Neuro-ophthalmology: Pupils, CN Palsies & Papilledema
A high-yield, boards-focused neuro-ophthalmology lesson covering pupillary abnormalities (RAPD, Horner, Adie, Argyll Robertson), CN III/IV/VI palsies, and papilledema/IIH, organized around localization, buzzwords, and next-best-step decisions. Emphasizes the exam-critical fork of pupil-involving (aneurysm) vs pupil-sparing (microvascular) CN III palsy — noting that pain does not distinguish them — and the image-before-LP rule for raised ICP.
Why this section is free points
Neuro-ophthalmology on the boards reduces to three testable buckets: pupillary abnormalities, ocular motor cranial nerve palsies (CN III, IV, VI), and optic disc edema. The one skill that unlocks all of them is localization. Pupils separate an afferent problem (optic nerve/retina — think RAPD) from an efferent one (sympathetic → Horner; parasympathetic → a blown pupil). For the ocular motor nerves, the pattern of misalignment plus a single pupil finding tells you whether a CN III palsy is a benign microvascular infarct or a compressive aneurysm about to rupture. And papilledema — bilateral disc swelling from raised intracranial pressure — is an emergency until neuroimaging says otherwise. Learn the buzzwords (down-and-out, light-near dissociation, anisocoria worse in the dark) and the reflexive next-best-step, and these questions answer themselves.
- RAPD (Marcus Gunn pupil): afferent lesion — optic neuritis, ischemic optic neuropathy, large retinal detachment/CRAO. Swinging-flashlight test: the affected pupil paradoxically dilates when light swings to it. No resting anisocoria (efferent path intact).
- Horner syndrome: partial ptosis, miosis, anhidrosis; anisocoria worse in the DARK. Painful + arm/hand findings → Pancoast tumor; sudden + ipsilateral neck pain/headache → carotid dissection.
- Adie tonic pupil: large pupil, poor light reaction, slow tonic near response (light-near dissociation). Constricts to dilute 0.1% pilocarpine (denervation supersensitivity). Young women; ± absent DTRs = Holmes-Adie.
- Argyll Robertson pupil: small, irregular, accommodates but does not react to light (light-near dissociation) → neurosyphilis ("prostitute's pupil").
- Blown pupil + ptosis + down-and-out eye: pupil-involving CN III palsy = compressive (PCoM aneurysm / uncal herniation) until proven otherwise.
Ocular motor cranial nerve palsies
| Feature | CN III (oculomotor) | CN IV (trochlear) | CN VI (abducens) |
|---|---|---|---|
| Muscle(s) | All EOMs except SO & LR; levator; pupil sphincter | Superior oblique | Lateral rectus |
| Resting eye | Down and out, ptosis, ± mydriasis | Slight hypertropia (affected eye higher) | Esotropia (adducted) |
| Diplopia | Variable | Vertical, worse on downgaze (stairs, reading) | Horizontal, worse at distance / on gaze toward lesion |
| Classic clue | Pupil-involving = aneurysm; pupil-sparing = diabetic microvascular | Compensatory head tilt away from lesion; Bielschowsky + (tilt toward worsens hypertropia); trauma | Long intracranial course → false localizing sign in ↑ICP |
A 55-year-old woman has sudden severe headache, right ptosis, and the right eye resting "down and out" with a fixed, dilated 7 mm pupil.
- Diagnosis: compressive CN III palsy from a posterior communicating artery (PCoM) aneurysm — parasympathetic fibers travel on the nerve's periphery/surface, so external compression hits the pupil first.
- Next best step: urgent CT angiography (or MRA) of the head/circle of Willis. This is a neurosurgical emergency (impending subarachnoid hemorrhage), not "watch and wait."
Contrast: a 60-year-old diabetic/hypertensive with a pupil-sparing, complete CN III palsy → microvascular ischemic palsy (core fibers infarct via vasa nervorum; peripheral pupil fibers spared by their pial blood supply). Control glucose/BP and observe; near-full recovery in ~3 months.
Key caveat: microvascular palsies are often painful too, so pain does NOT distinguish them — the pupil does. Image for any pupil involvement, incomplete/partial pupil-sparing, young age or no vascular risk factors, additional neuro signs, or no recovery by ~3 months.
An obese 28-year-old woman reports daily headaches worse in the morning and with Valsalva, brief transient visual obscurations, pulsatile tinnitus, and horizontal diplopia. Exam: bilateral papilledema with a CN VI palsy (false localizing). Visual acuity is normal; blind spots are enlarged. She takes doxycycline for acne.
- Diagnosis: idiopathic intracranial hypertension (pseudotumor cerebri).
- Next best step: MRI brain + MR venography FIRST — exclude a mass, hydrocephalus, and cerebral venous sinus thrombosis before LP. Then LP shows elevated opening pressure >250 mm H2O with normal CSF composition.
- Management: stop the offending drug, weight loss, acetazolamide (↓ CSF production). Progressive vision loss → optic nerve sheath fenestration (protects vision) or CSF diversion/shunt.
- Papilledema = BILATERAL disc edema from raised ICP. Acuity and color vision are preserved early; the danger is chronic edema → optic atrophy and permanent loss.
- Rule: neuroimaging before LP whenever a mass lesion is possible (herniation risk).
- Papillitis (anterior optic neuritis): usually unilateral, ↓ acuity, pain on eye movement, RAPD, red desaturation, central scotoma; MS association. Treat with IV methylprednisolone (per ONTT — speeds recovery; oral prednisone alone is contraindicated — it increases recurrence).
- Pseudopapilledema (optic disc drusen) mimics true swelling but has no ↑ICP and no obscured vessels/hyperemia.
- Key discriminator: papilledema does NOT cause acute vision loss or an RAPD — if either is present, think optic neuropathy instead.
- "SO4, LR6, all others 3": Superior Oblique = CN IV, Lateral Rectus = CN VI, all remaining extraocular muscles = CN III.
- "Down and out": the resting eye in a CN III palsy — the unopposed LR (CN VI) and SO (CN IV) pull the globe down and abducted.
- Horner = "PAM is horny": Ptosis, Anhidrosis, Miosis.
- Argyll Robertson = "Accommodation Reflex Present, Pupillary Reflex Absent" → neurosyphilis.
- Light-near dissociation trio: Argyll Robertson, Adie tonic pupil, and dorsal midbrain (Parinaud) syndrome.
Anisocoria: light vs dark + pupil pharmacology
| Test / question | Finding | Interpretation |
|---|---|---|
| Worse in the dark? | Abnormal pupil is the small one | Sympathetic lesion → Horner |
| Worse in the light? | Abnormal pupil is the large one | Parasympathetic (CN III) or Adie |
| Apraclonidine | Horner pupil dilates, reversing anisocoria | Confirms Horner (cocaine test: Horner pupil fails to dilate) |
| Hydroxyamphetamine | Dilates → central/preganglionic; fails → postganglionic | Localizes Horner (3rd-order) |
| 0.1% pilocarpine | Constricts the large pupil | Adie (denervation supersensitivity) |
| 1% pilocarpine | No constriction | Pharmacologic dilation (atropine); a CN III pupil would constrict |
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