A 52-year-old man presents with acute-onset diplopia, ptosis, and pupillary dilation on the left. Examination shows the left eye is 'down and out'. Which single finding best distinguishes this oculomotor nerve (CN III) lesion from an abducens nerve (CN VI) lesion?

Explanation

## Distinguishing CN III from CN VI Lesions ### Anatomical Basis **Key Point:** CN III innervates four extraocular muscles (medial rectus, superior rectus, inferior rectus, inferior oblique) PLUS the levator palpebrae and pupillary sphincter. CN VI innervates only the lateral rectus and has no parasympathetic fibers. ### Clinical Feature Comparison | Feature | CN III Lesion | CN VI Lesion | |---------|---------------|-------------| | **Eye position** | Down and out | Abducted (lateral) | | **Adduction** | **Lost** | Intact | | **Ptosis** | **Present** | Absent | | **Pupil** | **Dilated (mydriasis)** | Normal | | **Vertical gaze** | **Impaired** | Intact | | **Lateral rectus** | Intact | **Paralyzed** | ### Why Pupillary Involvement Distinguishes CN III **High-Yield:** Pupillary dilation (mydriasis) with motor deficit is pathognomonic for CN III lesion because: 1. CN III carries preganglionic parasympathetic fibers to the ciliary ganglion 2. These fibers innervate the pupillary sphincter muscle 3. CN VI has NO parasympathetic fibers — pupil is always normal in CN VI lesion 4. A dilated pupil + ophthalmoplegia = CN III until proven otherwise **Clinical Pearl:** In CN III palsy, the pupil is typically dilated and fixed to light because parasympathetic fibers (which are superficial in the nerve) are often affected first. This is especially true in compressive lesions (e.g., posterior communicating artery aneurysm). **Warning:** A CN III palsy with a normal pupil suggests a microvascular (medical) cause rather than a compressive lesion. Always ask: "Is the pupil involved?" — this single question narrows the differential dramatically. ### Why Other Options Are Misleading - **Preserved adduction** — adduction is LOST in CN III, not preserved - **Inability to depress the eye** — occurs in CN III (inferior rectus paralysis), but also can occur with CN IV lesion (superior oblique); not specific - **Intact lateral rectus function** — lateral rectus is intact in CN III but paralyzed in CN VI; this is true but less specific than pupil involvement **Mnemonic:** **CN III = 3 + 3**: 3 rectus muscles (medial, superior, inferior) + 3 other functions (levator, pupil, inferior oblique)