Introduction

Pachygyria and polymicrogyria are congenital disorders caused by abnormal neuronal migration in utero.  Neuroblasts migrate to form the 6 cell layers of the normal cerebral cortex.  Any disruption in this process can lead to one of the conditions collectively called neuronal migration disorders.

Etiology

Pachygyria (also termed incomplete lissencephaly) results from neuronal migration anomalies at 12 to 24 weeks gestation.  It has been suggested this could result from laminar necrosis of the third cortical cell layer.  Congenital CMV infection has been associated with pachygyria.  Pachygyria as well as complete lissencephaly has also been linked to two chromosomes.  X-linked pachygyria tends to affect the frontal lobe preferentially while chromosome 17 linked pachygyria is more associated with the parietal lobe.

Polymicrogyria is thought to be a result of ischemic laminar necrosis of the fifth cortical layer after the 20th gestational week.  Again, this disorder is linked to congenital CMV, but other infections, in utero vascular insufficiency, and chromosomal abnormalities are other possible causes.

Clinical manifestations

There is a wide variation of the clinical manifestations of both pachygyria and polymicrogyria.  The degree of cortical involvement does seem to correlate to severity of syptoms.  Developmental delay, focal neurological signs, epilepsy, and hypotonia are all characteristics of these disorders among others.

Radiological findings

As the name would suggest, numerous small gyri with shallow sulci most commonly effecting the frontal and parietal lobes are characteristic of polymicrogyria.  Large infoldings of thickened cortex is also characteristic.  However, the radiological appearance can vary greatly depending on such things as slice thickness.  In fact, often polymicrogyria can not be distinguished from pachygyria (which has the appearance of few broad gyri with thickend cortex) on radiological studies.  This distinction is usually of no clinical significance.