(1- 2mmHg) than ICP. Intracranial hypertension also induces changes at the cellular or axonal level such as the swelling of the fibers of the optic nerve that form the innermost layer of the retina (so-called nerve fiber layer – NFL). The information provided by the classic ophthalmoscopy is however only qualitative and may be inconclusive during early phases of intracranial hypertension since it usually takes between two and four hours from the onset of ICP elevation for a papilledema to develop. A patented method that utilizes optical coherence tomography to measure the thickness of the nerve fiber layer and infers ICP from it laid claims of being able to detect the IH-induced thickening of the retina shortly after the onset of IH, but there has been no data that would support the claims or clarify the relationship between the NFL thickness and levels of ICP. Optic nerve sheath diameter The use of optic nerve sheath diameter (ONSD) for the assessment of ICP dates back to 1987 when Cennamo and colleagues demonstrated a linear relationship between ICP and the sheath diameter measured with a trans-orbital ultrasound probe in an A-scan mode (principally equivalent to the time-of-the-flight measurements of the cranium diameter). The original measurement method was technically difficult and unreliable because of the nearly coaxial alignment of the optic nerve and propagation axis of the ultrasound wave, but the precision was significantly improved with the use of B-scan (or planar) ultrasound which provided longitudinal cross-section images of the optic nerve and its sheath. Since then, the method has been successfully validated in several relatively large studies that included patients with severe head trauma, hydrocephalus, intracranial hemorrhage or stroke, liver failure, and climbers with acute mountain sickness. While the ONSD can at any given point along the optic nerve be measured with a precision of <1mm, the reliability of derived ICP levels is plagued by inter-individual variability and the dependence of ONSD magnitude on the point along the nerve at which the measurement was taken. Almost all validation studies so far have recommended that ONSD to be used for identification of patients with intracranial hypertension that requires treatment (ICP>20mmHg, i.e. ONSD>5mmHg) rather than for a measurement of ICP. ONSD measurement by ultrasound is not as precise as invasive ICP measurement but may be useful if invasive measurement is not available. Ophthalmodynamometry or the measurement of the retinal venous outflow pressure (VOP) Performed