Horner syndrome is a set of three signs – a droopy eyelid, a small pupil, and sometimes reduced sweating – that together point to an interrupted nerve pathway. The signs themselves are rarely dangerous. What causes them sometimes is.
Horner syndrome is not a disease in itself but a pattern of signs resulting from disruption of the sympathetic nerve pathway that supplies the eye. This three-neuron chain runs from the hypothalamus in the brain, down through the neck, around the lung apex, and into the orbit – a remarkable anatomical journey that explains why so many different conditions can cause the same clinical picture. The classic triad is partial ptosis (a drooping upper eyelid), miosis (a constricted pupil), and anhidrosis (reduced sweating on the same side of the face). Understanding what has interrupted this pathway is the central task. In some patients the cause is benign; in others it warrants urgent investigation.
What You Need to Know About Horner Syndrome
- Horner syndrome is caused by any lesion along the three-neuron sympathetic pathway from hypothalamus to eye
- The classic signs are partial ptosis (2–3mm), miosis, and anhidrosis – though all three are not always present
- The ptosis of Horner syndrome is mild, not the dramatic drooping seen in third nerve palsy; the lower lid is often slightly elevated too (upside-down ptosis)
- Carotid artery dissection is one of the most important causes to identify quickly – it presents with acute Horner syndrome, often with neck or face pain, and carries stroke risk
- Pancoast tumor at the lung apex is the classic preganglionic cause and must be excluded in adults with new-onset Horner of unknown cause
- Pharmacological testing with apraclonidine eye drops confirms the diagnosis and helps localise the lesion
The Anatomy Behind the Signs
To understand Horner syndrome you need to follow the nerve. The sympathetic pathway to the eye is a three-neuron chain. A lesion at any of the three levels produces the same external signs, but the underlying cause – and the urgency – differs substantially depending on where along the pathway the interruption has occurred.
- Runs from hypothalamus to ciliospinal centre of Budge in spinal cord (C8–T2)
- Causes: stroke, multiple sclerosis, syringomyelia, brainstem lesions
- Associated neurological signs often present
- Anhidrosis affects the entire ipsilateral half of the body
- Exits spinal cord, loops around subclavian artery and lung apex, up to superior cervical ganglion
- Causes: Pancoast tumor, thyroid surgery, thoracic aortic aneurysm, brachial plexus injury
- Anhidrosis limited to face and neck
- Always requires chest imaging in adults to exclude malignancy
- Travels with the internal carotid artery, then enters the orbit via the ophthalmic branch of the trigeminal nerve
- Causes: internal carotid artery dissection, cluster headache, middle ear infections, cavernous sinus lesions
- Anhidrosis often absent – sweat fibres diverge earlier in the pathway
- Carotid dissection at this level is an acute emergency requiring same-day vascular imaging
Symptoms and Signs
The triad in practice
Partial ptosis is often the most noticeable feature. The upper lid droops 1–3mm. The lower lid simultaneously elevates very slightly – this “upside-down ptosis” of the lower lid narrows the palpebral fissure from above and below, giving a characteristic sleepy or sunken appearance to the eye. This is distinct from the more dramatic ptosis of a third nerve palsy, where the lid can be nearly completely closed.
Miosis – a small pupil – results from loss of the sympathetically driven pupil dilator muscle. The affected pupil dilates poorly in the dark, making the difference between the two pupils more apparent in dim illumination. In bright light, both pupils constrict and the anisocoria (size difference) is less obvious. This light-dependent difference in the degree of anisocoria is a helpful clinical clue.
Anhidrosis
Reduced or absent sweating on the affected side of the face – anhidrosis – is present in many cases but is not always reported because patients rarely notice it unless they are looking for it. The distribution of anhidrosis helps localise the level of the lesion, with postganglionic lesions often sparing sweating entirely.
Heterochromia in congenital Horner
Congenital Horner syndrome, whether present from birth or developing in infancy, is associated with heterochromia iridis – different coloured irises. The iris on the affected side remains lighter because the sympathetic signals that promote iris pigmentation never arrived. This finding in a child with Horner syndrome raises the question of a congenital cause but also occasionally indicates a neuroblastoma, which must be excluded.
