Embryogenesis as a key to understanding malformations
Normal development of the nervous system goes through several strictly sequential stages, with disruption at each leading to specific pathologies.
- Dorsal induction (3–4 weeks of gestation). During this period, the neural tube forms and closes. Disruption of this process causes cranial dysraphism (e.g., cephalocele or Chiari malformations).
- Ventral induction (5–8 weeks of gestation). The embryonic prosencephalon (forebrain) divides into two distinct hemispheres. Arrest of this process leads to a spectrum of holoprosencephalies (holoprosencephalies).
- Neuronal proliferation and migration (from week 7 to month 5). Neurons are generated in the germinal matrix (along the ventricular walls) and migrate peripherally along radial glia to form the six-layered cortex.
Disorders of dorsal induction
This group of malformations is associated with defects of the cranial vault and improper formation of posterior fossa structures.
Cephalocele (cephalocele) is a herniation of intracranial contents through a congenital bony defect. If the hernial sac contains only cerebrospinal fluid and meninges, it is a meningocele. If it contains abnormal and non-functional brain tissue, it is an encephalocele. Hernias most commonly occur along the midline (occipital, frontonasal).
Chiari II malformation (Arnold-Chiari) is a complex congenital malformation of the rhombencephalon. It is characterized by:
- A small posterior cranial fossa with an enlarged foramen magnum.
- Downward displacement (herniation) of the cerebellar tonsils, pons, and medulla oblongata.
- Beaking deformation of the tectum of the midbrain.
- An elongated and slit-like IV ventricle.
- Interdigitation of hemispheric gyri through a fenestrated falx cerebri.
- Hydrocephalus requiring shunting.
Disorders of ventral induction: Holoprosencephaly
Occurs due to incomplete cleavage of the cerebral hemispheres. The severity depends on the degree of fusion:
- Alobar form — the most severe. The hemispheres are not separated at all. MRI visualizes a single holoventricle and a massive "pancakes-like" brain tissue mass anteriorly. The thalami and basal ganglia are fused, and the Circle of Willis is not formed. Often accompanied by severe facial dysmorphism (up to cyclopia).
- Semilobar form — the interhemispheric fissure is indicated posteriorly, but the frontal lobes remain fused.
- Lobar form — brain volume is nearly normal, hemispheres are separated, but cortical continuity is preserved deep in the frontal lobes.
- Syntelencephaly (interhemispheric variant) — frontal and occipital poles are separated, while parietal and posterior frontal regions are fused together.
Anomalies of the corpus callosum and midline structures
The corpus callosum forms from anterior to posterior: first the genu, then the body and splenium. Its dysgenesis or complete agenesis is rarely isolated and often requires screening for other midline malformations (lipomas, cephaloceles).
Imaging appearance of corpus callosum agenesis:
- Lateral ventricles run parallel to each other.
- Colpocephaly (marked dilatation of the occipital horns).
- High-riding III ventricle projecting into the interhemispheric fissure.
- Probst bundles (bundles of Probst) — longitudinal white matter tracts that failed to cross the midline. They create a characteristic indentation on the medial walls of the lateral ventricles.
- Absence of the cingulate sulcus, causing cortical sulci to run vertically, descending straight to the III ventricle.
Isolated absence of the septum pellucidum (septum pellucidum) may be a developmental variant, but it is often part of septo-optic dysplasia (de Morsier syndrome), in which it is combined with underdevelopment of the optic chiasm/nerves and pituitary hormone deficiencies.
Briefly: answers to topical questions
In what sequence do the anatomical parts of the corpus callosum form?
Maturation of the corpus callosum begins with the posterior part of the genu, the body, and the splenium; then the anterior part of the genu develops.
Which brain malformations arise from impaired neuronal migration?
Cortical malformations associated with abnormal proliferation, migration, or organization of neurons and glia include:
- focal cortical dysplasia;
- agyria/pachygyria;
- polymicrogyria;
- gray matter heterotopias, including subcortical band heterotopia;
- schizencephaly.
Sources also list lissencephaly, hemimegalencephaly, tuberous sclerosis, and hypothalamic hamartoma among cortical malformations.
What types of Chiari malformation exist besides type II?
Besides type II, there are type I and type III Chiari malformations.
- Chiari I malformation (Chiari I malformation) — mentioned in sources without a detailed description of clinical and morphological manifestations.
- Chiari III malformation (Chiari III malformation) — a combination of brain anomalies characteristic of type II, along with an occipital or cervical encephalocele and an occipital bone defect.
The triad of de Morsier syndrome (septo-optic dysplasia) is easily remembered by the rule "H-H-O": Hypopituitarism, Hypoplasia of the optic nerves, Absence of the septum pellucidum.
Class and exam questions
What is the difference between meningocele and encephalocele?
Meningocele contains only the arachnoid/pia mater and cerebrospinal fluid, whereas encephalocele includes fragments of brain tissue (usually abnormal and non-functioning).
What spinal pathology is associated with Chiari II malformation?
Almost all patients with Chiari II malformation present with an open neural tube defect — lumbosacral myelomeningocele.
How to distinguish alobar holoprosencephaly from severe hydrocephalus?
In severe hydrocephalus, the separation of the hemispheres is preserved, and a thin but continuous cortical mantle is visible at the periphery of the fluid. In the alobar form, the hemispheres are fused into a single anterior mass, and midline structures are absent.
What are Probst bundles?
These are white matter tracts that, due to agenesis of the corpus callosum, failed to cross the midline and run longitudinally, deforming the medial wall of the lateral ventricles.
What else to study on this topic
- Chiari III malformation: a combination of Chiari II features and occipital encephalocele.
- Neuronal migration disorders: cortical layer formation and cortical dysplasias.
- Hydranencephaly: intrauterine destruction of the hemispheres with preservation of the posterior cranial fossa.
- Congenital intracranial lipomas and their association with midline malformations.
- Interhemispheric cysts in agenesis ofstartTimestamp the corpus callosum and indications for shunting.
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