AJNR Case Collection

Background:
Colloid cysts are rare and benign intracranial neoplasms that occur almost exclusively within the third ventricle near the foramen of Monro. They have a variable appearance likely related to the presence of cholesterol.

Clinical perspective:
Colloid cysts may present as an incidental finding. However, they have the potential to obstruct CSF flow, resulting in obstructive hydrocephalus. Imaging features in patients younger than 65 that may suggest high risk for symptoms include anterior location at the roof of the third ventricle (anterior to the mass intermedia), cyst size ≥7 mm, and hyperintensity on T2-FLAIR imaging.

Imaging perspective:
The pathognomonic location of a colloid cyst at the anterosuperior aspect of the third ventricle is helpful in diagnosis. It contrasts with a subependymoma, which is typically located in the fourth or lateral ventricles. Colloid cysts are well-defined and unilocular masses, while intraventricular hemorrhage will have multiple transient areas of involvement. Although colloid cysts are hyperdense on CT, they will not have the focal hyperattenuation seen with choroid plexus calcifications.

Outcome:
Symptomatic colloid cysts require neurosurgical intervention to restore CSF flow. Incidental or asymptomatic lesions, especially those with low-risk features, may be followed clinically and/or with MRI. While no definite consensus exists for the frequency of such monitoring, annual visits for 2–3 years and then discharge with red flag advice have been used previously.

Teaching Points:
Colloid cysts are an important finding on brain imaging with possibly severe consequences. They may be the cause of acute symptoms or eventually lead to them in the future. It is important to bring them to attention and appropriately detail relevant anatomy for neurosurgical intervention or follow-up.