The three nerves that move the eye are inseparable in practice. Together they orchestrate one of the most elegant systems in the body — a real-time tracking, aiming, and depth-judging apparatus that runs continuously, mostly beneath conscious awareness, and whose failure modes are diagnostic gold. Few parts of the neurological examination yield as much localizing information per minute as a careful look at the eyes: the pupils, the lids, and the way the globes move when the patient follows your finger. A pupil that will not constrict, a gaze that cannot cross midline, an eyelid that droops only on lateral gaze — each is the nervous system telling you exactly where the lesion is.

The oculomotor (III), trochlear (IV), and abducens (VI) nerves share an examination because their work is shared. You cannot test one without recruiting the others. The trick is to know which nerve is asked to do what in any given position of gaze, and then to read the eye’s behavior as a report on each nerve in turn.

The Three Nerves at a Glance

Three peculiarities are worth remembering, because each one shapes how disease presents:

• CN IV is the only cranial nerve that decussates before exit. The right superior oblique is innervated by the left trochlear nucleus. A nuclear lesion therefore weakens the contralateral eye.
• CN IV is the only cranial nerve that exits dorsally. It has the longest intracranial course of any nerve, which is why head trauma — even mild — is the leading acquired cause of trochlear palsy.
• CN VI runs a long course over the petrous ridge and through the cavernous sinus. Anything that raises intracranial pressure can stretch it, making sixth-nerve palsy a classic false localizing sign.

The Pupillary Examination

Of all the maneuvers in the ocular exam, the pupillary light reflex is the most informative — and the most often performed sloppily. The pathway is short, hard-wired, and crosses cleanly enough that almost every abnormality can be localized to a precise segment of the loop.

Anatomy of the Light Reflex

The afferent limb begins at the retina. Light → retinal ganglion cells → optic nerve → optic chiasm → optic tract. Unlike the rest of the visual pathway, the fibers carrying pupillary information leave the tract before the lateral geniculate, synapsing instead in the pretectal nucleus of the dorsal midbrain. From the pretectal nucleus, fibers project bilaterally (via the posterior commissure) to both Edinger-Westphal nuclei — the parasympathetic component of CN III. This bilateral relay is why a light shone in one eye constricts both pupils: the direct response (same eye) and the consensual response (other eye).

The efferent limb is the parasympathetic outflow of CN III. Preganglionic fibers travel on the outer surface of the third nerve to the ciliary ganglion in the orbit, where they synapse. Postganglionic fibers (the short ciliary nerves) then reach the iris sphincter. The outer course of these pupil fibers along CN III is a clinical fact of enormous consequence: it makes them vulnerable to compression (aneurysm, uncal herniation) and relatively resistant to ischemia (microvascular diabetic palsy). This anatomy underlies the entire “pupil-involving vs pupil-sparing” rule discussed below.

How to Examine the Pupils

Use a bright, focused light in a dimly lit room. Record three things for each pupil: size in dim light, size in bright light, and reactivity. Then perform the swinging flashlight test (described below) to look for an afferent defect, and finally test the near response. The whole exam takes about ninety seconds. Documentation should include both pupil sizes (e.g., “OD 4→2 mm, OS 4→2 mm, brisk, no RAPD”) rather than the unhelpful “PERRL.”

🔍 Did You Know?

Physiologic anisocoria — a benign asymmetry of pupil size — is present in about 20% of normal adults. The difference is usually ≤0.4 mm and, critically, is the same in light and in dark. If the anisocoria magnitude changes with ambient light, the pupils are not behaving normally.

