A colloid cyst is a collection of gelatinous material contained within a fibrous epithelial-lined wall. It is also named a neuroepithelial cyst. These lesions are slowly growing benign tumors. They most frequently origin from the roof of the third ventricle near its connection with the lateral ventricles (the foramen of Monroe). However, they may unfrequently be encountered elsewhere.

Symptoms
Commonly reported symptoms include headache, nausea and vomiting, gait disturbance, altered mentation, blurred vision, urinary incontinence. Symptoms are usually related to obstruction of the ventricular system. Progressive tumor growth results in obstruction of the lateral ventricles’ outlet, the foramen of Monroe. This translates in progressive accumulation of cerebrospinal fluid (CSF) within the lateral ventricles referred to as chronic hydrocephalus. The tumor might also move on its pedicle to episodically obstruct the foramen of Monroe. This results in intermittent acute hydrocephalus and secondary intracranial hypertension. Sudden death has been described with such tumors possibly due to an acute blockage of CSF flow. Although most symptomatic cyst measure more than 1.5 cm in diameter however, some may be clinically manifest at a smaller size.

Diagnosis
Imaging is required to establish the diagnosis. CT scanner or MRI scanner may be performed. Importantly, some colloid cyst may not be seen on CT scanner if there appearance is similar to surrounding brain structures. Therefore, MRI is the preferred imaging modality.

The first step is to establish if the cyst must be treated. Symptomatology and size are two factors taken into consideration to decide if treatment is required.

The optimal surgical approach remains to be established. Numerous factors must be taken into consideration including the location of the cyst, the ventricles’ size and the cyst’s viscosity.

Previously, bilateral ventricular shunts would be offered to bypass the lateral ventricles’ outlet. However, approaches aiming tumor removal are preferred. The interhemispheric transcallosal approach enables to access the third ventricle without incising the cortex. It requires a frontal craniotomy. More recently, the endoscopic approach has provided an excellent corridor to treat intraventricular lesions. Using intra-operative neuronativation, the optimal trajectory to the foramen of Monroe is determined. After the ventricle is penetrated, .a small cylindric sheat is installed, allowing sufficient space to enter the endoscope and microinstruments. Removal of the colloid cyst is performed respecting microsurgical principles. Another option is drainage of the cysts content using image guidance. Although cyst aspiration might be sufficient in some patients, recurrence rate following this procedure is higher than with surgical removal of the tumor.