Abstract
Background and Importance:
Arachnoid cysts are common and account for approximately 1% of intracranial mass lesions. Most are congenital, clinically silent, and remain static in size. On occasion, an arachnoid cyst will increase in size over time and produce clinical symptoms due to mass effect or obstructive hydrocephalus. The mechanism of progressive enlargement of arachnoid cysts is controversial. A one-way slit or ball valve is often hypothesized as the mechanism responsible for enlargement.
Clinical Presentation:
We present four cases of communicating congenital suprasellar prepontine arachnoid cysts in which a slit-valve was identified in the wall of the cyst. All four patients presented with hydrocephalus due to enlargement of the arachnoid cyst. The slit valve was located in the arachnoid wall of each arachnoid cyst and was located in the midline directly over the main trunk of the basilar artery. We believe this slit valve was responsible for the net influx of cerebrospinal fluid (CSF) into the cyst and thus responsible for cyst enlargement over time. We also present one case of an arachnoid cyst in the middle cranial fossa that had a small circular opening in the wall of the cyst but lacked a slit-valve. This cyst did not enlarge and required surgery because of cyst wall rupture and the development of a subdural hygroma with headaches.
Conclusion:
One-way slit-valves exist and are the mechanism of enlargement of suprasellar prepontine arachnoid cysts. The location of the slit valve opening in each of our arachnoid cyst cases was directly over the basilar artery trunk. Caudal to rostral CSF flow in the prepontine cistern during the cardiac cycle lifted the arachnoid wall of the cyst off the basilar trunk and increased the size of the opening of the valve, whereas rostral to caudal CSF flow during the remainder of the cardiac cycle pushed the slit opening against the basilar artery and decreased the size of the opening. Arachnoid cysts that communicate CSF via circular, non-slit valves are probably more likely to remain stable in size.
Key words: arachnoid cyst, hydrocephalus, one-way valve, slit-valve
Running Title: Arachnoid Cyst Slit Valves
Introduction:
Arachnoid cysts are non-tumorous CSF collections encased within a membrane of split arachnoid matter and account for approximately 1% of intracranial mass lesions. 1-3 Arachnoid cysts are usually incidentally found on MRI and do not normally enlarge. They are typically clinically silent and require surgical intervention when sufficient enlargement occurs, in the rare case of cyst wall rupture with the development of a subdural hygroma, or when an overlying vein tears causing bleeding. Several theories regarding the mechanism of enlargement of arachnoid cysts exist and this continues to be a point of debate. 4-8

Case Presentations:
With IRB approval, we present four cases of communicating congenital prepontine arachnoid cysts in which a slit-valve was identified in the wall of the cyst (Table 1). Each of these cases presented with a constellation of symptoms related to hydrocephalus including: a history of headaches, increasing head circumference, irritability, progressive memory loss, or progressive coordination problems. All patients had obstructive hydrocephalus due to progressive enlargement of their cyst demonstrated on pre-operative CT and/or MRI imaging. Operative management via an endoscopic cyst fenestration was recommended and accomplished in all cases. In all of these cases, a “valve” was directly visualized. Representative intraoperative images are displayed for these cases. In all cases, the “slit-valve” can be seen in the wall of the arachnoid cyst over the caudal portion of the basilar artery trunk. We also present a case of a left middle cranial fossa arachnoid cyst found incidentally. This cyst did not enlarge over 15 years but then ruptured requiring operative management. A left temporal craniotomy was performed and a small circular opening in the cyst wall was visualized with the operative microscope around one of the sylvian arteries. However, no “slit-valve” phenomenon was observed through the circular opening with equal influx and egress of CSF during the cardiac cycle.

