🧠 Optic Nerve Diameter
The optic nerve diameter refers to the measurable width of the optic nerve, assessed by imaging techniques such as MRI, ultrasound, or fundoscopy.
📏 Normal Values
| Location | Normal Diameter | Notes |
|---|---|---|
| Intraorbital (ultrasound) | 3.0 – 5.0 mm | Measured 3 mm behind the globe (standard site) |
| Intracanalicular (MRI) | ~2.5 – 4.5 mm | Depends on age and myelination |
| Intracranial (MRI) | ~4 – 6 mm | Includes nerve and sheath |
🔍 Clinical Significance
- Increased diameter:
- Papilledema
- Optic neuritis
- Optic nerve glioma or meningioma
- Decreased diameter:
- Optic atrophy
- Glaucoma
- Optic nerve hypoplasia
- Post-radiation changes
🛠️ ONSD in Neurocritical Care
Optic Nerve Sheath Diameter (ONSD) is a useful non-invasive marker of intracranial pressure (ICP).
- Standard site: 3 mm posterior to globe (ultrasound)
- ONSD > 5.0–5.7 mm → suggests ICP > 20 mmHg
This measurement is increasingly used in emergency and intensive care settings for rapid screening.