Introduction

Arachnoid cysts (ACs) are described as congenital anomalies that occur within the cerebrospinal fluid (CSF) cisterns and major cerebral fissures. They are intimately covered by arachnoid membranes and generally contain clear and colorless fluid that is nearly identical to CSF [13]. Although they have been described as congenital lesions, few ACs are diagnosed during early neonatal life or infancy. The prevalence of these cysts is higher in the first 2 decades of life. Nearly three quarters of ACs become symptomatic in children. The AC, a relatively common lesion encountered in neurological practice, accounts for approximately 1% of all intracranial space-occupying lesions. It is likely, however, that this underestimates their true prevalence, as the increased use of magnetic resonance (MR) images and computed tomographic (CT) scans has revealed an increasing number of asymptomatic lesions [10].

Despite their common occurrence, little is actually known about the natural history and pathogenesis of ACs. The majority of cysts are dormant fluid compartments that can remain static for many years of serial neuroimaging studies. Some cysts, however, progressively enlarge, exerting an increasing mass effect on adjacent neural structures and leading to spontaneous intracyst or subdural bleeding; rarely, a cyst involutes and disappears over time [13]. Spontaneous regression of an AC has been reported in the literature on rare occasion [1, 2, 4, 8, 12, 14, 15, 17, 19, 20, 24, 25, 27, 28, 30]. Until now, only two cases of spontaneous disappearance of suprasellar AC have been described in the literature [8, 25]. We present a case of spontaneous regression of a suprasellar AC persisting after ventriculoperitoneal (VP) shunt performed 10 years earlier.

Case report

A 5-year-old boy suffering from impaired visual acuity and urinary incontinence was admitted to a local clinic in October 1988. Plain skull radiographs showed prominent convolution markings suggesting chronic increased intracranial pressure. Brain MR images showed a large suprasellar cystic lesion extending upward from the suprasellar cistern and displacing the third ventricle superiorly with dilatation of the lateral ventricles (Fig. 1). A two-step surgical procedure was planned. The goal was to resolve the noncommunicating hydrocephalus first and then treat the cystic lesion subsequently. A VP shunt was put in place 5 days after admission to alleviate the symptoms associated with hydrocephalus. Two months later, he was referred to our neurosurgery department for further evaluation. On admission, a neurological examination revealed no focal deficits. A repeat CT scan revealed no change in the cystic mass of water attenuation in the suprasellar cistern or further deformity of brainstem (Fig. 2). Although there was no obvious neurological dysfunction, surgical exploration and decompression were recommended due to the persistent size of the cyst and signs of brainstem compression. However, the boy’s parents declined any further surgical treatment, and the patient was subsequently lost to follow-up for 10 years. When the patient returned to our clinic at the age of 15 years, he was neurologically intact. Repeat MR images showed a complete disappearance of the suprasellar AC and a marked reduction in the hydrocephalus (Fig. 3). His family denied any significant interval history such as head injury, intracerebral hemorrhage, degenerative disease, or CNS infection. The patient did not report any symptoms that could be related to a spontaneous rupture of the cyst wall. The patient remained neurologically intact, attended school, and was without symptoms.

Fig. 1
figure 1

Preoperative MR images at age of 5 years. Axial (a), sagittal (b), and coronal (c) T1-weighted images demonstrating a large-sized suprasellar arachnoid cyst. Note that upward displacement of the mammillary bodies, effacement of the ventral surface of the pons and hypothalamus, and noncommunicating hydrocephalus were prominent

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Fig. 2
figure 2

Follow-up CT scans. CT scans (a, b) performed 2 months after placement of the ventriculoperitoneal shunt showing no change in the size of the cyst and signs of brainstem compression

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Fig. 3
figure 3

Follow-up MR images at the age of 15 years. Axial (a) and sagittal (b) T1-weighted images reveal complete disappearance of suprasellar arachnoid cyst. Note that previous abnormal displacements have returned to normal and noncommunicating hydrocephalus has disappeared with minimal bilateral subdural hygroma

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Discussion

The incidence of suprasellar AC is rare, constituting 10% of all ACs [6]. The frequency with which this finding is identified is increasing because of the widespread use of current neuroimaging techniques. Many aspects of this disorder remain poorly defined. The origin of ACs has recently been elucidated. A slit-valve mechanism is created when the basilar artery pierces the prepontine arachnoid membrane; this process is responsible for the formation and enlargement of the ACs [22]. Although some cysts appear centered in the suprasellar region, both MRI and neuroendoscopy show that the cysts originate in the prepontine space and extend upward, rather than originating in the suprasellar region [9]. Accordingly, the term prepontine or suprasellar-prepontine AC has been proposed.

