Polymicrogyria can arise from multiple different causes. Sometimes problems during pregnancy, such as infections or episodes in which there was a lack of oxygen to the fetus, can lead to polymicrogyria. Sometimes, polymicrogyria is due to a genetic problem (i.e., an abnormality in the genes, or DNA). Genetic problems include both those that run in families and those that have appeared for the first time in the child who is affected. Sometimes, we cannot find a specific cause for polymicrogyria in a particular child.

-From Wikipedia

Polymicrogyria (PMG) is a developmental malformation of the human brain characterized by an excessive number of small convolutions (gyri) on the surface of the brain. Either the whole surface or parts of the surface can be affected.

Clinical presentation
The diagnosis of PMG is merely descriptive and is not a disease in itself, nor does it describe the underlying etiology or cause of the brain malformation.

Polymicrogyria may in fact be just one piece of a syndrome of developmental abnormalities, because children born with it may suffer from a wide spectrum of other problems, including: global developmental disabilities, mild to severe mental retardation, motor dysfunctions including speech and swallowing problems, respiratory problems, and seizures. Each child is unique in their presentation of the disorder, thus it is difficult to make a predictable prognosis for children with the diagnosis of PMG

With increased use of imaging techniques such as MRI and 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.

Polymicrogyria is a disorder of neuronal migration resulting in structurally abnormal cerebral hemispheres. The Greek 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).

Types of PMG
PMG is generally seen with different areas of the brain affected. These types are described below:

1. 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.

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.

2. 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.

3. Bilateral Posterior Parietal PMG (BPPP)

This may be a mild form of BPP.

4. 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.