Perioperative Variation in Optic Nerve Sheath Diameter - A Prospective Observational Study of Traumatic Brain Injury Patients Undergoing Decompressive Craniectomy

Background: Measuring optic nerve sheath diameter (ONSD) by transbulbar ultrasonography (TBUS) can suffice non-invasive ICP measurement with considerable accuracy. Objective: The primary objective of this study was to evaluate the perioperative variation in ONSD by TBUS in Traumatic Brain Injury (TBI) patients undergoing emergency craniectomy. Methods: We prospectively compared bilateral ONSD measurements in 45 consecutive TBI cases undergoing decompressive craniectomy under general anesthesia; before and after surgery. A total of 180 ONSD images were obtained and measurements were done by the same investigator blinded to the pre/postoperative nature of the image. Results: Based on preoperative Glasgow Coma Scores, 34 cases (75.5%) had severe TBI; 10 cases (22.2%) moderate TBI; and 1 case (2.2%) mild TBI. Preoperative ONSD in the study population were as 6.625 +/- 0.414mm. Average ONSD reduced significantly by 0.249 +/- 0.148 mm (P < 0.001) after craniectomy. On pooled analysis of cases undergoing right versus left sided craniectomy average ONSD reduced significantly by 0.252 +/- 0.173 mm (P < 0.001) and 0.259 +/- 0.139 mm (P < 0.001), respectively. ONSD of right eye with left eye and vice-versa were strongly correlated both pre/postoperatively with Pearson correlation coefficients (r)=0.879 (P < 0.001) and r = 0.827 (P < 0.001), respectively. Conclusions: In TBI cases undergoing decompressive craniectomy ONSD is bilaterally increased preoperatively. ONSD reduces significantly immediately after craniectomy; however, the diameters did not near the normal range. There hold a strong correlation between right/left ONSD measurements irrespective of the laterality of injury or side of surgery. Variable elastic properties of ONS in an injured brain can possibly explain our findings.