EXTERNAL HYDROCEPHALUS AFTER ANEURYSM SURGERY - PARADOXICAL RESPONSE TO VENTRICULAR SHUNTING

Object. The authors sought to investigate the mechanisms and pathophys iological effects of subdural fluid collection after surgery for aneur ysmal subarachnoid hemorrhage (SAH). Methods. The authors retrospectiv ely analyzed the medical records of 76 patients who had undergone cran iotomy. The patients included 55 with aneurysmal SAH (SAH group) and 2 1 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 a ppeared to be at greater risk for subdural fluid collection in both gr oups (p < 0.05), this condition developed even in relatively young pat ients with SAH. In the SAH group most subdural fluid collection was as sociated with ventricular dilation (81%), and a significant correlatio n 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 colle ction. The course of patients with subdural fluid collection varied fr om spontaneous resolution to normal-pressure hydrocephalus. Seven pati ents with persistent subdural collections underwent shunt placement (v entriculoperitoneal [VP] shunt in six and lumboperitoneal in one), whi ch resulted ill resolution of fluid collection in all seven. Conclusio ns. The results indicate that for most patients in the SAH group, subd ural fluid collection represented ''external hydrocephalus'' rather th an simple ''subdural hygroma.'' Decreased absorption of cerebrospinal fluid because of SAH and surgically created tears in the arachnoid mem brane communicating with the subdural space were factors in the develo pment of external hydrocephalus. The authors believe that differentiat ing external hydrocephalus from subdural hygroma is extremely importan t, because VP shunt placement can be used to treat the former but coul d worsen the latter.