[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Central nervous system](https://durgical.com/en/general-pathology/areas/cns/) › Brain malformations: forebrain and posterior fossa # Brain malformations: forebrain and posterior fossa - Lesion Brain malformations arise when cells are too few, move to the wrong place or fold wrongly; many cause epilepsy and developmental delay. Updated Nov 12, 2026 ## Key points - Microcephaly: small brain and head from reduced neuron production; causes include congenital Zika and CMV, alcohol, chromosomal and genetic disorders. - Lissencephaly (agyria): smooth brain with few or no gyri and a thick four-layer cortex from failed neuronal migration. - Polymicrogyria: many small fused gyri; can follow in utero injury such as CMV or ischemia. - Holoprosencephaly: incomplete separation of the hemispheres, with midline facial defects; linked to trisomy 13 and to Sonic hedgehog pathway mutations. - Chiari I: low cerebellar tonsils, often found in adults, with headache and syrinx; Chiari II: small posterior fossa, vermis and medulla pulled down, with myelomeningocele. - Dandy-Walker: missing or small cerebellar vermis with a big midline cyst (the enlarged fourth ventricle). ## What it is Beyond neural tube defects, the developing brain can fail in three main ways: too few neurons are made (microcephaly), neurons do not migrate or organise properly (lissencephaly, polymicrogyria, heterotopia), or the forebrain does not split into two hemispheres (holoprosencephaly). The posterior fossa has its own anomalies, Chiari and Dandy-Walker, and a fluid cavity can form in the cord (syringomyelia). Genetic, infectious (Zika, CMV) and toxic (alcohol) causes are all important. ## Structure - Syrinx: a slit-like cavity, usually in the cervical cord, lined by glia rather than ependyma, which interrupts crossing pain and temperature fibres. - Hydromyelia is a dilated central canal lined by ependyma; the two often overlap. - Joubert syndrome is a related hindbrain anomaly with a small vermis and the ‘molar tooth’ shape of the midbrain on imaging. ## The main malformations by mechanism | Malformation | What went wrong | Typical associations | | --- | --- | --- | | [Microcephaly](https://durgical.com/en/conditions/zika-virus-infection/) | Too few neurons produced | Congenital Zika, CMV, fetal alcohol, trisomies | | Lissencephaly | Neurons fail to migrate to the cortex | Seizures, severe delay; genes such as LIS1 | | Polymicrogyria | Disorganised late cortical layering | In utero infection or ischemia, genetic causes | | [Neuronal heterotopia](https://durgical.com/en/conditions/epilepsy-and-seizures/) | Nests of neurons left in the white matter | Epilepsy | | [Holoprosencephaly](https://durgical.com/en/conditions/trisomy-18-and-trisomy-13/) | Forebrain fails to divide | Trisomy 13, Sonic hedgehog pathway, maternal diabetes; cyclopia in the extreme | | Agenesis of the corpus callosum | Missing commissure | Often with other anomalies; can be silent | | [Chiari II](https://durgical.com/en/conditions/spina-bifida/) | Small posterior fossa, hindbrain displaced downward | Myelomeningocele, hydrocephalus | | [Dandy-Walker](https://durgical.com/en/conditions/hydrocephalus/) | Vermis absent, fourth ventricle cystic | Hydrocephalus | | [Syringomyelia](https://durgical.com/en/conditions/syringomyelia-and-chiari-malformation/) | Fluid cavity in the central cord | Chiari I, trauma, tumours | ## Clinical notes - A syrinx gives a cape-like loss of pain and temperature over the shoulders and arms with preserved touch, and later hand weakness. - The congenital Zika epidemic in Latin America (2015–2016) showed severe microcephaly with calcifications and cortical malformations. ## Memory aid Count, move, split: too few cells (microcephaly), cells that will not move (lissencephaly), a