The morphological features of disproportionately enlarged subarachnoid-space hydrocephalus (DESH), the core type of idiopathic normal pressure hydrocephalus, can not be explained by the classical theory of CSF absorption at the arachnoid villi and the hypothesis of CSF flow blocking at the convexity subarachnoid spaces. By using MRI Time-SLIP CSF flow imaging, we demonstrated that CSF freely move in the subarachnoid spaces below and in the Sylvian fissures. CSF does not move in the convexity subarachnoid spaces in healthy individuals and patients with DESH, indicating the presence of flow obstacles in the convexity subarachnoid spaces by nature. If so, CSF absorption must take place below the Sylvian fissures. CSF would retain in the ventral subarachnoid spaces and ventricles, once the absorption below the Sylvian fissures is impaired. This hypothesis enables to explain the morphological features of DESH. We proposed an alternative hypothesis of CSF circulation and absorption including absorption from the cranial and spinal nerve roots to the lymphatic system.