Supratentorial Malformations
Abnormalities of Neuronal / Grey Matter development.
Mature cerebral cortex has 6 layers. Cells start in germinal matrix of cerebral cortex along the margins of the lateral ventricles in 3 stages:
- Neuronal proliferation = cells form in the germinal matrix.
- Neuronal migration = cells move to the periphery.
- Neuronal organization = cells form normal cortex.
Abnormalities of Neuronal Proliferation.
1) Hemimegalencephaly
S+S: Seizures, mostly severe.
Path: Hamartomatous / dysplastic overgrowth of whole of one cerebral hemisphere.
Radiology:
- Increase in size of one hemi and skull
- Venticle is LARGER on affected side.
- ++ Blood flow -> More rapid myelination.
- Heterotopia / Polymicrogyria
Variants: Can affect only part of a hemisphere and thus cross over with FCD.
2) Focal Cortical Dysplasia (FCD)
S+S: Seizures, incidental, Delayed development.
Type I
Path: Focal abnormal proliferation of abnormal cells.
Radiology: Best seen on T1.
- Type Ia: Abnormal Radial Lamination
- Type Ib: Abnormal Tangential Lamination
- Type Ic: Both.
Type II
Path: Dysmorphic neurons.
- Type IIa: Rad= Not obvious as IIb.
- Type IIb: Balloon cells = 'Taylor-type' = Tubers in TSC.
- Rad: 'Transmantle' sign = High T2 in blurred cortex which tapers in the direction of the lateral ventricle margin (mostly sup-lat).
Type III
Path: Associated with other lesions.
- Type IIIa: with Hippocampal sclerosis.
- Type IIIb: with Tumour.
- Type IIIc: with Vascular Injury.
- Type IIId: with Parenchymal Injury.
Abnormalities of Neuronal Migration.
1) Heterotopia
Path: Neurons fail to migrate to periphery of brain. Rad: Abnormal site of grey matter, mostly nodular. T1 or DWI : Best.
a) Periventricular Nodular Heterotopia (PVNH)
- Most common type. Mostly around trigones
- Uni or bilateral.
- Multi or unifocal.
- SS= Incidental , seizures.
b) Subcortical Heterotopia
- Clusters or multifocal.
- Often with PVNH.
- Occasionally creates line from ventricle to cortex (DD: Schizencephaly)
c) Band-like Heterotopia
- Parallels cortex.
- Typically bilateral and symmetrical.
- Complete or Partial (posterior parts mostly incomplete)
- Often with lissencephaly.
Abnormalities of Neuronal Organization.
Normal cortex is 6 layers and has adequate sulcation.
Path: Signalling mechanism disruptions due to genetics, infection, ischaemia in
utero.
a) Polymicrogyria
- Symmetrical tends to be genetic. Ca2+ tends to be TORCH esp CMV.
- Rad: Multiple but smaller +/- nodular gyri.
b) Schizencephaly
- Follows from a transmantle (ventricle->periphery) parenchymal injury.
- Rad: Cleft from ventricle to subarachnoid space, ++/- polymicrogyria lined.
- Can be bilateral, and subtle therefore LOOK!
- Types:
-- Closed Lip: thin volume of tissue (DD:Transmantle heterotopia but the CLS
will have irregular contour in the lateral ventricle).
-- Open Lip: large volume of tissue loss and thus obvious.
Disorders Corpus Callosal Formation
~ 130 Genetic syndromes associated with CC abns. Most will cause severe SS.
Agenesis of Corpus Callosum
Radiology:
- Absence of CC, Cingulate gyrus.
- Radiating gyri from III (Sag) with III in direct continuity with interhemispheric fissure.
- Parallel configuration of lateral ventricles.
- Large atria and occcipital horns of lateral ventricles (colphocephaly)
- +/- Interhemispheric arachnoid cyst / cystic meningeal dysplasia.
Probst Bundles = fibres that would have crossed the midline instead run in AP plane along medial margins of ventricles.
DD: in F look for coloboma ~ AICARDI SYNDROME
Dysgenesis of Corpus Callosum
Path: Broad spectrum = Partial absence, abnormal morph, short AP dimension, general thinning.
NB: General thinning associated with WM injury or loss ( associated Wallerian degeneration ).
Meninx primitiva (Neural Crest Mesenchyme) remnants => Midline Lipoma. These are often on dorsal surface of posterior CC, which is often dysplastic and assoc'd CHIARI II
Holoprosencephaly Spectrum
Path: Failure of cleavage of Prosencephalon (embryologic forebrain) into hemispheres.
Spectum from ALOBAR -> SEMILOBAR -> LOBAR.
All at risk of pituitary / endocrine abn , cleft palate and other midlines.
| Type | Interhemispheric Fissure | Thalami | Corpus Callosum | Ventricles | Septum Pellucidum | Other |
|---|---|---|---|---|---|---|
| Alobar | None / Minimal | Fused | Absent / Dysplastic | Dysmorphic Monoventricle | Absent | Azgous ACA, Single incisor |
| Semilobar | Minimal | Variable fusion vs dysmorphia | Dysplastic | Dysmorphic monoventricle | Absent | |
| Lobar | Partial | Separate, usu N. | Mild dysplasia | Dysmorphic Ventricles | Absent | |
| Syntelencephaly | Mostly present | Normal | Usu N, occ mid-CC dysplasia | Mildly dysmorphic | Absent | Vertically orientated sylvian fissures. |
| Septo-optic Dysplasia | Present | Normal | Normal / Mild dysplasia | Mostly normal | Absent | ON Hypoplasia, ectopic neurohypophysis, schizencephaly |
Syntelencephaly
Mild end of spectrum. aka "Middle Hemispheric variant"
Rad/Path: Only abnormality in Telencephalon
- Fully formed CC +/- mid-CC dysplasia.
- Interhemispheric bridge of parenchyma ( = Telencephalic bridge)
- Vertical orientation of sylvian fissure.
Septo-Optic Dysplasia
Some consider to be the mildest of the Holoprosencephalies.
Rad/Path:
- Absence of Septum Pellucidum
- +/- ON hypoplasia (often the indication for the exam )
- +/- Ectopic neurohypophysis.
Lissencephaly / Pachygyria / Agyria Spectrum 1
Spectrum of conditions with smooth relatively featureless contour of brain
NB: Diff = Premature!!
Now viewed as Lissencephaly Spectrum subsuming pachygyria and agyria. And with
Subcortical band heterotopia at the other end.
See 1 for upto date classification system and correlation with clinical features = at end of the pdf.
Lissencephaly Smooth 4 layered cortex.
Pachygyria Thick, featureless cortex.
Agyria No gyri. Thus, no sulci except sylvian fissure.