Acute Horner syndrome presents with the classic triad of ipsilateral miosis, ptosis, and anhidrosis due to disruption of the oculosympathetic pathway. The most critical diagnosis to exclude in the ED is carotid artery dissection, which is the most common nonprocedural cause and carries risk of catastrophic stroke if missed. [1-2]
1. History
- Onset and timing: Exact time of symptom onset; sudden vs. gradual; duration of ptosis/anisocoria
- Pain: Ipsilateral headache, neck pain, facial pain, or orbital pain — present in ~65% of carotid dissections (headache) and ~50% (neck pain) [2]
- Trauma: Any recent head/neck/chest trauma, even minor (chiropractic manipulation, sports, MVA, coughing/vomiting) [1][3]
- Recent procedures: Neck/chest/spine surgery, central line placement, epidural, interscalene block [4-5]
- Neurological symptoms: Weakness, numbness, speech difficulty, vision changes, vertigo, dysphagia
- Associated symptoms: Pulsatile tinnitus (~8% of dissections), visual obscurations, monocular vision loss (amaurosis fugax) [2]
- Important negatives: No prior anisocoria, no history of eye drops, no prior episodes
2. Alarm Features
- Ipsilateral headache or neck pain + acute Horner → carotid dissection until proven otherwise [3]
- Focal neurological deficits (hemiparesis, aphasia, hemisensory loss) → stroke from dissection or brainstem infarction [1][6]
- History of trauma → carotid dissection, brachial plexus injury, cervical cord injury [1]
- Cranial nerve palsies (especially CN XII, IX, X) → compression from enlarged dissected artery [3]
- Weight loss, smoking history, shoulder/arm pain → Pancoast tumor (second-order neuron)
- Brainstem signs (ataxia, vertigo, diplopia, crossed deficits) → lateral medullary syndrome (Wallenberg)
- Pediatric acute Horner → neuroblastoma, mediastinal mass must be excluded [7]
3. Medications
- Relevant contributors: Topical eye drops (brimonidine, pilocarpine) can mimic or mask anisocoria; recent anesthesia (interscalene/stellate ganglion block) [4]
- Treatment of underlying cause (carotid dissection):
- Antithrombotic therapy: antiplatelet (aspirin 81–325 mg or clopidogrel 75 mg) OR anticoagulation (heparin → warfarin, INR 2.0–3.0) for 3–6 months [8-9]
- The CADISS trial found no significant difference between antiplatelet and anticoagulant therapy [10]
- Anticoagulation may be preferred with vessel occlusion; antiplatelet may be preferred with large infarct or hemorrhagic risk [11]
- Contraindicated: Anticoagulation if intracranial extension of dissection or large hemorrhagic infarct [12]
4. Diet
- No specific dietary triggers or recommendations for Horner syndrome itself
- If anticoagulated with warfarin for dissection, counsel on consistent vitamin K intake
- Adequate hydration during acute hospitalization
5. Review of Systems
- Neuro: Headache, neck pain, vision changes, weakness, numbness, speech changes, vertigo, dysphagia, tinnitus
- Ophthalmologic: Droopy eyelid, pupil asymmetry, facial flushing, decreased sweating on one side
- Pulmonary: Cough, hemoptysis, chest pain, shoulder pain (Pancoast tumor)
- Constitutional: Weight loss, fevers, night sweats (malignancy, infection)
- MSK: Arm/hand weakness or numbness (brachial plexus, C8-T2 involvement)
6. Collateral History and Family History
- Confirm timing of anisocoria onset — old photographs can help determine chronicity vs. acuity
- Witnesses to trauma or mechanism of injury
- Family history: Connective tissue disorders (Ehlers-Danlos, Marfan, fibromuscular dysplasia) predispose to spontaneous dissection [3]
- History of migraines or cluster headaches (can cause transient Horner) [1]
7. Risk Factors
- Carotid dissection: Recent cervical trauma (even minor), connective tissue disorders, fibromuscular dysplasia, recent chiropractic manipulation, hypertension [2-3]
- Stroke/brainstem infarction: Hypertension, diabetes, atrial fibrillation, smoking
- Malignancy (Pancoast tumor): Smoking, age >40, occupational exposures
