Horner Syndrome

Neuro-OphthalmologyTriadHigh YieldAlso: Horner's Syndrome, Oculosympathetic Palsy, Bernard-Horner Syndrome

Key Points

•Acute painful Horner syndrome is carotid dissection until proven otherwise — urgent MRA/CTA neck is mandatory.
•Apraclonidine (not cocaine) is now the preferred first-line pharmacological test for confirming Horner syndrome due to denervation supersensitivity reversal of anisocoria.
•Hydroxyamphetamine differentiates postganglionic (3rd order) from preganglionic/central lesions by testing the integrity of the 3rd neuron's norepinephrine stores.
•In congenital Horner, iris heterochromia (lighter affected iris) is a key finding — rule out neuroblastoma in all pediatric cases.

Hallmark Features(Triad)

Miosis (pupil constriction)

Ptosis (partial, 1-2mm)

Anhidrosis (ipsilateral facial)

Finder Clues

Miosis (ipsilateral)Partial ptosis (1-2 mm)Anhidrosis (ipsilateral facial)Apparent enophthalmosIris heterochromia (congenital Horner)Upside-down ptosis (lower lid elevation)Dilation lag of pupil

1. Definition

Disruption of the oculosympathetic pathway causing ipsilateral miosis, partial ptosis, and anhidrosis. Three-neuron arc: central (1st order), preganglionic (2nd order), postganglionic (3rd order).

2. Genetics

Not a genetic disorder. Acquired from disruption of the three-neuron sympathetic pathway at any level (central, preganglionic, or postganglionic). Congenital Horner may result from birth trauma (brachial plexus injury) or neuroblastoma.

3. Pathogenesis

The oculosympathetic pathway is a three-neuron arc:

1st order (central) neurons:

Travel from the posterolateral hypothalamus to the ciliospinal center of Budge (C8-T2)
Located in the intermediolateral column of the spinal cord

2nd order (preganglionic) neurons:

Exit the spinal cord and travel over the lung apex
Synapse at the superior cervical ganglion

3rd order (postganglionic) neurons:

Travel along the internal carotid artery, through the cavernous sinus
Reach the eye via the long ciliary nerves (to the iris dilator)
Sympathetic fibers supply Mueller's muscle (upper lid) and inferior tarsal muscle (lower lid)

Disruption at any level causes loss of sympathetic tone to the ipsilateral eye and face.

Anhidrosis pattern by lesion level:

1st order — affects the entire ipsilateral body (face, arm, trunk)
2nd order — affects the ipsilateral face
3rd order — affects only the medial forehead (or no anhidrosis, as facial sweat glands receive sympathetic supply via the external carotid plexus)

4. Clinical Features

Classic triad:

Miosis — small pupil from unopposed parasympathetic tone
Ptosis — partial, 1-2 mm, from loss of Mueller's muscle tone
Anhidrosis — pattern depends on lesion level

Additional signs:

Apparent enophthalmos — narrowed palpebral fissure (not true globe recession)
Dilation lag — affected pupil dilates more slowly in darkness; best observed with flash photography at 5 and 15 seconds
Lower lid reverse ptosis — loss of inferior tarsal muscle tone
Iris heterochromia — in congenital Horner, affected iris is lighter due to impaired melanocyte stimulation

5. Ocular Manifestations

Pupil findings:

Miosis — 1-2 mm smaller than contralateral pupil, more pronounced in dim illumination (anisocoria increases in the dark)
Dilation lag — affected pupil dilates more slowly when lights are turned off

Lid findings:

Ptosis — mild, 1-2 mm, from denervation of Mueller's muscle
Lower lid elevation (reverse ptosis) — loss of inferior tarsal muscle tone
Apparent enophthalmos — result of the narrowed fissure, not true posterior globe displacement

Other findings:

Iris heterochromia — lighter iris on affected side in congenital Horner (sympathetic innervation required for melanocyte development)
Decreased IOP on affected side — due to reduced aqueous production

6. Systemic Manifestations

1st order (central) — anhidrosis of ipsilateral face, arm, and trunk:

Brainstem stroke
Syringomyelia
Multiple sclerosis
Tumors

2nd order (preganglionic) — anhidrosis of ipsilateral face:

Pancoast tumor (lung apex)
Thyroid surgery
Chest tube, trauma
Lymphoma, cervical rib

3rd order (postganglionic) — medial forehead only or absent:

Carotid dissection — most important to exclude
Cavernous sinus lesion
Cluster headache
Middle ear infection

Systemic features depend entirely on the underlying cause, not the Horner syndrome itself.