Causes and Investigation
The cause of Horner syndrome determines urgency. This is not a condition where it is reasonable to just treat the cosmetic appearance and move on – the underlying cause must be identified.
Carotid artery dissection
Acute-onset Horner syndrome with ipsilateral neck, jaw, or face pain should be assumed to be carotid dissection until proven otherwise. Dissection of the internal carotid artery disrupts the sympathetic fibres running alongside it and carries a significant risk of stroke from thrombus formation at the tear site. Same-day vascular imaging – MRI/MRA or CTA – is the standard of care. Anticoagulation or antiplatelet therapy may be started while imaging is arranged. Do not defer investigation on this presentation.
Pancoast tumor and thoracic causes
Any adult with new Horner syndrome not explained by a known recent surgery or procedure requires chest imaging, typically CT of the chest, to look for an apical lung tumor or mediastinal pathology. Pancoast tumours at the lung apex invade the brachial plexus and sympathetic chain, classically presenting with shoulder pain, arm weakness, and Horner syndrome. This combination is specific enough to warrant prompt investigation regardless of smoking history.
Post-surgical, cluster headache, congenital
Horner syndrome following neck surgery (thyroidectomy, carotid endarterectomy) or thoracic surgery is a known complication that is often transient. Cluster headache, an intensely painful unilateral headache condition, frequently produces a transient Horner on the affected side during attacks. Congenital Horner, identified at birth or in infancy, requires investigation for neuroblastoma in children but often has a benign birth-related cause. These cases still need appropriate assessment but carry a different degree of urgency than an unexplained new-onset case in an adult.
Horner Syndrome and the Neuro-Ophthalmology Workup
Horner syndrome is a paradigmatic neuro-ophthalmology condition – it uses the eye as a window to understand the nervous system. The ophthalmologist or neurologist will often use pharmacological testing to confirm the diagnosis and help identify the level of the lesion.
Apraclonidine drops, instilled in both eyes, cause the affected pupil to dilate (because its sympathetic receptors are hypersensitive from denervation) while having little effect on the normal pupil. This reversal of anisocoria confirms Horner syndrome. Cocaine drops, where available, also confirm the diagnosis by showing failure of dilation in the affected eye. Hydroxyamphetamine drops help distinguish preganglionic from postganglionic lesions, though this distinction is increasingly supplemented or replaced by imaging.
The full workup – including MRI of the brain and neck, vascular imaging, and chest CT in appropriate cases – is guided by the clinical presentation. Most patients with Horner syndrome end up being seen jointly by ophthalmology and neurology. Our neuro-ophthalmology section covers more conditions at this interface.
Seek Urgent Assessment For
- Sudden-onset drooping of one eyelid with a smaller pupil on the same side, especially with any neck, jaw, or face pain – this is carotid dissection until proven otherwise
- Horner syndrome with shoulder pain, arm weakness, or hand wasting – Pancoast tumor
- Horner syndrome in a child with a neck or abdominal mass – neuroblastoma
- Any new Horner syndrome without a clear and benign explanation
The signs of Horner syndrome – a small pupil and a slightly droopy lid – can look quite innocent. The urgency is entirely about the cause, not the appearance of the eye itself.
Frequently Asked Questions About Horner Syndrome
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Is the droopy eyelid in Horner syndrome the same as a lazy eye?
No. Amblyopia (lazy eye) is a developmental condition affecting visual acuity (visual acuity) in children. The ptosis of Horner syndrome is a mechanical drooping of the eyelid caused by loss of the muscle responsible for lifting the upper lid – Muller’s muscle, which is sympathetically innervated. It is typically mild (1–3mm) and does not usually obstruct vision in adults. In infants with congenital Horner, however, significant ptosis can interfere with visual development and needs monitoring.
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Does Horner syndrome go away?