- Iatrogenic: Recent neck/chest/spine surgery, central venous catheterization, epidural/regional anesthesia [5]
8. Differential Diagnosis
- Carotid artery dissection — most dangerous and most common nonprocedural cause of acute Horner; pain + Horner is highly suggestive [1][3]
- Brainstem stroke (lateral medullary/Wallenberg syndrome) — first-order neuron; crossed findings, ataxia, vertigo, dysphagia
- Pancoast tumor — second-order neuron; shoulder/arm pain, weight loss, smoking history
- Cluster headache — episodic, severe unilateral periorbital pain with autonomic features; Horner resolves between attacks [1]
- Iatrogenic/post-procedural — most common overall cause; recent surgery or regional anesthesia [5]
- Cavernous sinus lesion — third-order neuron; associated CN III, IV, V1, V2, VI palsies
- Physiological anisocoria — up to 1 mm difference, no ptosis, no dilation lag; must be excluded [13]
Mimics: Pharmacologic miosis (pilocarpine), Adie tonic pupil (opposite: dilated pupil), CN III palsy (opposite: dilated pupil with ptosis), mechanical ptosis
9. Past Medical History
- Prior stroke or TIA
- Connective tissue disorders
- Known vascular disease or fibromuscular dysplasia
- Prior episodes of anisocoria or ptosis
- History of malignancy (lung, thyroid, lymphoma)
- Prior neck/chest surgery
- Migraine or cluster headache history
10. Physical Exam
- Eyes: Measure pupil size in light AND dark — anisocoria greater in dark is classic for Horner (dilation lag). Assess ptosis (typically 1–2 mm), lower lid "reverse ptosis" (upside-down ptosis), pseudoenophthalmos [1][13-14]
- Face: Ipsilateral facial anhidrosis (central/preganglionic) vs. limited to forehead (postganglionic); facial flushing
- Neurological exam: Full cranial nerve exam (especially CN XII — tongue deviation), motor/sensory exam, cerebellar testing, gait
- Neck: Carotid bruit, tenderness, palpable mass
- Chest: Auscultation, supraclavicular lymphadenopathy
- Vital signs: Hypertension or hypotension, heart rate asymmetry
11. Lab Studies
- Routine: CBC, BMP, coagulation studies (PT/INR, PTT) — especially if anticoagulation is planned
- If stroke suspected: Glucose, lipid panel, HbA1c
- ESR/CRP: If vasculitis or inflammatory etiology considered
- Urine catecholamines/VMA: In pediatric patients to rule out neuroblastoma
- Labs are primarily to guide treatment of the underlying cause rather than to diagnose Horner syndrome itself
12. Imaging
- First-line (acute Horner): CT head without contrast + CTA head and neck with contrast — this is the recommended initial imaging per ACR Appropriateness Criteria [1]
- CTA has high resolution and short acquisition time for carotid dissection
- NCCT head evaluates for infarction and hemorrhage before starting antithrombotics
- Coverage from carina to vertex also assesses lung apex, mediastinum, neck soft tissues
- MRI/MRA head and neck: More sensitive for brainstem infarction, demyelination, intramural hematoma (T1 fat-sat showing crescent sign); may be used as next-step if CT is nondiagnostic [1][3]
- Chest X-ray or CT chest: If Pancoast tumor or mediastinal mass suspected
- When imaging is unnecessary: Known iatrogenic cause (e.g., post-interscalene block with rapid resolution) [4]
13. Special Tests
- Apraclonidine 0.5% test: Reversal of anisocoria confirms Horner syndrome (sensitivity ~93%); the affected pupil dilates due to denervation supersensitivity. Can be positive as early as 48 hours after onset, though may have reduced sensitivity in the hyperacute period [15-16]
- Cocaine 4–10% test: Failure to dilate the affected pupil confirms Horner; less sensitive (~40%) and harder to obtain [15]
- Hydroxyamphetamine 1%: Differentiates preganglionic from postganglionic lesions (fails to dilate in postganglionic); limited availability
- Automated pupillometry: Dilation lag measurement can differentiate Horner from physiological anisocoria with ~95% sensitivity [13]
- Note: In the acute ED setting, do not delay imaging for pharmacologic testing if clinical suspicion is high [1]
14. ECG
