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Oculomotor (CN III) Nerve Palsy
Overview
The oculomotor nerve (CN III) is the workhorse motor nerve of the orbit. It innervates four of the six extraocular muscles (superior rectus, medial rectus, inferior rectus, and inferior oblique) plus the levator palpebrae superioris, which it elevates via its superior division. It also carries the parasympathetic fibers responsible for pupillary constriction and accommodation; these synapse at the ciliary ganglion after leaving the nerve. The oculomotor motor nucleus sits in the midbrain, rostral to the CN IV nuclear complex, at the level of the superior colliculus.
Because CN III supplies so much of the eye's motor and autonomic function, a complete palsy is one of the most recognizable findings in ophthalmology. It is also one of the highest-stakes: the differential includes a posterior communicating artery (PCoA) aneurysm, which can be fatal if it ruptures.
Clinical Features
A complete CN III palsy leaves only the two muscles it does not innervate still functioning: the lateral rectus (CN VI) and the superior oblique (CN IV). With every adducting, elevating, and depressing muscle paralyzed, the unopposed pull of these two muscles rotates the eye into the classic "down and out" position: abducted by the lateral rectus, depressed and intorted by the superior oblique. Complete ptosis from levator paralysis usually accompanies this and can mask the diplopia until the lid is lifted.
Pupil-Involving vs. Pupil-Sparing: Why It Matters
The parasympathetic pupillomotor fibers travel along the outer, superficial surface of the oculomotor nerve for most of its intracranial course, while the fibers supplying the extraocular muscles run through the nerve's central core, which is nourished by small penetrating vessels from within.
This anatomic arrangement explains the clinical split between the two major causes of acquired CN III palsy. Ischemic (microvascular) third nerve palsy, typically seen in older patients with vascular risk factors such as diabetes and hypertension, damages the nerve's internal microvasculature first and spares the superficial pupillomotor fibers; the pupil stays normal-sized and reactive, hence "pupil-sparing." Compressive third nerve palsy works the opposite way. An external mass, most classically a posterior communicating artery (PCoA) aneurysm at its junction with the internal carotid artery, presses on the nerve from outside and compresses the superficial fibers first. The pupil becomes dilated and poorly reactive early, often before the motility deficit is even complete, earning this presentation the label "surgical third" nerve palsy.
An acute CN III palsy with a dilated, non-reactive pupil in any patient, especially if abrupt in onset, painful, or accompanied by a severe headache, is a neurosurgical emergency until proven otherwise. Urgent neuroimaging (CT/MR angiography, with catheter angiography if non-invasive imaging is equivocal) to exclude a PCoA aneurysm cannot wait for outpatient workup. Do not reassure a patient with a pupil-involving third nerve palsy that it's "probably just diabetic": that reasoning only applies once a compressive lesion has been actively excluded.
Subnuclear & Fascicular Anatomy
The oculomotor nuclear complex in the midbrain is not a single homogeneous pool of motor neurons. It is organized into distinct subnuclei, each supplying one specific muscle, with two exceptions relevant to localizing a nuclear-level lesion (see brainstem nuclear localization for how this fits into the broader cranial nerve map). The Edinger-Westphal nucleus is the parasympathetic component of the complex: it sends preganglionic fibers via CN III to the ciliary ganglion, which in turn supplies the iris sphincter for pupillary constriction and the ciliary muscle for accommodation (see efferent/autonomic pupillary disorders for what happens when this pathway fails). A single central caudal subnucleus, by contrast, provides bilateral innervation to both levator palpebrae superioris muscles. This is a textbook example of Hering's law of equal innervation operating within one nuclear structure, and it explains why a unilateral nuclear CN III lesion can still produce bilateral ptosis. The superior rectus subnucleus is likewise unusual: its fibers decussate to innervate the contralateral superior rectus, unlike every other subnucleus in the complex, which projects ipsilaterally.
Aberrant Regeneration
After a compressive (though not typically ischemic) CN III palsy, damaged axons can regenerate along the wrong pathways and reinnervate the wrong muscle, a phenomenon called aberrant regeneration. The classic finding is paradoxical eyelid retraction on downgaze: fibers originally destined for the inferior rectus, a depressor, misroute into the levator instead, so attempting to look down actually elevates the lid rather than letting it follow the eye. Other patterns include pupillary constriction on attempted adduction, from misrouted fibers reaching the pupillary sphincter. Aberrant regeneration almost never follows a true ischemic microvascular palsy. Its presence points back to a compressive, not vascular, cause.
- Pupillomotor fibers run superficially on CN III: compressive lesions (PCoA aneurysm) hit them first ("surgical third"); ischemic lesions spare them (pupil-sparing).
- A pupil-involving acute CN III palsy = urgent neuroimaging to exclude a PCoA aneurysm, full stop.
- Complete CN III palsy → eye rests "down and out" (unopposed lateral rectus + superior oblique) with ptosis.
- Aberrant regeneration (e.g., lid retraction on downgaze) points to a prior compressive, not ischemic, insult.
- The superior rectus subnucleus is the only one that innervates the contralateral eye; the central caudal subnucleus innervates both levators.
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