Nonneoplastic Cysts

Main Text

Preamble

There are many types of intracranial cysts. Some are incidental and of no significance. Others may cause serious—even life-threatening—symptoms.

In this chapter, we consider a number of different intracranial cysts: Cystic-appearing anatomic variants that can be mistaken for disease, congenital/developmental cysts, and a variety of miscellaneous cysts. Parasitic cysts, cystic brain malformations, and cystic neoplasms are excluded as they are discussed in their respective chapters.

While there are many different ways to classify cysts, an imaging-based approach to classification of intracranial cysts is the most practical, as most of these lesions are discovered on CT or MR examination.

An imaging-based approach takes into account three easily defined features: (1) Anatomic location, (2) imaging characteristics (i.e., density/signal intensity of the contents, presence/absence of calcification &/or enhancement), and (3) patient age. Of these three, anatomic location is the most helpful.

While many types of intracranial cysts occur in more than one location, some sites are “preferred” by certain cysts. In this chapter, we discuss cysts by location from the outside in, beginning with scalp and intracranial extraaxial cysts before turning our attention to intraaxial (parenchymal and intraventricular) cysts.

The following are four key anatomy-based questions to consider about a cystic-appearing intracranial lesion. A summary chart based on these simple questions, together with the cysts discussed throughout the text, is also included (Table 32-1).

4 KEY ANATOMY-BASED QUESTIONS

Scalp Cysts

Preamble

Nonneoplastic scalp cysts are sometimes identified on imaging studies intended to visualize intracranial structures. The most common incidental scalp masses in adults are epidermoid cysts (50%), trichilemmal (“sebaceous”) cysts (40%), and dermoid cysts (10-20%). Epidermoid and dermoid cysts are discussed later in this chapter. We discuss trichilemmal (pilar or “sebaceous”) cysts of the scalp here.

Trichilemmal (“Sebaceous”) Cyst

Terminology

Although the term “sebaceous cyst” is commonly used by radiologists, this type of cyst does not actually contain sebaceous material. These cysts are more accurately called trichilemmal cysts (TCs) or pilar cysts.

Etiology

TCs are derived from the outer root sheath of hair follicles, not sebaceous glands.

Pathology

Location, Size, and Number

Most TCs are found within the dermis or subcutaneous tissue. TCs can be single or multiple and vary in size from a few millimeters to several centimeters in diameter.

Gross and Microscopic Features

TCs are oval/round (75%) or lobulated (25%) and consist of a fibrous capsule lined by stratified squamous epithelium. Cyst contents consist of waxy desquamated keratin without a granular layer.

Clinical Issues

Demographics

TCs affect 5-10% of the population. Although they can occur at any age, most are found in middle-aged and older women.

Presentation and Natural History

TCs generally appear as hairless, mobile, slightly compressible subcutaneous scalp masses.

TCs grow slowly and have often been present for years. Up to 2% of TCs give rise to a proliferating trichilemmal tumor known as pilar or “turban” tumors. Rarely, TCs can become locally aggressive and even invade bone. Malignant degeneration (“proliferating trichilemmal cystic carcinoma”) is extremely rare.

Imaging

CT Findings

TCs are generally sharply delineated solid, cystic, or mixed solid/cystic masses that are heterogeneously hyperdense compared with subcutaneous fat. Calcification is seen in 80% of cases and can appear as punctate, curvilinear, or coarsely clumped aggregations (32-1A).

Soft tissue density and hyperdense foci are common. Typical TCs do not enhance. Calvarial remodeling or invasion is absent.

MR Findings

TCs are well-circumscribed scalp masses that appear incompletely surrounded by fat. They are generally homogeneously iso- to mildly hyperintense relative to brain on T1WI and inhomogeneously hypointense on T2WI and FLAIR (32-1). TCs do not suppress on FLAIR.

“Blooming” foci on T2* (GRE, SWI) are caused by calcifications, not hemorrhage. Simple, uncomplicated TCs do not enhance, although the proliferating variant may show significant enhancement with solid lobules interspersed with nonenhancing cystic foci.

Differential Diagnosis

In adults, the imaging differential diagnoses are benign and malignant scalp tumors. Basal cell carcinomas and scalp metastases are ill-defined, poorly delineated scalp masses that invade the subcutaneous soft tissues and may erode bone. Superficial ulceration is common. Dermoid and epidermoid cysts as well as hemangiomas are all much more common in the skull than in the scalp.

Extraaxial Cysts

Preamble

Extraaxial cysts lie between the skull and brain. With few exceptions, most are contained within the arachnoid membrane or in the subarachnoid space.

