**Question:** A Young girl with previous history of repeated pain over medial canthus and chronic use of decongestants now presents with intense chills, rigors, and diplopia on lateral gaze. Examination shows optic disc congested. Most likely diagnosis would be:

A. Toxic optic neuropathy
B. Optic neuritis
C. Angle closure glaucoma
D. Central retinal artery occlusion

**Correct Answer:**

**Core Concept:**

Toxic optic neuropathy (TON) is a clinical entity characterized by optic nerve dysfunction caused by exposure to certain drugs or chemicals. In this case, the patient's history of chronic decongestant use is a strong clue, as these drugs are known to cause toxic optic neuropathy.

**Why the Correct Answer is Right:**

The correct answer is Toxic Optic Neuropathy (TON) because the patient has a history of repeated pain over the medial canthus and chronic use of decongestants. Decongestants, specifically those containing sympathomimetic amines, can cause optic neuropathy due to their vasoconstrictive effect on the ophthalmic artery and its branches, including the central retinal artery (CRA) and the optic nerve. This leads to ischemia and optic nerve dysfunction, as seen in this patient's case.

**Why Each Wrong Option is Incorrect:**

A. Optic neuritis: This is an inflammation of the optic nerve, not caused by drug use. The patient's optic disc congestion is due to vasoconstriction from decongestant use, not inflammation.

B. Optic neuritis: Similar to the previous reason, this condition is caused by inflammation, not drug exposure.

C. Angle closure glaucoma: This is a glaucoma type characterized by closed angles, not optic disc congestion and neurologic symptoms.

D. Central retinal artery occlusion: This is a condition characterized by occlusion of the central retinal artery, leading to retinal ischemia. In this case, the patient's optic disc congestion is due to vasoconstriction from decongestant use, not arterial occlusion.

**Clinical Pearl:**

In cases of suspected optic neuropathy related to medication use, consider the possibility of Toxic Optic Neuropathy (TON). In these scenarios, the patient's symptoms and signs (oculodynia, optic disc congestion, and neurologic symptoms) are more consistent with the vasoconstriction from decongestant use, rather than the actual condition being tested (TON). This helps students understand the importance of considering medication-related side effects when assessing patients with neurological symptoms and signs.