Arachnoid Cysts

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CHAPTER 178 Arachnoid Cysts

Arachnoid cysts are benign, congenital, non-neoplastic, extra-axial, intra-arachnoid lesions filled with fluid similar to or exactly like cerebrospinal fluid (CSF).¹^–³ Bright’s⁴ early observation that these entities are “serous cysts forming in connection with the arachnoid, and apparently lying within its layers” has been confirmed in more recent pathologic evaluation.³

Arachnoid cysts constitute about 1% of all nontraumatic intracranial space-occupying lesions. As neuroimaging has become more widely available, incidentally discovered cysts are frequently found.⁵^–⁹ Although they may present at any age, 60% to 80% of arachnoid cysts are discovered in children¹⁰^,¹¹ and appear with a male-to-female ratio of 2:1 to 3:1.

Primary arachnoid cysts are congenital cysts lined by a single layer of flattened, arachnoid cells in a vascular collagenous membrane that lies entirely within the arachnoid layer. The cysts may be loculated, compartmentalized, or freely communicating with the surrounding CSF cisterns. Secondary arachnoid cysts develop as a result of another condition, although many of the cysts attributed to meningitis, trauma, or hemorrhage are probably congenital in origin.¹² Secondary arachnoid cysts may show signs of previous inflammatory changes such as gliosis or hemosiderin within the walls.¹³^,¹⁴ The cyst fluid may be xanthochromic, proteinaceous, or hemorrhagic.¹⁵

Epidemiology

The incidence of arachnoid cysts in the general population is estimated to be around 0.1% from autopsy series.¹⁶ The incidence is similar in computed tomography (CT) imaging series (0.2%)⁶ and greater in magnetic resonance imaging (MRI) series (0.8% to 1.7%).⁵^,¹⁷ The greater incidence in autopsy and imaging series than suggested by clinical symptoms indicates that arachnoid cysts are asymptomatic and incidental in most cases. Most arachnoid cysts are found in the supratentorial space (90%). Rengachary and Watanabe¹⁸ first reported on the distribution of 208 arachnoid cysts reported between 1831 and 1980, and other authors have found a similar distribution.¹⁹^–²² Abtin and Walker²³ summarized the distribution of arachnoid cysts from several series (Table 178-1). These figures from academic centers represent the distribution of symptomatic arachnoid cysts referred to neurosurgical centers rather than the true distribution.

TABLE 178-1 Distribution of Intracranial Arachnoid Cysts in Pediatric Series *

LOCATION	INCIDENCE (%)
Sylvian fissure/middle fossa	42
Posterior fossa	24
Suprasellar	10
Quadrigeminal cistern	7.5
Interhemispheric	7.3
Cerebral convexity	5.7
Other	2.5
Supratentorial/infratentorial	1.1

* Data from references 18 to 22.

Pathology

The sine qua non of arachnoid cysts is splitting of the arachnoid layers at the margins of the cyst, and these cysts are therefore best described as intra-arachnoid cysts. They usually arise within and expand the margins of CSF cisterns rich in arachnoid (i.e., the sylvian fissure, suprasellar and quadrigeminal cisterns, cerebellopontine angle, interhemispheric fissure, and posterior fossa midline cisterns).²⁴^,²⁵

Appearance

Large cysts may produce thinning of the overlying adjacent dura and bone. The membranes of the cyst are delicate, filmy, and translucent and may blend imperceptibly with the surrounding leptomeninges. The arachnoid layers of primary arachnoid cysts have a normal histologic appearance and consist of laminated collagen bundles. The presence of islands of mesenchymal cells that occasionally have a whorled appearance is useful in identifying arachnoid cysts. The inner arachnoid layer is apposed to the pia mater, and the subarachnoid space is virtually obliterated by the compression of the cyst. The underlying cortex is usually normal, but some gliosis may be present. Agenesis or dysgenesis of the underlying brain is notably absent in most cases. The distinguishing features of the arachnoid cyst wall versus a normal arachnoid membrane include the split of the arachnoid layer at the margin of the cyst, the increased thickness of the collagen layer, and the absence of the cobweb-like trabeculations of normal arachnoid. The cyst fluid is clear and devoid of cellular or proteinaceous material. Inflammatory cells and hemosiderin are absent from the fluid or cyst walls.

