13 Colloid Cysts
13.1 Introduction
Colloid cysts are relatively rare benign intracranial lesions, representing 0.5 to 2.0% of brain tumors and 15% of all intraventricular masses.1,2 Most colloid cysts are identified in patients between 30 and 40 years old, though colloid cysts can be encountered at any age, and 8% of cases are diagnosed in the pediatric age group.3 In children, colloid cysts may have a more aggressive clinical course and appear more acutely radiologically.4 Some theorize that rapid development of clinical manifestations in this age group may be attributed to rapid enlargement of the cysts due to more water in their contents.5
13.2 Pathophysiology
Previously, colloid cysts were believed to be neuroepithelially derived6; however, more recent studies have concluded that they are endodermal in origin.7,8,9,10,11 Histologically, colloid cysts are composed of an outer thin fibrous capsule over an inner epithelium. The epithelium is usually a single layer of mucin-producing or ciliated cells. The interior of the cysts contains mucoid and gelatinous material, which can vary in consistency from thin and mucinous to solid. The material is positive for periodic acid–Schiff (PAS) and mucicarmine staining.6,12 Familial occurrence is extremely rare; in such cases, clinical and radiographic evaluation is recommended in all first-degree relatives.10,13 Although colloid cysts are most frequently located in the rostral aspect of the third ventricle near the foramen of Monro,13,14,15 they are also found throughout the ventricular system. Extraventricular location is rare; however, cerebral hemisphere, velum interpositum, suprasellar, optic chiasm, cerebellar hemisphere, fourth ventricle, cavernous sinus, olfactory groove, and brainstem colloid cysts all have been reported.12,16,17,18,19
13.3 Clinical Features
The most common presenting symptoms are headache, altered level of consciousness, ataxia (or gait instability), and memory difficulties. When the tumor enlarges rapidly causing cerebrospinal fluid (CSF) obstruction, ventriculomegaly, and increased intracranial pressure (Video 13.1 and Video 13.2), sudden death, although rare, can occur. Some cysts enlarge more gradually, allowing the patient to accommodate the enlarging mass without disruption of CSF flow, in which case the patient remains asymptomatic.20,21,22,23 Prediction of which cysts will enlarge and when remains challenging. Most patients are believed to have headaches before they descend into depressed consciousness.
13.4 Diagnosis and Neuroimaging
Pneumoencephalography was among early radiologic techniques used in the diagnosis of colloid cysts of the third ventricle, by which the mass was outlined in almost all cases. Lateral “hanging head” projections were most successful at demonstrating the lesion. The radiologist could manipulate the air placed in during a ventriculographic examination; as it moved through the foramina of Monro into the third ventricle, it would clearly outline the mass, whose characteristic round shape and foraminal location yielded the likely diagnosis. In carotid angiography, displacement of the anterior segment of the internal cerebral veins where they transition from the roof of the third ventricle to the lateral ventricles also served as supporting evidence.24 In modern times, the mainstay of diagnosis is computed tomography (CT) and magnetic resonance imaging (MRI).25 On CT scans, the lesions are most often clearly seen as hyperdensities at the foramen of Monro (Fig. 13.1a). In contrast, the MRI features are highly variable, and such cysts may even be overlooked by the unwary26 (Fig. 13.2b–e). This is largely a function of the quality of the contents of the cysts. Those colloid cysts that are rich in protein and cholesterol are usually hyperdense on CT, and on MRI are hyperintense on T1-weighted and hypointense on T2-weighted sequences.27 Watery cysts are more likely to be bright on T2-weighted sequences. Intensity and density are correlated with difficulty of aspiration during the endoscopic procedure; hyperdensity on CT and low-intensity on T2-weighted sequences are consistent with high-viscosity intracystic contents, which may affect the surgeon′s ability to aspirate them intraoperatively.28,29
13.5 Treatment
With a 10% risk of sudden death due to acute hydrocephalus, surgical resection is recommended for colloid cysts that are large, symptomatic, or associated with hydrocephalus, whereas close observation may be considered for small asymptomatic lesions.23 Cysts greater than 9 mm in diameter are more likely to present with hydrocephalus and be symptomatic.23 The traditional treatment for colloid cysts has been open craniotomy through a transcallosal or transcortical approach. Increasingly, less invasive endoscopic approaches are employed to address these lesions in an effort to minimize perioperative morbidity.30,31,32
The optimal approach to colloid cysts remains controversial. Microsurgeons point to reported 100% rates of complete resection with open microsurgery to favor open surgery, while advocates of endoscopy report shorter hospital stays and minimal morbidity with endoscopic approaches.33,34 One reported drawback of endoscopic approaches is a lower rate of complete cyst wall resection and a theoretically higher risk of cyst recurrence.35,36,37,38,39 However, an important caveat is that previous reports demonstrate a significant learning curve associated with endoscopic techniques.35,38 For this reason, earlier endoscopic series may report lower rates of complete resection than are achievable via endoscopic approaches today. Furthermore, the employment of technical modifications, using a bimanual dual-instrument endoscopic technique, and anterolateral approach can produce more favorable results compared to traditional endoscopic approaches.
