= AGENETIC PORENCEPHALY = TRUE PORENCEPHALY

= “split brain”

= rare CNS malformation consisting of a full-thickness CSF-filled parenchymal cleft lined by gray matter extending from subarachnoid space to subependyma of lateral ventricles

Frequency: 1÷1,650

Cause: ? vascular ischemia of portion of germinal matrix / genetic mutation → segmental developmental failure of neuronal cell migration to form cerebral cortex; NOT porencephaly

Time of injury: 30–60 days of gestation

Often associated with: polymicrogyria, microcephaly, gray matter heterotopia, septooptic dysplasia

Types:

1. Closed-lip schizencephaly (type 1) = gray matter-lined lips in contact with each other (may be missed in imaging planes parallel to the plane of cleft)
   • walls appose one another obliterating CSF space
2. Open-lip schizencephaly (type 2) = separated lips
   • CSF-cleft from wall of lateral ventricle to pial surface

• seizure disorder
• motor + mental deficiencies correlate with extent of defect:
  • mild / moderate developmental motor delay
  • range of normal mentation to severe mental retardation
• blindness possible (optic nerve hypoplasia in 33%)

Location: most commonly near pre- and postcentral gyri (sylvian fissure); uni- / (mostly) bilateral; in middle cerebral artery distribution

• cleft from ependyma of lateral ventricle to pial surface of cortex:
  • cleft lined by gray matter (PATHOGNOMONIC)
  • full-thickness cleft through hemisphere with irregular margins
  • asymmetrical dilatation of lateral ventricles with midline shift
  • wide separation of lateral ventricles + squaring of frontal lobes
  • ventricle wall may be tented pointing to defect
• absence of cavum septi pellucidi (66%)
• absence / focal thinning of corpus callosum
• cleft lined by polymicrogyria (66%) + heterotopias (common)
  • abnormal gyral pattern adjacent to cleft = “gyri dive into cleft”
  • polymicrogyria / pachygyria adjacent / remote to cleft
• bilateral often symmetric intracranial cysts, usually around sylvian fissure

Prognosis: severe intellectual impairment, spastic tetraplegia, blindness

DDx:

1. Pseudoporencephaly = acquired porencephaly = local parenchymal destruction ← vascular / infectious / traumatic insult (almost always unilateral, lined by gliotic white matter on MRI)
2. Arachnoid cyst
3. Cystic tumor