Polymicrogyria is a focal brain abnormality characterized by excessive abnormal small cerebral gyri with cortical over-folding, creating an irregular cortical surface and ill-defined grey-white matter junction 14. Counter-intuitively, it often appears as cortical thickening and simplification when the small gyral folds are below the resolution of the imaging modality or cortical surfaces are fused.
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Epidemiology
Polymicrogyria may be genetic (sporadic or with heterogenous inheritance patterns), or secondary to infection, metabolic, or vascular causes (listed below). Over 50 genes have been associated and genetic testing is often low yield 15-17.
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genetic
ADGRG1/GPR56 mutation 16q2.2-21 (causes bilateral frontoparietal polymicrogyria) 8,9
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infectious
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vascular
cerebral ischemia
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metabolic
Associations
schizencephaly (the cleft is lined by polymicrogyria)
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intellectual disability syndromes:
Clinical presentation
The clinical presentation varies depending on the degree of involvement, bilaterality, and associated syndromes. While some patients may have normal neurology, others can have epilepsy and severe neurodisability 6.
Pathology
Polymicrogyria may represent both a malformation and disrupted development 14. It is thought to result from an insult in utero occurring toward the end of neuronal migration and early phase of cortical organization (between 17 and 26 weeks gestation) 1,10,13. There is an abnormal arrangement and excessive folding of cerebral cortical cell layers which 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 but there is a predilection for the perisylvian region, (80% of patients), and bilateral involvement is common (60% of patients).
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 can only resolve thickened poorly formed gyri, the microgyri are too small to identify. Associated abnormalities may be seen 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 may be hyperintense on T2 weighted images (20-27%) possibly relating to dilated perivascular spaces. Occasionally (<5%), and perhaps more 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 sometimes able to be resolved on thin-section high-resolution MRI, but are often difficult to distinguish from pachygyria. Sulci in the affected cortex are typically shallow 11.
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
Treatment is symptomatic and often aimed at controlling epilepsy, a common sequela.
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