External hydrocephalus after aneurysm surgery: paradoxical response to ventricular shunting

The authors sought to investigate the mechanisms and pathophysiological effects of subdural fluid collection after surgery for aneurysmal subarachnoid hemorrhage (SAH).The authors retrospectively analyzed the medical records of 76 patients who had undergone craniotomy. The patients included 55 with aneurysmal SAH (SAH group) and 21 with unruptured aneurysms (non-SAH group) who were used as controls. Subdural fluid collection was more common in the SAH than in the non-SAH group (38% compared with 14%, p < 0.05). Although older patients appeared to be at greater risk for subdural fluid collection in both groups (p < 0.05), this condition developed even in relatively young patients with SAH. In the SAH group most subdural fluid collection was associated with ventricular dilation (81%), and a significant correlation was seen between fluid collection and the need for subsequent shunt placement (48% compared with 21%, p < 0.05). These results point to an association between hydrodynamic dysfunction and subdural fluid collection. The course of patients with subdural fluid collection varied from spontaneous resolution to normal-pressure hydrocephalus. Seven patients with persistent subdural collections underwent shunt placement (ventriculoperitoneal [VP] shunt in six and lumboperitoneal in one), which resulted in resolution of fluid collection in all seven.The results indicate that for most patients in the SAH group, subdural fluid collection represented "external hydrocephalus" rather than simple "subdural hygroma." Decreased absorption of cerebrospinal fluid because of SAH and surgically created tears in the arachnoid membrane communicating with the subdural space were factors in the development of external hydrocephalus. The authors believe that differentiating external hydrocephalus from subdural hygroma is extremely important, because VP shunt placement can be used to treat the former but could worsen the latter.