13.1 Introduction

Arachnoid cysts (ACs) are benign, cerebrospinal fluid (CSF) filled expansions of the normal arachnoid mater. The first known report of an intracranial AC was published by the English physician Richard Bright in 1831. He described these as “serous cysts forming in connection with the arachnoid and apparently between its layers.” ¹ Ultrastructural analysis conducted by Rengachary and Watanabe demonstrated four characteristic features of ACs: splitting of the arachnoid membrane at the cyst margin, hyperplastic arachnoid cells in the cyst wall, a thick collagen layer within the cyst wall, and the absence of traversing trabecular processes within the cyst. ² ACs are believed to be congenital lesions that develop in utero, although there are rare reports of them having occurred after trauma, infection, or even de novo. Clinically, they typically have a benign natural history.

In the pediatric population, they are often found incidentally on imaging upon workup for other symptoms or head trauma. A recent study that analyzed over 12,000 pediatric MRI scans suggested that overall incidence of ACs in children is approximately 2.6%. ³ Their study also revealed a male and left-sided predominance. The middle fossa (47%) and posterior fossa (38%) were the two most common places ACs were observed in, with the remainder of the cysts seen in the quadrigeminal plate (6%), cerebral convexity (4%), anterior fossa (2%), sellar/suprasellar (2%) and interhemispheric (1%). Intrasellar arachnoid cyst (IAC) and suprasellar arachnoid cyst (SAC) deserve special attention due to their unique location and proximity to the pituitary gland, optic apparatus, hypothalamus, and third ventricle. These variants often produce clinical symptoms, including obstructive hydrocephalus, endocrine dysfunction, visual disturbances, and neurocognitive deficits. As a result, they require some form of neurosurgical intervention and the introduction of neuroendoscopy has shifted management from CSF diversion devices and open microsurgical approaches, to a minimally invasive approach. This chapter will focus on IACs and their endoscopic skull base management.

13.2 Epidemiology and Pathogenesis

The differential diagnosis for a cystic lesion in the sellar region should include Rathke’s cleft cyst, craniopharyngioma, IAC, dermoid or epidermoid cysts, and empty sella syndrome. IACs are infrequently encountered and represent approximately 3% of all intracranial ACs. ² Overall, there have been less than 100 cases reported in the English literature. ⁴ Moreover, the majority of those published have shown an adult predominance. Not only does this make these lesions exceedingly rare in the pediatric population, but it also suggests that the exact incidence and natural history of IACs in children remain unknown. Interestingly, this is in stark contrast to SAC, which have been well documented in children, and the reason for this discrepancy is unclear.

The development of any ACs may be due to a combination of mechanisms, and the prevailing theories include a one-way valve mechanism where CSF enters the cyst but cannot escape, an osmotic gradient enables fluid to collect in the cyst, and the normal arachnoid membranes may actively secrete fluid into the cyst. Within the sella, there is typically no arachnoid membrane below the diaphragma sellae and one theory for the formation of IACs involves the herniation of the basal arachnoid membranes through the diaphragma aperture. ⁵ The diaphragmatic aperture has been shown to have significant variation in size, with the aperture found to be larger than the pituitary stalk in approximately 40% of patients. ⁶ Hornig and Zervas postulated that the development of IACs must also involve a slit-valve mechanism from a diaphragmatic defect that allows unilateral transgression of CSF from the suprasellar area into the sella turcica. ⁷ The mechanism of a one-way valve was directly visualized by the authors during transsphenoidal fenestration of a sellar AC and they suggested that even a 1-mm slit perforation of the diaphragma could lead to the development of an IAC (▶ Fig. 13.1). Dubuisson et al observed that approximately 50% of their IACs in their series did not communicate with the suprasellar arachnoid space (“noncommunicating” cysts). They hypothesized that the site of CSF transgression may eventually close when intrasellar pressure counteracts the intracranial pressure due to apposition of the arachnoid membranes. ⁵ This closure has been speculated to be accelerated by infection, hemorrhage, or inflammatory events.

A histopathologic examination of an IAC was reported by Güdük et al, and they observed that the cyst wall was fibrous and contained a monolayer of flattened arachnoid cells on a reticulin rich basement membrane ⁴ (▶ Fig. 13.2). Immunohistochemistry was positive for epithelial membrane antigen (EMA) but negative for glial fibrillary acidic protein (GFAP), Ki-67, synaptophysin, and S-100. The combination of the histological and immunohistochemical findings aided in differentiating IACs from other epithelial cysts.

