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Colloid cyst of the third ventricle

colloid cyst of the third ventricle is a benign epithelial lined cyst with characteristic imaging features. Although usually asymptomatic, they can present with acute and profound hydrocephalus. 

Epidemiology

Colloid cysts account for 0.5-3% of primary brain tumours and 15-20% of intraventricular masses 1-3.  They are located at the foramen of Monro in 99% of cases 1.

The majority of cases are identified in early middle age (30-40 years of age) although 8% of cases may be diagnosed in paediatric age 3

Clinical presentation

In the vast majority of cases, colloid cysts are found incidentally and are asymptomatic. Their position in the roof of the third ventricle immediately adjacent foramen of Monro can on occasion result in sudden obstructive hydrocephalus and can present with a thunderclap headache or unconscious collapse.  The headaches tend to be positional, and patients may learn how to relieve symptoms. 

Pathology

Colloid cysts originate from abnormal folding of the primitive neuro-epithelium (the paraphysis elements2-3. They contain mucin, old blood (haemosiderin), cholesterol, and various ions, accounting for the wide range of imaging appearance. 

These cysts are lined by a single layer of columnar epithelium which produces mucin, which appears as a thick yellow green fluid when the cyst is open. 

Radiographic features

Cross-sectional imaging usually enables the diagnosis to be made with confidence. MRI is superior to CT in fully characterising the lesion. 

On all modalities colloid cysts appear as a rounded, sharply demarcated lesion at the foramen of Munro, which range in size from a few millimetres to 3-4cm 3.  

CT

Typically seen as a well defined, rounded lesion at the roof of the 3rd ventilcle

  • unilocular
  • typically hyperdense
  • isodense and hypodense cysts are uncommon
  • calcification is uncommon 2
MRI

MR signal characteristics include

  • T1: variable
    • ~50% high signal 4
    • the rest are hypointense or isointense to adjacent brain
  • T1 C+ (Gd): only rarely demonstrates thin rim enhancement, but usually this represents enhancement of the adjacent and stretched septal veins 3
  • T2: variable
    • most are low T2/T2* signal (short T2) is associated with thick "motor oil" consistency fluid 4,5
    • some have central low T2 and high peripheral T2 signal 4
    • some are homogeneously high signal 
  • FLAIR: cysts which are of low signal on T2 will appear similar to attenuated CSF on FLAIR, and are thus difficult to appreciate 4 

Treatment and prognosis

They tend to gradually increase in size over time and if resection is required, this can be performed via a transcallosal approach or more recently endoscopically or stereotactically.

When large and or symptomatic the decision to operate is relatively straight forward, as it can be life saving. For small lesions without symptoms attributable to them, the careful discussion between the patient and treating surgeon is required to weigh up the pros and cons of surgical intervention.  

The options are open or endoscopic resection or endoscopic or stereotactic aspiration. Aspiration may fail if the content of the cyst is too viscous, which can be predicted by appreciating low signal on T2 weighted sequences 5

Differential diagnosis

There are usually no differential diagnoses for a colloid cyst. In atypical cases it is worth considering other masses which arise in the region of the foramen of Monro, including: 

For a more complete list please refer to masses in the region of the foramen of Monro

Pearls and pitfalls

  1. Colloid cysts are most easily seen on CT.
  2. MRI signal is variable, and it is difficult to predict which sequence will best demonstrate the cyst.
  3. Beware of low T2 signal cysts (most common) as they will appear CSF like on FLAIR and be difficult to see - thus it is essential to have a standard T2 sequence in your protocol.
  4. Key features to include in a report of a study with a colloid cyst:
    • size of cyst
    • T2 signal intensity of the cyst (low signal=difficult to aspirate 5
    • size of ventricles
    • presence of cavum septum pellucidum et vergae
    • presence and location of internal cerebral veins
    • presence of any abnormal vascular structures (e.g. DVA) which may be present along the surgical tract

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