Posterior fossa malformations are developmental anomalies involving the cerebellum and brainstem that can lead to obstructive hydrocephalus and neurological dysfunction. These malformations vary in severity, timing of presentation, and associated spinal cord abnormalities.
Major Posterior Fossa Malformations
| Malformation | Pathologic Features | Clinical Presentation | Associated Conditions |
|---|---|---|---|
| Chiari I Malformation | Downward displacement of cerebellar tonsils through the foramen magnum (1 structure) | Often asymptomatic in childhood; manifests in adulthood with occipital headaches, neck pain, or cerebellar symptoms (e.g., ataxia) | Syringomyelia (cystic cavity in spinal cord) |
| Chiari II Malformation (Arnold–Chiari II) | Downward displacement of cerebellar vermis, tonsils, and medulla through foramen magnum (2 structures) | Infant presentation: dysphagia, stridor, apnea, limb weakness due to brainstem compression | Myelomeningocele (usually lumbosacral), noncommunicating hydrocephalus |
| Dandy–Walker Malformation | Agenesis of cerebellar vermis, with cystic dilation of the 4th ventricle, enlarging the posterior fossa | Delayed motor development, hypotonia, or signs of increased intracranial pressure | Noncommunicating hydrocephalus, spina bifida |
Key Points to Remember
- Chiari I: mild, presents later, associated with syringomyelia
- Chiari II: severe, early presentation, associated with myelomeningocele.
- Dandy–Walker: absent vermis + cystic 4th ventricle expansion.
- Noncommunicating hydrocephalus is a common feature of Chiari II and Dandy–Walker.
- MRI is the preferred imaging modality for diagnosis.
Learning Objective
By the end of this session, medical students should be able to:
Differentiate between Chiari I, Chiari II, and Dandy–Walker malformations based on their anatomical defects, clinical features, and associated conditions.
