Cerebellar Hemispheric Malformations
Overview of Cerebellar Hemisphere Anomalies
- Types of anomalies:
- Hypoplasia: Reduced cerebellar hemisphere volume (underdevelopment).
- Dysplasia: Abnormal neuronal architecture.
- Disruptions: Secondary injuries affecting cerebellar growth.
- Predominantly hemispheric underdevelopment is uncommon:
- Can be seen in pontocerebellar hypoplasia.
- Occasionally observed in survivors of extreme prematurity.
- Unilateral cerebellar hypoplasia:
- Typically results from developmental disruptions (e.g., cerebellar hemorrhage).
- Less commonly due to primary dysgenesis.
Cellular Mechanisms of Cerebellar Hypoplasia
- Cerebellum development involves two germinal zones:
- Primary ventricular neuroepithelium (Purkinje cells, Bergmann glia).
- Rhombic lip (Granule cell precursors).
- Rhombic lip-derived cells vastly outnumber ventricular-derived cells.
- Purkinje cells (ventricular zone origin) support granule cell precursor proliferation via Sonic Hedgehog (SHH) signaling.
- Dysfunctional SHH signaling causes global cerebellar hypoplasia (equal vermian and hemispheric involvement).
- Earlier isthmic organizer defects cause disproportionate vermian hypoplasia.
- Timing:
- Granule cell proliferation continues into late gestation and postnatally.
- Cerebellar hypoplasia may become apparent late in gestation or after birth.
- Neuronal migration disruptions:
- Reelin gene mutations disrupt Purkinje cell migration, commonly leading to cerebellar hypoplasia.
- Normal Bergmann glia function is essential for Purkinje cell migration.
Rhombencephalosynapsis
- Rare cerebellar malformation characterized by:
- Fusion of cerebellar hemispheres (80% complete, 20% partial).
- Fusion of superior cerebellar peduncles.
- Midline continuity of deep cerebellar nuclei.
- Consistent absence of superior vermis.
- Associated anomalies (variable):
- Fused thalami and fornices.
- Absent septum pellucidum.
- Corpus callosum agenesis.
- Hydrocephalus with aqueductal stenosis.
- MRI hallmark:
- "Diamond-shaped" 4th ventricle.
- Cerebellar folia oriented transversely across midline without vermian interruption.
- Clinical spectrum broad, typically includes neurodevelopmental impairment.
- Associated patterning disruption:
- Possible association with holoprosencephaly, suggesting dorsal-ventral patterning disruption.
Pontocerebellar Hypoplasias (PCH)
Overview
- Group of rare, autosomal recessive disorders.
- Characterized by:
- Small, hypoplastic pons.
- Cerebellar hypoplasia (hemispheres more severely affected than vermis).
- Progressive neurodegeneration following initial developmental defects.
- Prognosis:
- Usually severe with profound developmental delays.
- Often progressive, resulting in early mortality.
PCH: Classic Imaging Description
- Dragonfly Appearance:
- Flattened cerebellar hemispheres ("wings") and preserved vermis ("body").
- Supratentorial involvement common (cortical atrophy, ventriculomegaly, microcephaly).
Pontine Tegmental Cap Dysplasia
- Posterior fossa anomaly caused by disrupted axonal guidance.
- Imaging findings:
- Flat ventral pons.
- Dorsal pontine "cap/beak" protrudes into 4th ventricle.
- Severe hypoplasia of middle/inferior cerebellar peduncles.
- Associated cranial neuropathies:
- Typically 8th nerve (hearing loss).
- Facial paralysis/anesthesia.
- Swallowing abnormalities.
- Motor and cognitive deficits common.
Clinical Implications
- Early fetal MRI assessment critical:
- Identify concurrent brainstem anomalies, significantly worsening prognosis.
- Genetic counseling essential due to autosomal recessive inheritance of many PCH subtypes.
- Management is supportive/palliative, tailored to individual patient needs and complications.