Colloid cysts are relatively unusual (1.5% neurosurgically treated tumours) benign intracranial masses located in the rostral aspect of the IIIrd ventricle. As they grow they obstruct the foramen of Munro producing hydrocephalus of the lateral ventricles.¹ This can be acute and catastrophic because of the pedunculated nature of the tumour resulting in brain herniation and death.² Both computed tomography (CT) and magnetic resonance imaging (MRI) will usually show the lesion and its effects on the ventricular system, but the multiplanar capability and higher contrast resolution of MRI will usually define its relations better.

Unless the tumour is an incidental finding³ patients usually present with intermittent headache and less commonly with nausea, short term memory loss, gait disturbance, blurred vision or coma. Non-specific neurological symptoms and signs are common but may be suggestive of raised intracranial pressure. The cause and origin of the cysts are still debated. The cyst size ranges from 2–50 mm and they contain colloidal material including cholesterol fats that are largely responsible for their appearances on MRI. Thin (≤5 mm) axial sections on MRI or CT will show the lesion hanging from the roof of the IIIrd ventricle, while sagittal and/or coronal sections will show its relation with the foramen of Munro⁴ (fig 1). On CT the mass is hyper- or isodense with grey matter while on MR T1 weighted imaging it is usually hyperintense, reflecting the colloidal matrix and fatty content. T2 weighted sections show it as hyperintense or isointense with grey matter while FLAIR (fluid attenuated inversion recovery) sequences will show it as hyperintense against a background of hypointense cerebrospinal fluid. Surgical treatment used to be carried out via a transcallosal approach in an attempt to avoid damage to the fornix and resultant memory loss, but more recently stereoscopic and endoscopic approaches to the IIIrd ventricle have proved superior in many cases^4,5 with less neurological and neuropsychological sequelae.

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Figure 1

(A) Axial non-contrast CT showing an 8 mm diameter round high attenuation tumour, a colloid cyst in the IIIrd ventricle at the level of the foramen of Munro—as yet no hydrocephalus. (B) T2 weighted axial and (C) T1 weighted sagittal sections showing a 10 mm diameter tumour hanging from the roof of the IIIrd ventricle obstructing the foramen of Munro, causing hydrocephalus of the lateral ventricles with periventricular oedema.