Pathogenesis

The pathologic hallmarks of arachnoid cysts have generally been established, but their pathogenesis is still debated. The occurrence of cysts in neonates and siblings, the anatomic relationship of cysts to the cisterns, and the association of arachnoid cysts with other developmental anomalies all lend credence to the supposition that these cysts are usually developmental in origin and not acquired from other pathologic conditions. This theory is the generally accepted explanation of arachnoid cyst pathogenesis.

Arachnoid cysts may coexist with other genetic disturbances in collagen formation. Several genetic disorders are now associated with a higher incidence of arachnoid cysts, including Marfan syndrome,²⁶ neurofibromatosis type I,²⁷^–²⁹ glutaric aciduria,³⁰^–³² acrocallosal syndrome,³³ other hereditary disorders,³³^–⁴¹ and autosomal dominant polycystic kidney disease (PKD).⁴²^–⁴⁵ Shievink and coworkers⁴⁵ found intracranial arachnoid cysts in 8% of patients with PKD who underwent prospective radiologic screening for intracranial aneurysms.

Embryology

The origin of arachnoid cysts has been a controversial subject, and the details remain unclear. The formation of arachnoid cysts is hypothesized to result from abnormal embryologic development of the subarachnoid space. Early in normal embryonic development, a loose layer of connective tissue, called the meninx primitiva, or perimedullary mesh, a precursor to the pia mater and arachnoid, lines the surface of the dura and surrounds the neural tube. At about 15 weeks’ gestation, the rhombic roof ruptures, CSF pulses through this mesh, and the pia mater and arachnoid separate incompletely,⁴⁶ resulting in the cobweb-like appearance of the arachnoid. The leading hypothesis for arachnoid cyst formation proposes that this separation of the superficial arachnoid and deep pia is aberrant, and enclosed, loculated chambers form and develop into a cystic mass. Alternatively, the meninx primitiva formation is aberrant, which may also lead to cyst formation.⁴⁷ If this hypothesis is correct, arachnoid cysts should be in close proximity to an arachnoid cistern, and this has been found to be true in most cases.

This embryologic model may explain the presence or formation of the cyst but does not explain how and why these cysts expand. There have been several unsatisfactory or experimentally unverifiable explanations as to why some cysts may generate sufficient intracystic pressure to cause parenchymal compression. Although Na⁺,K⁺-ATPases on the luminal membrane and alkaline phosphatases on the extraluminal lamina membrane have been discovered, there has been no direct evidence of fluid production within the cyst. Further, most cysts remain stable in size and rarely disappear, demonstrating that secretion is neither universal nor the only mechanism involved.

More likely, a slit-valve or ball-valve communication may exist between the subarachnoid space and the cyst, allowing entry but not egress of CSF.⁴⁸^,⁴⁹ CT cisternography or cine-phase contrast-enhanced MRI sequences often demonstrate slow filling or emptying of the arachnoid cyst.⁴⁸ Normal CSF pulsation or Valsalva maneuvers and the local geometry of the split membrane may provide the pressure gradient to drive fluid into the cyst. Slit valves have been directly observed with endoscopy and currently are the most direct explanation for why the cysts expand.^12,^48,^50,⁵¹

Clinical Presentation

Arachnoid cysts most commonly present during childhood. About 75% of intracranial arachnoid cysts presented before 3 years of age in one series,²² and the median age at presentation was 2.2 years in another series.¹⁹ In the European cooperative study, the average age at presentation was slightly older (6 years).⁵²

Clinical presentation varies primarily by age and location. Some arachnoid cysts remain asymptomatic throughout life.⁵³ The most common presenting symptom is headache, which may be due to local mass effect, high intracranial pressure (ICP), or hydrocephalus, or may be unrelated to the coexisting cyst. Infants may present with macrocephaly, enlarged tense fontanelle, and splayed sutures with irritability, failure to thrive, and developmental delay.