Anisocoria — A Two-Question Workflow

Confronted with unequal pupils, ask two questions in order:

1. Which pupil is abnormal? Compare the anisocoria in bright light versus dim light. If the difference is greater in the light, the larger pupil is failing to constrict — the problem is in the parasympathetic outflow (CN III, ciliary ganglion, or iris). If the difference is greater in the dark, the smaller pupil is failing to dilate — the problem is in the sympathetic chain (Horner). If the anisocoria is equal in light and dark, it is physiologic.
2. Are there accompanying findings that localize? A ptotic lid with the small pupil → Horner. A ptotic lid with the large pupil plus diplopia → CN III. A pupil that constricts poorly to light but well to near → light-near dissociation (Argyll-Robertson, Parinaud, Adie).

The Relative Afferent Pupillary Defect (RAPD, “Marcus Gunn pupil”)

The swinging flashlight test is the single most sensitive test in clinical neurology for a lesion in front of the lateral geniculate. Hold the light steadily on the right eye for two to three seconds, then swing it briskly to the left eye and back. Both pupils should constrict equally when either eye is illuminated; the consensual response keeps the unlit pupil at the same diameter as the lit one. When the test eye has an afferent defect — less light reaches the brain — its pupils (both of them) constrict less briskly. The clinical sign is therefore a paradoxical dilation of the pupil on the diseased side when the light swings to it, because the consensual constriction is suddenly weaker than the dilation forced by removing direct illumination from the healthy side.

An RAPD localizes anywhere from the retina through the optic tract. It is preserved even when the pupil itself is abnormal — the test compares the relative afferent input from the two eyes, and the efferent limb is identical for both. The most common causes are optic neuritis, non-arteritic anterior ischemic optic neuropathy, retinal detachment, and dense vitreous hemorrhage. Asymmetric papilledema and severe glaucoma can also produce a measurable RAPD.

Light-Near Dissociation

A pupil that constricts poorly to light but well to a near target has light-near dissociation. Because the near response uses a separate (more ventral) midbrain circuit, this pattern points to a lesion that disrupts the dorsal pretectal pathway while sparing the more ventral near pathway. The classic causes are:

• Argyll-Robertson pupils — small, irregular, no light response, brisk to near, classically bilateral. The traditional association is with tertiary neurosyphilis, but diabetes, sarcoid, and other midbrain lesions can produce the same pattern.
• Parinaud’s syndrome — light-near dissociation accompanies upgaze palsy, lid retraction, and convergence-retraction nystagmus from a dorsal midbrain lesion (pineal tumor, midbrain infarct, multiple sclerosis plaque).
• Adie’s tonic pupil (see below) — light response is sluggish; the near response, when present, is even slower but tonic.
• Severe optic nerve disease — when light input is profoundly reduced bilaterally, the light response disappears while the proprioceptive-driven near response can persist.

Adie’s Tonic Pupil

Adie’s pupil is a postganglionic parasympathetic lesion — most often at the ciliary ganglion itself. The affected pupil is large, with a sluggish or absent light reaction. The near response, when patiently elicited, is slow and tonic, and the redilation that follows is even slower. Sector palsies of the iris produce the characteristic “vermiform” movements seen under the slit lamp.

The diagnosis is confirmed pharmacologically. Because postganglionic denervation produces receptor supersensitivity, the affected pupil constricts to dilute pilocarpine (0.1% or 0.125%) — concentrations far too weak to affect a normal pupil. Adie’s syndrome (Holmes-Adie) adds depressed deep tendon reflexes, classically at the knees and ankles. It is benign, more common in young women, and the only treatment usually needed is reassurance.

Horner’s Syndrome

The triad — ptosis, miosis, and ipsilateral anhidrosis — reflects loss of the sympathetic supply to the eye and face. The ptosis is partial (the sympathetically innervated Müller’s muscle elevates the lid by only about 2 mm; the levator, supplied by CN III, does the heavy lifting). The anhidrosis is variable and depends on where in the three-neuron sympathetic chain the lesion sits.

The pathway has three neurons. First-order neurons descend from the hypothalamus through the lateral brainstem to the ciliospinal center of Budge-Waller in the C8-T2 intermediolateral cell column. Second-order (preganglionic) fibers leave the cord, pass over the lung apex and the subclavian artery, and ascend to the superior cervical ganglion. Third-order (postganglionic) fibers ride the internal carotid artery into the cavernous sinus, joining CN V₁ to reach the orbit. Sweat fibers for the face follow the external carotid; lesions distal to the bifurcation therefore spare facial sweating.

