Ophthalmology – Horner Syndrome

Basics

Description

Horner syndrome (HS) results from interruption of the oculosympathetic pathway supplying the eye and face.

The classic clinical features are:

• Miosis — due to paralysis of the iris dilator muscle
• Mild ptosis — due to denervation of Müller’s superior tarsal muscle
• Anhidrosis — variable, depending on the level of the lesion

Other findings include:

• Dilation lag in darkness
• Mild reverse ptosis of the lower eyelid
• Transient conjunctival hyperemia in acute cases
• Iris heterochromia in congenital or very early-onset Horner syndrome

Horner syndrome may be congenital or acquired.

Clinical priority: New painful Horner syndrome should raise immediate concern for internal carotid artery dissection.

Oculosympathetic Pathway

Understanding the three-neuron sympathetic pathway is essential for localizing Horner syndrome.

First-Order Neuron — Central

The pathway begins in the posterolateral hypothalamus.

Fibers descend ipsilaterally through the:

Hypothalamus → midbrain → pons → medulla → cervical spinal cord

They terminate in the ciliospinal center of Budge, approximately C8–T2.

Causes of First-Order Horner Syndrome

• Brainstem stroke
• Demyelinating disease
• Brainstem tumor
• Syringomyelia
• Cervical spinal cord tumor
• Cervical spinal cord trauma

Associated neurologic findings frequently help localize the lesion.

Second-Order Neuron — Preganglionic

Preganglionic fibers leave the spinal cord, pass over the pulmonary apex, and ascend through the cervical sympathetic chain.

They eventually synapse in the superior cervical ganglion, near the carotid bifurcation.

Causes of Second-Order Horner Syndrome

Think particularly about lesions of the chest and neck:

• Pancoast/apical lung tumor
• Neck tumors
• Mediastinal disease
• Cervical trauma
• Brachial plexus injury
• Neck or thoracic surgery
• Neuroblastoma in children

Because the pathway passes near the lung apex, an otherwise unexplained Horner syndrome may be the presenting manifestation of an apical pulmonary malignancy.

Third-Order Neuron — Postganglionic

Postganglionic fibers leave the superior cervical ganglion and travel with the internal carotid artery into the skull.

They pass through the cavernous sinus, then enter the orbit and ultimately reach:

• Iris dilator muscle
• Müller’s superior tarsal muscle

Causes of Third-Order Horner Syndrome

Important causes include:

• Internal carotid artery dissection
• Internal carotid aneurysm
• Cavernous sinus lesions
• Skull-base lesions
• Cluster headache
• Other trigeminal-autonomic cephalalgias

A cavernous sinus lesion usually causes additional cranial neuropathies rather than an isolated Horner syndrome.

High-Yield Localization

A useful anatomical framework is:

1st order: hypothalamus → brainstem → C8–T2

2nd order: spinal cord → lung apex → cervical sympathetic chain → superior cervical ganglion

3rd order: internal carotid artery → cavernous sinus → orbit

This anatomy determines both the differential diagnosis and the required imaging.

Pathophysiology

Loss of sympathetic innervation produces the characteristic ocular findings.

Miosis

Paralysis of the iris dilator leaves parasympathetic pupillary constriction relatively unopposed.

Ptosis

Denervation of Müller’s muscle causes approximately 1–3 mm of upper-lid ptosis.

Reverse Ptosis

Loss of sympathetic tone to the lower eyelid can produce slight elevation of the lower lid.

Together, upper and lower lid changes create an apparent narrowing of the palpebral fissure.

Anhidrosis

Loss of sympathetic innervation to sweat glands causes reduced sweating.

Its distribution depends strongly on lesion location.

Diagnosis

History

The most common presenting complaint is new unilateral ptosis.

Patients usually do not notice the miosis itself.

The history should establish:

• When the ptosis or anisocoria began
• Whether it was sudden or gradual
• Whether old photographs show the same findings
• Previous head, neck, or chest trauma
• Recent neck manipulation or surgery
• Previous central venous procedures
• Smoking history
• Pulmonary disease
• Previous malignancy
• Neurologic symptoms

Old photographs are particularly valuable for distinguishing a longstanding Horner syndrome from a newly acquired one.

Pain Is a Critical Red Flag

The most important associated symptom in an acute Horner syndrome is:

Head, orbital, facial, or neck pain.

A patient with:

Acute Horner syndrome + ipsilateral head/neck pain

should be considered to have internal carotid artery dissection until appropriately excluded.

The pain may be severe, but it can also be described simply as a dull ache or discomfort.

This presentation requires urgent vascular imaging.

