01OverviewDefinition, clinical context and the essential points that orientate the chapter.
Horner syndrome reflects interruption of the sympathetic supply to the eye and surrounding structures. The familiar combination of miosis, mild ptosis and reduced sweating is often incomplete. In practice, a subtle change in lid position or a pupil difference noticed in photographs may be the only presenting complaint, and the patient may retain normal acuity and eye movements.
The clinical task is to decide whether the syndrome is present, establish its likely duration and identify features that localise a potentially serious lesion. Sympathetic fibres traverse a long route, so causes include brainstem or spinal disease, apical thoracic lesions, neck injury and carotid dissection. A quiet-looking eye and normal vision do not establish that the cause is benign.
Key points
- Horner syndrome produces a smaller pupil and usually mild upper-lid drooping on the affected side.
- Anisocoria is more conspicuous in darkness because the affected pupil dilates poorly; absence of obvious facial anhidrosis does not exclude the diagnosis.
- The sympathetic pathway extends from the brain through the spinal cord, chest and neck to the orbit, so investigation must follow the relevant anatomy.
- Painful acute Horner syndrome should be treated as possible internal carotid dissection until appropriately excluded.
- Pharmacological confirmation can be falsely negative early and must not delay urgent imaging.
- Old photographs can establish chronicity, but newly noticed and definitely new are different histories.
- Management targets the underlying cause; cosmetic pupil or lid treatment is secondary to diagnosing vascular, neurological or malignant disease.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Central pathway disease
Lesions affecting descending sympathetic fibres in the hypothalamus, brainstem or upper spinal cord can cause Horner syndrome. Additional neurological findings often indicate the central site and help determine the required imaging.
Thoracic and cervical lesions
The sympathetic pathway passes near the lung apex and through the neck. Apical masses, trauma and surgery can interrupt these fibres, sometimes accompanied by shoulder pain, arm symptoms or a relevant procedural history.
Carotid-associated disease
Postganglionic sympathetic fibres travel with the internal carotid artery towards the orbit. Dissection can therefore produce ipsilateral painful Horner syndrome before any retinal or cerebral ischaemic deficit becomes apparent.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Three-neuron pathway
Sympathetic signals descend from the hypothalamus to the spinal ciliospinal centre, ascend to the superior cervical ganglion and then accompany arterial pathways to the eye. Interruption at different levels produces overlapping ocular signs.
- 2Impaired pupil dilation
Reduced sympathetic activity weakens the iris dilator response while parasympathetic constriction remains functional. The affected pupil stays relatively small, making the size difference more evident when both eyes should dilate in darkness.
- 3Lid and sweat changes
Loss of sympathetic input to smooth eyelid muscle causes mild ptosis rather than complete levator paralysis. Sweating changes depend on the lesion's position and may be incomplete, subtle or clinically undetectable.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Measure both pupils in light and darkness and observe their reaction. In Horner syndrome the smaller pupil is abnormal and the asymmetry generally increases in darkness. Early dilation lag may be subtle, so one hurried inspection in a bright room can miss the finding.
Upper-lid ptosis may be only one or two millimetres, with slight elevation of the lower lid narrowing the palpebral opening. The globe can appear sunken without true enophthalmos. Severe complete ptosis or restricted eye movements requires consideration of an additional or different lesion.
Determine whether there is new neck, orbital, facial or head pain, recent trauma or a neck procedure. Ask about transient monocular visual loss, weakness, sensory disturbance, imbalance and speech change. The combination of acute pain and Horner signs warrants emergency assessment even without stroke symptoms.
Brainstem signs suggest a central lesion, while shoulder or arm pain, hand weakness and chest symptoms may suggest a lesion near the lung apex or lower cervical pathway. Previous surgery can explain some cases, but an assumed procedural cause should match the timing and documented findings.
Review earlier photographs, previous examinations and the patient's recollection of lid asymmetry. A newly discovered pupil difference may be longstanding. Conversely, absence of an old diagnosis is not evidence that a current painful presentation can safely await outpatient testing.
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
Immediate arterial imaging in a painful acute presentationFirst step - Why
- Identify carotid dissection or another urgent lesion before ischaemic complications occur.
- Interpretation and limitations
- The emergency team should arrange appropriate CTA or MRA of the head and neck. The UK combined imaging algorithm uses CTA coverage from the aortic arch to the intracranial circulation with assessment of the apices and orbits; local neuroradiology determines the precise protocol.
- 02
Focused neurological and ophthalmic examination - Why
- Identify central signs or additional cranial-nerve involvement that changes localisation.
- Interpretation and limitations
- Assess acuity, fields, pupils, eye movements, facial sensation, coordination and limb function. Normal visual acuity does not exclude dissection. Central neurological findings may require MRI of the brain, brainstem or spinal cord in addition to emergency vascular assessment.
- 03
Specialist pharmacological confirmation - Why
- Support a diagnosis of sympathetic denervation when the clinical examination remains uncertain.
- Interpretation and limitations
- Apraclonidine or other specialist testing may confirm Horner syndrome, but early denervation can produce a false-negative response. These tests do not reliably replace anatomical localisation or urgent imaging and should never be a prerequisite for investigating a painful acute case.