It depends entirely on the cause. Horner syndrome from carotid dissection often partially or fully resolves over months as the vessel heals. Post-surgical Horner frequently recovers. Horner from a tumor will persist until the cause is treated, and even then may not resolve completely. The signs themselves – the ptosis and miosis – are not dangerous, but they reflect whatever is happening to the nerve pathway.
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Can Horner syndrome affect vision?
The Horner syndrome signs themselves rarely affect vision significantly. The mild ptosis doesn’t usually block the visual axis in adults. The miosis means slightly less light enters the eye, which may make vision marginally less comfortable in dim conditions, but this is rarely a significant functional problem. Vision symptoms, if present, are usually from the underlying cause rather than the Horner syndrome itself.
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My child was born with different-coloured eyes and a slight ptosis. Could this be Horner syndrome?
Heterochromia and mild ptosis together in a child are classic features of congenital Horner syndrome. This is worth evaluating by an ophthalmologist and paediatrician. The most important thing is to exclude neuroblastoma, a tumor of neural crest tissue that can cause congenital Horner – urine catecholamine tests and imaging are standard. Most congenital Horner syndrome turns out to have a benign cause (birth trauma, for example), but it needs proper assessment.
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I’ve been told I have Horner syndrome but feel completely well. Do I still need investigations?
Yes. Feeling well doesn’t reliably exclude serious underlying causes, particularly Pancoast tumor, which may be silent for some time. The standard recommendation for adults with new, unexplained Horner syndrome is imaging of the head, neck, and chest. The investigation is to characterise the cause, not because the Horner signs themselves are dangerous.
The American Academy of Ophthalmology’s neuro-ophthalmology resources cover related conditions at the brain-eye interface. For a thorough review of the anatomy and clinical localisation of Horner syndrome, this review in the Journal of Clinical Neuroscience provides detailed clinical guidance. For more on conditions affecting the optic nerve and visual pathways, visit our neuro-ophthalmology section.
Horner syndrome is the clinical triad of ipsilateral ptosis, miosis, and anhidrosis resulting from interruption of the three-neuron sympathetic pathway that supplies the eye and face. The sympathetic chain runs from the hypothalamus through the brainstem and spinal cord (first-order neuron), across the thoracic inlet and around the subclavian and carotid arteries (second-order/preganglionic neuron), and along the internal carotid artery to the orbit (third-order/postganglionic neuron). Identifying which segment of this pathway is interrupted determines the urgency and direction of investigation. A new postganglionic Horner syndrome with ipsilateral head, neck, or face pain may represent internal carotid artery dissection , a stroke-risk emergency requiring immediate imaging. A new preganglionic Horner syndrome in an adult smoker may represent a Pancoast tumor. The clinical signs themselves cause little visual disability; the danger of Horner syndrome lies in the underlying cause.
Clinical Overview: Horner Syndrome
- Classic triad: Ptosis (2-3 mm , partial, from superior tarsal muscle [Müller’s muscle] denervation, not levator), miosis (pupil smaller on the affected side, more prominent in dim light , the darkness should dilate the normal pupil but the denervated pupil cannot), anhidrosis (of the face/forehead on the affected side , may be partial; depends on the level of the lesion)
- Additional signs: Lower lid elevation (upside-down ptosis, from inferior tarsal muscle denervation, narrows the palpebral fissure by approximately 1 mm inferiorly); iris heterochromia (congenital Horner only , the affected iris is lighter, from failure of melanin deposition during development); enophthalmos (apparent, from narrowed palpebral aperture, not true orbital recession)
- Pharmacological testing: Cocaine 4-10% dilates the normal pupil (blocks noradrenaline reuptake , requires noradrenaline in the synapse); fails to dilate in Horner (no noradrenaline released from denervated terminal). Now largely replaced by apraclonidine 0.5-1%: in Horner, the denervated pupil shows supersensitivity and dilates; the normal pupil slightly constricts. Result reversal of anisocoria on apraclonidine confirms Horner syndrome.