Associated Neurologic Symptoms

Ask specifically about:

• Vertigo
• Ataxia
• Dysarthria
• Dysphagia
• Nystagmus
• Facial sensory abnormalities
• Hemisensory loss
• Limb weakness
• Diplopia

These findings may indicate a central lesion such as a brainstem stroke.

Physical Examination

Ptosis

Upper-eyelid ptosis is generally mild, approximately 1–3 mm.

This is because the major eyelid elevator—the levator palpebrae superioris supplied by CN III—remains functional.

Thus:

Horner ptosis = mild

whereas a complete third-nerve palsy can cause profound ptosis.

Reverse Ptosis

Slight elevation of the lower eyelid may occur from loss of sympathetic innervation to the inferior tarsal muscle.

The combination of:

Upper-lid ptosis + lower-lid reverse ptosis

creates an apparent enophthalmos.

The globe itself is generally not truly enophthalmic.

Miosis and Anisocoria

The affected pupil is smaller.

The key examination principle is:

Anisocoria is greater in darkness.

In bright illumination, both pupils constrict relatively normally.

In darkness, the normal pupil dilates promptly while the Horner pupil cannot dilate normally.

Therefore, the difference between the pupils becomes more obvious.

Dilation Lag

Dilation lag is characteristic of Horner syndrome.

After moving from bright illumination into darkness, the Horner pupil dilates more slowly than the normal pupil.

The anisocoria may therefore be most prominent during the first several seconds of darkness.

Dilation lag supports the diagnosis but its absence does not exclude Horner syndrome.

Pupillary Light Reaction

Because the parasympathetic pathway is intact, the Horner pupil generally has a normal direct and consensual light response.

This is an important distinction from disorders involving the parasympathetic pupillary pathway.

Anhidrosis

Anhidrosis varies according to the anatomical level of the lesion.

Central and preganglionic lesions may produce more extensive facial anhidrosis.

In many postganglionic lesions involving the internal carotid sympathetic plexus, facial sweating is relatively preserved because sudomotor fibers to much of the face travel predominantly with the external carotid artery.

Therefore:

Absence of anhidrosis does not exclude Horner syndrome.

Iris Heterochromia

Congenital or very early-onset sympathetic denervation can interfere with iris melanocyte development.

The affected iris becomes less pigmented and therefore lighter.

Thus:

Congenital Horner syndrome → lighter iris on the affected side

Heterochromia is much less useful in adult-onset disease.

Pharmacologic Confirmation

Apraclonidine Test

Apraclonidine testing is now commonly used to confirm Horner syndrome.

Chronic sympathetic denervation produces denervation supersensitivity of α₁ receptors in the iris dilator.

After apraclonidine:

• Normal pupil → little dilation or slight constriction
• Horner pupil → significant dilation

This produces a characteristic:

Reversal of anisocoria

The previously smaller Horner pupil becomes similar in size to, or larger than, the normal pupil.

The Horner ptosis may also improve temporarily.

Important Limitation in Acute Horner Syndrome

Denervation supersensitivity requires time to develop.

Consequently, very acute Horner syndrome may occasionally produce a false-negative apraclonidine test.

More importantly:

Pharmacologic testing must never delay urgent imaging when carotid dissection or another dangerous cause is suspected.

Apraclonidine in Infants

Apraclonidine requires particular caution in young children because systemic absorption can cause:

• Profound lethargy
• Bradycardia
• Hypotension
• Respiratory depression

It should generally be avoided in very young infants, with pediatric testing protocols determined by the treating specialist.

Cocaine Test

Cocaine blocks norepinephrine reuptake at sympathetic nerve terminals.

Normal Eye

Norepinephrine accumulates → pupil dilates.

Horner Eye

Little norepinephrine reaches the terminal → minimal dilation.

Thus, after cocaine:

Anisocoria increases.

Historically, cocaine was the classic confirmatory test, but practical limitations have made apraclonidine more commonly used in many settings.

Hydroxyamphetamine

Hydroxyamphetamine was historically used to distinguish preganglionic from postganglionic Horner syndrome by stimulating norepinephrine release from intact postganglionic terminals.

However, it is difficult to obtain and has important practical limitations.

Modern imaging has substantially reduced the need for pharmacologic lesion localization.

Laboratory Investigation

Routine laboratory testing is generally not necessary to establish Horner syndrome.

Laboratory studies should instead be directed toward the suspected underlying disease.

The central diagnostic priority is usually appropriate imaging.

Imaging

Imaging is determined by:

• Age
• Acute versus chronic onset
• Pain
• Associated neurologic abnormalities
• Trauma
• Suspected anatomical localization
• Whether a benign cause has already been established

Unexplained acquired Horner syndrome generally requires investigation of the relevant oculosympathetic pathway.

Acute Painful Horner Syndrome

This is the most important emergency presentation.

