Brain Imaging in Colloid Cyst 

Colloid cysts are benign, congenital epithelium-lined cysts that almost always arise in the anterior third ventricle. However, rare reports describe cysts in other locations. The cysts are believed to derive from either primitive neuroepithelium of the tela choroidea or from endoderm.^[1] See the images of colloid cysts below.

The diagnosis is usually made by assessing the typical location and appearance of the cyst. Colloid cysts account for approximately 1% of all intracranial tumors and are the most common type of the neuroepithelial cysts, as well as the most common tumor in the third ventricle. Typically, patients are asymptomatic, although colloid cysts may cause symptoms by obstructing the foramen of Monro, which results in sudden death in rare cases.^{[2, 3]}

Shapiro et al described the long-term results of an interhemispheric, transcallosal, subchoroidal, fornix-sparing approach to gross-total resection of colloid cysts. In 57 colloid cysts, total removal was achieved via a 3 x 3-in paramedian craniotomy flap and a microscopic interhemispheric, transcallosal, subchoroidal approach sparing the ipsilateral fornix. According to the investigators, at 1 year after surgery, computed tomography (CT) scanning or magnetic resonance imaging (MRI) confirmed gross-total resection, with no infection, hemiparesis, seizures, or disconnection syndrome. There were no deaths or recurrences. The authors noted that the results were superior to those seen with endoscopy.^[4]

Preferred examination

Either CT scanning or MRI may help in diagnosing a colloid cyst, although MRI has a few advantages.^[5] The multiplanar capabilities of MRI optimally demonstrate the location of the cyst, and typical signal intensities in the cyst help to confirm the diagnosis. T2-weighted MRI sequences may be useful to assess the nature of the intracystic contents and to predict the difficulty of aspiration during endoscopic or stereotactic procedures.^{[6, 7, 8, 9]}

Typically, colloid cysts are clinically silent and are found incidentally when patients are imaged for other reasons. When patients are symptomatic, they typically experience chronic headaches, which may be intermittent and positional because of transient CSF obstruction. On rare occasions, a colloid cyst may completely and irreversibly obstruct the foramen of Monro, resulting in sudden loss of consciousness and, if patients are not treated, in coma and death.^{[2, 3]}

Pollock et al reviewed 155 cases of colloid cyst and found 4 factors that were associated with colloid cyst-related clinical symptoms^[10] : (1) younger patient age, (2) increased cyst size (average of 13 mm in symptomatic patients vs 8 mm in asymptomatic patients), (3) ventricular dilatation (although 31% of asymptomatic patients had this finding), and (4) increased signal intensity on T2-weighted MRIs. These findings suggest that slowly growing colloid cysts may allow for compensation by the brain, thus avoiding symptoms. In addition, findings of high T2 signals in symptomatic patients suggest that the more-serous colloid cysts enlarge the fastest.

The appearance of colloid cysts on CT and MRI scans is important in planning treatment, because the success rate after percutaneous aspiration is lower in colloid cysts that have high attenuation on CT scans and decreased MRI T2-signal intensity than in the cysts that have fluid characteristics.^[11] Interestingly, the cysts with high T2-signal intensity are easier to treat with minimally invasive surgical techniques; however, these cysts are also the ones that are most likely to cause clinical symptoms.

Preferred examination

Although MRI has the advantage of multiplanar imaging, CT scanning is usually adequate for the diagnosis of a colloid cyst.

Limitations of techniques

Intraventricular hemorrhage occasionally mimics a colloid cyst on CT scans and MRI.

Subependymomas, central neurocytomas, and subependymal astrocytomas may occur in the same area as a colloid cyst on imaging studies, but these entities can be distinguished by their shape, contrast enhancement, and signal intensity.

CT scan findings include a round mass with high attenuation at the anterior third ventricle. Rarely, colloid cysts may be isoattenuating or hypoattenuating relative to brain parenchyma. The typical high attenuation likely results from proteinaceous fluid. See the images below.

Hydrocephalus, which can be severe, results when the cyst obstructs the lateral ventricles at the foramen of Monro. This finding can be intermittent, as the cyst can act as a ball valve. Patients may then present with intermittent positional headaches.

Degree of confidence

A nonenhancing area of high attenuation in the typical location is almost diagnostic of a colloid cyst, but most clinicians confirm the diagnosis with MRI.

False positives/negatives

No normal variants are commonly confused with a colloid cyst.

Colloid cysts are well demonstrated on MRI scans and are found near the foramen of Monro, just posterior to the fornices in the anterosuperior third ventricle. (See the images below.)

The MRI signal intensity of colloid cysts is notoriously variable, with any combination of T1- and T2-signal intensities described.^{[5, 12, 13, 14, 15]} The most common appearance is hyperintensity with T1-weighted sequences and isointensity to hypointensity with T2 sequences.^[16] This variation is believed to be a result of the proteinaceous fluid, as well as the paramagnetic effects of the metal ions in the fluid and hemorrhage.

El Khoury et al found that low signal intensity of colloid cysts on T2-weighted images corresponded to difficult percutaneous aspiration in 100% of cases, whereas most of the cysts with high T2-signal intensity were considered easy to aspirate.^[17]

Rarely, inspissated debris may form a dependent nodule that can be seen on CT and MRI scans.

Colloid cysts do not have intrinsic enhancement; the presence of enhancement suggests a solid tumor. Peripheral enhancement may be present because of vascularity in the outer wall.^[16]

Degree of confidence

The diagnosis of a colloid cyst is made based on the classic location, appearance, and signal intensity. Central contrast enhancement suggests an alternative diagnosis, but most other pathologies that occur in this location are easily distinguishable from colloid cysts.

False positives/negatives

No normal variants are commonly confused with a colloid cyst.