01OverviewDefinition, clinical context and the essential points that orientate the chapter.
Pupil size reflects a balance between parasympathetic constriction and sympathetic dilation. Light entering either eye sends bilateral pretectal input to the Edinger–Westphal nuclei, creating direct and consensual responses; the third nerve then carries parasympathetic fibres to the ciliary ganglion and sphincter. Sympathetic fibres take the three-neuron Horner pathway. Comparing lighting conditions therefore identifies which autonomic action is failing before a lesion is localised.
The oculomotor nerve innervates levator palpebrae and all extraocular muscles except superior oblique and lateral rectus. A nuclear lesion may have complex bilateral patterns because subnuclei differ; a fascicular midbrain lesion can include cerebral-peduncle or red-nucleus signs; the subarachnoid nerve lies close to the posterior communicating artery; the cavernous sinus groups third, fourth, sixth and trigeminal fibres; the orbital divisions separate superior and inferior functions. A partial palsy should expand—not narrow—the differential.
Compression tends to affect superficial parasympathetic fibres, whereas ischaemic injury within the nerve more often spares them, creating the familiar teaching distinction. Real presentations overlap. Aneurysm can initially spare the pupil, diabetes can coexist with aneurysm and pupil size is difficult to judge after ocular surgery or drugs. Modern practice bases urgency on onset, pain, completeness, age and accompanying signs, with low threshold for vascular imaging rather than pupil status alone.
Anisocoria may be physiological, congenital, ocular, traumatic, pharmacological or neurological. Determine whether it is new using photographs, whether vision is affected and whether ptosis, eye-movement deficit, red eye, pain or focal neurology is present. A seemingly isolated abnormal pupil in an alert well patient follows a different pathway from anisocoria discovered during decline after head injury, where repeat CT and neurosurgical action must not wait for a detailed outpatient pharmacological test.
Key points
- Describe each pupil's size in bright and dim light, shape and direct and consensual reaction before attaching a syndrome label. Record previous eye surgery, trauma and topical or transdermal drug exposure.
- If anisocoria increases in bright light, the larger pupil is failing to constrict; if it increases in darkness, the smaller pupil is failing to dilate. Equal asymmetry across conditions may be physiological.
- The afferent pathway runs from retina through optic nerve and midbrain; the parasympathetic efferent pathway travels superficially in the third nerve to constrict the pupil. Relative afferent defects and anisocoria answer different questions.
- A complete third-nerve palsy causes ptosis and impaired adduction, elevation and depression, leaving the eye typically down and out; partial fascicular or divisional palsies can be much less tidy.
- Pupillary involvement, severe headache, pain, progressive deficit or incomplete pattern must be assumed compressive until urgent arterial and structural imaging is reviewed by the appropriate specialist.
- Pupil sparing can occur in microvascular third-nerve palsy, but it does not guarantee a benign cause—particularly early, incomplete or in younger patients and when pain or other neurology is present.
- Third-nerve palsy with contralateral weakness, tremor, ataxia or altered consciousness localises to the midbrain or adjacent structures rather than an isolated peripheral diabetic neuropathy.
- A relative afferent pupillary defect reflects asymmetric retinal or optic-nerve input and usually does not cause anisocoria because both pupils can constrict when the healthier eye is stimulated.
- A tonic Adie pupil is typically large with poor light reaction, better near constriction and slow redilation, often with sectoral iris movement; confirmation is specialist-led after dangerous causes are excluded.
- Pharmacological mydriasis may produce a very large non-reactive pupil without ptosis or ophthalmoplegia; enquire about nebulised anticholinergics, scopolamine patches, plant exposure and accidental drop contact.
- A small irregular pupil, posterior synechiae, red painful eye or previous intraocular procedure points toward local ocular disease and needs slit-lamp assessment rather than purely neurological localisation.
- Do not perform or interpret dilute pilocarpine, cocaine or apraclonidine tests casually in an acute emergency; drug concentration, denervation timing and ocular contraindications can mislead.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Aneurysmal or compressive third-nerve palsy
A posterior communicating artery aneurysm, tumour or other mass can compress superficial parasympathetic fibres and ocular-motor fascicles.
Microvascular nerve ischaemia
Diabetes, hypertension and vascular risk can infarct the central third nerve, often producing painful ophthalmoplegia with relative pupil sparing.
Midbrain and herniation lesions
Stroke, inflammation, tumour and transtentorial pressure injure third-nerve nuclei or fascicles and usually add contralateral or consciousness abnormalities.
Ocular and pharmacological disease
Acute glaucoma, iris trauma, previous surgery and topical or transdermal drugs can alter pupil size without a primary neurological nerve lesion.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Parasympathetic pathway dysfunction
Damage from midbrain to ciliary ganglion reduces sphincter activity, producing a large poorly reactive pupil on examination.
