Authors: Christina Sarris, MD; Tyler Cole, MD; Ruth Bristol, MD (Phoenix, AZ)

Introduction: Lumbar puncture has long been considered a reliable method of measuring intracranial pressure.The development of the intraparenchymal pressure monitor calls the accuracy of those measurements into question.The effects of general anesthesia on LP opening pressures should not be underestimated. The flurane anesthetics, ketamine, patient’s BMI, and elevated end-tidal CO2 have all been found to increase ICP. Methods: An IRB-approved retrospective review of all patients who had undergone ICP wire placement and also had prior LP opening pressure measurements between 2012 and 2017 was carried out. Results: 25 patients met inclusion criteria. The majority of patients were undergoing workup for chronic headache or idiopathic intracranial hypertension. Preoperative LP opening pressures were recorded in cm H2O and converted to mm Hg for comparative analysis. Mean LP opening pressure was 29.6 cm H2O, or 21.9 mmHg (Range 16-37 mmHg). Intraoperative mean ICP measured via wire was 13.2 mmHg (Range 6-26 mmHg), and postoperative mean ICP measured via wire was 8.1 mmHg (Range -2.5 to 18.5 mmHg). Results of a paired t-test demonstrated that both LP opening pressures and intraoperative ICP measurements were significantly higher by 14 and 5 points, respectively, compared to postoperative ICP measurements (p<0.001). End-tidal CO2 was not found to significantly correlate with intraoperative ICP (p=0.515). Conclusion: Lumbar puncture pressure was not an accurate measurement of intracranial pressure in our series. As physicians often incorrectly use mmHg and cm H2O interchangeably, the discrepancy in using cm H2O for lumbar puncture and using mmHg for ICP should be accounted for by conversion to the same units for comparison. Anesthetic protocols should avoid inhalational agents and ketamine. Interventions, such as shunting, cranial vault remodeling, or medication management, should be undertaken with caution if LP opening pressure measurement is the only available estimate of ICP.