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Primary Nonneoplastic Cysts Overview

General Approach to Brain Cysts

General Considerations

Overview: Cysts are common findings on MR and CT brain

scans. There are many types of intracranial cysts, some

significant, some incidental. In this section, we exclude cystic

neoplasms (such as pilocytic astrocytoma and

hemangioblastoma), solid neoplasms that commonly have

intratumoral cysts (such as ependymoma), and tumors that

often display central necrosis (e.g., glioblastoma).

We also exclude parasitic cysts (neurocysticercosis, hydatid

disease) and cystic brain malformations (Dandy-Walker

spectrum) from the discussion. Thus, the focus of this

particular section is primary nonneoplastic cysts.

Because the etiology, pathology, and clinical importance of

nonneoplastic cysts is so varied, classifying them presents a

real challenge. Some neuropathologists typically classify cysts

according to the histology of the cyst wall. Others group them

according to putative origin or pathogenesis.

In a schema based on pathogenesis, cysts may occur as normal

anatomic variants [e.g., enlarged perivascular (Virchow-Robin)

spaces] (PVSs), congenital inclusion cysts [e.g., dermoid and

epidermoid cysts (ECs)], or lesions derived from embryonic

ecto-/endoderm [colloid cysts (CCs) and neurenteric cysts

(NCs)]. Of course, there is a group of miscellaneous cysts [such

as choroid plexus cysts (CPCs) and nonneoplastic tumorassociated cysts (TACs)] that does not fit nicely into any

category.

Neurologists and their neuroradiology colleagues face a very

real dilemma: A cystic-appearing lesion is identified on MR or

CT. What is it? What else could it be? Histopathology of the

cyst wall isn't a practical consideration. What is readily

apparent is (1) the anatomic location of the cyst, (2) its

imaging characteristics (density/signal intensity, presence or

absence of calcification, enhancement, etc.), and (3) the

patient's age. The recommended initial approach to analyzing

brain cysts is anatomy based.

Anatomy-Based Approach to Brain Cysts

General Considerations

Key features: Four features help the diagnostic approach to

cystic-appearing intracranial lesions. The 1st step is to

determine whether the cyst is intra- or extraaxial. If it is

extraaxial, is the cyst supra- or infratentorial? Is it midline or off

midline? If a cyst is intraaxial, is it supra- or infratentorial? Is it

parenchymal or intraventricular? Although many intracranial

cysts certainly may occur in > 1 location, some sites are

"preferred" by certain cysts.

Extraaxial Cysts

Supratentorial extraaxial cysts: Nonneoplastic, noninfectious

extraaxial cysts can occur in the midline or off midline. Pineal

and Rathke cleft cysts occur only in the midline. Although

dermoid cysts seem to prefer a midline location like the

suprasellar cistern, they also occur off midline. Look for

rupture with fatty "droplets" in the subarachnoid cisterns.

Arachnoid cysts (AC) are usually off midline. In the

supratentorial compartment, midline ACs are relatively rare.

The most frequent midline locations are the suprasellar

cistern, followed by the quadrigeminal cistern and velum

interpositum. Large suprasellar ACs usually present in children

and may cause obstructive hydrocephalus.

The most common off midline extraaxial supratentorial cyst is

an AC. Although these can occur virtually anywhere, the

middle cranial fossa is the location of at least 50% of all ACs.

Occasionally, ACs occur over the cerebral convexities, most

commonly over the parietal lobe. ACs follow cerebrospinal

fluid (CSF) on all sequences and are differentiated from ECs