Arachnoid cysts of the middle cranial fossa may be associated with hemorrhage, but this complication has been considered very infrequent. Of just 42 cases reported in the literature, half were associated with a blunt head trauma. In a series reported by Parsch and colleagues,¹⁷ 2.43% of the personally observed subdural hematomas and hygromas were related to the rupture of a sylvian fissure arachnoid cyst. In a series of 168 temporal arachnoid cysts,⁵⁴ 6.5% were found to have intracystic or subdural hematomas. There has been no longitudinal study to date monitoring patients with asymptomatic arachnoid cysts to determine the rate of hemorrhage.

Supratentorial Arachnoid Cysts

Sylvian fissure cysts compose about one third of all arachnoid cysts in children and about half of those presenting in adulthood. Presentation is more common in children than in adults, and there is a nearly 3:1 male-to-female predominance. The left side is affected more often than the right. The common middle fossa location may be explained by meningeal maldevelopment if the arachnoid covering of the temporal and frontal lobes fails to merge when the sylvian fissure is formed in early fetal life.⁵⁵

The most common symptom is a unilateral headache in the supraorbital or temporal region that may be exacerbated with physical exertion. The headaches may rarely be associated with other signs and symptoms of increased ICP such as nausea, vomiting, and papilledema.²¹ The next most frequent associated symptom is focal, complex-partial, or generalized seizures occurring in up to one third of patients.^19,^22,⁵⁶ The cause of seizures in patients with arachnoid cysts is unknown and the relationship between seizure focus and cyst location is unclear^57,⁵⁸ (see Koch 1994, Yalcin 2002).

Middle fossa and sylvian fissure cysts were classified by Galassi and associates ⁵⁷ on the basis of their CT appearance and apparent communication with adjacent normal CSF spaces (Fig. 178-1). Type I cysts are small, lenticular, biconvex collections located at the anterior pole of the middle fossa directly posterior to the sphenoid ridge that appear to communicate freely with the adjacent cisterns. There is little associated mass effect, and they usually do not have associated calvarial deformities. They do not require treatment.

FIGURE 178-1 A, T2-weighted magnetic resonance image (MRI) showing Galassi type I temporal arachnoid cyst. B, T1-weighted MRI showing Galassi type II temporal arachnoid cyst. C, T2-weighted MRI showing Galassi type III temporal arachnoid cyst.

Type II cysts are larger triangular- or quadrangular-appearing cysts and involve the proximal to mid aspect of the sylvian fissure with a medial border along the margin of the insula. They are less likely to communicate with adjacent basal cisterns with delayed contrast uptake on cisternography and are more likely than type I cysts to require treatment.

Type III cysts are large, rounded cysts that involve the entire length of the sylvian fissure. They typically present with marked mass effect and midline shift with thinning, scalloping, and expansion of the middle fossa cranial bones (thinning of temporal squama or displacement of the wings of the sphenoid bone) or splaying of cranial sutures in younger children. About 30% of arachnoid cysts fall within this category. They typically occupy the whole middle fossa and sometimes extend into the anterior fossa to compress the convexity of the frontal lobe. They usually do not have communication with adjacent cisterns and often require surgical treatment.

Parasellar intra-arachnoid cysts are subdivided into intrasellar and suprasellar subgroups according to their relationship to the diaphragma sella. Suprasellar cysts are much more common and found almost exclusively in children. Nearly 50% of suprasellar cysts are diagnosed in children younger than 5 years old and 20% in children younger than 1 year old.⁶⁰ They typically present with hydrocephalus by extending into the third ventricle and obstructing the aqueduct of Sylvius (Fig. 178-2). They may also present with visual abnormalities, including hemianopia or decline in acuity. Gait ataxia and opisthotonos may occur in patients with large cysts that displace the midbrain posteriorly. The “bobble-head doll syndrome” has been described in relation to suprasellar arachnoid cyst.⁶¹^–⁶⁸ Ten to 60% of patients with suprasellar arachnoid cysts may present with endocrinopathy. Precocious puberty and growth hormone deficiency are the most common.⁶⁹^–⁷²

FIGURE 178-2 A, CT scan showing suprasellar arachnoid cyst with associated hydrocephalus. B, T2-weighted magnetic resonance image showing suprasellar arachnoid cyst with associated hydrocephalus.