13.3 Clinical Symptoms and Diagnostic Radiology Findings

The clinical symptoms of IAC are mainly due to their mass effect on the pituitary gland and the optic apparatus. Therefore, they may mimic those of a nonfunctional pituitary adenoma. Given the paucity of literature regarding IACs, Dubuisson et al conducted an extensive review and reported the clinical symptoms from 14 published papers on IACs between 1980 and 2007. A total of 51 patients (age range: 16–80 years; mean age: 47 years) were identified in their review and the main presenting symptoms were visual disturbances (55%) and headache (41%). ⁵ While the visual symptoms may be explained by mass effect against the optic nerves and chiasm, the symptoms of headache are less specific, although some authors have attributed it to the dural distension caused by the cyst. Endocrine abnormalities were less common (22%) and symptomatic patients mainly had gonadotropic dysfunction (menstrual irregularity, decreased libido, impotence, and infertility), growth hormone insufficiency, or mild hyperprolactinemia. Bordo et al described severe hyponatremia (mean sodium level 115 ± 6 mmol/L) in three of eight patients with IACs as well as hypoadrenalism and hypothyroidism. ⁸

ACs have a distinct appearance on both CT and MRI (▶ Fig. 13.3). On CT, they are well-circumscribed, nonenhancing masses that are typically hypodense. CT cisternography may also demonstrate communication with the subarachnoid space. MRI provides a definitive diagnosis, as they are well-demarcated fluid collections that should parallel CSF on all sequences. They are nonenhancing, isointense with CSF on T1- and T2-weighted imaging, exhibit T2 suppression on fluid attenuation inversion recovery (FLAIR) sequences, and lack diffusion restriction on diffusion-weighted imaging (DWI). In some instances, the IAC may appear slightly hyperintense to CSF on T1 sequences due to stagnation of fluid and elevated protein concentration. ⁹ IACs are isolated sellar lesions that may distort the pituitary gland, medial cavernous sinus walls, or the optic apparatus. Some studies suggest that the IAC may displace the pituitary gland posteroinferiorly, and this may be a differentiating factor from Rathke’s cleft cyst, in which the pituitary gland may be distorted anteriorly. ^{5 ,​ 9 ,​ 10} Extension of an IAC into the suprasellar region has also been described. ⁵

13.4 Management

The management of IACs is analogous to the management of a nonfunctioning pituitary adenoma. Asymptomatic lesions rarely require any form of intervention and can be followed with serial imaging. ¹¹ Indications for surgical intervention include an enlarging cyst on serial imaging, visual compromise from mass effect on the optic nerves/chiasm, pituitary dysfunction, and severe headaches. The main goal of surgery is to decompress the cyst and relieve mass effect. Secondary goals include excision of all or part of the cyst membranes as well as fenestration of the cyst into the suprasellar cistern (cyst cisternostomy) in order to prevent recurrence. Given the location of IACs in the sella, the transsphenoidal approach to the cyst is favorable over a transcranial route and this can be accomplished via either the microscope or endoscope. Intraoperative findings should include egress of clear, CSF-like fluid after the cyst wall is opened. A normal pituitary gland, stalk, and diaphragma sellae may also be observed. ⁴ As part of the transsphenoidal approach, the sellar floor must be thoroughly packed and reconstructed at the end of the operation to prevent postoperative CSF leak. To our knowledge, there have not been any reports of cyst shunt placement for the treatment of IACs.

The outcomes after transsphenoidal approach to IAC have been favorable, and several reports suggest that recurrence is exceedingly rare, ⁴ although the exact rate remains unknown. The few reports that have documented recurrence suggest that it occurs in a delayed fashion and several years after the original operation. ^{4 ,​ 12} Symptomatic IAC recurrence entails another operation through either a repeat transsphenoidal approach or transcranial approach. Murakami et al suggested that a craniotomy and transsylvian approach may be advantageous for reoperations, as it enabled wide cyst wall excision and communication between the cyst and the suprasellar cistern. ¹² Visual deficits and partial pituitary dysfunction have both been reported to improve postoperatively. ⁵ Nonetheless, long-term follow-up after treatment of IACs has been recommended.

A delayed CSF leak following the transsphenoidal approach is a common postoperative complication, and the incidence has been reported to be as high as 21.4%. ¹³ CSF rhinorrhea typically presented 5 to 7 days after surgery; however, it was not uncommon outside this window. Any CSF leak is accompanied by an increased risk of meningitis and can be upward of 8% after the transsphenoidal approach for IACs. ⁴ Temporary CSF diversion with a lumbar drain is often employed as first-line treatment of CSF rhinorrhea; however, the current literature suggests that patients will often require a second operation to repair the site of the CSF fistula. Persistent CSF leaks may require a permanent CSF diversion device. ¹³ A rare, but feared, complication is one in which the optic apparatus prolapses into an empty sella with resultant blindness. ¹⁴ This has been reported, and the authors postulated that this could have been prevented if the sella was more efficiently packed at time of closure.