feature: many [poly] small [micro] gyri (convolutions in the surface of the brain). It is also characterized by shallow sulci, a slightly thicker cortex, neuronal heterotopia and enlarged ventricles. When many of these small folds are packed tightly together, PMG may resemble pachygyria (a few "thick folds" - a mild form of lissencephaly).
Etiology
- Polymicrogyria is increased numbers of small gyri with two, four or more lamination.
- It can be sporatic due to intrauterine anoxia or ischemia or intrauterine infection.
- It can also be familial . What are the different types of PMG?
PMG is generally seen with different areas of the brain affected. These types are described below:
Bilateral Perisylvian PMG (BPP) The sylvian fissure is a feature found on either side of a normal brain. In BPP there are areas of polymicrogyria around these fissures. Of all the PMG conditions, this type appears most commonly.
Individuals with BPP may have learning difficulties, cerebral palsy, and seizures. What is more distinctive about this condition is that individuals often have problems using the muscles of the face, throat, jaws and tongue – when this is mild it may lead to just a speech impediment or a tendency to drool but if more severe it can lead to difficulties with feeding as a baby.
Consistent familial recurrence has been reported for BPP, but is sporadic in the great majority of patients. It has been reported in children born from identical twin pregnancies which were complicated by twin-twin transfusion syndrome. For more information on BPP click here.
BPP is also known as Congenital Bilateral Perisylvian Syndrome (CBPS) and more recently it has been found that Worster-Drought Syndrome (WDS) is also the same condition.
Bilateral Frontoparietal PMG (BFPP)
MRI shows symmetric polymicrogyria affecting the frontoparietal regions most severely, as well as ventriculomegaly, bilateral white matter signal changes, and small brainstem and cerebellar structures.
The syndrome is characterized by global developmental delay of at least moderate severity, seizures, dysconjugate gaze (the eyes do not align), and bilateral pyramidal and cerebellar signs.
BFPP shows an autosomal recessive inheritance pattern that has been mapped to a locus on chromosome 16q12-21. It appears to be more common than previously recognized and may be frequently misdiagnosed.
Other types of PMG
- Bilateral mesial occipital polymicrogyria
- Bilateral diffuse polymicrogyria
- Bilateral parieto-occipital polymicrogyria
- Bilateral parasagittal parietooccipital polymicrogyria,
- Bilateral frontal polymicrogyria
- Unilateral perisylvian or multilobar polymicrogyria.
References:
http://en.wikipedia.org/wiki/Polymicrogyria http://pathweb.uchc.edu/eAtlas/CNS/662.htm http://www.lissencephaly.org.uk/aboutliss/pmg.htm