Determining sublocation of an extraaxial cyst (supra- vs. infratentorial, midline vs. off-midline) is helpful in establishing a meaningful differential diagnosis (Table 32-1). For example, an arachnoid cyst is the only type that commonly occurs in the posterior fossa. Some extraaxial cysts are usually (although not invariably) off-midline. Others—pineal and Rathke cleft cysts—occur only in the midline.

Arachnoid Cyst

Terminology and Etiology

Arachnoid cyst (AC) is also known as a meningeal cyst. ACs are CSF-containing cysts that arise as an anomaly of meningeal development. The embryonic endomeninges fail to merge and remain separated, forming a “duplicated” arachnoid. CSF is secreted by cells in the cyst wall and accumulates between the layers.

Most ACs are solitary, isolated sporadic, nonsyndromic lesions. Syndromic ACs have been reported in association with acrocallosal, Aicardi, and Pallister-Hall syndromes. A rare familial form of ACs with FOXC2 and RERE mutations has been reported.

Pathology

Location

Most ACs are supratentorial. They are usually off-midline and are the most common off-midline, extraaxial, supratentorial cyst. Nearly 2/3 are found in the middle cranial fossa, anteromedial to the temporal lobe (32-2). 15% of ACs are found over the cerebral convexities, predominantly over the frontal lobes. Between 10-15% of ACs are found in the posterior fossa, predominately the cerebellopontine angle (CPA) cistern.

Size and Number

ACs are almost always solitary. Size ranges from small incidental cysts to large space-occupying lesions.

Pathology

ACs are well-marginated cysts filled with clear, colorless fluid that resembles CSF. They are devoid of internal septations and are completely encased by a delicate translucent membrane lined by a single layer of mature, histologically normal arachnoid cells (32-3). Focal inflammatory infiltrates may occur but are rare.

ARACHNOID CYST: PATHOLOGY

Location

Gross Pathology

Clinical Issues

ACs are the most common of all congenital intracranial cysts. They account for ~ 1% of all space-occupying intracranial lesions and are identified on imaging studies in 1-2% of patients. ACs can be seen at any age. Most (nearly 75%) are found in children and young adults. There is no significant sex predilection.

More than 94% of ACs are asymptomatic and found incidentally, remaining stable over many years. Enlargement—if any—is very gradual. Some ACs—especially in the sylvian fissure—often reach a large size without causing symptoms. Large cysts in locations compressing CSF pathways (e.g., suprasellar ACs) may be candidates for surgical fenestration.

Hemorrhage—either traumatic or spontaneous—into an intracranial AC is rare but may cause sudden enlargement. ACs carry a slightly increased risk of subdural hematoma (SDH) (32-6).

Reduction or disappearance of an AC is rare. Some cases occur spontaneously while others have been reported following inciting events, such as trauma. Cyst wall rupture with CSF flow perturbation seems to be the most applicable pathophysiologic mechanism in so-called triggered spontaneous AC resolution.

Imaging

ACs vary in size, ranging from small incidental cysts to large space-occupying lesions. Uncomplicated ACs behave exactly like CSF on CT and MR (32-4). FLAIR and DWI are the best sequences to distinguish cystic-appearing intracranial masses from one another.

There is a relationship between SDHs and ACs, although whether this is causative or coincidental is unclear. Traumatic SDHs can rupture into ACs. The converse—AC rupture causing a spontaneous SDH—occurs but is rare.

CT Findings

Uncomplicated ACs are CSF density (32-6A). If intracystic hemorrhage has occurred, the cyst fluid may be moderately hyperdense compared with CSF (32-6B). Large middle cranial fossa ACs expand the fossa and cause temporal lobe hypoplasia or displacement (32-3).

With moderately large ACs, bone CT may show pressure remodeling of the adjacent calvarium. ACs do not cause frank bone invasion. ACs do not enhance.

MR Findings

ACs are sharply marginated, somewhat scalloped-appearing lesions that parallel CSF signal intensity on all sequences. They are therefore isointense with CSF on T1- and T2-weighted images. ACs cause moderate focal mass effect, displacing but not engulfing adjacent brain, vessels, and cranial nerves (32-7).

The internal appearance of an AC is intrinsically featureless, containing neither septations, vessels, nor cranial nerves (32-8).

ACs suppress completely with FLAIR (32-9). Occasionally, CSF pulsations within large lesions may cause spin dephasing, producing heterogeneous signal intensity and significant propagation of phase artifact across the scan (32-10). ACs do not restrict on DWI and do not enhance.