- Obtain ECG if stroke is suspected — evaluate for atrial fibrillation as an alternative embolic source
- No specific ECG findings for Horner syndrome itself
- Consider telemetry monitoring if admitted for dissection-related stroke
15. Assessment
Acute Horner syndrome is an emergency presentation that demands urgent exclusion of carotid artery dissection and stroke. The classic triad (miosis, ptosis, anhidrosis) may be incomplete — partial Horner with only miosis and mild ptosis is common, especially with postganglionic (third-order) lesions where anhidrosis is limited or absent. [14][17] Cerebral or retinal ischemia develops in 50–95% of carotid dissection cases after initial warning symptoms, making early diagnosis critical. [6] At least 40% of all Horner syndrome cases remain idiopathic, but acute onset with pain should never be assumed benign. [1][18]
16. Treatment Plan
- Initial stabilization: ABCs, IV access, continuous monitoring, NPO if stroke suspected
- If carotid dissection confirmed:
- Antithrombotic therapy for 3–6 months: antiplatelet (aspirin or clopidogrel) OR anticoagulation (heparin bridge → warfarin, target INR 2.0–3.0) [8][12]
- AHA scientific statement notes similar efficacy between antiplatelet and anticoagulant therapy; anticoagulation may be preferred with vessel occlusion [10-11]
- Blood pressure management — avoid extremes; no strong evidence for specific agents [9]
- If acute ischemic stroke: Activate stroke protocol; thrombolysis (IV tPA) or thrombectomy per standard criteria
- If Pancoast tumor/malignancy: Oncology and thoracic surgery consultation
- If brainstem stroke: Neurology consultation, supportive care, secondary prevention
- Iatrogenic Horner: Observation; typically self-resolves (e.g., post-anesthesia block) [4][19]
- Horner syndrome itself does not require specific treatment — management targets the underlying cause
17. Disposition
- Admit: Carotid dissection (especially with ischemic symptoms or stroke), brainstem infarction, new malignancy, expanding neck hematoma, any unstable neurological exam
- Observation: Isolated painful Horner with negative initial imaging — consider admission for serial neuro exams and possible MRI/MRA
- Discharge: Clear iatrogenic cause with resolution, known cluster headache with typical pattern, confirmed physiological anisocoria
- Specialist consultation triggers: Neurology (stroke, dissection), neurosurgery (if surgical intervention needed), ophthalmology/neuro-ophthalmology (diagnostic confirmation, pharmacologic testing), oncology (malignancy), vascular surgery (refractory dissection) [9]
18. Follow Up / Return Precautions
- Follow-up timing: Neurology/neuro-ophthalmology within 1–2 weeks if discharged; vascular imaging follow-up at 3 months for dissection (MRA to assess recanalization) [12]
- Return immediately for: New weakness or numbness, speech difficulty, worsening headache, vision loss, difficulty swallowing, worsening ptosis or pupil changes
- Patient counseling: Most carotid dissections heal spontaneously with medical therapy; prognosis is generally favorable. Recurrent dissection is rare. Avoid high-risk activities (contact sports, chiropractic manipulation) during recovery [11-12]
- Expected recovery: Horner syndrome from dissection may persist for weeks to months; iatrogenic cases typically resolve within hours to days [4][19]
References
1. ACR Appropriateness Criteria® Horner Syndrome. — Expert Panel on Neurological Imaging, Dubey P, Shekhrajka N, et al. Journal of the American College of Radiology : JACR. 2025.
2. Treatment and Outcomes of Cervical Artery Dissection in Adults: A Scientific Statement From the American Heart Association. — Yaghi S, Engelter S, Del Brutto VJ, et al. Stroke. 2024.
3. Cervical-Artery Dissections: Predisposing Factors, Diagnosis, and Outcome. — Debette S, Leys D. The Lancet. Neurology. 2009.
4. Horner's Syndrome After Scalene Block and Carotid Dissection. — Padaki AS, Fitch RW, Stack LB, Thurman RJ. The Journal of Emergency Medicine. 2016.