Unfortunately, there is no pathognomonic sign for a suprasellar AC and various pathologic entities can cause a similar clinical presentation. Therefore, neuroimaging studies play a crucial role in accurately diagnosing these lesions. The AC is often described as a cystic mass with similar signal intensity to CSF on all sequences of MR images. The walls of cysts do not enhance after the administration of intravenous contrast [13]. If the cysts reach a sufficiently large size in the suprasellar cistern, a cyst-induced mass effect can obstruct the third ventricle and lead to severe noncommunicating hydrocephalus. In these cases, patients may occasionally undergo VP shunt placement to treat the hydrocephalus while the AC remains obscure. According to Wang et al. [26], three distinguishing preoperative neuroimaging characteristics are consistently observed on sagittal T1-weighted MR images: (1) vertical deflection of the optic chiasm, (2) upward displacement of the mammillary bodies, and (3) effacement of the ventral surface of the pons. These findings are in contradiction to those of noncommunicating hydrocephalus caused by aqueduct stenosis in which the reverse is true, i.e., the mammillary bodies and the third ventricular floor are inferiorly displaced.

Because the natural history of suprasellar AC remains poorly defined, few treatment guidelines exist. The cyst may progress, stabilize, or spontaneously regress [8, 25]. In the past, this ambiguity has led to various treatment options that include (1) subfrontal/pterional craniotomy with fenestration to the basal arachnoid cisterns with the option of an additional cystoperitoneal shunt, (2) transcallosal fenestration of the cyst into the lateral ventricles with optional VP shunt, (3) primary cystoperitoneal shunt in patients without hydrocephalus, and (4) percutaneous ventriculocystostomy [13, 21]. VP shunt may be initially performed because of the failure to appreciate the presence of the cyst and clinically urgent symptoms associated with hydrocephalus [21]. However, progression of the cyst size after VP shunt may occur in nearly 40% of suprasellar ACs, especially with noncommunicating hydrocephalus. This paradoxical evolution of the cyst most likely results from the development of a postshunt pressure gradient between the drained ventricles and the cyst [18]. In addition, the success rate of the VP shunt as a definitive treatment for cysts is only reported to be 10%. The majority of the cases require second or third procedures [21]. In our case, we performed VP shunt for the control of severe noncommunicating hydrocephalus and then planned to fenestrate the cyst into the ventricular or cisternal space via a transcallosal or subfrontal craniotomy. More recently, advances in endoscopy have brought endoscopic fenestration to the forefront of the management algorithm for suprasellar AC. Endoscopic ventriculocystostomy at the apical membrane and cystocisternostomy at the basal membrane are accepted as safe and successful modalities for symptomatic and radiographic improvement [26].

Subsequent reports of arachnoid cyst regression have been described following minor head trauma [16, 23, 29], meningitis, and hemorrhage [3, 31]. Beltramello and Mazza [2] in 1985 first reported the spontaneous disappearance of an AC in the middle cranial fossa. To date, 17 case reports published, including this one, have described spontaneous regression of ACs without antecedent specific events (Table 1) [1, 2, 4, 8, 12, 14, 15, 17, 19, 20, 24, 25, 27, 28, 30]. After review of the reported cases, it was observed that the detection of disappearance of an AC usually occurred during the pediatric or adolescent period. The cyst disappeared before the age of 20 years in all pediatric cases. Nine patients had cysts in the middle cranial fossa, three in the posterior fossa, three in the suprasellar region, and two in the frontal convexity. The interval between initial diagnosis and resolution of cyst ranged from 1 month to 11 years. Actually, the possible start time of this rare event was from 1 month to 7.5 years after initial diagnosis, according to serially followed-up or sudden symptomatic cases.