Differential Diagnosis

The major differential diagnosis of AC is epidermoid cyst (EC). ECs are often almost—but not quite—exactly like CSF. They have a cauliflower-like, lobulated configuration instead of the sharply marginated borders of an AC. ECs engulf vessels and nerves, insinuating themselves along CSF cisterns. ECs do not suppress completely on FLAIR and typically show moderate to marked hyperintensity on DWI.

Enlarged subarachnoid spaces caused by brain volume loss are usually more diffuse CSF collections and do not cause mass effect on adjacent structures.

A subdural hygroma or chronic SDH (cSDH) is not precisely like CSF and is usually crescentic, not round or scalloped. cSDHs usually show evidence of prior hemorrhage, especially on T2* sequences, and may have enhancing encasing membranes.

A porencephalic cyst looks just like CSF, but it is intraaxial and lined by gliotic white matter that is often hyperintense on FLAIR.

ARACHNOID CYST: CLINICAL ISSUES, IMAGING

Clinical Issues

Imaging

Differential Diagnosis

Choroid Fissure Cyst

The C-shaped choroid fissure is an infolding of CSF positioned centrally between the fornix and thalamus with the former forming its outer margin and the latter forming its inner margin.

The choroid fissure is normally a shallow, inconspicuous, C-shaped cleft that curves posterosuperiorly from the anterior temporal lobe all the way to the atrium of the lateral ventricle. The choroidal arteries and choroid plexus lie just medial to the choroid fissure.

A CSF-containing cyst can form anywhere in the plane of the choroidal fissure. These “choroid fissure cysts” (CFCs) are probably caused by maldevelopment of the embryonic tela choroidea, a double layer of pia that invaginates through the choroid fissure to reach the lateral ventricles. CFCs may thus have a neuroglial, neuroepithelial, or arachnoidal origin.

Imaging

Most CFCs are discovered incidentally on imaging studies. They lie just medial to the temporal horn of the lateral ventricle between the hippocampus and diencephalon and thus will shift the lateral ventricle and its choroid plexus laterally. Patients are usually asymptomatic or exhibit symptoms that do not correlate with the anatomic location of the cyst.

CT scans show a well-delineated, homogeneously hypodense mass with CSF-like attenuation. Calcification and contrast enhancement are absent. CFCs follow CSF signal intensity on all MR sequences, suppressing completely on FLAIR (32-11).

On axial and coronal images, CFCs are round to ovoid in shape, but on sagittal images, they have a distinctive, pathognomonic, elongated spindle configuration (32-11D).

Epidermoid Cyst

Both congenital and acquired epidermoid cysts (ECs) are found in the CNS. Although spinal ECs are often acquired lesions, intracranial ECs are always congenital in origin.

Terminology

An intracranial EC is a congenital, developmental, nonneoplastic inclusion cyst. ECs arise when the embryonic neural and cutaneous ectoderm fail to separate completely during neural tube closure, resulting in transplantation of epithelial cell rests into the developing neural tube.

ECs have incorrectly been called “tumors,” but they are not neoplastic. The term “cholesteatoma” should be reserved for an acquired lesion arising as a complication of chronic otitis media.

Pathology

Location

Extracranial ECs commonly involve the scalp, face, and neck. Over 90% of intracranial ECs are intradural and are almost always extraaxial. They have a distinct predilection for the basilar cisterns and are usually off- or paramidline. ECs insinuate themselves around cranial nerves and vessels (32-12).

The CPA cistern is the single most common site, accounting for 50% of all intracranial ECs. ECs are the third most common CPA mass (after vestibular schwannoma and meningioma).

The middle cranial fossa (sylvian fissure) and parasellar region together account for 10-15% of ECs. Less common locations are the cerebral ventricles, most commonly the fourth ventricle. Intradiploic ECs account for 5-10% of cases.

Gross Pathology

The outer surface of an EC is often shiny, resembling mother of pearl (32-13). Multiple “cauliflower” excrescences are typical. The cyst wall consists of an outer fibrous capsule lined by stratified squamous epithelium. The cyst itself is filled with soft, waxy, creamy, or flaky material with keratinaceous debris and solid crystalline cholesterol (32-14). Dermal appendages, such as hair follicles (a characteristic of dermoid cysts), are absent.

Clinical Issues

Epidemiology

ECs are the most common intracranial developmental cyst and are 4-9x more common than dermoid cysts. Peak age of presentation is 20-50 years. Symptomatic ECs are rare in children.

Natural History

ECs may remain clinically silent for many years. They grow very slowly via progressive accumulation of epidermal cells and accretions of desquamated keratin. ECs often reach considerable size before becoming symptomatic. Headache and cranial neuropathy are common features.

