113 Ultrasound of the Optic Nerve Sheath as an Indicator of Mild Traumatic Brain Injury: Animal Model Provides Anatomical Basis

Abstract

With no definitive method for diagnosis, many cases of mild traumatic brain injury (mTBI) receive suboptimal clinical management. Since emergency physicians and emergency medical services personnel are on the front lines with regard to mTBI, a tool for more accurate and timely indication of mTBI could improve diagnosis and management of mTBI in acute settings. Clinically, we have begun to validate that ultrasound measurements of optic nerve sheath dilation can be used to indicate mTBI. The objective of this study was to investigate the anatomical mechanism behind what has been observed clinically. More specifically, we evaluated the effect of increased intracranial pressure (ICP) on the fiber bundles connecting the optic nerve to the optic sheath in a swine model. Four experimental, 2 sham, and 4 normal swine were used in this study. In the experimental swine models, increased ICP was induced by passing a Foley catheter through a drilled opening into the epidural space and inflating a balloon. ICP was monitored continuously. For all animals, a section of optic nerve sheath with the intact nerve was removed after euthanasia and enucleation. The tissue was fixed overnight in paraformaldehyde and glutaraldehyde in a sodium cacodylate buffer, postfixed in osmium tetroxide in a sodium cacodylate buffer, dehydrated with a graded ethanol series, and critical point dried with carbon dioxide. The dried control and experimental model sections were mounted and imaged using a scanning electron microscope (SEM). Images of the trabecular meshwork were taken radially around the optic nerve at several magnifications. The images were segmented and analyzed using the ImageJ plug-in DiameterJ. Segmentation quality was confirmed using image subtraction of the original image from the segmented image and vice versa. All images had false positive and false negative total percentages of less than 5% of all pixels. Image overlays of the segmented and original images confirmed this qualitatively. This is an ongoing study. Image processing and analysis is still in progress. For a normal and an experimental animal, mean and median fiber bundle diameters were recorded for a total of 61 images. Thirty (49.1%) of the images analyzed were from the control sample and 31 (50.9%) were from the experimental sample. An analysis of variance found that both the mean (.52 μm) and median (.62 μm) fibrous bundle diameters were significantly different (ie, smaller) for the experimental model as compared to the mean (.73 μm) and median (.85 μm) fibrous bundle diameters for the control (p<.0001). The relationship between mean and median fibrous bundle diameters and sample (control or experimental) was not significantly correlated to radial location (relative to the optic nerve) or magnification of the image based on analyses of covariance. Thus far, this research demonstrates that in a swine model the fibrous bundles of the optic sheath’s connections to the optic nerve are statistically different (smaller) after exposure to increased ICP. We anticipate similar results from the other samples. Future research will evaluate if the thinning of fibrous bundles between optic sheath and optic nerve is caused by increased ICP. This will be accomplished by increasing the sample size and using other methods to induce increased ICP.