A CSF–venous fistula is a direct communication between the spinal subarachnoid space, usually a nerve root sleeve or meningeal diverticulum, and an adjacent foraminal, epidural or paraspinal vein, through which CSF drains unregulated into the venous system. First described in 2014, it is now a recognised major cause of spontaneous intracranial hypotension, typically without an extradural CSF collection on spine MRI. Brain MRI signs are summarised by the Bern score and, in patients without an extradural collection, the Mayo score, but the fistula is shown only by dedicated decubitus myelography, digital subtraction or CT, with the hyperdense paraspinal vein as the key sign. Definitive options are transvenous liquid embolic embolisation of the draining veins, surgical disconnection of the nerve root sleeve and CT-guided fibrin glue occlusion; epidural blood patching is often only temporising.