Which of the following is the most common site of demyelination in optic neuritis?

Explanation

## Anatomical Distribution of Optic Neuritis **Key Point:** Retrobulbar optic neuritis (inflammation of the optic nerve behind the globe) is the most common form, accounting for approximately 60–70% of cases. ### Classification by Location | Site | Frequency | Clinical Features | |------|-----------|-------------------| | **Retrobulbar** | 60–70% | No disc edema initially; relative afferent pupillary defect (RAPD) prominent; pain with eye movement | | **Papillitis** | 20–30% | Disc edema visible on fundoscopy; optic disc hyperemia | | **Perineuritis** | Rare | Inflammation extends to nerve sheath; may show enhancement on MRI | ### Clinical Correlation **High-Yield:** In retrobulbar neuritis, the optic disc appears normal initially because the inflammation is posterior to the lamina cribrosa. This is why the term "the patient sees nothing, and the doctor sees nothing" is classically taught — vision is severely affected but the fundus looks normal. **Clinical Pearl:** Papillitis (optic disc swelling) is more common in children and in non-demyelinating causes (e.g., viral infections, syphilis, tuberculosis). In adults with demyelinating optic neuritis, retrobulbar disease predominates. ### Why Retrobulbar is Most Common The retrobulbar segment is the longest portion of the optic nerve and has the highest concentration of myelin, making it the most vulnerable to demyelinating attack in multiple sclerosis (MS)-associated optic neuritis. [cite:Harrison 21e Ch 435]