Pharmacological testing helps localize the level:

• Cocaine 4-10% dilates a normal pupil by blocking norepinephrine reuptake. A Horner pupil — at any level — fails to dilate because there is no norepinephrine being released to begin with. This confirms Horner but does not localize it.
• Hydroxyamphetamine 1% releases stored norepinephrine from the postganglionic terminal. A first- or second-order lesion (with intact postganglionic neuron) dilates; a third-order (postganglionic) lesion does not. This localizes the level.
• Apraclonidine 0.5-1% — a weak α1 agonist — has become the practical bedside test because it is widely available. The Horner pupil shows denervation supersensitivity and dilates; the normal pupil is unaffected. Apraclonidine reverses the anisocoria, which can be visually dramatic.

The causes vary by level. First-order: brainstem stroke (Wallenberg), syringobulbia, demyelination. Second-order: Pancoast tumor, chest or neck surgery, neuroblastoma in children. Third-order: carotid dissection (a painful Horner is dissection until proven otherwise), cavernous sinus pathology, cluster headache.

The Examination of Eye Movements

The Six Cardinal Positions (the “H” Pattern)

Ask the patient to follow your finger as you trace a wide, slow H in the air. The six end positions — far right, right-up, right-down, far left, left-up, left-down — each isolate one or two muscles. In primary gaze, multiple muscles share the work; only at the extremes does a single muscle bear the full load. That is why we look at extremes.

• Lateral gaze tests the lateral rectus (CN VI) of the abducting eye and the medial rectus (CN III) of the adducting eye.
• Up-and-out isolates the superior rectus (CN III).
• Down-and-out isolates the inferior rectus (CN III).
• Up-and-in isolates the inferior oblique (CN III).
• Down-and-in isolates the superior oblique (CN IV). This is the position in which a fourth-nerve palsy produces its maximal vertical diplopia.

Saccades versus Smooth Pursuit

The brain generates two fundamentally different eye movements, and they can fail independently because they use different circuits. Saccades — fast, ballistic shifts of fixation — are driven by the frontal eye fields (contralateral) and the superior colliculus, with the burst generators in the paramedian pontine reticular formation (PPRF, for horizontal) and the rostral interstitial nucleus of the MLF (for vertical). Smooth pursuit — keeping a moving target on the fovea — is driven by parieto-occipital cortex and the cerebellum. Test saccades by asking the patient to alternate gaze between two targets (“look at my finger… now my nose… now my finger”). Test smooth pursuit by having them follow a slowly moving target.

The patterns are useful localizers. Slow saccades point to brainstem disease (PSP, Huntington’s, spinocerebellar ataxias, brainstem stroke). Saccadic dysmetria — overshoot or undershoot followed by a corrective saccade — points to the cerebellum. Saccadic pursuit — a target tracked in a series of small catch-up movements rather than smoothly — is nonspecific but exaggerated in cerebellar and basal-ganglia disorders.

Vergence

Convergence is part of the near triad (convergence + accommodation + miosis). Test it by bringing your finger from 50 cm to the bridge of the patient’s nose; both eyes should adduct and both pupils should constrict. A patient who cannot adduct on lateral gaze but converges normally has an internuclear ophthalmoplegia rather than a medial rectus palsy — a clinical pearl with enormous localizing weight.

Patterns of Cranial Nerve Palsy

Third-Nerve Palsy

A complete CN III palsy gives a strikingly recognizable picture: the eye is fixed “down and out” (unopposed superior oblique and lateral rectus), the lid is ptotic (loss of levator), and the pupil is dilated and unreactive to light (loss of parasympathetic supply). The patient sees diplopia in every direction except the field of the paretic eye.

