Herniation secondary to critical postcraniotomy cerebrospinal fluid hypovolemia.

OBJECTIVE: Cerebrospinal fluid hypovolemia resulting in postural headaches is a well-known clinical entity, but severe forms of cerebrospinal fluid hypovolemia with altered mental status and signs of transtentorial herniation ("brain sag") have rarely been reported. This article describes the clinical features of brain sag after craniotomy in an attempt to increase recognition of this syndrome.
METHODS: Between April 2001 and January 2003, 220 consecutive patients with subarachnoid hemorrhage were prospectively enrolled in the Columbia Subarachnoid Hemorrhage Outcomes Project; 137 underwent craniotomy for aneurysm clipping. Among these patients, the diagnosis of brain sag was made when all three of the following criteria were present: clinical signs of transtentorial herniation, head computed tomographic scans revealing effacement of the basal cisterns with an oblong brainstem, and improvement of symptoms after placement of the patient in the Trendelenburg position (-15 to -30 degrees). For each patient, the symptoms, clinical course, and subsequent response to treatment were characterized. In addition, brainstem dimensions were measured on computed tomographic scans taken before, during, and after resolution of brain sag. A "sag ratio" was generated for these time points by dividing the maximum anteroposterior distance by the maximum bipeduncular distance.
RESULTS: Eleven (8.0%) of 137 aneurysmal subarachnoid hemorrhage patients treated by craniotomy and an intraoperative spinal drain met the criteria for brain sag. Signs of transtentorial herniation developed most commonly between 2 and 4 days postoperatively. Pupillary asymmetry was noted in 10 (91.0%) of 11 patients, whereas the other patient demonstrated extensor posturing. The Trendelenburg position reversed the symptoms in all patients. The mean sag ratios before, during, and after resolution of brain sag were 0.91 +/- 0.03 (mean +/- standard error), 1.18 +/- 0.03, and 0.91 +/- 0.03, respectively. This represented a 30.9% elongation of the brainstem during sag (P < 0.001) and a 23.6% change back to baseline after resolution of the syndrome (P < 0.002). There was no significant difference between the presag and postsag ratios.
CONCLUSION: Severe cerebrospinal fluid hypovolemia after craniotomy may produce a dramatic herniation syndrome that is completely reversed by the Trendelenburg position. Brain sag should be included in the differential diagnosis for acute postoperative clinical deterioration in this patient population.