A 68-year-old woman from Mumbai presents with acute-onset diplopia, ptosis of the right eyelid, and inability to adduct the right eye. On examination, the pupil on the right side is dilated and non-reactive to light. The left eye and all other cranial nerve functions are intact. Which cranial nerve is most likely affected?

Explanation

## Diagnosis: CN III (Oculomotor) Palsy with Pupillary Involvement ### Clinical Features Matching CN III Involvement **Key Point:** The oculomotor nerve (CN III) innervates four of the six extraocular muscles (medial rectus, superior rectus, inferior rectus, inferior oblique), the levator palpebrae superioris (eyelid elevation), and parasympathetic fibers to the pupil and ciliary muscle. ### Anatomical Basis of Presentation The patient's constellation of findings is diagnostic of CN III palsy: | Feature | CN III Involvement | Mechanism | |---------|-------------------|----------| | Ptosis (drooping eyelid) | Levator palpebrae paralysis | CN III innervates levator palpebrae superioris | | Diplopia (double vision) | Extraocular muscle paralysis | CN III innervates 4 of 6 EOM | | Inability to adduct eye | Medial rectus paralysis | CN III innervates medial rectus | | Dilated, non-reactive pupil | Parasympathetic fiber loss | CN III carries preganglionic parasympathetic fibers | | "Down and out" position | Unopposed action of CN IV & VI | Superior oblique (CN IV) and lateral rectus (CN VI) still functional | **Clinical Pearl:** The **"down and out" eye position** (eye deviated inferiorly and laterally) is the classic resting position in CN III palsy because the unopposed actions of the superior oblique (CN IV) and lateral rectus (CN VI) muscles push the eye downward and outward. ### CN III: Functional Components **Mnemonic: SOMI-P** (CN III innervation) - **S**uperior rectus - **O**culomotor (medial rectus) - **M**edial rectus - **I**nferior rectus - **I**nferior oblique - **P**upil (parasympathetic) and **P**alpebra (levator palpebrae) **High-Yield:** Pupillary involvement in CN III palsy indicates a **complete CN III lesion** (typically compressive, such as from aneurysm or tumor). **Pupil-sparing CN III palsy** (medical CN III palsy from diabetes, hypertension, vasculitis) presents with eye movement abnormalities but normal pupil reactivity — this distinction is critical for determining urgency of imaging. ### Differential: CN III vs CN IV vs CN VI | Feature | CN III | CN IV | CN VI | |---------|--------|-------|-------| | Ptosis | Yes (levator paralysis) | No | No | | Pupil dilation | Yes (parasympathetic loss) | No | No | | Medial rectus paralysis | Yes (cannot adduct) | No | No | | Superior oblique paralysis | No | Yes (intorsion loss) | No | | Lateral rectus paralysis | No | No | Yes (cannot abduct) | | Vertical diplopia | No | Yes (head tilt sign) | No | **Warning:** Do not confuse CN III palsy with CN VI palsy. CN VI causes inability to **abduct** the eye (lateral rectus paralysis), whereas CN III causes inability to **adduct** the eye (medial rectus paralysis). ```mermaid flowchart TD A[Acute diplopia + ptosis + pupil dilation]:::outcome --> B{Pupil involved?}:::decision B -->|Yes, dilated & non-reactive| C[CN III with parasympathetic involvement]:::action B -->|No, pupil normal| D[Pupil-sparing CN III palsy]:::action C --> E{Compressive lesion likely?}:::decision E -->|Yes - aneurysm, tumor| F[Urgent imaging: CT/MRI + CTA]:::urgent E -->|No - medical cause| G[Diabetes, HTN, vasculitis]:::action A --> H{Eye position?}:::decision H -->|Down and out| I[CN III palsy confirmed]:::outcome H -->|Cannot abduct| J[CN VI palsy]:::outcome H -->|Intorsion loss| K[CN IV palsy]:::outcome ```