17.1 Introduction

Intraventricular cysts are one of the best surgical indications for neuroendoscopy. These lesions are usually located deep in the brain, and the working space for the neuroendoscope is secured by the cysts themselves or the enlarged ventricle.

Various types of pathology are responsible for the development of intraventricular cysts (Fig. 17.1). Cysts can be classified by the origin of tissue they arise from as follows1:

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  Mesenchymal origin: arachnoid cyst.

  
  
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  Endodermal origin: neuroenteric cyst and colloid cyst.

  
  
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  Exodermal origin: ependymal cyst, choroid plexus cyst, interhemispheric cyst, Rathke’s cyst, pineal cyst, cyst of septum pellucidum, etc.

Arachnoid cysts are the most common intracranial cysts, accounting for 1% of the intracranial mass lesions. A recent report revealed the prevalence of arachnoid cysts in the pediatric population as 2.6% on MRI.2 Patients with intraventricular cysts may have signs and symptoms of increased intracranial pressure (headache, enlarged head circumference, developmental delay, seizure, and hemiparesis), but for most part their intracranial cysts are found incidentally. Controversy remains about when surgery is indicated and which procedures to chose.

Although nearly 30 to 50% of the arachnoid cysts are found in the middle fossa, there seems to be some relationship between age and their location. The intraventricular (lateral ventricle), suprasellar, and interhemispheric cysts are diagnosed more often in infants or fetuses.3

The most common form of intraventricular cyst is a choroid plexus cyst which is commonly observed on prenatal sonograms during the second trimester.4,5 Choroid plexus cysts present in ~ 30% in the general population at autopsy and are unlikely to be symptomatic due to their small size.6

From the surgical point of view, location of the cyst is more important than the pathology of the cyst because surgical approaches and techniques differ based on the cyst location. Intraventricular cysts are divided into the following categories based on their location:

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  Cysts in the lateral ventricle: arachnoid cyst, choroid plexus cyst, ependymal cyst.

  
  
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  Cysts in the third ventricle: suprasellar arachnoid cyst (SSAC), quadarigeminal cistern arachnoid cyst.

  
  
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  Cysts in the periventricular region: middle fossa arachnoid cyst (MFAC), ependymal cyst, interhemispheric cyst.

In this chapter, we will describe these cysts and their neuroendoscopic management.

17.2 Pathophysiology

SSACs (Fig. 17.1e) are estimated to comprise ~ 2% of the all intracranial arachnoid cysts2,7 and ~ 10 to 20% of surgically treated arachnoid cysts, as patients with SSACs are more likely to have clinical symptoms than those with other arachnoid cysts.8 The SSAC originates from the suprasellar cistern and develops toward the third ventricle and to the foramen of Monro on the midline. Thus, the SSAC forms a third ventricular cyst, expanding the third ventricle roundly and compressing the brainstem dorsally.

Hydrocephalus is the most common clinical presentation. It is caused by obstruction of both foramen of Monro. Cerebral aqueduct obstruction, or stenosis, due to cyst compression of the midbrain is another factor in the development of hydrocephalus. Intraoperative neuroendoscopic findings demonstrate obstruction of the entrance of the cerebral aqueduct by the expanded cyst wall in the third ventricle. The optic apparatus and the hypothalamo-pituitary system are also stretched and compressed by the SSAC, leading to visual and endocrinologic dysfunction. Although most SSACs are congenital in origin, a recent report reveals the presence of acquired SSACs after birth.9

Among cysts in the brain, cysts in the lateral ventricle (Fig. 17.1a–d) compose a relatively small number. Shim et al. reported of 209 cases of intracranial arachnoid cysts only 1.4% were found in the lateral ventricle. Pradilla and Jallo reported on their series that surgically treated arachnoid cysts were in the intraventricular location in 20% of the cases.10 The embryonic origin of arachnoid cysts in the lateral ventricle is unclear, but they may migrate from the periventricular cistern in the early embryonic period. Fig. 17.2 suggests that arachnoid cysts in the lateral ventricle originate from the velum interpositum cistern. Most lateral ventricular cysts are congenital in origin and can be diagnosed in utero (Fig. 17.3). Choroid plexus cysts are generally small and asymptomatic. They may become symptomatic if they are larger than 2 cm in diameter4,5 (Fig. 17.1a). Because those cysts are located in the ventricle, they grow subclinically. Symptomatic lateral ventricular ependymal cysts are quite rare; there has been only 4 cases reported.11,12