- 2Somatic third-nerve failure
Loss of levator and extraocular motor fibres causes ptosis and impaired adduction, elevation and depression on examination.
- 3Afferent pathway asymmetry
Retinal or optic-nerve disease creates a relative afferent pupillary defect but generally preserves equal pupil size.
- 4Pressure-mediated progression
Expanding aneurysm or herniation may recruit pupil and movement fibres over time, so an incomplete early pattern is not reassuring.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Sudden painful ptosis and ophthalmoplegia with a dilated or sluggish pupil strongly suggests compression by a posterior communicating artery aneurysm and requires emergency CTA or equivalent neurovascular imaging.
Acute diplopia and ptosis with complete external third-nerve weakness but normal pupillary responses in an older person with vascular risk can be microvascular, only after urgent dangerous alternatives and atypical features are assessed.
Third-nerve dysfunction with contralateral pyramidal weakness, tremor, sensory loss or ipsilateral ataxia suggests a fascicular midbrain lesion from stroke, demyelination, tumour or inflammation.
A newly enlarging poorly reactive pupil with falling consciousness, progressive hemiparesis or abnormal posturing indicates critical mass effect until proven otherwise and needs immediate airway and neurosurgical management.
Severe ocular pain, red eye, blurred vision or haloes, headache, nausea and a mid-dilated poorly reactive pupil suggests acute angle closure, an ophthalmic pressure emergency rather than an isolated cranial neuropathy.
A usually unilateral large pupil with weak light reaction, slow segmental near constriction and slow redilation, often in an otherwise well younger adult, supports postganglionic parasympathetic denervation after specialist assessment.
A fixed large pupil without ptosis, extraocular weakness or other neurology may follow anticholinergic contact. Exposure history and specialist testing distinguish it from denervation after emergency causes are considered.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Standardised pupil and ocular-motor examinationFirst step - Why
- Identify the abnormal pupil, light response, afferent asymmetry and muscles affected by a third-nerve or orbital lesion.
- Interpretation and limitations
- Measure in millimetres under bright and dim conditions, assess near response, lids, position and movements and check acuity, fields and fundi. Photographs require consent and secure clinical storage.
- 02
CT angiography of head and intracranial arteries - Why
- Detect posterior communicating or other aneurysm, haemorrhage and acute arterial disease in sudden painful third-nerve palsy.
- Interpretation and limitations
- A dedicated vascular study is required; a routine non-contrast CT alone does not exclude an unruptured aneurysm. Equivocal or high-risk cases need neuroradiology and neurosurgical review and sometimes further angiography.
- 03
MRI brain, cavernous sinus and orbits - Why
- Characterise midbrain, nerve, meningeal, cavernous-sinus, orbital and infiltrative causes of partial or multiple ocular-motor palsies.
- Interpretation and limitations
- Thin-section coverage and contrast depend on localisation. MRI complements rather than delays emergency aneurysm imaging and cannot be assumed to cover every orbital or arterial question.
- 04
Slit-lamp examination and intraocular pressure - Why
- Evaluate painful red eye, iris abnormality, synechiae, trauma, surgical change and suspected acute angle closure.
- Interpretation and limitations
- Pressure and angle assessment require appropriate equipment and expertise. Avoid pharmacological dilation when angle closure is suspected until ophthalmology directs care.
- 05
Glucose, HbA1c and vascular-risk assessment - Why
- Identify diabetes and modifiable microvascular risk after urgent compressive causes have been considered.
- Interpretation and limitations
- A raised glucose supports risk but does not establish causation. Follow-up must document recovery; progression, pupil change or non-resolution prompts imaging and diagnostic review.
- 06
Specialist pharmacological pupil testing - Why
- Differentiate tonic denervation, pharmacological blockade and sympathetic failure in stable, appropriately selected patients.
- Interpretation and limitations
- Dilution, timing and prior ocular disease affect sensitivity and specificity. Results are interpreted with the full examination and should never postpone imaging of an acute painful or neurological syndrome.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Horner syndrome
A small pupil with mild ptosis and normal extraocular movement indicates sympathetic failure rather than third-nerve palsy.
Adie tonic pupil
A large pupil with poor light response, better near constriction and slow redilation supports tonic postganglionic parasympathetic denervation.
Pharmacological mydriasis
A very large non-reactive pupil without ptosis or ophthalmoplegia and relevant exposure suggests anticholinergic contamination from local exposure.
Local ocular disease
A red painful eye, irregular pupil, synechiae or previous ocular procedure requires slit-lamp assessment rather than neurological localisation alone.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Painful third-nerve palsyExclude aneurysm immediatelyFirst stepNew ptosis and impaired adduction, elevation or depression with headache, ocular pain, pupillary abnormality or uncertain completeness.+
- 1Perform ABCDE, document exact onset, acuity, fields, pupils and all eye movements and examine the remaining cranial nerves and limbs for subarachnoid or midbrain features.
