Porencephaly

Porencephaly is a rare congenital disorder that results in cystic degeneration and encephalomalacia and the formation of porencephalic cysts. The term is used variably among radiologists with its broadest definition being a cleft or cystic cavity within the brain ⁹, its more narrow definition being a focal cystic area of encephalomalacia that communicates with the ventricular system and/or the subarachnoid space. 

Terminology

The necessity of a cyst communicating with the ventricular system and/or subarachnoid space to earn the designation 'porencephaly' is a point of contention amongst neuroradiologists. Some authors use the term for cysts, whether or not communication exists, and thus there is significant overlap with encephalomalacia. Others reserve the term for cysts that communicate with at least one space, in which case they divide porencephalic cysts into internal (communicating with the ventricle) or external (communicating with the subarachnoid space). Still, others require communication with both. 

For the purpose of this article, the term porencephaly is taken to mean the following: a cystic lesion of the brain due to an encephaloclastic insult (e.g. intrauterine infections and ischemia), lined by white matter, which communicates with the ventricles and/or the subarachnoid space. 

Clinical presentation

Clinical manifestations are variable, as can be expected from the fact that porencephalic cysts vary widely in size and location. Patients with porencephalic cysts range from asymptomatic, to profoundly impaired. 

Signs and symptoms often become evident in the first year of life, with spasticity and seizures being common early manifestations. Language impairment, intellectual disability, and motor deficits are also frequently encountered ¹⁰. 

Head circumference is variable. It may be normal or small, or alternatively, synechiae can create a one-way valve effect with progressive enlargement of the cyst and skull expansion or hydrocephalus, resulting in an enlarged head, often asymmetrically.

Pathology

Porencephalic cysts are an uncommon congenital finding. The cysts are typically lined by white matter. They are thought to occur from focal encephalomalacia due to a localized cerebral insult, most frequently during late gestation, as etiologically similar insults before 24 weeks gestational age generally result in schizencephaly with polymicrogyria lining the cleft. Gliosis will develop if the insult occurs late enough, usually thought to be after the start of the third trimester, although perhaps insults as early as 20 weeks' gestation may result in gliosis ¹¹.

Etiology

• perinatal cerebral ischemia

• trauma

• infection

• antenatal intraparenchymal hemorrhage ⁴

• familial porencephaly: mutations in the COL4A1 gene (a COL4A1-related disorder) leading to fragile blood vessels causing antenatal/perinatal hemorrhagic strokes

Radiographic features

Ultrasound

On an antenatal ultrasound scan, these may be seen as one or more intracranial cysts that communicate with the ventricular system and/or subarachnoid space. There may also be asymmetrical ventricles with displacement of the midline ventricular echo ⁸.

CT

Porencephalic cysts appear as an intracranial cyst with a well-defined border and central attenuation equal to that of CSF. There is usually no mass effect on the adjacent parenchyma, although occasionally enlarging cysts do result in local mass effect. There is no enhancement after contrast injection and no solid component.

MRI

As with CT, the cyst appears well-defined and often corresponds to a vascular territory. The cyst is lined by white matter, which may or may not demonstrate evidence of gliosis (this depends on the age at which the insult occurred (see Pathology above), and is common in familial porencephaly). Importantly, the cyst is not lined by grey matter, helpful in distinguishing them from arachnoid cysts and schizencephaly. Typically, the cyst is seen to communicate with the ventricles and/or the subarachnoid space. 

Signal characteristics

The content of the cyst follows CSF signal on all sequences: 

• T1: low signal intensity

• T2: high signal intensity

• FLAIR: suppression of fluid signal intensity

• DWI: no restricted diffusion

Treatment and prognosis

Management is essentially supportive. The extent of impairment is somewhat related to the size and location of the lesion.

History and etymology

Porencephalic cysts were originally described by R Heschl in 1859 ^4,5.

Differential diagnosis

General imaging differential considerations include:

• neuroglial cyst: no communication with the ventricles or the subarachnoid space

• arachnoid cyst: extra-axial, with intact underlying grey matter

• interhemispheric cyst

• schizencephaly: it is lined with grey matter 

  • it is worth noting that some authors refer to schizencephaly as "true porencephaly" ¹²

• holoprosencephaly: failure of normal hemispheric separation