- Hydroxyamphetamine (paredrine) test: Localizes to pre- vs post-ganglionic: 1% hydroxyamphetamine releases stored noradrenaline from the post-ganglionic terminal. Dilates in first- and second-order Horner (terminal is intact); fails to dilate in third-order Horner (terminal is denervated). Not widely available in UK.
- Urgency of investigation: Acute onset + ipsilateral pain (head/neck/face) = carotid dissection until proven otherwise; immediate MRI/MRA. Acute onset + neurological signs (arm weakness, brainstem signs, Wallenberg syndrome) = first-order Horner from brainstem infarct; immediate stroke workup. Chronic Horner in a child = investigate for neuroblastoma. New Horner in an adult with no explanation = chest CT to exclude Pancoast tumor.
Anatomy of the Sympathetic Pathway
The oculosympathetic pathway has three neurons. The first-order (central) neuron originates in the posterolateral hypothalamus, descends through the midbrain tegmentum and lateral pons, and synapses in the ciliospinal center of Budge-Waller (C8-T2 in the spinal cord). Lesions here cause Horner with other brainstem or cord signs (Wallenberg syndrome, Brown-Sequard). Visual field testing may show hemianopia in first-order central lesions. The second-order (preganglionic) neuron exits at C8-T2, passes over the apex of the lung and subclavian artery, and synapses in the superior cervical ganglion (SCG) at the bifurcation of the common carotid artery. Lesions here include Pancoast tumor, lung apex disease, thyroid surgery, neck dissection, thoracic aortic aneurysm. The third-order (postganglionic) neuron travels along the internal carotid artery (for pupil and upper lid) and the external carotid artery (for lower lid and sweat glands). Internal carotid dissection is the critical emergency diagnosis at this level.
Investigation by Level
New Horner + ipsilateral pain (head, neck, face): Internal carotid artery dissection is the priority. MRI brain with DWI (to exclude ischemic stroke) and MRA/CTA of the neck (to identify dissection , the crescent sign of mural hematoma in the carotid wall). Onset typically follows neck trauma, chiropractic manipulation, or spontaneous in a young patient with connective tissue disorder (Marfan syndrome, Ehlers-Danlos). Treat with antiplatelet or anticoagulation; neurological or vascular surgery referral.
Horner + neurological signs (brainstem, cord, brachial plexus): First-order or second-order neuron pathology. MRI brain and cervical spine. Wallenberg syndrome (lateral medullary infarct): ipsilateral Horner, ipsilateral facial numbness, contralateral body numbness, dysphagia, vertigo , a recognizable cluster. Pancoast syndrome (second-order preganglionic): ipsilateral Horner + arm pain/weakness + wasting of hand muscles, from superior sulcus lung tumor compressing C8-T1 roots and the preganglionic sympathetic chain.
Isolated Horner, no pain or neurological signs: Localization is less clear clinically. If acute onset in an adult smoker: chest CT to exclude Pancoast tumor (apical lung mass). If chronic and incidentally discovered: likely old; document and review history for prior chest surgery, neck trauma, or thoracic interventions. In a child with no prior trauma or surgery: neuroblastoma workup (urine catecholamines, abdominal/chest CT/MRI).
Congenital Horner Syndrome
Congenital Horner presents at birth or in early infancy with the triad plus iris heterochromia (lighter iris on the affected side). Causes: birth trauma (brachial plexus injury at C8-T1, from shoulder dystocia), neuroblastoma (adrenal or posterior mediastinal), congenital vascular malformations, or idiopathic. All new Horner syndromes in children require investigation for neuroblastoma: urine catecholamines (homovanillic acid [HVA] and vanillylmandelic acid [VMA]), chest and abdominal MRI. Iris heterochromia is permanent even after the sympathetic pathway recovers , it is a marker of early-onset sympathetic denervation during melanocyte development, not of current disease activity.
Clinical Decision Points
- New Horner + headache or neck pain, young patient: Carotid dissection emergency. MRI DWI + MRA neck immediately. Do not defer to next-day outpatient clinic.
