====== 🧠 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.


===== 🔬 Ultrasound Measurement of the Optic Nerve Diameter =====

[[Ultrasound Measurement of the Optic Nerve Diameter]]