83 Time Correlation of Optic Nerve Sheath Diameter to Increasing Intracranial Pressure in a Cadaveric Model

Abstract

Ultrasound (US) evaluation of the optic nerve sheath width (diameter) has been shown to correlate with intracranial pressure. Normal optic nerve sheath diameter as measured 3 mm distal to the posterior globe is less than 5 mm. As intracranial pressure increases, the optic nerve sheath dilates. However, the speed at which the optic nerve sheath dilation occurs is unknown. Our objective was to determine the speed and the extent of dilation of the optic nerve sheath using a cadaveric model with increasing intracranial pressure. This was a prospective observational cadaveric trial comparing 2D US measurements of the optic nerve sheath to invasive intracranial monitoring of the intracranial space. Using an intracranial monitoring device, 5 ml of saline was instilled in the intracranial space while simultaneous measurements of the optic nerve sheath diameter were made using a SonoSite Micromaxx US system and a L38 (10 - 5 MHz) linear transducer. Measurements of the intracranial pressure were noted as changes from the baseline intracranial pressure. US measurements of the optic nerve sheath were taken in a single plane 3 mm distal to the posterior globe. US measurements were made every 5 minutes after fluid instillation until no further changes in the optic nerve sheath were detected. Three measurements of the optic nerve sheath were averaged for each measurement trial. The intracranial pressure increased immediately after fluid instillation into the intracranial space. However, as the fluid redistributed within the intracranial space, the pressure would decrease and would reach a steady-state baseline within the first 5 minutes. The optic nerve sheath diameter changed immediately with instillation of fluid into the intracranial space. There was little change in the optic nerve sheath diameter as the fluid redistributed within the intracranial space. However, the amount of change in the optic nerve sheath diameter was not proportional to the amount of fluid instilled into the intracranial space or to the intracranial space measurements. Figure 1 demonstrates the change in optic nerve sheath diameter and intracranial pressure values for 1 hour duration of the experiment. Although the change in optic nerve sheath dilation is simultaneous with intracranial pressure increase, the behavior of optic nerve sheath dilation (varies from 4.2 mm to 5.3 mm) is less dynamic than change in intracranial pressure levels (4 cm/H¬2O to 34 cm/ H¬2O). The change in the optic nerve sheath diameter is simultaneous with the increase in the intracranial pressure. However, the optic nerve sheath dilation is less dynamic compared to change in the intracranial pressure level.