In contrast to dermoid cysts, rupture of an EC is rare. Malignant transformation occurs but is very rare.

EPIDERMOID CYST: PATHOLOGY AND EPIDEMIOLOGY

Pathology

Epidemiology

Imaging

ECs resemble CSF on imaging. Irregular frond-like excrescences and an insinuating growth pattern within CSF cisterns are characteristic.

CT Findings

The vast majority (> 95%) of ECs are hypodense (“black epidermoids”) and appear almost identical to CSF on NECT scans (32-16A). Calcification is present in 10-25%. Hemorrhage is very rare as is enhancement on CECT sequences.

An atypical hyperdense variant, the so-called white epidermoid, is uncommon, representing between 1.5 and 5.6% of all intracranial ECs. The hyperdensity of white epidermoids (32-19A)has been variously attributed to hemorrhage, formation of calcium soaps, or high protein content.

MR Findings

Classic (“black”) ECs are iso- or slightly hyperintense compared with CSF on both T1-(32-17A)and T2-weighted (32-17B)sequences. Slight heterogeneity in signal intensity is often present.

Typical ECs either do not suppress at all or suppress incompletely on FLAIR (32-17C). They restrict on DWI (32-17D)and are therefore moderately to strikingly hyperintense. Enhancement is generally absent, although mild peripheral enhancement can be seen in 25% of cases (32-18).

“White” ECs generally display reversed MR signal intensities (i.e., hyperintense on T1WI, hypointense on T2WI) (32-19). Most articles attribute the T2 hypointensity to high protein level and high viscosity of the cystic content, not intracystic hemorrhage. Partial FLAIR suppression may be present and has been termed the shading sign. DWI hypointensity is common and may be due to a T2 “blackout” effect.

Differential Diagnosis

The major differential diagnosis is arachnoid cyst (AC). ACs are smoothly marginated, behave exactly like CSF on all sequences, suppress completely on FLAIR, and do not restrict on DWI. ACs displace nerves and vessels while ECs surround and engulf them. Dermoid cysts should not be confused with ECs. Dermoid cysts contain fat and dermal appendages and do not resemble CSF on imaging studies. Neurenteric (endodermal) cysts are rare; the most common intracranial site is the prepontine medullary cistern.

Parasitic cysts, such as neurocysticercosis (NCC), are usually multiple and comparatively small. NCC cysts often contain a discernible scolex. Cystic neoplasms are rarely mistaken for ECs, as the cyst wall &/or nodule typically enhances.

EPIDERMOID CYST: IMAGING FEATURES

General Features

Imaging

Differential Diagnosis

Dermoid Cyst

Pathology

Dermoid cysts (DCs) are congenital inclusion cysts. The cyst wall contains mature squamous epithelium, keratinous material, and adnexal structures (hair follicles and sebaceous and sweat glands). DCs typically contain a thick, greasy sebaceous material with lipid and cholesterol elements (32-21).

DCs are usually extraaxial lesions that are most often found in the midline. The suprasellar cistern is the most common site (32-20) followed by the posterior fossa and frontonasal region.

Clinical Issues

DCs are much less common than epidermoid cysts (ECs) (ECs are 4-10x more common). DCs grow slowly secondary to the production of hair and oils from the internal dermal elements.

Presentation occurs at significantly younger ages compared with epidermoids, peaking in the second to third decades. DCs often remain asymptomatic until they rupture. Chemical meningitis with seizure, coma, vasospasm, infarction, and even death may ensue as a consequence.

Malignant transformation of intracranial DCs into squamous cell carcinoma is extremely rare.

Imaging

CT Findings

DCs are quite hypodense on NECT scans. With rupture, hypodense fatty “droplets” disseminate in the CSF cisterns and may cause discernible fat-fluid levels in the ventricles (32-23A).

MR Findings

Signal intensity varies with fat content in the cyst. Most DCs are heterogeneously hyperintense on T1WI. T1WI is also the most sensitive sequence to detect disseminated fat “droplets” in the subarachnoid space, diagnostic of ruptured dermoid (32-23C). Fat suppression is helpful to confirm the presence of lipid elements.

Standard PD and T2 scans show increasingly more pronounced “chemical shift” artifact in the frequency-encoding direction as the time of repetition is lengthened. Fat is very hypointense on standard T2WI (32-23D)but is “bright” (hyperintense) on fast spin-echo T2-weighted sequences. DCs demonstrate heterogeneous hyperintensity with linear or striated laminations if hair is present within the cyst.