13.5.1 Operative Procedure
The patient is placed in a three-point Mayfield head fixation frame, with the head in a neutral position with the sagittal plane perpendicular to the floor. By utilizing the preoperative images or image guidance, the optimal point of entry and trajectory can be identified. This point is typically ~ 5 cm anterior to the coronal suture (8 cm behind the nasion) and 5 to 7 cm lateral to the midline, depending on the degree of ventriculomegaly present.40
In general, the most anterolateral trajectory that can be achieved without violating the caudate head is preferred to provide the best angle of attack to the point of attachment of the colloid cyst to the roof of the third ventricle (Video 13.1 and Video 13.2).40 A right frontal approach is favored; however, a left-sided approach can be chosen in the presence of ventricular asymmetry and if the lesion is predominantly located on the left, taking care not to make the entry through Broca′s area. The ventricle is cannulated under image guidance or with a guide needle. A peel-away sheath is optional. If the ventricles are small, no peel-away sheath is used so as to maintain the volume of the ventricle. Once the ventricle has been accessed, a 30-degree rigid endoscope is passed into the frontal horn of the lateral ventricle also under stereotactic guidance. Cyst resection should proceed without CSF aspiration and under gentle intermittent hand-pump irrigation with room temperature lactated Ringer solution. Care is taken to identify normal structures and be sure that the correct side has been entered. Choroid plexus overlying the cyst is coagulated (Video 13.1 and Video 13.2). The exposed area of the cyst wall is then initially coagulated with bipolar electrocautery, then punctured with sharp scissors, and aspirated utilizing a 6-Fr pediatric endotracheal suction catheter.30,41 Following aspiration, a dual-instrument technique is used to resect the cyst wall (Video 13.1), with a flexible pediatric grasping forceps placed in a side-channel of the endoscope and a second instrument in the traditional straight working channel (Fig. 13.2, Video 13.1). The resection is performed in a bimanual fashion, with one hand controlling each instrument, while an assistant holds the endoscope. The small forceps is used to grasp the cyst so that scissors or bipolar can be applied under tension. Using this dual-instrument technique, cyst walls are pulled up into the lateral ventricle and resected with a combination of gentle traction, coagulation, and sharp dissection (Fig. 13.3a–d, Video 13.1).
If a flexible neuroendoscope is used, a standard Kocher′s entry point (2.5 to 3 cm off the midline and 1 cm in front of coronal suture) approach is sufficient. The surgical technique is very similar to the one done with a rigid endoscope, starting with fenestration of the cystic wall followed by drainage of the cystic content. The capsule should be resected, if possible. The disadvantage of a flexible endoscope is the lack of bimanual instrumentation; therefore, the procedure may consume more time. Ideally colloid cyst resection should be performed with a rigid endoscope.
The flexible neuroendoscope can be used at the end of the cystic resection to inspect the occipital horn and posterior portion of the third ventricle to look for colliod cyst fragments that have fallen inadvertently during the resection. These free fragments can occlude the entrance of the cerebral aqueductal and cause hydrocephalus.
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