Appearance of the Optic Nerve Sheath Diameter (ONSD) using Higher Frequency Linear Probes in Detection and Monitoring of Raised Intracranial Pressures-A Cadaveric Study

Abstract

Introduction: Trans-bulbar ultrasonography of Optic Nerve Sheath Diameter (ONSD) is increasingly used to detect raised Intracranial Pressures (ICP). Current guidelines for measuring ONSD are based on earlier descriptions of anatomy of optic nerve sheath using a 7.5 MHz linear probe. There is wide variation in the cut-off value of an abnormal ONSD, mostly due to an edge artefact around the dura and uncertainty over placement of measurement cursors. Aim: To describe the detailed anatomy of the appearance of ONSD on cadaveric simulated models using a higher frequency probe. Materials and Methods: A 20G intravenous cannula was inserted into the subarachnoid space around optic nerve of nine embalmed cadavers and the space gradually insufflated with saline to mimic raised ICP. Axial and lateral ultrasonography of ONSD was done using a high frequency 13 MHz transducer, before and after insufflation of the space. The ultrasonic appearance of optic nerve sheath at normal and increased intracranial pressures was studied. Results: At normal pressures, the true subarachnoid CSF space was either not visible or seen as a thin sliver of anechoic space. At elevated pressures, this space appeared as an anechoic triangular or semi lunar space with scattered trabecular echoes on either side of anterior optic nerve in axial and longitudinal scans. Edge artefacts are easily appreciated. Conclusion: High frequency ultrasound probes (13 MHz) easily guide identification of duramater, distinguish edge artefacts and observe changes in appearance of subarachnoid space in elevated ICP. This would help in precise bed side monitoring of raised ICP especially in resource poor settings. Images described here can be a useful tool to educate and train ICU and emergency care providers.

Keywords

• subarachnoid space
• trans-ocular
• ultrasound