7. Diagnosis

Step 1 — Confirm Horner syndrome:

Cocaine 4-10% drops — blocks norepinephrine reuptake; normal pupil dilates, Horner pupil fails to dilate (absent norepinephrine at nerve terminal)
Apraclonidine 0.5-1% — weak alpha-1 agonist; Horner pupil dilates via denervation supersensitivity, normal pupil constricts slightly — reversal of anisocoria
Apraclonidine is now preferred first-line due to cocaine availability issues

Step 2 — Localize the lesion:

Hydroxyamphetamine 1% (Paredrine) — releases stored norepinephrine from intact postganglionic terminals
Pupil dilates → 3rd order neuron intact → 1st or 2nd order lesion
Pupil fails to dilate → 3rd order (postganglionic) lesion

Step 3 — Imaging:

MRI/MRA of brain, neck, and chest based on suspected level
CTA or MRA neck is critical to rule out carotid dissection in acute, painful 3rd order Horner

8. Differential Diagnosis

Key differentials:

Physiological anisocoria — 20% of population; anisocoria is constant in light and dark, no ptosis or anhidrosis
Argyll Robertson pupil — bilateral miosis, light-near dissociation, associated with neurosyphilis
Pharmacological miosis — pilocarpine, opiates
Mechanical miosis — posterior synechiae, iris sphincter spasm
Third nerve palsy — causes mydriasis, not miosis (opposite finding)
Adie tonic pupil — dilated pupil with light-near dissociation

9. Management

Treatment is directed at the underlying cause, not the Horner syndrome itself.

Urgent workup:

Carotid dissection — MRA/CTA neck in acute, painful postganglionic (3rd order) Horner — most important emergency to exclude

Imaging by lesion level:

1st order — MRI brain (brainstem stroke, demyelination, tumor)
2nd order — CT chest (Pancoast tumor), MRI spine (syringomyelia)
Congenital Horner in children — rule out neuroblastoma with urine catecholamines and MRI/CT of chest and abdomen

Symptomatic:

Ptosis and miosis rarely require treatment
Phenylephrine drops can temporarily improve lid position if cosmetically desired

10. Prognosis

Prognosis depends entirely on the underlying etiology:

Carotid dissection — requires urgent anticoagulation/antiplatelet therapy to prevent stroke
Pancoast tumor — carries an oncological prognosis
Idiopathic Horner — excellent prognosis
Congenital Horner from birth trauma — typically benign course

The miosis, ptosis, and anhidrosis are generally well-tolerated cosmetically and functionally.

Clinical Pearls

Anisocoria that increases in dim illumination (dark) suggests a sympathetic defect (Horner); anisocoria that increases in bright light suggests a parasympathetic defect (CN3 palsy or pharmacological mydriasis).

The dilation lag test is a simple bedside test: photograph both pupils at 5 seconds and 15 seconds after lights off — the Horner pupil dilates more slowly and the anisocoria is greatest at 5 seconds.

Apraclonidine test may yield false-negative results in acute Horner syndrome (typically within the first 1–2 weeks) because denervation supersensitivity takes variable time to develop; positive results have been reported as early as 48 hours, but false negatives have been documented up to 2 weeks after onset.

Mnemonics

Horner Triad

M—Miosis

P—Ptosis

A—Anhidrosis

Classic triad of Horner syndrome

Horner Three-Neuron Arc Causes

1st order: Stroke, Syringomyelia, MS (central)

2nd order: Pancoast, Surgery, Trauma (preganglionic)

3rd order: Carotid Dissection, Cavernous sinus, Cluster headache (postganglionic)

Common etiologies organized by neuron order

Pharmacological Testing Order

C—Confirm (Cocaine or Apraclonidine)

L—Localize (Hydroxyamphetamine)

I—Image (MRI/CTA based on level)

Stepwise diagnostic approach for Horner syndrome