Uncomplicated DCs are heterogeneously hyperintense on FLAIR. Ruptured DCs demonstrate subtle FLAIR sulcal hyperintensity (32-23E)and “bloom” on T2* GRE or SWI (32-23F).

Most DCs do not enhance, although ruptured DCs may cause significant chemical meningitis with extensive leptomeningeal reaction and enhancement.

Spectroscopy may show an elevated lipid peak at 0.9-1.3 ppm.

DERMOID CYST

Pathology

Clinical Findings

Imaging

Differential Diagnosis

The major differential diagnosis of DC is EC. ECs behave more like CSF on both CT and MR imaging, whereas DCs resemble fat. Lipoma may resemble a DC, but it is generally much more homogeneous on MR imaging, and it is often associated with other congenital malformations, such as callosal dysgenesis.

Craniopharyngioma is often multicystic, extends into the sella, calcifies, and enhances. Teratoma may resemble a DC but most commonly occurs in the pineal gland and is much more heterogeneous on imaging than the typical DC.

DERMOID vs. EPIDERMOID CYST

Pathology

Clinical Issues

Imaging

Endodermal (Neurenteric) Cyst

Endodermal/neurenteric (NE) cysts are rare endodermal-derived developmental CNS lesions. Spinal NE cysts are significantly more common than their intracranial counterparts.

Terminology

Endodermal cysts are also called enterogenous cyst, enteric cyst, NE cyst, and neuroendodermal cyst (32-26). Similar lesions can occur in the bronchi and foregut.

Etiology

NE cysts, along with Rathke cleft and colloid cysts, are endodermally derived developmental lesions of the CNS. Embryonic multipotent endodermal cells freely migrate along the embryonic neuroectoderm, which lies just dorsal to the developing notochord. If the neuroectoderm fails to separate from the notochord, displaced nests of respiratory or alimentary tissue may ultimately form an NE cyst.

Pathology

Location

The most common CNS site is the spine; intracranial NE cysts are rare, accounting for 25% of all cases. The majority of these occur as extraaxial lesions in the posterior fossa, most commonly in/near the midline at the pontomedullary junction or the CPA cistern (32-24). Between 15-20% of intracranial cysts are supratentorial.

Size and Number

NE cysts vary widely in size. Most are relatively small, but occasionally, these cysts can become very large. NE cysts are almost always solitary lesions. Rare cases of intracranial cyst dissemination following surgical excision have been reported.

Gross Pathology

NE cysts are typically well delineated with a thin, translucent wall. Contents vary from clear colorless fluid that resembles CSF to thick viscous mucoid secretions.

Microscopic Features

NE cysts are lined with pseudostratified columnar epithelium with ciliated cells and mucous-secreting goblet cells. Neoplastic metaplasia is rare.

Clinical Issues

NE cysts occur in patients of all ages. Posterior fossa NE cysts typically present with waxing and waning neck pain, occipital headaches, and gait disturbance. NE cysts grow very slowly and are often stable for years.

Imaging

General Features

NE cysts are all well-circumscribed round to ovoid masses. Imaging characteristics—density and signal intensity—vary according to protein content of the cyst fluid and do not exactly follow CSF characteristics.

CT Findings

Most NE cysts are iso- to slightly hyperdense compared with CSF. Calcification and intracystic hemorrhage are absent. Bony anomalies are rare.

MR Findings

Signal intensity varies. Cyst contents are usually iso- to hyperintense compared to CSF on T1WIs (32-25). Most are hyperintense on T2WI and do not suppress on FLAIR. NE cysts with inspissated cyst contents may appear hypointense on T2WI. Most do not exhibit enhancement on T1 C+ sequences and do not restrict on DWI.

Differential Diagnosis

The major differential diagnosis of NE is epidermoid cyst (EC). ECs are insinuating lesions with lobulated, frond-like surfaces and are usually off-midline in the cerebellopontine angle cistern. Most ECs restrict strongly on DWI. A “white” epidermoid (32-19) (32-19) (hyperdense on NECT and hyperintense on T1WI) may be indistinguishable from EC.

Arachnoid cyst follows CSF signal intensity on all sequences. Ecchordosis physaliphora is a gelatinous-appearing notochordal remnant that typically occurs in the prepontine cistern and is attached to a visible defect in the dorsal clivus by a thin, stalk-like pedicle.

Pineal Cyst

Cystic-appearing lesions in the pineal gland are common incidental findings on MR scans, seen in 1.5-11% of cases.

