Differentiation of the subdural and the subarachnoid spaces is a matter of debate in the extracerebral fluid collections. Visualization of the cortical veins, so-called cortical vein sign was proposed as an index of the subarachnoid space. We examined the validity of this sign.

We reviewed the anatomy of cerebral meninges in the literature. We also examined the cortical vein sign in some patients with extracerebral fluid collections, evaluated by magnetic resonance imaging and computed tomography.

The subarachnoid space is a space below the arachnoid barrier cell layer, above the pia mater, and between arachnoid trabeculae. Since the trabeculae are not elastic, the subarachnoid space cannot be enlarged unless the trabeculae are torn. Before tearing of the trabeculae, dural border cell layer of the dura will be separated. Anatomically, the subarachnoid space cannot expand beyond the limit of the trabeculae. The cortical veins lie on the surface of intima pia anchored by arachnoid trabeculae. As they approach the convexity, cortical veins cross the arachnoid and dura, draining into the sinus. Near the sinuses, theses vessels lie between the dura and the arachnoid, and they can be seen even in patients with subdural hygroma.

The cortical vein sign is not useful to differentiate subdural hygroma and atrophy.