brain that will not split (holoprosencephaly). ## Test yourself Think of the answer, then tap the question to see it. ## What is the cortical defect in lissencephaly? Failed neuronal migration: the surface is smooth with few or no gyri and the cortex is thick with an abnormal layering. ## Which signalling pathway is classically mutated in holoprosencephaly? The Sonic hedgehog pathway, which patterns the midline of the forebrain and face. ## How does Chiari I differ from Chiari II? Chiari I is a mild downward tonsillar displacement, often adult and with syrinx; Chiari II is a severe hindbrain displacement with myelomeningocele and hydrocephalus. ## Why does a cervical syrinx cause cape-like loss of pain and temperature? The cavity first destroys the crossing spinothalamic fibres in the anterior commissure at the levels it spans. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/general-pathology/brain-malformations-forebrain-and-posterior-fossa/) [More Pathology questions →](https://durgical.com/en/practice/?f=pathology) ## Elsewhere in DurgiPedia - [Brain vesicles: from three to five and what each becomes](https://durgical.com/en/embryology/brain-vesicles-from-three-to-five-and-what-each-becomes/) - [Spinal cord: alar and basal plates and the ascent of the cord](https://durgical.com/en/embryology/spinal-cord-alar-and-basal-plates-and-the-ascent-of-the-cord/) - [Alcohol: fetal alcohol spectrum disorder](https://durgical.com/en/embryology/alcohol-fetal-alcohol-spectrum-disorder/) - [Molecular regulation: Shh, Wnt, FGF, BMP, Hox genes and retinoic acid](https://durgical.com/en/embryology/molecular-regulation-shh-wnt-fgf-bmp-hox-genes-and-retinoic-acid/) ## Connections ## Related 5 - [Neural tube defects](https://durgical.com/en/general-pathology/neural-tube-defects/) - [Cerebral edema, hydrocephalus and herniation](https://durgical.com/en/general-pathology/cerebral-edema-hydrocephalus-and-herniation/) - [Perinatal brain injury](https://durgical.com/en/general-pathology/perinatal-brain-injury/) - [Trisomies 21, 18 and 13, and the 22q11.2 deletion](https://durgical.com/en/general-pathology/trisomies-21-18-and-13-and-the-22q11-2-deletion/) - [Alcohol: how it injures the liver, brain, heart and fetus](https://durgical.com/en/general-pathology/alcohol-how-it-injures-the-liver-brain-heart-and-fetus/) ## Related conditions 8 - [Syringomyelia and Chiari malformation](https://durgical.com/en/conditions/syringomyelia-and-chiari-malformation/) - [Hydrocephalus](https://durgical.com/en/conditions/hydrocephalus/) - [Zika virus infection](https://durgical.com/en/conditions/zika-virus-infection/) - [Congenital (TORCH) infections: syphilis, toxoplasmosis, CMV and Zika](https://durgical.com/en/conditions/congenital-infections-syphilis-toxoplasmosis-cmv-and-zika/) - [Trisomy 18 (Edwards) and trisomy 13 (Patau)](https://durgical.com/en/conditions/trisomy-18-and-trisomy-13/) - [Fetal alcohol spectrum disorder](https://durgical.com/en/conditions/fetal-alcohol-spectrum-disorder/) - [Epilepsy and seizures](https://durgical.com/en/conditions/epilepsy-and-seizures/) - [Cerebral palsy](https://durgical.com/en/conditions/cerebral-palsy/) ## Microorganisms 2 - [Zika virus](https://durgical.com/en/microbiology/zika-virus/) - [Cytomegalovirus](https://durgical.com/en/microbiology/cytomegalovirus/) [← All of general pathology](https://durgical.com/en/general-pathology/) Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given. Sources: [Chiari Malformation | CMs | MedlinePlus](https://medlineplus.gov/chiarimalformation.html) (MedlinePlus, US National Library of Medicine, 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/general-pathology/brain-malformations-forebrain-and-posterior-fossa/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/brain-malformations-forebrain-and-posterior-fossa/