5. Causes of Horner Syndrome: A Study of 318 Patients. — Sabbagh MA, De Lott LB, Trobe JD. Journal of Neuro-Ophthalmology : The Official Journal of the North American Neuro-Ophthalmology Society. 2020.
6. 2011 ASA/ACCF/AHA/AANN/AANS/ACR/ASNR/CNS/SAIP/SCAI/SIR/SNIS/SVM/SVS Guideline on the Management of Patients With Extracranial Carotid and Vertebral Artery Disease: A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines, and the American Stroke Association, American Association of Neuroscience Nurses, American Association of Neurological Surgeons, American College of Radiology, American Society of Neuroradiology, Congress of Neurological Surgeons, Society of Atherosclerosis Imaging and Prevention, Society for Cardiovascular Angiography and Interventions, Society of Interventional Radiology, Society of NeuroInterventional Surgery, Society for Vascular Medicine, and Society for Vascular Surgery. — Brott TG, Halperin JL, Abbara S, et al. Journal of the American College of Cardiology. 2011.
7. Horner Syndrome Secondary to Rapid Enlargement of a Mediastinal Lymphatic Malformation. — Pickup E, Spagnolo K, Kornbluh AB. Pediatric Neurology. 2025.
8. 2011 ASA/ACCF/AHA/AANN/AANS/ACR/ASNR/CNS/SAIP/SCAI/SIR/SNIS/SVM/SVS Guideline on the Management of Patients With Extracranial Carotid and Vertebral Artery Disease: Executive Summary: A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines, and the American Stroke Association, American Association of Neuroscience Nurses, American Association of Neurological Surgeons, American College of Radiology, American Society of Neuroradiology, Congress of Neurological Surgeons, Society of Atherosclerosis Imaging and Prevention, Society for Cardiovascular Angiography and Interventions, Society of Interventional Radiology, Society of NeuroInterventional Surgery, Society for Vascular Medicine, and Society for Vascular Surgery. — Brott TG, Halperin JL, Abbara S, et al. Journal of the American College of Cardiology. 2011.
9. The Society for Vascular Surgery Implementation Document for Management of Extracranial Cerebrovascular Disease. — AbuRahma AF, Avgerinos ED, Chang RW, et al. Journal of Vascular Surgery. 2022.
10. Surgical and Radiological Interventions for Treating Symptomatic Extracranial Cervical Artery Dissection. — Hynes N, Kavanagh EP, Sultan S, Jordan F. The Cochrane Database of Systematic Reviews. 2021.
11. Carotid Dissection: Pathophysiology and Treatment. — Bakradze E, Yaghi S. Seminars in Neurology. 2025.
12. Spontaneous Dissection of the Carotid and Vertebral Arteries. — Schievink WI. The New England Journal of Medicine. 2001.
13. Differentiation of Horner Syndrome and Physiological Anisocoria by Automated Pupillometry. — Disse LR, Bockisch CJ, Weber KP, Fierz FC. Journal of Neuro-Ophthalmology : The Official Journal of the North American Neuro-Ophthalmology Society. 2024.
14. Horner Syndrome: A Clinical Review. — Martin TJ. ACS Chemical Neuroscience. 2018.
15. Apraclonidine Is Better Than Cocaine for Detection of Horner Syndrome. — Bremner F. Frontiers in Neurology. 2018.
16. Apraclonidine for the Pharmacologic Confirmation of Acute Horner Syndrome. — Nguyen MTB, Farahvash A, Zhang A, Micieli JA. Journal of the Neurological Sciences. 2020.
17. Neuro-Ophthalmological Manifestations of Horner's Syndrome: Current Perspectives. — Maamouri R, Ferchichi M, Houmane Y, Gharbi Z, Cheour M. Eye and Brain. 2023.
18. Invited Commentary: Evaluation of Horner Syndrome in the MRI Era. — Kawasaki A. Journal of Neuro-Ophthalmology : The Official Journal of the North American Neuro-Ophthalmology Society. 2018.
19. Neuro-Ophthalmic Observation and 16-Month Follow-Up of Horner Syndrome After Thyroidectomy: A Case Report. — Li XM, Yang B, Yang S, Wang B, Liu SX. Medicine. 2026.