A Novel Radiographic Method for Predicting Increased Intracranial Pressures in Severe Traumatic Brain Injury Using Optic Nerve Sheath Diameter Measured on CT Head

Introduction: Traumatic brain injury (TBI) is a major cause of disability, death, and economic cost to our society. Prevention of secondary brain insult in TBI is dependent upon optimization of intracranial pressures and cerebral perfusion. Changes in optic nerve sheath diameter(ONSD) can be used to predict increasing ICPs in a reliable noninvasive way. We propose a novel radiographic method to predict increased ICPs using ONSD measured on initial CTH along with the admission Marshall score.

Methods: Patients were retrospectively selected from the neurosurgery database of a busy level II trauma center in southern California. Study subjects included patients aged 18–90, who sustained a severe TBI requiring placement of an ICP monitor upon admission. All patients had a noncontrast CTH prior to placement of an ICP monitor. Patients receiving any intervention for decreasing suspected elevated ICPs prior to ICP monitor placement and documented orbitofacial trauma were excluded. PACs was used as the software for measuring ONSDs. Using standard criteria the ONSD was measured 3mm behind the posterior aspect of the globe on a noncontrast CTH.

Results: A total of 242 patients met the inclusion criteria. The control group consisted of 100 patients. Mean ONSD in the control group was 5.73 ± 0.58 mm. The ONSD range was 4.25 to 7.15 mm. The average optic nerve sheath diameter of both eyes was 6.72 ± 0.79 mm. The average opening pressure at the time of ICP monitor placement was 16.6 ± 10 mm Hg. Correlation analysis showed a trend in increasing opening intracranial pressure with increasing optic nerve sheath diameter, particularly when ONSD was >6.5 mm (p = 0.0001, r = 0.36). ONSD value alone had a sensitivity of 76.4% and specificity of 47.2%. The Marshall score, when >2 had a specificity of 77.8% and a positive predictive value of 87%. When combined with the admission Marshall score >2, it provided a specificity of 89.7% and positive predictive value of 90.4% that a patient would have ICPs >20 mm Hg.

Conclusion: Our findings indicate that ONSD of 6.5 mm and Marshall score >2 were both sensitive markers of elevated ICP. When combined, they provide reliable data points that can determine whether an ICP monitor should be placed. Our specificity of 89.7% and positive predictive value of 90.7% are the highest in the literature utilizing ONSD measured on CT in the setting of TBI. Further studies need to be undertaken in the future in an effort to better stratify ONSD values in combination with individual parameters, which may include the utilization of both dynamic ONSD monitoring (US) as well as precise ONSD measurements (CT). This combination of dynamic, objective and validated criteria will have the potential to create an effective clinical tool that changes the way we determine which patients need invasive ICP monitoring.