Holoprosencephaly

• 1 in 10,000 births
• Disturbance in separation of embryonic prosencephalon into two separate cerebral hemispheres
• Associated with chromosomal abnormalities, maternal diabetes
• 3 types based on severity: alobar, semilobar, interhemispheric variant (syntelencephaly)

Alobar holoprosencephaly

• Complete or near complete failure of separation of cerebral hemispheres
• Often seen in stillborn
• Infants that survive show abnormal reflexes, tone and seizures
• Severe midline facial deformities – cyclops, single eye on a stalk, midline clefts, hypotelorism
• Medial and ventral brain not formed
• Septum pellucidum is absent
• Imaging
  • Crescent shaped holoventricle continuous with large dorsal cyst
  • Cerebrum as a pancake like mass with no interhemispheric fissure, corpus callosum or falx cerebri
  • Fused basal ganglia and hypothalamus
  • Arterial supply directly from internal carotid arteries
  • Hydrocephalus eventually develops and shunt is required

Semilobar holoprosencephaly

• Less severe
• Posteriorly interhemispheric fissure partially formed
• Anteriorly hemispheres fused
• Midline facial abnormalities mild
• Children present late with delayed milestones or hydrocephalus
• Frontal lobes fused
• Thalami partially separated
• Single ventricle
• Posterior corpus callosum is partially present
• Rudimentary temporal horns
• Hippocampi underdeveloped

Interhemispheric variant (syntelencephaly)

• Mild learning and visual impairments
• Anterior frontal and posterior occipital lobes separated
• Posterior frontal and parietal lobes fused with absent interhemispheric fissure, falx and corpus callosum