Case 1
A 3-year-old boy presented to the emergency room one day after falling out of a wagon and hitting the back of his head in the left parietal region. Clinically, he was normal except for one episode of vomiting. His head circumference was 55.5 cm which was well above the 98th percentile. He had no evidence of papilledema. On CT scan he was noted to have a left parietal skull fracture, a 5-mm subdural hematoma, and ventriculomegaly due to an obstructive suprasellar arachnoid cyst. An MRI was performed and showed a suprasellar arachnoid cyst causing obstruction of the third ventricle (Fig. 1). Minimal trans-ependymal fluid was noted around the walls of the ventricles. No enhancing lesion was seen. Endoscopic fenestration of both the superior wall of the cyst at the foramen of Monro, as well as the inferior wall of the cyst in front of the lower pons was performed through a precoronal burr hole. An arachnoid slit valve was identified at the inferior aspect of the cyst directly over the basilar artery, in front of the inferior pons (Fig. 2A+B). The cyst wall lifted up slightly off the basilar artery during the cardiac cycle due to caudal to rostral CSF flow, increasing the size of the slit opening. The cyst wall and slit opening then pushed down against the basilar artery trunk minimizing the opening during the cardiac cycle, in which there is caudal to rostral CSF flow (Supplemental Video 1, Case 1). The cerebral ventricles decreased in size. Approximately two years later, intermittent vomiting developed. A lumbar puncture showed an opening pressure of 24 cm of CSF. A right frontal VP shunt was inserted with resolution of the vomiting.

Case 2
A 23-year old woman presented to our clinic with reports of headaches since the 4th grade. These were felt to be migraines and improved over a few years without treatment or imaging of the head. Two-years prior to presentation, she redeveloped headaches and had a CT scan which demonstrated a suprasellar arachnoid cyst. At the time of presentation, the patient noted a two-month history of difficulty with memory and feeling off-balance. Her neurologic examination was normal. An MRI demonstrated a suprasellar arachnoid cyst and obstructive hydrocephalus. Endoscopic fenestration of the cyst wall at the superior aspect of the cyst at the foramen of Monro as well as at the inferior cyst margin at the lower pons was performed through a precoronal burr hole. A slit valve opening was noted along the inferior aspect of the cyst, directly over the basilar artery, in front of the lower pons. (Supplemental Video 1, Case 2) The slit valve opening was directly visualized to change size during the cardiac cycle. Her symptoms resolved.

Case 3
A 53-year-old man presented to our clinic who reported a large head for the majority of his life. His head circumference measured 61 cm. Over the past six months, he reported progressive memory loss so much so that his employer had threatened to fire him.
One week prior to presentation he had had a syncopal episode without cardiac cause. MRI of the head demonstrated a suprasellar arachnoid cyst causing obstructive hydrocephalus. Endoscopic fenestration through a precoronal burr hole of this cyst into the cerebral ventricles and the prepontine basal cisterns was accomplished. Multiple small holes in the arachnoid cyst wall were present directly over the basilar artery and constituted a slit-valve mechanism with preferential CSF flow during rostral to caudal CSF flow in the prepontine cistern during the cardiac cycle. (Supplemental Video 1, Case 3) His symptoms stabilized after the fenestration.

Case 4
An 8-month-old female infant was admitted to the hospital after a regular well-child visit noting irritability as well as rapidly increasing head circumference from 44.5 cm at six months to the 49 cm at 8 months. A head CT scan showed ventriculomegaly. An MRI demonstrated a suprasellar arachnoid cyst and obstructive hydrocephalus. Endoscopic fenestration was performed of the superior and inferior aspect of the cyst, as in the previous three cases. A slit valve was visualized along the inferior margin of the cyst wall directly over the basilar artery, similar to Case 1 and 2. The video from this case was lost. The hydrocephalus resolved.

