1. Malformations due to abnormal neuronal and glial proliferation or apoptosis.

A. Decreased Proliferation/Increased Apoptosis: Microcephalies

What is it:

• Reduced proliferation of neurons and glia in the germinal zones
• The circumference of the head is at least 3 standard deviations below the mean

Best imaging clue:

• Few gyri and abnormally shallow sulci (50% of normal depth) may be present

B. Increased Proliferation/Decreased Apoptosis (normal cell types): Megalencephalies

What is it:

• Increased proliferation of neurons and glia in the germinal zones
• The circumference of the head is at least 3 standard deviations above the mean

Best imaging clue:

• Enlarged brain with normal-sized ventricles and subarachnoid spaces

C. Abnormal Proliferation (abnormal cell types)

1. Non-neoplastic

a. Cortical hamartomas of Tuberous sclerosis (Fig. 5-8).

What is it:

• Spectrum of CNS hamartomas containing giant balloon cells. Associated brain tumor (subependymal giant cell astrocytoma) in 15%, mostly close to Monroe’s foramen.

Best imaging clues:

• Calcified subependymal nodules Multiple T2 hyperintense parenchymal tubers = hamartomas

b. Cortical dysplasia with balloon cells (Fig. 9-14)

What is it:

• Presence of abnormal cells from the wall of the ventricle to the cortex: ‘transmantle dysplasia’.
• The abnormal cells constist of dysplastic neurons and large cells with large nuclei and abundant eosinophilic cytoplas (balloon cells).

Best imaging clues:

• Abnormal heterogenous signal intensity extends radially inward form the cortex to the ventricular surface: ‘transmantle dysplasia’

c. Hemimegalencephaly (HMEG)

What is it:

• A rare hamartomatous malformation of a cerebral hemisphere, considered as a primary neuroblast migratory disturbance

Best imaging clues:

• Abnormal gyration, ventriculomegaly, colpocephaly, displacement of the occipital bone across the midline, and increased volume of the white matter, in addition to the overall increased size of the involved hemisphere.

2. Neoplastic (associated with disordered cortex)

a. DNET (dysembryoplastic neuroepithelial tumor) (Fig. 30-32)

What is it:

• A benign mixed intracortical glial- neuronal neoplasm consisting of a heterogenous population of neurons, astrocytes and oligodendroglia- like cells. Cystic components may be present.

Best imaging clues:

• Wedge- shaped cortically based mass with bubbly appearence exceeding normal cortical thickness. Lack of surrounding edema, no contrast enhancement.

b. Ganglioglioma (Fig. 33-34)

what is it:

• A benign mixed intracortical glial- neuronal neoplasm consisting of a heterogenous population of neurons, astrocytes and fibrovascular stroma cystic components may be present.

Best imaging clue:

• Well circumscribed enhancing cortically based mass, often partially cystic. May be large in younger patients.

2. Malformations due to abnormal neuronal migration.

A. Lissencephaly/Subcortical Band Heterotopia Spectrum. (Fig. 18-20)

What is it

• Arrest in neural migration resulting in three layered cortex: thin outer cortical zone – cell sparse zone – thick inner cortical zone
• When inner cortical zone is thin: band heterotopia (mild form of lissecephaly)

Best imaging clue:

• Sulcation is reduced an depht of sulcation is more shallow.
• When sulcation is completely absent: complete lissencephaly = agyria Incomplete lissencephaly: pachygyria

B. Cobblestone complex

1. Congenital muscular dystrophy syndromes

2. Syndromes with no involvement of muscle

C. Heterotopia (Fig. 15-17)

What is it:

• Heterotopic clusters of gray matter consisting of arrested/ disrupted neurons along the migration path from the periventricular germinal zone to the cortex

Best imaging clue:

• Isointense with gray matter on all sequences
• 1. Subependymal (periventricular)
• 2. Subcortical (other than Band Heterotopia)
• 3. Marginal glioneuronal

3. Malformations due to abnormal cortical organization (including late neuronal migration).

A. Polymicrogyria and schizencephaly

1. Polymicrogyria syndromes (Fig. 21-25)

What is it:

• Abnormal cortical organization consisting of 4 layers vs. normal 6.

Best imaging clue

• Multiple tiny indentations along the brain surface in the child.
• There is a decreased number of broad, thick gyri, often with anomalous venous drainage in the adult.
• May be unilateral or bilateral (e.g. in congenital bilateral perisylvian syndrome (CBPS))

2. Schizencephaly (polymicrogyria with clefts) (Fig. 26-29)

What is it:

• Presence of a gray matter- lined clefts extending from the pial surface of cerebral mantle to the ventricle and lined by polymicrogyric cortex The cleft may be uni- or bilateral , the lips may be open or closed

Best imaging clue:

• Transmantle gray matter –lined cleft

B. Cortical dysplasia* (Fig. 9-14)

* For the classification of cortical dysplasia in the spectrum of malformations of cortical development, we refer to the more recent classification of Palmini et al.

1. Mild MCD (may represent the abandoned term of ‘microdysgenesis’

What is it:

• Subtle derangements of focal cortical architecture, with ectopically placed neurons adjacent to or outside layer 1, without the presence of dysmorphic neurons or balloon cells. This entity represents the mildest form of Focal Cortical Development (FCD).

Best imaging clue:

• The lesions are not detectable with current structural MR imaging techniques

2. Focal Cortical Dysplasia (FCD).

What is it

• A. Type I: Subtle derangements of focal cortical architecture without the presence of dysmorphic neurons or balloon cells
• B. Type II (Taylor-type FCD): Subtle derangements of focal cortical architecture with dysmorphic neurons Type IIa no balloon cells Type IIb with balloon cells

Best imaging clue

• Type I: the cortical lesions are not detectable with current structural MR imaging techniques
• Type II: Focal areas of increased cortical thickness
• Blurring of the grey/white matter junction Increased signal on T2- weighted or FLAIR images (more in balloon cells (Type IIb)
• Extension of cortical tissue with increased signal from the cortical surface to the ventricle in transmantle dysplasia

4. Malformations of cortical development, not otherwise classified.

Simplified classification of malformations of cortical development according to Barkovicz, 2001.