Which cranial nerve is involved in a patient presenting with acute diplopia, ptosis, a right eye deviated down and out, and a 7 mm poorly reactive pupil?

CN III (Oculomotor Nerve) Palsy

The answer is B. CN III. This patient's presentation of acute diplopia, ptosis, "down and out" eye deviation, and a dilated poorly reactive pupil is pathognomonic for oculomotor nerve palsy with pupillary involvement.

Clinical Reasoning

The constellation of findings definitively localizes to CN III:

• Ptosis: Loss of levator palpebrae superioris innervation (CN III)
• "Down and out" position: Unopposed action of lateral rectus (CN VI) and superior oblique (CN IV), with loss of medial rectus, superior rectus, inferior rectus, and inferior oblique function (all CN III)
• Dilated, poorly reactive pupil (7 mm): Involvement of parasympathetic fibers traveling with CN III
• Diplopia: Result of ocular misalignment from extraocular muscle dysfunction

Critical Distinction: Pupil-Involving vs. Pupil-Sparing

This is a pupil-involving third nerve palsy, which is a medical emergency 1, 2. The presence of pupillary involvement (dilated, poorly reactive pupil) suggests external compression of the oculomotor nerve rather than microvascular ischemia 1.

Urgent Differential Considerations:

The primary concern is vascular compression from a posterior communicating artery aneurysm, which requires immediate exclusion 2. Other compressive etiologies include:

• Tumors (meningioma, schwannoma, metastatic lesions)
• Trauma
• Subarachnoid hemorrhage
• Uncal herniation

Why Pupil Involvement Matters:

Parasympathetic fibers traveling with CN III are located peripherally in the nerve bundle, making them vulnerable to external compression. In contrast, pupil-sparing third nerve palsies typically result from microvascular ischemia (diabetes, hypertension) affecting the central nerve fibers while sparing peripheral parasympathetic fibers 1, 2.

Immediate Management Algorithm

Neuroimaging with vascular assessment is mandatory 2:

1. First-line: MRI brain with gadolinium + MRA, OR CT angiography (CTA) 2
2. If aneurysm suspected but MRA/CTA negative: Proceed to catheter angiography (DSA) after brain MRI with specific attention to the third nerve 2
3. If imaging negative: Consider serologic testing (syphilis, Lyme) and lumbar puncture (glucose, protein, cell count, cytology, culture) 2

Why Not the Other Nerves?

• CN II (Optic): Would cause vision loss, not diplopia or ptosis
• CN IV (Trochlear): Causes vertical diplopia worse on downgaze and head tilt, but no ptosis or pupillary changes
• CN VI (Abducens): Causes horizontal diplopia with inability to abduct the eye laterally ("inward" deviation), no ptosis or pupillary involvement

Common Pitfall

Do not assume microvascular etiology based solely on patient age or diabetes history when the pupil is involved 2. While vasculopathic third nerve palsies can occasionally have mild pupillary involvement, a 7 mm poorly reactive pupil represents significant parasympathetic dysfunction and mandates urgent vascular imaging to exclude aneurysm 2.