Measurement of optic nerve sheath diameter will be performed with a linear transducer placed superior and lateral to the eye, above the upper eyelid. The patient will be positioned supine, with a 30 degree head elevation. After visualization of the entry of the optic nerve into the globe, we will freeze the image. Optic nerve sheath diameter will be measured 3mm behind the globe in a perpendicular axis to the nerve. Two measurements will be taken per eye, and the mean of the 4 values will represent the optic nerve sheath diameter. A nerve sheath diameter of more than 5.8mm is defined as equivalent with an intracranial pressure (ICP) of more than 20mmHg.