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Porencephaly

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

Definition

In the broadest definition of the term, porencephaly encompasses a number of conditions, and can then be divided into developmental and congenital types 9

  • developmental porencephaly
    • schizencephaly (true porencephaly)
    • simple porencephaly
    • congenital midline porencephaly (usually considered part of the holoprosencephaly spectrum - the so-called dorsal cyst)
  • congenital encephaloclastic porencephaly (acquired porencephaly)

This is not the most common everyday usage of the term however, and if unqualified most radiologits and clinicians alike will assume porencephaly refers to acquired encephaloclastic porencephaly. 

The necessity of a cyst communicating with the ventricular system and / or subarachnoid space to earn the designation porencephaly is also a point of contention. Some authors use the term for cysts whether or not communication exists. Others reserve the term of 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, lined by white matter, which communicates with the ventricules and/or the subarachnoid space. 

Epidemiology 

Porencephalic cysts are uncommon, and usually congential. Aetiologies include:

  • perinatal cerebral ischaemia
  • trauma
  • infection
  • antenatal intraparenchymal haemorrhage 4

Clinical presentation

As can be expected from the knowledge that porencephalic cysts vary widely in size and location, clinical manifestations are also protean. Patients with porencephalic cysts range from asymptomatic, to profoundly impaired. 

Often signs and symptoms become evident in the first year of life, with spasticity and seizures being common early manifestations. Language impairment, mental retardation, 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. 

Pathology

The cysts are typically lined by white matter. They are thought to occur from focal encephalomalacia due to a localised cerebral insult most frequently in during early gestation. Gliosis will develop if the insult is late enough, usually thought to be after the start of the third trimester, although perhaps insults as early as 20 weeks of gestation may result in gliosis 11

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 ventricular with displacement of the midline ventricular echo 8.

CT

Porencephalic cysts appear as an intracranial cyst which has a well defined border and central attenuation the same as CSF. There is usually no mass effect on the adjacent parenchyma, although occasionally the cysts are enlarging and do result in local mass effect. There is no enhancement with contrast and no solid component. 

MRI

As with CT, 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). Importantly the cyst is not lined by grey matter, helpful in distinguishing them from arachnoid cysts and schizencephaly. Typically the cyst seen to communicate with the ventricles and/or the subarachnoid space. 

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

Treatment and prognosis

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

Etymology

Porecephalic cysts were originally described by R Heschl in 1859 4-5.

Differential diagnosis

General imaging differential considerations include:

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