The pupil distinction is the single most important fact in the assessment of a third-nerve palsy. As noted above, the pupillary fibers travel on the outer surface of CN III. A compressive lesion — most classically a posterior communicating artery aneurysm, or uncal herniation from a mass — pushes on the outside of the nerve first and produces a pupil-involving palsy. A microvascular lesion — diabetic or hypertensive small-vessel ischemia — strikes the core of the nerve and spares the surface fibers, producing a pupil-sparing palsy. The rule applies most strictly to complete palsies of acute onset: any patient with a pupil-involving third nerve palsy should be imaged urgently (CTA or MRA) to exclude an aneurysm. The converse rule has exceptions, and pupil-sparing palsies that fail to resolve in three months also deserve imaging.

Fourth-Nerve Palsy

The superior oblique depresses and intorts the eye. Its dominant clinical action is most obvious when the eye is adducted — try to look down at a book on the lap while the eye is turned in toward the nose, and the superior oblique is doing all the work. Vertical diplopia worse on down-and-in gaze is therefore the cardinal complaint. Patients spontaneously tilt the head away from the affected side (because head tilt toward the affected side would require intorsion the paretic eye cannot perform, increasing the diplopia).

The Parks-Bielschowsky three-step test formalizes the diagnosis at the bedside:

1. Step 1: Which eye is higher in primary gaze? In a right CN IV palsy, the right eye is higher (hypertropia of the affected eye).
2. Step 2: Is the hypertropia worse on left gaze or right gaze? In a right CN IV palsy, the right hypertropia worsens on left gaze (because the right eye is adducted and the superior oblique is being asked to do the depression).
3. Step 3: Is the hypertropia worse with the head tilted right or left? In a right CN IV palsy, tilting the head to the right makes the hypertropia worse (intorsion is required, which the paretic muscle cannot supply).

Trauma — sometimes minor — is the most common acquired cause, reflecting the trochlear nerve’s long dorsal course. A congenital or “decompensated congenital” trochlear palsy is common in adults presenting in middle age, and old photographs often reveal a longstanding subtle head tilt.

Sixth-Nerve Palsy

Loss of lateral rectus function gives horizontal diplopia, worse on gaze toward the affected side, with the eye failing to abduct past midline at rest. Of the three ocular motor palsies, CN VI is the most likely to be a false localizing sign: raised intracranial pressure can stretch the nerve along its long course over the petrous ridge and inside the cavernous sinus, producing a sixth-nerve palsy that is not localizing at all. Look for papilledema in any case of “isolated” CN VI palsy without obvious cause.

Internuclear and Supranuclear Syndromes

Internuclear Ophthalmoplegia (INO)

The medial longitudinal fasciculus (MLF) links the abducens nucleus on one side to the contralateral oculomotor (medial rectus) subnucleus. When you look right, the right abducens fires the right lateral rectus, and sends a yoked signal up the left MLF to the left CN III medial rectus subnucleus, pulling the left eye into adduction so the two eyes move together. A lesion of the MLF breaks that coupling.

The clinical picture is unmistakable once you have seen it. On attempted lateral gaze away from the side of the lesion, the contralateral eye abducts normally — often with abducting nystagmus — but the ipsilesional eye fails to adduct. Convergence, which does not use the MLF, is preserved. That preserved convergence is the key bedside test that an INO is not a CN III medial rectus palsy.

The clinical context narrows the etiology more than any other single sign:

• Bilateral INO in a young patient is multiple sclerosis until proven otherwise.
• Unilateral INO in an older patient is brainstem ischemia until proven otherwise.

One-and-a-Half Syndrome (Fisher)

A lesion in the paramedian pons that takes out the abducens nucleus (or the PPRF) and the adjacent MLF on the same side produces a striking pattern: total horizontal gaze palsy in the direction of the lesion (the “one”), plus an INO when looking the other way (the “half”). The only intact horizontal movement is abduction of the contralateral eye. Causes are the same as for an INO at this level: stroke, demyelination, brainstem tumor.

