Bilateral subdural collections invisible on a CT brain scan.

We describe a patient who presented with headaches and vomiting. A computerised tomogram (CT) of brain failed to demonstrate bilateral subdural collections subsequently shown on magnetic resonance imaging (MRI). This case illustrates that an essentially normal CT scan does not always exclude significant intracranial pathology. CASE REPORT A 44 year old female presented with a four month history of headaches and vomiting. No abnormality was detected on neurological examination and routine investigations were normal. A CT scan of brain was performed before and after intravenous contrast. No focal abnormality was demonstrated but there was poor visualisation of the third ventricle and basal cisterns (fig 1). In particular the left ambient cistern was completely obscured a sign suggestive of supratentorial pathology. In view of these findings the possibility of a low grade infiltrating lesion such as a glioma within the basal ganglia was considered, and the patient was referred for magnetic resonance imaging. Axial T2-weighted MRI scans demonstrated the presence of significant chronic bilateral subdural collections which were invisible on the axial CT scan (fig 2a & b). Sagittal TI-weighted images showed tonsillar herniation below the level ofthe foramen :s,' '1 ~~~~~~~~~~~~~~~~~~.