Terminology and Etiology

A pineal cyst (PC) is a benign glia-lined, fluid-containing cyst within the pineal gland parenchyma. The precise etiology of PCs is unknown. Persistent coalescing embryonic pineal cavities and glial degeneration with cavitation have been cited as possible etiologies.

Pathology

Up to 40% of autopsied pineal glands in the general population contain cysts. PCs are well-demarcated, round or ovoid expansions within an otherwise normal-appearing pineal gland (32-29). Most are < 10 mm in diameter. The largest reported PC is 4.5 cm. PCs are usually unilocular, but lesions containing multiple smaller cysts do occur.

Gross Pathology

The general appearance is that of a smooth, soft, tan-yellow pineal gland that contains a uni- or multilocular cyst (32-30). PCs do not have ependymal or epithelial lining, so the cyst “wall” is actually compressed pineal parenchyma. The inner surface of the cyst cavity is often hemosiderin stained as the result of intralesional hemorrhage. Cyst fluid is clear to yellowish.

There are no gross pathologic or histologic features that distinguish symptomatic from asymptomatic PCs.

Clinical Issues

Demographics

PCs can occur at any age, although they are more often discovered in middle-aged and older adults.

The overall F:M ratio is 2:1. The incidence among women ages 21-30 years is significantly higher than in any other group.

Presentation

PCs rarely cause symptoms. Most are clinically benign and discovered incidentally at imaging or autopsy. Large PCs may compress or obstruct the cerebral aqueduct, resulting in hydrocephalus and headache. Parinaud syndrome (tectal compression) is less common.

Pineal “apoplexy” occurs with sudden intracystic hemorrhage. Acute worsening of headaches combined with visual symptoms can occur. A “thunderclap” headache may mimic symptoms of aneurysmal subarachnoid hemorrhage. Pineal “apoplexy” can result in acute intraventricular obstructive hydrocephalus.

Natural History

Only 5% of PCs (mainly in patients younger than 50 years of age) grow; cyst stability or shrinkage is more likely across all age groups. Routine follow-up of PCs is unnecessary in the absence of unusual radiologic characteristics or related clinical symptoms. Surgery is rarely needed or indicated for PCs.

Follow-up of indeterminate cystic lesions of the pineal region usually shows no significant change over time intervals varying from months to years. Patients with growing lesions, atypical contrast enhancement, or hemorrhage on MR are more likely to develop hydrocephalus or exhibit malignant pathology, so follow-up serial MRs are recommended.

Imaging

CT Findings

At least 25% of PCs show calcification within the cyst wall (32-31A). The cyst fluid is iso- to slightly hyperdense compared with CSF (32-35). A very hyperdense PC in a patient with severe headache should raise suspicion of hemorrhage with cyst “apoplexy.” Rim, crescentic, or nodular enhancement patterns have all been described with PCs.

The ventricles are usually normal. Large ventricles with “blurred” margins indicate acute obstructive hydrocephalus.

MR Findings

As with other cysts, PC signal intensity varies with imaging sequence and cyst contents. Between 50% and 60% of PCs are slightly hyperintense compared with CSF on T1WI (32-31B). Approximately 40% are isointense with CSF. Approximately 1-2% are very hyperintense, which may indicate intracystic hemorrhage. A blood-fluid level may be present (32-36).

Most PCs are small and cause minimal or no mass effect. Large cysts may cause obstructive hydrocephalus. In such cases, T2/FLAIR scans show “fingers” of hyperintensity extending into the periventricular white matter due to subependymal accumulation of brain interstitial fluid. These are especially well demonstrated on sagittal scans.

The vast majority of PCs are iso- to slightly hyperintense on T2WI (32-35)and do not suppress completely on FLAIR (32-32). Internal septations are visible in 20-25% of cases, and up to 60% are multicystic on CISS or FIESTA sequences. If acute hemorrhage has occurred, intracystic blood may appear very hypointense on T2WIs and “bloom” on T2* (GRE, SWI). PCs typically do not restrict on DWI.

Between 1/3 and 2/3 of PCs enhance. The most common pattern is a thin (< 2-mm), circumferential rim of enhancement (32-31B). Less common patterns include nodular or crescentic enhancement. Enhancing internal septa, rim thickness > 2 mm, or irregular enhancing nodules can be seen in 15-20% of PCs and may make differentiation between a nontumoral PC and cystic pineal neoplasm challenging.

On delayed imaging, contrast may accumulate within the cystic component and can be confused with a solid lesion.