Suspect: Internal Carotid Artery Dissection

Urgent imaging generally involves:

• CTA head and neck, or
• MRI/MRA head and neck

The carotid arteries must be adequately visualized.

Carotid Doppler ultrasonography alone is insufficient to exclude many dissections, particularly lesions near the skull base.

Suspected Central Horner Syndrome

When associated with:

• Ataxia
• Vertigo
• Dysarthria
• Dysphagia
• Nystagmus
• Sensory abnormalities
• Other focal neurologic deficits

perform appropriate brain and brainstem MRI, often with vascular imaging depending on the presentation.

Suspected Preganglionic Horner Syndrome

Evaluate the:

• Neck
• Cervical sympathetic chain
• Thoracic inlet
• Lung apex

Chest and neck imaging is particularly important when there is:

• Shoulder or arm pain
• Smoking history
• Brachial plexopathy
• Neck mass
• Known malignancy

An apical lung tumor must be excluded when clinically appropriate.

Pediatric Horner Syndrome

The evaluation differs from that in adults.

Possible causes include:

• Birth trauma
• Neck/chest surgery
• Congenital abnormalities
• Neuroblastoma
• Other neck or thoracic masses

When there is no definite benign explanation, investigation for neuroblastoma and other mass lesions may be required.

This can include appropriate imaging of the neck, chest, abdomen, and pelvis and measurement of urinary catecholamine metabolites such as:

• VMA
• HVA

Importantly, normal urinary catecholamine metabolites do not completely exclude neuroblastoma.

Differential Diagnosis

Physiologic Anisocoria

Physiologic anisocoria is common.

Unlike Horner syndrome, there is:

• No characteristic ptosis
• No convincing dilation lag
• No pharmacologic evidence of sympathetic denervation

The degree of physiologic anisocoria is generally relatively similar under different illumination conditions, although some variability can occur.

Third Cranial Nerve Palsy

Both disorders may produce ptosis.

The key pupillary distinction is:

Horner syndrome → abnormal pupil is SMALL

Compressive CN III palsy → abnormal pupil is typically LARGE

Third-nerve palsy may additionally produce:

• Marked ptosis
• Ophthalmoplegia
• “Down-and-out” eye position

Adie Tonic Pupil

In Adie syndrome, the abnormal pupil is usually larger, particularly in bright light.

There may be:

• Poor light reaction
• Better near response
• Segmental iris sphincter palsy
• Slow redilation after near effort

Thus, it represents essentially the opposite anisocoria pattern from Horner syndrome.

Pharmacologic Anisocoria

Consider exposure to:

• Pilocarpine or other miotics
• Anticholinergic agents
• Sympathomimetics
• Scopolamine
• Certain inhaled or nebulized medications

Medication and occupational exposure history can prevent unnecessary investigation.

Treatment

There is no specific treatment required for the miosis itself.

Treatment is directed at the underlying cause.

Examples include management of:

• Carotid artery dissection
• Stroke
• Neoplasm
• Neuroblastoma
• Infection
• Inflammatory disease
• Cervical or thoracic pathology

Management of carotid dissection is individualized according to neurologic and vascular findings and generally involves specialist-directed antithrombotic therapy.

Ptosis Treatment

If the underlying disorder has been appropriately investigated and the ptosis remains cosmetically or functionally significant, surgical correction can be considered.

Options may include procedures targeting Müller’s muscle or other ptosis repair techniques depending on eyelid measurements and function.

Prognosis

The ocular manifestations of Horner syndrome themselves usually do not threaten vision.

Prognosis depends almost entirely on the underlying cause.

For example:

• Congenital or longstanding benign Horner syndrome may remain stable indefinitely.
• Horner syndrome associated with carotid dissection may improve as sympathetic function recovers.
• Tumor-associated Horner syndrome depends on the prognosis of the underlying malignancy.

High-Yield Clinical Pearls

Horner syndrome = mild ptosis + miosis ± anhidrosis.

Anisocoria is greatest in the DARK.

Dilation lag supports Horner syndrome.

The affected pupil retains a normal light reaction because the parasympathetic pathway is intact.

Apraclonidine can reverse the anisocoria because of denervation supersensitivity.

Acute Horner syndrome + ipsilateral head/neck/orbital pain = carotid artery dissection until excluded.

Preganglionic Horner syndrome → think lung apex and neck.

Central Horner syndrome + neurologic deficits → think brainstem/spinal cord disease.

Congenital Horner syndrome can cause ipsilateral iris hypopigmentation.

Unexplained pediatric Horner syndrome warrants consideration of neuroblastoma.

Do not delay urgent vascular imaging merely to obtain pharmacologic confirmation in a clinically suspicious acute painful Horner syndrome.