Neuroepithelial Cysts, Porencephaly, and Perivascular Spaces

CHAPTER 28 Neuroepithelial Cysts, Porencephaly, and Perivascular Spaces

Many pathologic processes present as cystic lesions of the brain. This chapter focuses on the noninfectious, non-neoplastic cysts of the brain parenchyma and ventricular system in children. These benign fluid spaces most commonly arise in utero, from developmental defects or acquired insults. Infectious cysts, cystic neoplasms, and cystic lesions of the extra-axial spaces are discussed elsewhere in this text.


These histologically benign, smooth-walled cysts are characterized by a glial wall and an epithelial lining resembling ependyma or choroid plexus (Fig. 28-1). Alternate names include epithelial cyst, ependymal cyst, choroid cyst, choroidal-epithelial cyst, and glioependymal cyst.1

Clinical Presentation

Neuroepithelial cysts are usually asymptomatic4 and discovered incidentally at imaging for unrelated conditions. Large cysts may cause symptoms related to mass effect on local structures. Symptomatic neuroepithelial cysts are usually intraventricular and present with obstructive symptoms.



On CT, neuroepithelial cysts appear as smooth, round to ovoid, nonenhancing lesions with a central low density resembling cerebrospinal fluid (CSF). They may be unilocular or display internal septations. Calcification is very rare.4 Intraparenchymal cysts appear well circumscribed with well-defined cyst margins. Because the cyst wall is typically thin, intraventricular cysts and cysts located within the cisterns may be difficult to distinguish from the surrounding CSF.


On MRI, the cyst fluid largely resembles CSF on both T1- and T2-weighted (T1W, T2W) images (Fig. 28-2).4 However, the cyst fluid may appear slightly hyperintense to CSF on T1W images, because the cyst fluid has a slightly higher protein content than CSF. The cyst fluid may also appear slightly hyperintense to CSF on T2W images, because the signal from the confined fluid is not dispersed by CSF pulsations (Fig. 28-3). There is little to no restriction of diffusion. The cyst walls do not enhance (Fig. 28-4). The surrounding brain parenchyma displays normal signal intensity with no sign of edema, gliosis, or other reaction to the cyst.

Neuroepithelial cysts commonly occur within the choroid fissure (Fig. 28-5). These choroidal fissure cysts may be recognized as CSF-intensity structures situated between the fimbria of the fornix and the diencephalon. They demonstrate a characteristic spindle shape when imaged in the sagittal plane.11 The cyst contents display no restricted diffusion on diffusion-weighted images (see Fig. 28-5).


In the most general definition, porencephaly refers to a CSF-containing cavity that replaces normal brain parenchyma. Heschl coined the term in 1859 (Gr. poros, meaning “opening” or “passage”) to describe full-thickness defects in the cerebral mantle that allowed for communication between the ventricular system and the subarachnoid space. That definition has since been modified to include cysts isolated from one or both of the ventricular and subarachnoid spaces. Loose usage of the term has caused confusion as to its meaning. Some authors classify porencephaly into two types. Porencephaly type I (synonyms: false porencephaly, pseudoporencephaly, and encephaloclastic porencephaly) results from an encephaloclastic insult that occurs in utero, in premature newborns, and in early infancy. The damaged area then evolves over time into a fluid-filled cavity (Fig. 28-6). Porencephaly type II (synonyms: schizencephaly, true porencephaly, and agenetic porencephaly) results from focal injury to the germinal matrix in early fetal life, disrupting the normal migration of subependymal neuroblasts to the surface of the developing hemisphere. This focal disruption creates a focal, often tubular cavity that (1) extends across the full thickness of the brain from the ventricular system to the brain surface and (2) is lined by dysplastic gray matter (Figs. 28-7 and 28-8).

Porencephaly and schizencephaly are closely related. Their differences most probably reflect the different gestational age at which the fetal brain is injured. Injury occurring before approximately 24 weeks of gestation may result in schizencephaly, whereas injury after that time may cause porencephaly.12 The gestational age at which the insult occurs will also determine the reaction in the surrounding brain. More mature brains have a greater capacity to develop a glial scar (gliosis) around the cyst.13 After approximately 30 weeks’ gestation, there is a greater chance of reactive glial scarring.14

Hydranencephaly represents an extreme form of bilateral porencephaly and is due to disruption of the fetal anterior circulation with destruction of the developing cerebral hemispheres. The tissue of each hemisphere is replaced by a large fluid-filled sac covered by a thin membrane representing the leptomeninges. Preservation of the vertebral arteries results in proper development of the brain stem, cerebellum, thalami, and occasionally portions of the temporal and occipital lobes.13 Alternate names include porencephalic cyst and encephaloclastic cyst.

Clinical Presentation

The clinical manifestations of porencephaly depend on the extent of the prenatal encephaloclastic event. The most common presenting signs are hemiparesis and epilepsy.15,16 When the insult is severe, spastic hemiplegia or tetraplegia may be present. There may be developmental delay, especially when the involvement is bilateral. When deeper midline structures are affected, there may be hypoplasia, resulting in visual deficits.17

Jan 22, 2016 | Posted by in NEUROLOGICAL IMAGING | Comments Off on Neuroepithelial Cysts, Porencephaly, and Perivascular Spaces
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