Table 1 Summary of reported cases with arachnoid cysts that disappeared spontaneously
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Resolution of the ACs can be divided into two patterns: slow and gradual disappearance on serial CT scans in six cases and sudden disappearance with specific symptoms in three cases. Remaining cases including ours might follow the slow and gradual route because they did not reveal any specific symptom or its changes. However, identification of the appropriate classification can be interrupted by patients lost to follow up and long intervals between CT scanning. The interval from reduction to disappearance of the cyst can range from 3 months to 3 years in cases with gradual resolution. Seven patients had a specific symptom associated with reduction or resolution of the cyst in repeated imaging studies. The symptoms included headache, blurred vision, increased frequency of seizure, and improved hydrocephalus symptom. In the remaining 10 cases, no triggering incident was evident to predict cyst change, although it may have been very mild and therefore gone unnoticed. The resolution of these cases could only be checked in routine CT scans for follow-up.

Different mechanisms have been suggested to explain the spontaneous resolution of AC. One possible cause might be the communication between the cyst and the subdural space. As a result of the observations that a subdural hematoma or hygroma is usually associated with disappearance of an AC following minor head trauma, it has been suggested that a tear in the cyst wall caused by trauma might establish a traumatic connection and result in consecutive absorption of the cyst fluid [16, 29]. Three of the nine cases with an identifiable course (gradual or sudden) revealed the presence of a subdural hematoma or hygroma in accordance with the above hypothesis. All of these ACs were in the middle cranial fossa and took a mean of 18 months to disappear. However, in the traumatic cases, it took 5 months [16] or less (only 45 days [29]) for the ACs to resolve. This difference may be explained by the extent of injured membrane.

Another possible mechanism for AC resolution is the formation of a communication between the cyst and the subarachnoid space or ventricle. In this case, the cyst would drain through the normal CSF pathways. Although this mechanism may explain some cases of resolution of the AC in the middle cranial fossa without any change in the subdural space [15, 27, 28], it is mainly accepted as a possible role in cases located in the posterior fossa or suprasellar-prepontine region [1, 8, 17, 24, 25]. Whether there is direct transport through the cyst wall or whether the CSF is released through a valve-like mechanism is still debated. The wall of an AC resembles normal arachnoid membrane and consists of lamellar collagen bundles [11]. Furthermore, multiple fenestrations are made in the AC wall membrane by using coagulation probes, microscissors, and blunt biopsy forceps during endoscopic surgery [13, 26]. Considering these characteristics of the cyst wall, a tearing of a weakened cyst wall or a widening of the slit-valve-like arachnoid membrane around the basilar artery may occur prior to the disappearance of the suprasellar AC. Additionally, other factors such as valsalva maneuver, straining, sneezing, coughing, or unnoticed trivial injury can exacerbate the disruption of the cyst membrane [1, 5, 20].

It is possibly postulated that normalization of the intraventricular pressure brought about by VP shunt may cause a reduction in the size of the cystic lesion. Actually, the reduction in pressure within the ventricular system could be followed by a reduction in size of the pineal cystic lesion. This can be explained by the semipermeability of the cyst wall or reversed direction of the fluid movement [7]. In our case, however, there was no reduction in size of the cyst after placement of the VP shunt on follow-up CT scans. Furthermore, the poor results with enlargement of the suprasellar cyst after the shunt procedures reported by several authors [18, 21] led us to believe that there is little relation between the decreased intraventricular pressure and reduction in the size of the cyst.

Since the patient was lost to follow-up for 10 years, we cannot exclude the possibility of an unreported head injury that might have caused a rupture of the cyst. However difficult to prove, we think it is likely that the cyst spontaneously ruptured when the characteristics of the cyst wall changed into more weakened or widened, with/without concomitant provocation factors such as sports activity, extreme breath holding, crying, or Valsalva maneuver. Furthermore, a persistent communication between the cyst and subarachnoid space or ventricle was established. Our case and a review of the literature show that ACs may spontaneously disappear, especially in pediatric patients and middle cranial fossa cases. This may occur more frequently than expected. Although the natural course of the suprasellar arachnoid cyst has not yet been clearly defined, we believe that spontaneous resolution can result from a communication between the cyst and the subarachnoid space or ventricle. In addition, this supports the use of the fenestration method as a preferred treatment modality for unresolved, symptomatic suprasellar arachnoid cyst.