**3. US optic nerve sheath diameter (ONSD)**

The optic nerve can be anatomically subdivided into an intraocular, intraorbital, canalicular, and intracranial segment [8]. The optic nerve, as part of the central nervous system, is covered by a leptomeningeal sheath, which is expandable in the anterior segment, behind the globe.

Optic nerve sheath ultrasound is a simple, safe, inexpensive, bedside diagnostic test analogous to the measurement of BP and has the potential to replace invasive ICP monitoring in cases of raised ICH. Ophthalmic ultrasound typically uses a frequency between 5 and 10.5 MHz to evaluate the eye and orbit [9, 10].

Two measurements are made for optic nerve:

One in the transverse plane, with the probe in horizontal, and one in the sagittal plane, with the probe in the vertical.

The final ONSD is the average of these measurements. ONSD is measured 3 mm behind the optical disc [11, 12]. The optic nerve appears as a sagittal hypoechoic structure, 4.5 to 5 mm thick, with 25 mm in length that runs from the outer part of the eyeball to the apex of the orbit.

The optical disc is seen as a hyperechoic line at the posterior pole of the globe. With high interobserver agreement, with a median difference of 0.2–0.3 mm [11].

Ultrasonography of the optic nerve sheath is easy to perform. Despite this, in-depth knowledge of the anatomy of ultrasound and the scanning technique is mandatory for the proper use of the technique in the appropriate clinical setting [11, 12].

Most authors have suggested that the reasonable upper value of ONSD is 5 mm. However, further studies suggest that the cutoff value of the ONSD that provides the best precision for the prediction of intracranial hypertension (ICP = 20 mmHg) is 5.7–6.0 mm and that the ONSD values above this limit should alert the doctor for the presence of raised ICP [9–11].

According to Geeraerts et al., a strong relationship was found between the ONSD average and the ICP. When using 5.8 mm values as a cutoff point, a very low probability of having a high ICP was observed when the ONSD had smaller dilations [13–18].

Despite the advantages, ultrasound of the optic nerve sheath has some limitations. In patients with ocular trauma and other diseases of the optic nerve complex, the assessment of ONSD can be challenging. Traumatic optic neuropathy is seen in a significant number of patients with severe head trauma, and the effects of eye trauma on ONSD are unclear [3, 18–24].