Case 5
A 47-year-old man presented to our clinic with headaches for one month. Fifteen years prior he had participated as a normal volunteer in an MRI research study and was diagnosed with a left temporal arachnoid cyst. The cyst was not treated and he remained well until one month prior to presentation when he developed a severe headache after jumping rope at the gym. At this time, he obtained another MRI which demonstrated the arachnoid cyst as well as a left subdural hygroma. The headaches were initially controlled with ibuprofen. Two-weeks later the headaches returned after riding in a space flight simulator at an amusement park. A follow-up head CT demonstrated slight enlargement of the subdural hygroma with minimal midline shift. On neurologic examination, he was normal except for very slight sixth nerve palsies bilaterally. Open microsurgical drainage (Supplemental Video1 Case 5) of the subdural hygroma was performed through a small temporal craniotomy with resection of the lateral wall of the arachnoid cyst and fenestration of the medial wall of the cyst to the internal carotid artery cistern. Under microscopic visualization, a small circular opening (Fig. 2C) in the wall of the cyst was visualized in the Sylvian fissure around a small arterial branch. The opening of the cyst wall was fixed in size and did not vary with CSF flow in either direction. This opening was left alone due to a small arterial branch along the wall opening, but the other walls of the cyst were fenestrated.

Discussion:
Arachnoid cysts can be congenital, which are the most common and referred to as “true” cysts, or they can be qualified as leptomeningeal (also referred to as “secondary”), which develop as a result of trauma, bleeding, or infection. 2, 5 Congenital cysts can be distinguished from leptomeningeal cysts by the absence of arachnoid scarring in the former. 2

The genesis of congenital arachnoid cysts is still debated. Starkman, et al., 9 considered early intrauterine splitting of the arachnoid membrane to be the cause of the cysts. This has led to the generally accepted theory that developmental anomalies, in which splitting or duplication of the arachnoid membrane during embryonal development, lead to a collection of CSF-like fluid and the formation of the cysts. 2, 5, 10, 11

It is important to note that the natural history of any particular arachnoid cyst is unclear. Most arachnoid cysts are static and clinically silent but have the potential to progressively enlarge or, on rare occasion, disappear spontaneously. 1, 2, 12 Intracranial arachnoid cysts that have symptomatic manifestations from mass effect make the mechanism of enlargement clinically important. There are many theories addressing potential mechanisms of enlargement. 1, 2, 13 Among these theories are the following: (1) active fluid secretion by identified microvilli on the inner surface of the cysts, 7 (2) an osmotic gradient between the cystic content and the cerebrospinal fluid, 2, 14 and (3) a slit-valve mechanism that functions as a one-way valve between the cyst and the subarachnoid space. 1, 5 However, the active fluid secretion theory has been disregarded due to the fact that the structural organization indicates constant fluid transport toward the lumen, which would not explain the fact that the cysts often remain static in size. 2, 7

The slit-valve mechanism of net inflow of CSF into an arachnoid cyst was obvious in the four cases presented here of congenital suprasellar arachnoid cysts. In each of the cases, a small opening in the arachnoid wall of the cyst was present directly over the basilar artery, forming a net one-way slit-valve for CSF flow. Caemart et al. were the first to observe and report on the presence of the slit-valve mechanism. 15 Schroeder and Gaab had also observed the one-way valve in a suprasellar prepontine arachnoid cyst. 5 Our observations clearly show that the valves were all slit-valves and, in conjunction with Schroeder, Gaab, and Caemart, may be the major contributor of enlargement of suprasellar arachnoid cysts. With pulsations of CSF in the prepontine cistern during the cardiac cycle, the flow of CSF from caudal to rostral, lifted the wall of the cyst up off the basilar artery, increasing the opening of the slit valve and resulted in net flow of CSF into the cyst — a finding similar to that observed by Schroeder and Gaab. 5

The flow of CSF into the cyst is attributable to the pulsatile flow of CSF and the normal pulsing of the brain, which creates transient pressure gradients for normal CSF flow. Transitory increases in CSF pressure can be concluded to create sufficient pressure gradients between the cyst and the rest of the subarachnoid space to allow net influx of CSF.16 In addition to CSF pulsations due to the cardiac cycle, Williams and Guthkelch 6 emphasized the responsiveness of the subarachnoid space to changes in venous pressure, which would indicate crying, exertion or Valsalva maneuver might also lead to increased filling of the cysts and contribute to enlargement.