Differential Diagnosis

The most common differential diagnosis is normal pineal gland. Normal pineal glands often contain one or more small cysts (32-33)and can have nodular, crescentic, or ring-like enhancement. Quadrigeminal cistern arachnoid cysts occur dorsal to the pineal gland and follow CSF signal intensity on all sequences.

The most important pathologic entity to be differentiated from a PC is pineocytoma. Pineocytoma is a CNS WHO grade 1 pineal parenchymal tumor that is usually solid or at least partially solid/cystic. Purely cystic pineocytomas are much less common and can be indistinguishable from PCs on imaging. Pineocytomas can remain stable for many years without significant change on serial imaging.

Atypical imaging findings, focal invasion, or significant interval change in a presumed PC or pineocytoma should raise suspicion for the more aggressive pineal parenchymal tumor of intermediate differentiation (PPTID), which is a CNS WHO grade 2 or 3 lesion.

PINEAL CYST

Pathology

Clinical Issues

Imaging

Differential Diagnosis

Nonneoplastic Tumor-Associated Cysts

Tumor-associated cysts (TACs)—also referred to as peritumoral cysts—are rare, benign cysts that are adjacent to, but not contained within, a neoplasm.

Most TACs represent trapped, encysted “pools” of CSF adjacent to a large benign extraaxial tumor, such as meningioma (32-37), schwannoma (32-39), pituitary macroadenoma (32-40), and craniopharyngioma. These trapped, encysted “pools” of CSF vary from clear, CSF-like liquid to turbid proteinaceous fluid. TACs are usually positioned at the tumor-brain interface between the extraaxial mass and adjacent cortex (32-41). Size varies from small insignificant collections to very large cysts.

Imaging findings vary with cyst content. Most are hypointense relative to brain on T1WIs and are very hyperintense on T2WI and FLAIR (32-39A). Enhancement is minimal or absent (32-39B).

TACs must be distinguished from cystic neoplasms (cysts are contained within the tumor), arachnoid cysts (not tumor associated), and enlarged perivascular (Virchow-Robin) spaces. The latter two behave like CSF on imaging studies.

Parenchymal Cysts

Preamble

Parenchymal (intraaxial) cysts are much more common than either their extraaxial or intraventricular counterparts. Once a cyst has been identified as lying within the brain itself, the differential diagnosis is limited. The most common parenchymal cysts—prominent perivascular spaces and hippocampal sulcus remnants—are anatomic variants. Neuroglial cysts and porencephalic cysts are relatively uncommon. All other nonneoplastic, noninfectious brain cysts are rare.

Enlarged Perivascular Spaces

By far, the most common parenchymal brain “cysts” are enlarged perivascular spaces (PVSs). They vary from solitary, small, inconspicuous, and unremarkable to multiple, large, bizarre, alarming-looking collections of CSF-like fluid. They are often asymmetric, may cause mass effect, and have frequently been mistaken for multicystic brain tumors.

Terminology

PVSs are also known as Virchow-Robin spaces (VRSs). PVSs are pia-lined spaces that accompany penetrating arteries and arterioles into the brain parenchyma (32-42). The PVSs do not communicate directly with the subarachnoid space.

The PVSs form an essential part of the brain’s “glymphatic system.” This system allows for dynamic exchange of interstitial fluid (ISF) and CSF along paravascular channels of the penetrating arteries and draining veins. The glymphatic system is essential in the clearance of neurotoxic solutes (such as amyloid) from the brain interstitium to the CSF efflux pathways.

Etiology

General Concepts

The brain PVSs are distributed throughout the cerebral hemispheres, midbrain, and cerebellum. The PVSs are filled with ISF, not CSF. In addition to clearing neurotoxic solutes, recent evidence suggests the PVSs also perform an essential role in maintaining brain fluid and intracranial pressure homeostasis.

Precisely why some PVSs become enlarged is unknown. Most investigators believe ISF egress is blocked, causing cystic enlargement of the PVSs. Extreme enlargement of the PVSs is unusual and sometimes referred to as tumefactive VRSs.

Pathology

Location

Although PVSs can be found virtually anywhere in the brain, they have a striking predilection for the inferior 1/3 of the basal ganglia, especially near the anterior commissure (32-42). They are also common in the subcortical and deep white matter as well as the midbrain and dentate nuclei of the cerebellum.

Size and Number

Enlarged PVSs tend to occur in clusters. Collections of multiple variably sized PVSs are much more common than solitary unilocular lesions.

Most PVSs are smaller than 2 mm. PVSs increase in size and prevalence with age. Giant so-called tumefactive PVSs measuring up to 9 cm in diameter have been reported.

Microscopic Features

Enlarged PVSs appear as collections of smoothly demarcated cysts filled with clear colorless fluid.