- Horner + contralateral motor or sensory deficit: First-order central lesion (brainstem or cord). Stroke pathway. Urgent MRI brain.
- Horner + shoulder/arm pain + apical lung opacity: Pancoast tumor (second-order preganglionic). CT chest and biopsy. Oncology referral.
- Isolated chronic Horner, adult, history of prior chest surgery or neck dissection: Iatrogenic. Document. No further workup required if history is clear and the Horner is stable and matches the surgical timeline.
- Horner in a child under 5 with no birth injury history: Neuroblastoma workup (urine HVA/VMA, cross-sectional imaging) before any other explanation is accepted.
- Apraclonidine test positive, no pain, no other signs, adult: Chest CT to exclude Pancoast tumor. If CT clear and no other explanation: label as idiopathic post-viral/post-traumatic Horner and monitor for 3-6 months with repeat clinical assessment.
Emergency Presentations
- Acute Horner + ipsilateral neck or face pain , carotid artery dissection; emergency MRI/MRA, stroke team assessment, and antiplatelet/anticoagulation
- Acute Horner + Wallenberg features (facial numbness, dysphagia, vertigo) , lateral medullary infarct; emergency stroke pathway
- New Horner in a child , neuroblastoma until proven otherwise; same-day paediatric referral
Internal carotid dissection classically presents to the eye clinic as a unilateral Horner syndrome, sometimes before the ischemic stroke event occurs. The ophthalmologist who identifies a new painful Horner and proceeds to a scheduled neuro-ophthalmology clinic appointment rather than emergency MRI/MRA may be seeing the patient during the window before stroke. Do not delay vascular imaging in any new painful Horner syndrome.
Clinical Pearls: Horner Syndrome
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Anisocoria in dim light, not bright light, is the distinguishing feature of Horner syndrome. Test in both conditions.
The sympathetic pathway dilates the pupil in darkness. In Horner syndrome, the affected pupil cannot dilate, so anisocoria is maximal in dim illumination (when the normal pupil is dilated wide and the Horner pupil is small). In bright light, both pupils constrict , the Horner pupil also constricts, reducing the anisocoria. Examining a patient with suspected Horner only in bright clinic lighting will minimize the anisocoria and may lead to dismissal of a real finding. Always examine with a dim flashlight from below, allowing the normal pupil to dilate, and observe the asymmetry.
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Apraclonidine reverses the anisocoria in Horner syndrome. Knowing this prevents misinterpretation.
When apraclonidine 0.5% is instilled bilaterally in a patient with Horner syndrome, the denervated pupil (supersensitive from upregulation of alpha-1 adrenoceptors) dilates, while the normal pupil constricts slightly (from alpha-2 agonism reducing aqueous production and causing mild mydriasis reversal). After 30-45 minutes, the previously smaller Horner pupil becomes larger , the anisocoria has reversed. This is a positive test. A clinician who does not know this interpretation may incorrectly conclude that the previously normal eye now has the abnormality. The direction of the anisocoria reversal is the positive result, not simply a change in pupil size.
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Iris heterochromia in Horner syndrome is always congenital or acquired very early in childhood. New-onset Horner does not cause heterochromia.
Melanocyte development in the iris stroma depends on trophic signals from sympathetic innervation during a critical developmental window in the first months of life. Sympathetic denervation during this window prevents normal melanin deposition, resulting in a permanently lighter iris on the affected side. An adult who develops Horner syndrome , from any cause , will not develop iris heterochromia. The finding of iris heterochromia in a Horner syndrome patient means the sympathetic denervation began in infancy, even if the clinical presentation seems recent. This has diagnostic implications for adult patients who present with “new” Horner and heterochromia , the lesion is longstanding, not acute.
Further reading: AAO Neuro-Ophthalmology PPP. For the anatomy context see the iris and pupil basics page. Related conditions: optic neuritis (other acute neuro-ophthalmic presentations), nystagmus (may co-occur with brainstem lesions). Subspecialty context: neuro-ophthalmology subspecialty page.