Polymicrogyria

Polymicrogyria is one of many malformations of cortical development (see classification system for cortical malformations), and along with grey matter heterotopias, falls under a bewildering group of conditions characterized by abnormalities both in the migration of neurons to the cortex and abnormal cortical organization. 

Epidemiology

Although often sporadic or genetic, polymicrogyria is also seen secondary to intrauterine cytomegalovirus (CMV) infection, vascular compromise in twins, or anomaly, intellectual disability syndromes including:

• Adams-Oliver syndrome: variant
• Arima syndrome
• Galoway-Mowat syndrome
• Delleman syndrome
• Zellweger syndrome
• Fukuyama muscular dystrophy

It is often associated with schizencephaly: the schizencephalic cleft is 'always' lined by polymicrogyric cortex. It is also encountered in patients with an absent septum pellucidum ⁷. 

Clinical presentation

The clinical presentation is very varied, depending on the degree of involvement, bilaterality, and associated syndromes. Some patients are essentially normal. Others have epilepsy of varying severity. Others still are severely disabled ⁶. 

Pathology

Thought to occur from a neuronal insult in late (after 20 weeks) gestation. There is an abnormal arrangement and excessive folding of cerebral cortical cell layers. This can be associated with fusion of the gyral surfaces ³. 

Some cases are genetic (e.g. as one of many heterogeneous manifestations of 22q11.2 deletion syndromes) and others form a distinct phenotype (e.g. bilateral frontoparietal polymicrogyria, mapped to a genetic mutation and GPR56 mutation 16q2.2-21)) ^8,9.

Radiographic features

Distribution is varied; however, there is a predilection for the perisylvian region which is involved in 80% of patients and bilateral involvement is common (60%). 

• frontal: ~70%, gyrus rectus and cingulate gyrus typically spared
• parietal: ~63%
• temporal: ~38%, hippocampus typically spared
• occipital: ~7%, visual cortex is typically spared

Recurring patterns of involvement have led to some morphological subtypes being described which include ⁴: 

• perisylvian: ~ 60%
• generalized: ~13%
• frontal: ~5%
• parasagittal parieto-occipital: ~3%

CT

CT is insensitive to the actual morphological changes, only able to resolve thickened poorly formed gyri. The microgyri are too small to identify. Associated abnormalities may be readily visible, however (e.g. schizencephaly). 

MRI

MRI is the modality of choice for assessing polymicrogyria. Both morphology and signal intensity may be abnormal. The best diagnostic clue is focal cortical thickening.

Signal intensity

Polymicrogyric cortex usually has signal characteristics similar to normal grey matter. The subjacent white matter is not infrequently hyperintense on T2 weighted images (20-27%) which may relate to dilated perivascular spaces. Occasionally (<5%), and perhaps more so in patients with congenital infection, the abnormal cortex demonstrates regions of calcification.

Morphology

The numerous small gyri that lend their name to the condition are very small and only seen on thin-section high-resolution MRI, and even then may be difficult to distinguish from pachygyria, as both are associated with broad enlarged and often thickened gyri.

The grey-white junction is often the best location to identify the 'bumpy' contour which on thicker slices may manifest as blurring.

Treatment and prognosis

No specific treatment of polymicrogyria per se is available. Treatment is symptomatic and particularly aimed at controlling epilepsy, a common sequela.