MRI flow studies show that the flow of CSF is oscillatory between the craniospinal compartments during one cardiac cycle. 17 In the case of the slit valve, upward flow of CSF from the spinal compartment lifts the arachnoid up off the basilar artery and opens the slit valve. This allows CSF to flow up into the cranial compartment, through the slit valve over the basilar, and into the cyst. Normal backflow of CSF into the spinal compartment is relatively blocked during the cardiac cycle due to apposition of the slit-valve against the basilar artery. The configuration of the slit-valve around the basilar artery limits complementary oscillatory backflow, creating a net influx of CSF into the cranial compartment greater than net egress back into the spinal compartment. The imbalance in oscillatory flow of CSF between the craniospinal compartments therefore further supports the slit-valve as being responsible for the increase in size of suprasellar prepontine arachnoid cysts.

Of equal importance is the case of the communicating arachnoid cyst found in the middle cranial fossa. This arachnoid cyst had a small circular opening through which the cyst communicated with the CSF of the Sylvian fissure. This cyst did not show progressive enlargement and lacked a slit-valve. The circular opening in the cyst wall allowed bidirectional flow of CSF with equal resistance to flow in both directions. The flow of CSF between the cyst and the surrounding Sylvian cistern was probably a net of zero. Furthermore, the lack of a slit-valve and absence of serial enlargement in this case strongly suggests that arachnoid cysts lacking a slit-valve are more likely to remain stable and static in size.

While the one-way valve is evident, it is imperative to note that there is also egress of fluid from the cyst. However, the net influx is greater than the net egress. This would lead to a gradual accumulation of fluid within the cyst. As most known cases of suprasellar prepontine arachnoid cysts have been reported to occur in children, 1, 5, 8, 15, 18 CSF accumulation and cyst enlargement typically lead to clinical sequelae within the first two decades of life. We believe it is this difference in inflow versus outflow of CSF through the slit-valve that leads to progressive enlargement of suprasellar prepontine arachnoid cysts.

Conclusion:
We believe that this series of cases documents a slit-valve over the basilar artery as one mechanism of net one-way flow of CSF that leads to arachnoid cyst enlargement. Arachnoid cysts that communicate via a non-slit valve may be more likely to remain stable in size, as evidenced by the middle fossa arachnoid cyst included here. It is interesting to note that the cases we found were prepontine suprasellar, as were those of Gaab, Schroeder and Caemart. Ultimately, the location of the slit valve over the basilar artery is a unique configuration that allows a net influx of fluid and leads to mass effect, obstructive hydrocephalus and clinical sequelae.

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Figure Legend:
Figure 1: MRI images are presented of Case 1. A.) Pre-operative sagital T2-weighted MRI demonstrating a suprasellar arachnoid cyst with dilation of the lateral ventricle. B.) Pre-operative axial T2-weighted MRI demonstrating dilation of the lateral ventricles. No trans-ependymal flow is observed. C.) Post-operative sagital T1-post contrast enhanced MRI demonstrating decompression of the cyst. D.) Post-operative axial T1-weighted MRI demonstrating some decrease in the size of the lateral ventricles.

Figure 2: Representative intra-operative images of Case 1 (A+B) and Case 5 (C). A.) Endoscopic still image of Case 1 demonstrating an open slit valve (white arrows) over the caudal basilar artery. This is during the portion of the cardiac cycle with caudal to rostral CSF flow into the cyst. B.) Endoscopic still image demonstrating this same valve (white arrows) now in a closed position during rostral to caudal CSF flow. C.) Intra-operative microscopic view of Case 5 demonstrating a small circular opening (white arrows) around a sylvian arterial branch, this opening did not vary in size or position during the cardiac cycle.

Supplemental Video 1: Intra-operative video showing Cases 1, 2, 3, and 5 and slit-valve openings. 4 min. 29 seconds; MB.