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.