Primary Nonneoplastic Cysts Overview
General Approach to Brain Cysts
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
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
Anatomy-Based Approach to Brain Cysts
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
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
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