Intracranial lipoma
An intracranial lipoma are not a tumours as such, but rather a result of "abnormal differentiation of embryologic meninx primitiva". They are frequently associated with abnormal development of adjacent structures, supporting the so called "brain dysraphism" theory.
They are usually asymptomatic, and either found incidentally or as a result of investigation of related malformations and thier presentation (e.g. mental retardation, epilepsy etc...).
Location
- pericallosal lipoma : 45%, associated with agenesis of the corpus callosum in approximately 50% of cases
- quadrigeminal cistern lipoma : 25%, associated with underdevelopment of the inferior colliculus
- suprasellar cistern lipoma : 15%
- cerebellopontine angle lipoma : 10% (the facial nerve and vestibulocochlear nerve often courses through the lipoma - see case 1).
- sylvian fissure : 5%
Radiographic features
The characteristic finding on both CT and MRI is of a mass which has appearances consistent with fat. CT (negative HU values) and MRI with and without fat saturation are able to make the diagnosis easily. In the absence of fat saturated images, then chemical shift artefact may be useful.
Often the lipomas are traversed by cranial nerves and adjacent vessels.
Differential diagnoses
The differential is essentialy that of masses which contain fat, and therefore includes:
- intracranial dermoid : if ruptured will often have multiple droplets scattered through the subarachnoid space. Usually midline.
- intracranial teratoma
- lipomatous transformation of neoplasm : PNET, ependymoma, glioma
On MRI, if no fat saturated sequences are available then a number of other possibilities should be entertained, which also have high T1 signal.
- thrombosed berry aneurysm : often will have calcified rim, and haemosiderin staining on gradient echo / SWI sequences.
- white epidermoid: rare, and will restrict on DWI
