Optic Nerve Sheath Diameter (ICP)

1. 1

   Patient supine with eyes closed. Use linear probe (7–15 MHz) with a generous standoff gel pad — do NOT apply pressure to the globe.

2. 2

   Place gel pad on closed eyelid. Orient probe transversely over the eye.

3. 3

   Identify the globe (anechoic sphere), lens (anterior hyperechoic arc), and optic nerve (hypoechoic tract exiting posteriorly).

4. 4

   Measure ONSD exactly 3mm posterior to the posterior wall of the globe, perpendicular to the nerve.

5. 5

   Normal ONSD ≤5.0–5.7mm (varies by reference). >5.7mm correlates with ICP >20 mmHg.

6. 6

   Rotate probe 90° for the longitudinal plane — repeat measurement. Average transverse and longitudinal.

7. 7

   Repeat on the contralateral eye. Bilateral elevation strengthens the correlation with raised ICP.

• Transverse ONSD (3mm posterior to globe)
• Longitudinal ONSD

• ONSD measurement
• Globe architecture
• Posterior globe flattening (severe ICP)

• Measure exactly 3mm behind globe posterior wall
• Use light pressure only — globe is fragile

ONSD >5.0–5.7mm (varies by study) correlates with ICP >20 mmHg. Use linear probe with standoff gel pad.