Intrasellar arachnoid cysts are uncommon and typically present in the fourth or fifth decade of life.⁷³ The most common presenting symptom is headache, and these cysts rarely present with visual abnormalities or endocrinopathy. Most of these intrasellar cysts are discovered incidentally, and they may be difficult to differentiate from an intrasellar craniopharyngioma or Rathke’s cleft cyst on imaging alone.

Arachnoid cysts of the cerebral convexities usually present with headaches or seizures, or both. Age, size, and location typically determine their clinical presentation. Small cysts may focally compress the underlying brain and thin the overlying bone, whereas large cysts may lead to asymmetric calvarial expansion, suture diastasis, and hemispheric distortion (Fig. 178-3). These cysts tend to occur more commonly in females than males.

FIGURE 178-3 T2-weighted magnetic resonance image showing a large hemispheric arachnoid cyst.

Interhemispheric cysts are often associated with agenesis of the corpus callosum. They most often present with macrocephaly and asymmetric growth of the calvaria. They tend to be associated with increased ICP, developmental delay, hypertonia or hypotonia, hemiparesis, ocular changes, and epilepsy (Fig. 178-4).⁷⁴^,⁷⁵

FIGURE 178-4 T1-weighted magnetic resonance image showing an interhemispheric arachnoid cyst.

Arachnoid cysts in the quadrigeminal cistern abutting the collicular plate may cause hydrocephalus by obstructing the sylvian aqueduct. Progressive macrocephaly in an infant is the most common presentation. Other symptoms and signs associated with quadrigeminal plate cistern arachnoid cysts include Parinaud’s syndrome, nystagmus, hearing deficits, trochlear nerve palsy, and apneic spells.^22,⁷⁶^–⁸⁰

Infratentorial Arachnoid Cysts

Arachnoid cysts of the cerebellopontine angle can present with tinnitus, vertigo, facial weakness, facial sensory loss, hearing loss, or ataxia.⁸¹ The presentation may be indistinguishable from Meniere’s disease. Other presentations have included trigeminal neuralgia and hemifacial spasm.⁸²^–⁸⁸ Large midline posterior fossa cysts often present with obstructive hydrocephalus (Fig. 178-5). This entity must be differentiated from mega cisterna magna, Dandy-Walker malformation, epidermoid cyst, and large cystic tumors. Infrequently, posterior fossa cysts may present with cerebellar signs such as ataxia, nystagmus, and cranial nerve dysfunction and progressive quadriparesis.

FIGURE 178-5 T2-weighted magnetic resonance image showing a large pineal region arachnoid cyst.

Imaging

Plain radiographs may show nonspecific thinning and deformity of the adjacent bone. Angiograms show shifts of the adjacent vasculature and venous phase. With large posterior fossa cysts, there may be elevation of the straight and transverse sinuses.

Intracranial arachnoid cysts are often diagnosed in utero by ultrasound. This is a noninvasive, inexpensive test and very effective screening tool for congenital brain malformations. Prenatal ultrasound has detected arachnoid cysts as early as 13 weeks’ gestation,⁸⁹ but most are diagnosed during the second trimester.⁹⁰^–⁹² Arachnoid cysts appear sonolucent with enhanced transmission of the ultrasound beam through the collection and are thus hypoechogenic to surrounding brain. Ultrasound may also be useful intraoperatively for placement of cystoperitoneal shunts, guidance of endoscopic fenestration, and detection and monitoring the progression of arachnoid cysts postnatally.

