Observations
As interhemispheric arachnoid cysts are rare lesions, information regarding their natural course and optimal management is limited. It is estimated that interhemispheric location of arachnoid cysts comprises approximately 5% of all cases [
10]. One retrospective study of arachnoid cyst prevalence in children found that of 309 incidentally detected cysts, only 4 were located in the interhemispheric region [
11]. In children, these lesions are typically concomitant with developmental anomalies of midline structures, such as partial or complete agenesis of the corpus callosum [
3,
6‐
9]. Our patient demonstrated dysplastic appearance of the splenium of the corpus callosum, aligning with this association. In spite of the large dimensions these lesions can attain, clinical manifestations may be subtle or absent. With the increased utilization of ultrasound prenatally, more of these lesions have been diagnosed prior to birth [
12,
13], as was the case in our patient.
Arachnoid cysts associated with neurological symptoms or increased ICP can be managed surgically with craniotomy for cyst resection, endoscopic fenestration, or cystoperitoneal shunting. There have been several prior case series on the surgical management of interhemispheric cysts using such techniques [
2,
6,
7,
9,
10]. This literature demonstrates that when encountered, interhemispheric cysts are often treated operatively.
Regression or disappearance of arachnoid cysts has been reported in prior literature. There have been several cited instances of cyst disappearance following head injury, in which cysts found in association with subdural or epidural hematomas following trauma decrease in size as the hematoma subsides [
14‐
16]. One proposed mechanism of this phenomenon is that cranial trauma may cause rupture of the outer membrane of the cyst, thus leading to cyst fluid drainage into the subarachnoid space [
17]. Thereby, cysts found incidentally on imaging done after trauma are found to decrease in size on subsequent imaging. Other types of intracranial irritation aside from trauma, such as meningitis, have also been associated with cyst disappearance [
18]. In the absence of trauma or other notable irritating event, some have suggested that a communication between the cyst and subarachnoid space can be created by sudden, transient increases in ICP during Valsalva maneuver [
19]. Numerous cases of seemingly spontaneous regression of arachnoid cysts have been discussed in the literature in both children and adults [
20‐
26]. The most common locations of these reported disappearing arachnoid cysts are the middle cranial fossa, posterior cranial fossa, temporal/temporo-frontal region, suprasellar region, and prepontine region [
19]. To date, there has been no cited case of spontaneously regressing interhemispheric cyst.
In the current case, we describe a pediatric patient with a multiloculated arachnoid cyst diagnosed in utero that was followed with serial imaging for several years and subsequently demonstrated spontaneous shrinkage. She did not have cranial trauma nor other intracranial irritation or pathology that preceded cyst regression. Of note, during the period of observation of the cyst, the patient experienced a fall from her tricycle without consequent loss of consciousness or neurological change. However, this occurred after radiological evidence of decrease in cyst size. As such, this fall likely was not the inciting event that led to cyst regression.
In cases of asymptomatic arachnoid cysts, most opt for conservative treatment with observation and serial imaging. Reports of shrinkage of these lesions are clinically relevant when counseling patients and families on the potential outcomes of arachnoid cysts. The currently reported case is the first cited description of such a shrinkage of an arachnoid cyst located within the interhemispheric fissure, adding to the collective knowledge of possible clinical course of a cyst in this specific location.