Polymicrogyria is a common malformation of cortical development characterised by the presence of numerous small gyri with abnormal cortical lamination typically in the perisylvian region.
Description
Polymicrogyria (PMG) is a condition related to abnormal development of the cerebral cortex, leading to the formation of numerous, excessively small convolutions, or gyri, on the surface of the brain. This abnormal cortical development results in a rough, irregular cortical surface. It is one of the most common cortical malformation conditions and can occur in isolation or as part of a syndrome. Although polymicrogyria can affect any part of the cerebral cortex, it has a predilection for the perisylvian region, especially in bilateral perisylvian polymicrogyria.
Pathogenesis
The exact pathogenesis of polymicrogyria is not completely understood, but it is thought to involve an early insult or injury to the developing cerebral cortex during the late first trimester or early second trimester of gestation. This could be due to various factors, including genetic mutations, intrauterine infection, ischemia, or other environmental factors. These insults interfere with the normal process of neuronal migration and cortical organisation, leading to the characteristic appearance of polymicrogyria.
Subtypes
Polymicrogyria can be classified based on its anatomical distribution:
- Bilateral frontal polymicrogyria: The most common subtype, predominantly affecting the frontal lobes.
- Bilateral perisylvian polymicrogyria: This subtype primarily involves the area around the Sylvian fissures.
- Bilateral parasagittal parieto-occipital polymicrogyria: This subtype involves the parasagittal parietal and occipital regions.
- Diffuse (or global) polymicrogyria: This subtype is less common and involves all cerebral lobes.
Epidemiology, Risk Factors & Associations
- Polymicrogyria is associated with various syndromes such as Aicardi syndrome, Zellweger syndrome, and congenital cytomegalovirus infection.
- In some cases, polymicrogyria is also associated with chromosomal abnormalities, including deletions and duplications.
- There is an increased risk in individuals with a family history of polymicrogyria, suggesting a genetic component.
Clinical Features
Clinical manifestations can vary widely, depending on the extent and location of the affected cortex. Symptoms can include:
- Developmental delay
- Motor dysfunction, such as hemiparesis or quadriparesis
- Intellectual disability
- Epilepsy
- Language and speech disorders
Complications
- Risk of recurrent seizures or epilepsy
- Risk of intellectual disability or learning difficulties
Pathological Features
Histopathology
- Macroscopic: The cortical surface appears irregular with numerous small gyri.
- Microscopic: There is disruption of the normal six-layered cortical lamination, with four-layered micro-laminated cortex often seen instead.
Serology
- Not applicable
Biochemistry
- Not applicable
Radiological Features
General Features
- Polymicrogyria characteristically demonstrates a “moroccan leather” appearance on imaging due to the numerous small gyri. It has a predilection for the perisylvian region, especially in bilateral perisylvian polymicrogyria.
- The cortical thickness is usually normal or slightly thickened, and the grey-white matter junction is irregular.
- The underlying white matter is often abnormal.
CT
- Non-contrast: May show irregular cortical surface and underlying white matter abnormalities. Calcifications can be seen in some cases, especially if associated with congenital infection.
- Contrast-enhanced: No significant enhancement is typically seen.
MRI
- T1: The cortical thickness may appear normal or slightly thickened with an irregular grey-white matter junction.
- T2: The cortex shows increased signal intensity, with the small gyri creating a complex, irregular pattern.
- T1 C+: No significant enhancement is typically seen.
- DWI/ADC: No restriction of diffusion.
US
- B-mode: May show irregularity of the cortical surface in neonates and young infants.
Grading and Staging
Not applicable as polymicrogyria is a congenital malformation and not a neoplasm.
Diagnosis
Diagnosis is usually based on the characteristic imaging findings, along with clinical correlation. Genetic testing can be useful in some cases, especially when a syndromic cause is suspected.
Differential Diagnosis
- Schizencephaly: Typically demonstrates a cleft extending from the ventricular surface to the pial surface of the brain. It may be associated with septa within the cleft, which is not seen in polymicrogyria.
- Cortical dysplasia: Typically presents with cortical thickening and abnormal signal on MRI, but without the numerous small gyri seen in polymicrogyria.
- Pachygyria: Characterised by fewer and thicker gyri compared to polymicrogyria.
Management
Management is primarily supportive and symptomatic, focusing on managing seizures and providing supportive therapies such as physical, occupational, and speech therapy. Referral to a neurologist is typically required for management of seizures, and a geneticist may be helpful in cases with a suspected syndromic cause.