Parinaud’s (Dorsal Midbrain) Syndrome

Compression or infiltration of the dorsal midbrain — most classically by a pineal region tumor — produces a cluster: upgaze palsy, light-near dissociation, convergence-retraction nystagmus, and bilateral lid retraction (Collier’s sign). The convergence-retraction nystagmus is best brought out by asking the patient to follow a rotating optokinetic drum upward; the eyes attempt to make upward saccades and instead jerk inward and back into the orbits. Hydrocephalus from aqueductal compression often accompanies the syndrome.

Pitfalls and Clinical Pearls

• Look before you touch. Spontaneous lid position, head tilt, eye alignment, and pupil size all convey information that an active exam can mask. A subtle head tilt should be looked for in old photographs in any patient with new diplopia.
• Use the patient’s own light. A bright ambient room flattens pupil findings. A dim room with a focused light reveals subtle anisocoria and sluggish responses that disappear under fluorescent lighting.
• The pupil that “won’t dilate” is as important as the one that won’t constrict. Dim the lights, watch for full dilation, and note any pupil that lags or fails. This is the bedside dynamic that catches Horner.
• “PERRL” is not an exam. Record both sizes in light and dark, the briskness of constriction, the swinging flashlight result, and the near response. Each of these tests a different part of the loop.
• A painful third-nerve palsy with a dilated pupil is an aneurysm until proven otherwise. CTA or MRA is non-negotiable. Do not be reassured by a long history of diabetes.
• Test convergence in any patient with an isolated adduction deficit. Preserved convergence with absent adduction localizes the lesion to the MLF.
• Slow saccades are pathognomonic for something. They are never a normal variant. Common culprits: progressive supranuclear palsy (vertical saccades), Huntington’s disease, Niemann-Pick type C, spinocerebellar ataxia.
• Cover-uncover and alternate cover tests distinguish a tropia (manifest deviation) from a phoria (latent deviation revealed only when fusion is broken). A small tropia is often the only sign of a partial cranial nerve palsy.

Bringing It Together

The ocular motor system is the easiest part of the nervous system to look directly at — there is no skull in the way — and yet it can take a career to read it fluently. The architecture is the key. Pupils tell you about the optic nerves, the dorsal midbrain, and the parasympathetic and sympathetic supplies to the eye. Saccades tell you about the cortex and the brainstem burst generators. Pursuit tells you about the cerebellum. Alignment tells you about CN III, IV, and VI individually. Convergence rules out an INO when adduction is missing. A patient who walks into clinic with diplopia has, hidden in the choreography of their eyes, a precise localization waiting to be read.

References

1. Campbell WW. DeJong’s The Neurologic Examination. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2013. Chapters 14, 21.
2. Kawasaki A, Borruat F-X. Pupillary disorders. In: Liu GT, Volpe NJ, Galetta SL, eds. Liu, Volpe, and Galetta’s Neuro-Ophthalmology. 3rd ed. Elsevier; 2019.
3. Frohman EM, Frohman TC, Zee DS, et al. The neuro-ophthalmology of multiple sclerosis. Lancet Neurol. 2005;4(2):111-121.
4. Walker MF, Daroff RB. Disorders of the ocular motor nerves. In: Jankovic J, Mazziotta JC, Pomeroy SL, Newman NJ, eds. Bradley and Daroff’s Neurology in Clinical Practice. 8th ed. Elsevier; 2022.
5. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Philadelphia: Wolters Kluwer; 2017.
6. Trobe JD. The evaluation of Horner syndrome. J Neuroophthalmol. 2010;30(1):1-2.
7. Fisher CM. Some neuro-ophthalmological observations. J Neurol Neurosurg Psychiatry. 1967;30(5):383-392.
8. Wray SH. Eye Movement Disorders in Clinical Practice. Oxford University Press; 2014.