Arterio-venous epidural shunting in epidural bleeding radiological and physiological characteristics. An experimental study in dogs.

In order to test the possibility suggested in previous studies that the long bleeding time and the large bleeding volume observed in experimental epidural bleeding can be explained by the development of an arterio-venous shunt, water soluble X-ray contrast was injected into the epidural space in dogs during an epidural bleeding in progress. It was found that the contrast medium left the epidural space through diploic veins in the cranial bone to the neck veins. By draining off epidural blood the arterio-venous shunt counteracts the intracranial pressure tamponade developing in intradural bleeds and thus prolongs bleeding. Perfusion experiments showed the absorption capacity for saline to be about 20 times as large in the epidural space as in the CSF space. While there was no apparent absorption limit for blood in the epidural space the absorption capacity for blood of the subarachnoid space became progressively saturated, leading to a continuously increasing CSF outflow resistance and CSF steady-state pressure. A theory for the formation of epidural haematoma is proposed which in essence implies that the epidural shunt is a major determinant of the clinical outcome of an epidural bleeding.