- 04
Imaging directed at chest, neck or central causes - Why
- Assess the relevant sympathetic pathway when history and examination suggest another lesion.
- Interpretation and limitations
- A chest radiograph alone can miss an apical mass, and carotid Doppler alone does not adequately exclude all dissections. Cross-sectional imaging should cover the suspected region; the investigation plan for painless longstanding cases is individualised after specialist review.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Physiological anisocoria
A small longstanding pupil difference can occur without neurological disease. Stability in old photographs and an otherwise normal examination help assessment, but new pain or ptosis requires investigation rather than assumption.
Third-nerve palsy
Oculomotor dysfunction may cause a larger pupil with more marked ptosis and impaired eye movements. Acute third-nerve palsy requires urgent investigation for compression, including when the pupil is initially unaffected.
Pharmacological or iris abnormalities
Accidental medication exposure, prescribed eye drops and local iris disease can alter pupil size. A careful exposure history and slit-lamp examination may identify these causes without explaining away associated neurological red flags.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01EmergencyAcute pain or neurological involvementFirst stepNew Horner signs occur with ipsilateral pain, trauma, visual ischaemia or focal neurological symptoms.+
- 1Send the patient for immediate emergency assessment, using emergency transport when stroke or other acute neurological compromise is suspected.
- 2Record onset and any transient visual or neurological episode, because resolution does not remove vascular risk.
- 3Discuss urgent head-and-neck arterial imaging with the receiving team and avoid delay for diagnostic eye drops.
- 4Let stroke or vascular specialists select antithrombotic treatment after evaluation of the cause and relevant contraindications.
02InvestigationPainless or uncertain-duration Horner syndromeThere is no acute pain or neurological red flag, but sympathetic dysfunction remains possible.+
- 1Arrange prompt ophthalmic or neuro-ophthalmic assessment and review old photographs, medical records and medication exposure.
- 2Confirm the clinical findings and decide whether pharmacological testing adds useful information to the examination.
- 3Plan imaging with attention to the whole relevant pathway rather than ordering a chest radiograph as a complete exclusion test.
- 4Provide clear instructions to seek emergency help for new neck pain, visual loss, weakness, speech disturbance or worsening headache.
03Cause specificTreat and monitor the responsible disorderImaging or further assessment has identified a structural, vascular, inflammatory or procedural explanation.+
- 1Coordinate management with the responsible neurological, vascular, respiratory, oncology or surgical service.
- 2Explain that pupil and lid findings may persist even when the underlying disease has been treated successfully.
- 3Assess whether persistent lid asymmetry affects function before considering elective ophthalmic treatment.
- 4Document the established baseline so later clinicians can distinguish residual signs from a new change.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Retinal or cerebral ischaemia
Carotid dissection may lead to embolic or haemodynamic ischaemia after local pain and sympathetic signs appear. Early recognition of the presenting syndrome is therefore important even when current visual and neurological function is preserved.
Delayed diagnosis of malignancy
A thoracic or cervical tumour may initially be recognised through sympathetic signs. Investigation restricted to the eye can delay diagnosis when shoulder pain, constitutional symptoms or pathway anatomy suggest a more distant lesion.
Persistent lid asymmetry
Ptosis and pupil asymmetry may remain after treatment of the underlying cause. Their cosmetic or functional impact should be addressed after the responsible disease has been assessed and a stable baseline established.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Record pupil sizes under comparable lighting and the degree of ptosis so follow-up comparisons have a consistent baseline.
- Confirm that recommended imaging and specialist referrals have been completed and that the patient understands who will communicate the results.
- Reassess any new headache, neck pain, transient visual symptom or focal neurological change urgently rather than attributing it to an established label.
- After a cause is identified, monitor according to that disease and review persistent lid symptoms separately from the vascular or neurological treatment plan.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Sweating does not settle the diagnosis
The distribution of sweating fibres varies along the sympathetic pathway, and reduced facial sweating can be difficult to notice in a cool clinical room. An apparently normal sweating history therefore provides little reassurance against Horner syndrome.
A negative drop test may be premature
Some pharmacological responses depend on denervation changes developing over time. An early negative test cannot outweigh a convincing acute painful presentation, and arranging repeated testing before imaging creates an avoidable delay.
Cluster headache needs careful context
Autonomic pupil and lid changes can accompany cluster headache, but a first painful Horner presentation should not be assigned that explanation without assessing vascular alternatives. The headache pattern and previous documented episodes help, but do not replace emergency judgement.
Anatomy determines the imaging field
A scan confined to the brain may omit an apical thoracic or cervical cause. Conversely, a chest film misses central and carotid disease. Discussing the localisation and uncertainty with radiology is more useful than requesting an undirected collection of tests.
11Common pitfallsFrequent interpretation and management errors.
- 01
Waiting for the full triad of ptosis, miosis and anhidrosis before considering Horner syndrome.
- 02
Reassuring a patient with acute painful Horner syndrome because visual acuity and limb strength are normal.
- 03
Using a negative early apraclonidine test or a normal carotid Doppler result to exclude dissection.
- 04
Assuming that a newly noticed pupil difference is either definitely acute or definitely congenital without examining the history.