 The optic nerve attaches to the globe posteriorly and is wrapped in a sheath that contains cerebrospinal fluid. The optic nerve sheath is contiguous with the dura mater and has a trabeculated arachnoid space through which cerebrospinal fluid slowly percolates. Pressure increases around the brain are therefore transmitted to around the optic nerve and increase the diameter of the optic nerve sheath. This optic nerve sheath diameter ONSD has been described as a highly sensitive 88-94 and specific 85 - 94 noninvasive technique of identifying increased intracranial pressures in patients with brain trauma, intracranial bleeding or stroke1-4. This method requires placement of an ultrasound probe on the patients closed eyelid and then a direct measurement of the diameter of the optic nerve sheath at a pre-specified distance from the globe. A value greater than 58 mm has been shown to correlate significantly with an increased ICP 20 cm H20; r 0.71, p0.001 5. 