STEPTRACKERSPECIAL SENSES + NERVOUS SYSTEM | ANATOMY + PATHOLOGY | HIGH-YIELD REVIEW

Optic Nerve, Visual Pathway & Pupils

Localize a visual field defect along the pathway, read the swollen or pale disc, and decode abnormal pupils and eye movements.

FOCUS ON

Field defect → site: one eye = optic nerve; bitemporal = chiasm (pituitary); homonymous = behind the chiasm; superior quadrant = temporal (Meyer loop); inferior = parietal; macular sparing = occipital (PCA). Pupils: RAPD (optic nerve), Horner (sympathetic), CN III (blown pupil), Argyll Robertson (syphilis), Adie (tonic).

LESION · SIGN

SITE · MECHANISM

FINDINGS

CLUE

Optic neuritis

Demyelination of the optic nerve; often MS, also NMO

Painful monocular vision loss worse with eye movement, ↓ color vision (red desaturation), RAPD; disc normal or swollen

IV steroids speed recovery · MRI brain

Papilledema

↑ intracranial pressure transmitted along optic nerve sheath

Bilateral disc swelling, blurred margins, venous engorgement; vision preserved early, enlarged blind spot

Tumor, IIH, hemorrhage, hypertensive emergency · image before LP

Ischemic optic neuropathy

Arteritic (GCA) or nonarteritic (small-vessel disease, "disc at risk")

Sudden painless monocular loss, pale swollen disc, altitudinal defect

GCA: ESR ↑, steroids now

Optic nerve lesion

One optic nerve (before chiasm)

Ipsilateral monocular blindness

Plus RAPD

Chiasm lesion

Crossing nasal fibers: pituitary adenoma, craniopharyngioma, ACom aneurysm

Bitemporal hemianopia

Pituitary: superior quadrants first

Optic tract / radiation lesions

Behind the chiasm (stroke, tumor)

Contralateral homonymous hemianopia · Meyer loop (temporal) → superior quadrantanopia · parietal → inferior

"Pie in the sky" = temporal

Occipital cortex lesion

PCA stroke

Contralateral homonymous hemianopia with macular sparing (MCA collaterals)

Bilateral → cortical blindness

Relative afferent pupillary defect

Asymmetric optic nerve/retina input

Swinging light: affected pupil dilates when light moves onto it

Marcus Gunn pupil

Horner syndrome

Sympathetic chain lesion: Pancoast tumor, carotid dissection, lateral medulla, cervical cord above T1

Ptosis (mild), miosis, anhidrosis; dilation lag

Neck pain + Horner → carotid dissection

CN III palsy

Compressive (PCom aneurysm, uncal herniation) vs ischemic (diabetes)

Eye down and out, ptosis; compressive → dilated pupil; ischemic → pupil spared

Parasympathetic fibers run on the outside

CN IV and CN VI palsy

CN IV: trauma (long course) · CN VI: ↑ ICP (long intracranial course), diabetes

CN IV: vertical diplopia on looking down/in (stairs), head tilts away · CN VI: eye can't abduct, horizontal diplopia

CN VI palsy = false localizing sign

Argyll Robertson & Adie pupils

AR: neurosyphilis (dorsal midbrain) · Adie: ciliary ganglion damage

AR: small irregular pupils, accommodate but don't react to light · Adie: large tonic pupil, slow constriction, ↓ reflexes

"Prostitute's pupil" · Adie → pilocarpine supersensitivity

Picture it — visual field defects (red = lost field, as the patient sees it)

Left optic nerveLeft eye blind.

Optic chiasmBitemporal hemianopia.

Left optic tractRight homonymous hemianopia.

Left Meyer loop (temporal)Right upper quadrantanopia — "pie in the sky".

Left parietal radiationRight lower quadrantanopia.

Left occipital cortex (PCA)Right homonymous hemianopia with macular sparing.

Schematic diagrams, not to scale. Red = abnormal.

Mechanism — the visual pathway

RetinaNasal fibers carry temporal field

→

Optic nerveMonocular loss

→

ChiasmNasal fibers cross → bitemporal

→

Optic tract → LGNHomonymous

→

RadiationsTemporal (upper field) · parietal (lower field)

→

Occipital cortexMacular sparing

Light reflex: retina → optic nerve → pretectal nuclei → both Edinger-Westphal nuclei → CN III → both pupils constrict.

Abnormal pupils

PUPIL

FINDING

CAUSE

Marcus Gunn (RAPD)

Paradoxical dilation on swinging light

Optic neuritis, severe retinal disease

Horner

Small, with ptosis and anhidrosis

Sympathetic lesion

CN III (compressive)

Large, nonreactive, eye down and out

PCom aneurysm, herniation

Argyll Robertson

Small; light-near dissociation

Neurosyphilis

Adie tonic

Large, slow, poor light reaction

Ciliary ganglion (young women)

Pinpoint, bilateral

Very small

Opioids, pontine hemorrhage, organophosphates

How it's tested

COMMON CONFUSIONPapilledema is bilateral with preserved acuity early; optic neuritis is usually unilateral with early, painful vision loss and an RAPD.

THE TRAPA painful CN III palsy with a dilated pupil is an aneurysm until proven otherwise — urgent vascular imaging, not "diabetic neuropathy."

YOUR APPROACHMonocular or binocular field loss → which half → congruous or not → lesion site → pupil and movement findings → cause (MS, tumor, stroke, aneurysm, syphilis).

If you see… think…

Young woman, painful vision loss in one eye, red desaturation → optic neuritis (MS)

Bilateral swollen discs with headache worse lying down → papilledema

Bitemporal hemianopia → pituitary adenoma at the chiasm

Contralateral superior quadrantanopia → temporal lobe (Meyer loop)

Homonymous hemianopia with macular sparing → PCA stroke

Pupil dilates when light swings onto it → RAPD (optic nerve)

Ptosis, miosis, anhidrosis + shoulder pain in a smoker → Horner from Pancoast tumor

Eye down and out with a blown pupil → compressive CN III palsy

Can't walk down stairs, head tilted → CN IV palsy

Pupils accommodate but don't react to light → Argyll Robertson (neurosyphilis)