The pathological process of extracerebral fluid collections in infancy includes subdural effusion and enlargement of the subarachnoid spaces. Both conditions have traditionally been investigated as a single clinical entity, because of difficulty in differentiating between them. The prognosis of subdural effusion is not as benign as that of enlargement of subarachnoid spaces, requiring differential diagnosis between these disorders. The present study was conducted to elucidate whether this differentiation could be made on magnetic resonance (MR) images. The series consisted of 16 infants aged 10 months or younger, including eight with verified subdural effusion and eight in whom a diagnosis of enlargement of the subarachnoid spaces was achieved by neuroimaging studies other than MR imaging. In all eight patients with subdural effusion, the intensity of the fluid was greater than that of cerebrospinal fluid (CSF) in at least one of the sequences using T-1-weighted, proton-density, and T-2-weighted MR images. The flow-void sign, indicating vessels in the fluid spaces, was not seen in any of these eight patients. On the other hand, in all eight patients with enlargement of the subarachnoid spaces, the fluid was isointense in relation to CSF, and vascular flow-void areas were seen in at least one of the MR imaging sequences. Based on these observations, it is concluded that differentiation between subdural effusion and enlargement of the subarachnoid spaces can be established by focusing on two aspects of MR imaging findings: 1) the intensity of the fluid, which is either iso- or hyperintense relative to CSF, and 2) the presence or absence of vascular now-void areas in the fluid spaces.