Other cystic lesions around the ventricle have a variety of pathologies. Apart from the middle fossa arachnoid cysts (MFAC), congenital periventricular arachnoid cysts are observed in the interhemispheric and quadrigeminal cisterns (Fig. 17.1g,h). Ependymal cysts are rare, and most of them are located in the supratentorial region12,13 (Fig. 17.1f). When a periventricular cistern, such as the vellum interpositum cistern, has been trapped by hemorrhage or inflammation, acquired periventricular cysts are observed.14

17.3 Clinical Features

Almost all cases of SSACs are associated with hydrocephalus or ventriculomegaly. In infants, rapid increase of head circumference is the common clinical presentation. In older children, headache and other symptoms due to increased intracranial pressure (ICP) can be the initial symptoms. Detailed study reveals endocrinologic abnormalities (25%), visual impairment (30%) and ataxia due to brainstem compression (30%) among those with SSACs. Bobbledoll head syndrome, although it is very rare and not specific to those with SSACs, is a well-known clinical sign of the SSAC.

Cystic lesions in the lateral ventricle and in periventricular locations are usually dormant. They remain asymptomatic until they grow large enough to cause signs and symptoms related to increased ICP.3 If a cyst is located near the foramen of Monro, it may cause intermittent obstructive hydrocephalus even if the cyst is small in size.4

Neuroimaging after minor head trauma or seizures may reveal an asymptomatic lesion. Recent advances in fetal imaging has made the diagnosis of intracranial cysts in utero possible.15

17.4 Diagnosis and Neuroimaging

Routine diagnostic axial CT and MRI images demonstrate SSAC. Because the third ventricle expands spherically and pushes its roof up above the foramen of Monro, the lateral ventricle, when imaged, appears separated from the anterior and posterior horns on each side by the enlarged third ventricle. As a result, the ventricles at the level of foramen of Monro appear as five isolated cavities, the “Mickey Mouse appearance.” Associated hydrocephalus is a common finding. In general, cysts are symmetric but could extend unilaterally in case of a large cyst. Rathke’s cysts, peri-hypothalamic cystic tumors, choroid plexus, and other congenital cysts need to be differentiated from the SSAC.

SSACs are divided into three types based on CT cisternography findings16 (Fig. 17.4). A type 1 cyst is completely isolated from the ventricle and basal cistern. A type 2 cyst is isolated from the ventricle, but there is delayed communication between the cyst and basal cistern (Fig. 17.5). A type 3 cyst is isolated from the ventricle but directly communicates with the basal cistern.

Arachnoid cysts in the quadrigeminal cistern are classified into three types based on the cyst extension: supratentorial and infratentorial extension with dumbbell-like appearance as type 1, infratentorial extension as type 2, and lateral extension as type 3.17

On neuroimaging, ventriculomegaly could take various sizes based on cyst location and the site of the CSF obstruction.3 An arachnoid or ependymal cysts in the lateral ventricle is commonly associated with focal enlargement of the lateral ventricle by the cyst with or without partial ventriculomegaly. The shape of the cyst is round or oval, not irregular. On the contrary, a choroid plexus cyst in the lateral ventricle is usually small and difficult to visualize on neuroimaging. A three-dimensional (3D) CISS MRI sequence is reported to be useful in identifying a choroid plexus cyst.4 Unilateral ventriculomegaly may develop with a choroid plexus cyst.6

Differential diagnosis of these cysts on neuroimaging alone is not always possible. Surgical specimens of the cyst wall may be required for precise pathologic diagnosis.11