PVSs are bounded by a single or double layer of invaginated pia. Cortical PVSs are lined by a single layer of pia, whereas two layers accompany lenticulostriate and midbrain arteries.

As a PVS penetrates into the subcortical white matter, it becomes fenestrated and discontinuous. The pial layer disappears completely at the capillary level.

The brain parenchyma surrounding enlarged PVSs is typically normal without gliosis, inflammation, hemorrhage, or discernible amyloid deposition.

Clinical Issues

Epidemiology

PVSs are the most common nonneoplastic parenchymal brain “cysts”(32-43). With high-resolution 3T or 7T MR (32-45), small PVSs are seen in nearly all patients (32-44), in virtually every location, and at all ages. Between 25-30% of children have identifiable PVSs on high-resolution MR scans.

Demographics

Enlarged PVSs are more common in middle-aged and older patients and increase in both size and number with age. Recent studies have linked enlarged PVSs with age, lacunar stroke subtype, and white matter lesions and consider them as an MR marker of cerebral small vessel disease.

Presentation

Most enlarged PVSs do not cause symptoms and are discovered incidentally on imaging studies or at autopsy. Neuropsychological evaluation is typically normal. Nonspecific symptoms, such as headache, dizziness, memory impairment, and Parkinson-like symptoms, have been reported in some cases, but their relationship to enlarged PVSs is unclear. Large PVSs in the midbrain may cause obstructive hydrocephalus and present with headache.

Natural History

Enlarged PVSs tend to be stable in size and remain unchanged over many years, although a few cases of progressively enlarging PVSs have been reported.

Treatment Options

Enlarged PVSs are “leave me alone” lesions that should not be mistaken for serious disease. If midbrain PVSs cause obstructive hydrocephalus, the generally accepted treatment is to shunt the ventricles, not the cysts.

Imaging

General Features

The common pattern of enlarged PVSs is one or more clusters of variably sized CSF-like cysts. They commonly cause focal mass effect. For example, if they occur in the subcortical white matter, the overlying gyri are enlarged with concomitant compression of adjacent sulci (32-49).

The clinical significance and MR appearance of tumefactive VRSs depends on their anatomic location. Type 1 tumefactive VRSs are located in the basal ganglia along the lenticulostriate arteries. Type 2 lesions are found in the subcortical white matter along perforating cortical arteries. Type 3 tumefactive PVSs occur in the mesencephalothalamic region (32-52). Recently, a fourth type (type 4) has been described in the subcortical white matter of the anterior temporal lobe adjacent to the middle cerebral artery.

CT Findings

Enlarged PVSs are groups of round/ovoid/linear/punctate CSF-like lesions that do not demonstrate calcification or hemorrhage (32-48A). PVSs do not enhance following contrast administration.

MR Findings

Even though they are filled with ISF, PVSs closely parallel CSF signal intensity on all imaging sequences. Focal mass effect is common. Enlarged PVSs in the subcortical white matter expand overlying gyri (32-48B). Enlarged tumefactive PVSs in the midbrain may compress the aqueduct and third ventricle, resulting in intraventricular obstructive hydrocephalus (32-51).

PVSs are isointense with CSF on T1-, PD, and T2WI (32-50). They suppress completely on FLAIR. Edema in the adjacent brain is absent, although 25% of tumefactive PVSs have minimal increased signal intensity around the cysts (32-47).

PVSs do not hemorrhage, enhance, or demonstrate restricted diffusion.

Differential Diagnosis

The major differential diagnosis is chronic lacunar infarction. Although they often affect the basal ganglia and suppress on FLAIR, lacunar infarcts do not cluster around the anterior commissure, are often irregular in shape, and frequently exhibit hyperintensity in the adjacent brain.

In some older patients, very prominent PVSs in the basal ganglia are present (32-46). This condition, called “état criblé” (cribriform state), should not be mistaken for multiple lacunar infarcts. PVSs are round/ovoid and regular in configuration, and the adjacent brain parenchyma is usually normal without gliosis or edema.

Infectious cysts (especially parenchymal neurocysticercosis cysts) are usually small. Although often multiple or multilocular, they typically do not occur in clusters of variably sized cysts as is typical for enlarged PVSs.

ENLARGED PERIVASCULAR SPACES

Terminology

Related posts:

Approach to Congenital Malformations Choroid Plexus Tumors Approach to Toxic, Metabolic, Degenerative, and CSF Disorders Vascular Neurocutaneous Syndromes Dementias and Brain Degenerations Arterial Anatomy and Strokes

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