- 2DefinitiveContact emergency neurovascular, stroke or neurosurgical services and obtain urgent CTA or the local definitive arterial imaging, not merely non-contrast CT or outpatient review.
- 3AlternativeMonitor consciousness, pupil size and focal findings while imaging is interpreted and follow specialist aneurysm, cavernous-sinus or alternative structural management.
02Anisocoria with deteriorationTreat possible herniationA pupil becomes enlarged or unreactive in a patient with head injury, haemorrhage, tumour, seizure or falling conscious level.+
- 1Call anaesthetic and neurosurgical help, secure oxygenation and circulation, elevate and position appropriately within the local neurocritical protocol and reassess GCS components and both pupils.
- 2Obtain immediate repeat or first CT imaging without delaying stabilisation and review anticoagulation, sodium, ventilation and seizure activity with the responsible team.
- 3DefinitiveUse temporising hyperosmolar or ventilation strategies only under the emergency neurocritical pathway while definitive decompression or cause-specific treatment is arranged.
03Stable isolated anisocoriaIdentify the abnormal autonomic actionPupil asymmetry is noticed without reduced consciousness, acute severe pain, ophthalmoplegia or other focal neurology.+
- 1Measure in bright and dim light, test direct, consensual and near responses and examine lids, acuity, fields, eye movements and the ocular surface; seek old photographs and exposure history.
- 2Route red or painful eye to ophthalmology, a small dark-abnormal pupil to Horner assessment and a large light-abnormal pupil to parasympathetic, tonic, traumatic or pharmacological evaluation.
- 3Arrange neuro-ophthalmic review and targeted imaging or pharmacological testing according to onset and phenotype, with emergency advice for new pain, ptosis, diplopia, vision loss or neurological change.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Aneurysm rupture and subarachnoid haemorrhage
Delayed recognition of a compressive third-nerve palsy can permit catastrophic aneurysmal bleeding, stroke or death without intervention.
Brain herniation
A newly dilating pupil with reduced consciousness may signal transtentorial shift and impending brainstem failure during deterioration.
Visual and corneal injury
Diplopia, ptosis and impaired eye protection restrict function, while acute glaucoma can cause irreversible visual loss.
False reassurance and delayed diagnosis
Pupil sparing or partial movement loss can occur early in dangerous disease, so rigid one-sign rules create diagnostic harm.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Repeat and time-stamp millimetre pupil size, reactivity, ptosis, ocular alignment and conscious level whenever an acute compressive or traumatic process is possible.
- In an observed presumed microvascular palsy, review pupil status and ocular movements early and confirm progressive recovery; worsening or non-resolution triggers diagnostic revision.
- Track visual acuity, pain, redness and corneal clarity in ocular causes and escalate any visual deterioration to the eye service immediately.
- Review final vascular and structural imaging, not only the initial verbal report, and document ownership of incidental aneurysm or mass findings.
- Record all relevant topical, inhaled, patch and procedural drug exposures so subsequent pupil assessments are not repeatedly misinterpreted.
- Provide urgent return advice for severe headache, neck pain, vomiting, reduced alertness, new diplopia, weakness or visual loss.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
RAPD does not enlarge one pupil
An afferent defect changes the relative response when light moves between eyes, yet both pupils share consensual efferent output and are often equal in resting size.
Near response can dissociate
Some tonic or midbrain pupils constrict better to a near target than to light. This light–near dissociation is a pattern, not a single diagnosis.
Partial palsies deserve caution
A lesion can affect only one third-nerve division or fascicle, and aneurysm may present incompletely. Failure to match a textbook complete palsy should lower reassurance.
Ptosis can hide diplopia
A completely closed eyelid prevents binocular double vision. Lifting it briefly during supervised examination may reveal marked ocular misalignment but should not be mistaken for symptom absence.
Surgery alters pupil anatomy
Cataract surgery, iris trauma and glaucoma procedures can create permanent irregularity or impaired reactivity. Prior records and slit-lamp findings prevent false neurological attribution.
Pupil sparing is temporal
A compressive lesion may affect motor fibres before superficial parasympathetic fibres become abnormal. Serial assessment and appropriate imaging matter more than one early normal measurement.
11Common pitfallsFrequent interpretation and management errors.
- 01
Calling anisocoria normal without comparing bright and dark illumination.
- 02
Assuming diabetes proves a pupil-sparing third-nerve palsy is microvascular.
- 03
Using a normal non-contrast CT to exclude posterior communicating artery aneurysm.
- 04
Confusing a relative afferent pupillary defect with a large efferent pupil.
- 05
Applying dilating or constricting drops to a painful red eye without ophthalmic assessment.
- 06
Ignoring a new fixed pupil because pre-existing cataract surgery is mentioned but undocumented.
- 07
Waiting for a complete down-and-out eye posture before escalating a painful partial palsy.