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 tumour 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

Ptosis degree 1–3mm Mild partial ptosis – much less than in third nerve palsy

Urgent cause Carotid dissection Acute-onset Horner with neck/face pain needs same-day vascular imaging

Congenital Heterochromia Different-coloured irises in congenital Horner reflect lack of pigmentation signals

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.

Anatomical diagram showing the three-neuron sympathetic pathway from the hypothalamus down through the brainstem and spinal cord, looping around the lung apex, traveling with the carotid artery, and terminating at the eye. Key lesion locations are marked along the pathway to illustrate where different causes of Horner syndrome interrupt the signal. — The three-neuron sympathetic pathway from brain to eye. A lesion anywhere along this route can produce Horner syndrome. Identifying where the interruption has occurred guides the investigation.

account_tree First-order neuron (central)

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

linear_scale Second-order neuron (preganglionic)

Exits spinal cord, loops around subclavian artery and lung apex, up to superior cervical ganglion
Causes: Pancoast tumour, thyroid surgery, thoracic aortic aneurysm, brachial plexus injury
Anhidrosis limited to face and neck
Always requires chest imaging in adults to exclude malignancy

route Third-order neuron (postganglionic)

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.

emergency

Urgent – same day

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.

radiology

Urgent – days

Pancoast tumour 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 tumour 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.

check_circle

Known or benign causes

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 tumour
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

Is the droopy eyelid in Horner syndrome the same as a lazy eye?
No. Amblyopia (lazy eye) is a developmental condition affecting 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.
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 tumour 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.
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.
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 tumour 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.
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 tumour, 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 comprehensive 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.