Introduction

Ocular ultrasound as used by the emergency or critical care physician has several applications. The diagnosis of lens disruption and/or retinal detachment can be made with ultrasound and has been well described in the ophthalmology and radiology literature (1, 2). Ultrasound can also diagnose the presence of ocular foreign bodies (1, 2). However, more recently, other research focuses for ocular ultrasound have included the use of ultrasound to measure the optic nerve sheath diameter. Here, the concept is based on the idea that increased intracranial pressure (ICP) is reflected through the nerve sheath, causing edema and swelling. Since increases in ICP are transmitted by the cerebrospinal fluid (CSF) down the perineural subarachnoid space of the optic nerve, this expansion of the nerve sheath that can be measured by ultrasound (3–5). In many ways, this technique parallels the concept of papilledema as a marker for increased ICP. However, optic nerve ultrasonography is arguably easier to perform and more quantifiable than the presence or absence of papilledema. It is this interest in ultrasound's potential to measure ICP noninvasively that has sparked so much research.

Many ocular ultrasound studies have been done to try to define normal optic nerve diameter ranges and where enlargement becomes pathologic and correlates with increased ICP. Although research is ongoing, to date most researchers have found that both pediatric and adult ocular nerve sheath diameters >5 mm have correlated with evidence of increased ICP as measured by degree of hydrocephalus, intrathecal infusions where lumbar pressures were measured, or CT evidence of increased ICP (6–9).