Imaging of congenital arachnoid cysts with CT or MRI reveals sharply circumscribed, smoothly marginated lesions having a homogeneous density very near to CSF. A noncontrast CT is usually sufficient to differentiate an arachnoid cyst from other cystic lesions. Arachnoid cyst walls are so thin that they are not visible on CT, whereas the walls of cystic tumors may be visible. The wall of an arachnoid cyst does not enhance if contrast agent is administered. Hyperdense calcium flecks are often visible in the walls of a parasellar craniopharyngioma but are never present in an arachnoid cyst. The fluid of an arachnoid cyst is identical to CSF on CT imaging unless there has been an intracystic hemorrhage, in which case it will appear hyperdense to CSF. CT ventriculography, cisternography, or cystography may show communication of the cyst with the basal cisterns and subarachnoid space.^11,^59,⁹³ CT alone is not capable of demonstrating CSF flow dynamics and communication of the arachnoid cyst to the surrounding subarachnoid space. CT with intrathecal or intraventricular injection of metrizamide can simultaneously provide anatomic detail of the subarachnoid and intraventricular space as well as physiologic information. In communicating arachnoid cysts, the cyst fills with metrizamide, but the clearance of contrast from the cyst is delayed with respect to the surrounding subarachnoid space and basal cisterns. In noncommunicating cysts, there is no early entry of contrast into the cyst (2 to 6 hours), but contrast accumulates around the cyst in the cisterns creating a halo demarcating the cyst boundary (about 24 hours). The contrast then clears from the subarachnoid space and slowly accumulates within the arachnoid cyst.

The three-dimensional relationship of arachnoid cysts to the basal cisterns is best visualized with the multiplanar acquisition of MRI. The mass effect on adjacent structures and associated anomalies can also best be visualized, particularly when combined with contrast imaging to highlight the surrounding vascular anatomy. The greater resolution of MRI allows for better detection of smaller cysts and cysts adjacent to bony structures. MRI can distinguish between the CSF-like fluid of most arachnoid cysts and the proteinaceous fluid of other cystic lesions. MRI sequences can also differentiate epidermoids from arachnoid cysts by their hyperintense appearance on proton density–weighted and diffusion-weighted sequences.⁹⁴ Cysts associated with parasites are also hyperintense on proton density–weighted images.

Like arachnoid cysts, ependymal and choroid plexus cysts have homogeneous, nonenhancing thin walls not visible with CT, but they can be distinguished from arachnoid cysts by their intraventricular location. The location of benign intraparenchymal cysts in the lobes of the cerebrum or cerebellum with walls of neuroglial tissue is also a distinguishing factor. Rathke’s cleft cysts, however, are almost impossible to distinguish from small intrasellar arachnoid cysts by imaging alone.

In the MRI era, metrizamide CT has in many cases been replaced by MRI sequences that are sensitive to CSF flow.⁹⁵^,⁹⁶ Electrocardiogram-gated cine-mode MRI sequences were able to demonstrate communication between intracranial arachnoid cyst and CSF spaces in 95% of patients in a recent study.⁹⁷ The identification of a jet-like flow void at a potential communication site is the most convincing evidence of communication between the arachnoid cysts and the adjacent subarachnoid space. These results compare favorably with CT cisternography and operative findings.

Magnetic resonance cisternography with T2-weighted imaging ⁹⁶^–¹⁰¹ or more recent three-dimensional Fourier transformation constructive interference in steady state or fast imaging employing steady-state acquisition can depict medial arachnoid cyst walls, thin cranial nerves, small vessels, and cortical sulci in detail.¹⁰⁰ Magnetic resonance cisternography was originally used to detect small tumors or vascular compression at the cerebellopontine angle¹⁰²^,¹⁰³ as well as small lesions in the ventricular system.¹⁰⁴^,¹⁰⁵ Fine detail such as the trochlear nerve and Lillequist’s membrane can be resolved using this technique.¹⁰⁵^,¹⁰⁶ The delineation of these structures has been confirmed during neuroendoscopic surgery and has proved useful for planning fenestration into the basal subarachnoid cisterns.