Introduction

 Intracranial pressure (ICP) monitoring is a vital element in the diagnosis and management of several neurological disorders, such as head injury, hydrocephalus, subarachnoid hemorrhage, and intracranial hematoma.

 The optic nerve sheath is in direct contact with the subarachnoid space. This relationship provides the physiological foundation for using the optic nerve sheath to assess intracranial pressure. Since the optic nerve sheath is loosely attached to the nerve, the subarachnoid space in this region more distensible, and appears bulbous on ultrasound. Dilation of the optic nerve sheath, in contrast to papilloedema, occurs faster, and can be a sign of increased intracranial pressure.

 The advancements of ultrasound modalities enabled researchers to enhance optic nerve sheath diameter (ONSD) measurements, with further research that concentrated on determining the best distance behind the globe to measure OSND. According to a 1996 study, ONSD increased by up to 60% at 3 mm behind the globe compared to only 35% at 10 mm. Later studies showed that the measuring should be performed 3 mm behind the globe, since ultrasound contrast is superior at this depth with a linear probe, allowing for reproducibility.