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Summary Table

Category	Detail
Risk Factors	History of lung apex tumor (Pancoast), neck trauma or surgery, carotid artery dissection, brainstem stroke, cluster headache, indwelling central venous catheters in the neck
Etiology	Disruption of the three-neuron oculosympathetic pathway at any level: central (first-order), preganglionic (second-order), or postganglionic (third-order)
Patient Presentation	Unilateral droopy eyelid with a small pupil on the same side; may note decreased sweating on the affected side of the face; often discovered incidentally or in the context of another presenting complaint (e.g., neck pain in carotid dissection, arm pain in Pancoast tumor)
Classic Physical Exam	Ipsilateral triad: miosis (constricted pupil), partial ptosis (1 to 2 mm lid droop), and anhidrosis (decreased sweating). Dilation lag of the affected pupil in dim lighting. Apparent enophthalmos (sunken eye appearance). In congenital cases, iris heterochromia (lighter iris on the affected side)
Key Diagnostic Results	Pharmacological testing: cocaine drops fail to dilate the affected pupil (confirms diagnosis); apraclonidine reverses the anisocoria (affected pupil dilates, normal pupil constricts slightly). Hydroxyamphetamine localizes the lesion to the third-order neuron if the pupil fails to dilate. Imaging directed by localization (MRI brain, CT chest, CTA/MRA neck)
Management	No direct treatment for the syndrome itself. Management is entirely directed at identifying and treating the underlying cause. Urgent workup is required when carotid dissection or malignancy is suspected
Keywords	"Miosis, ptosis, anhidrosis"; "Pancoast tumor with Horner"; "ipsilateral small pupil"; "cocaine eye drop test"; "dilation lag"; "carotid dissection with Horner"; "iris heterochromia in a child"

1. Pathophysiology

The sympathetic nervous system controls pupil dilation, smooth muscle tone of the upper eyelid, and sweat gland activity on the face. These functions are carried through a three-neuron relay chain called the oculosympathetic pathway. A lesion at any point along this chain produces Horner syndrome, and the location of the disruption determines both the clinical features and the underlying etiology.

The first-order (central) neuron originates in the posterolateral hypothalamus and descends through the brainstem, traveling close to the lateral medullary structures, before terminating at the ciliospinal center of Budge located at the C8 to T2 level of the spinal cord. Because this neuron descends through the brainstem, lesions here are almost always accompanied by other neurological signs. The classic association is Wallenberg syndrome (lateral medullary infarction), where Horner syndrome appears alongside crossed sensory loss, dysphagia, vertigo, and cerebellar ataxia. Other first-order causes include demyelinating disease and syringomyelia.

The second-order (preganglionic) neuron exits the spinal cord at T1 and ascends over the lung apex and along the neck to synapse at the superior cervical ganglion. The anatomical relationship with the lung apex is the reason a Pancoast tumor (superior sulcus tumor) is one of the most tested causes of Horner syndrome. The vignette will typically describe a smoker with shoulder or arm pain radiating along the medial arm (C8/T1 brachial plexus involvement) plus ipsilateral Horner findings. Thyroid and neck surgery, trauma, and chest tube placement are other second-order causes.

The third-order (postganglionic) neuron leaves the superior cervical ganglion and travels along the internal carotid artery before entering the skull, passing through the cavernous sinus, and reaching the orbit. It innervates the dilator pupillae muscle (pupil dilation), Muller's muscle (contributes about 2 mm of upper lid elevation), and facial sweat glands. Critically, the sudomotor fibers to the face branch off with the external carotid artery, not the internal carotid. This means a third-order lesion (which tracks along the internal carotid) produces minimal to no anhidrosis, a key localizing clue. The most important and most urgent third-order cause is carotid artery dissection, where the patient presents with sudden neck pain, ipsilateral Horner syndrome, and possible cerebral ischemic symptoms.

Understanding why each symptom occurs is essential:

Miosis results from loss of sympathetic input to the dilator pupillae muscle, leaving the parasympathetic-driven constrictor pupillae unopposed. The pupil becomes small but remains reactive to light.
Partial ptosis occurs because Muller's muscle, a sympathetically innervated smooth muscle, loses tone. It contributes only 1 to 2 mm of lid elevation, so the droop is subtle (unlike third cranial nerve palsy, which causes complete ptosis via levator palpebrae paralysis).
Anhidrosis reflects loss of sympathetic innervation to ipsilateral sweat glands. Its distribution depends on the lesion level: first-order lesions affect the entire ipsilateral body, second-order lesions affect the ipsilateral face, and third-order lesions produce little or no anhidrosis.
Dilation lag occurs because the Horner's pupil relies on slow passive dilation in darkness (parasympathetic relaxation) rather than active sympathetic-driven dilation. When lights are dimmed, the normal pupil dilates quickly while the affected pupil lags behind.

2. Classification and Clinical Manifestation

By Neuron Order

Neuron Order	Synonyms	Pathway Segment	Common Etiologies	Distinguishing Clinical Clues
First-order (Central)	Central Horner	Hypothalamus to ciliospinal center (C8-T2)	Wallenberg syndrome (lateral medullary stroke), demyelinating disease, hypothalamic/brainstem tumor, syringomyelia	Associated brainstem signs: crossed sensory deficits, ataxia, dysphagia, nystagmus. Anhidrosis of the entire ipsilateral body
Second-order (Preganglionic)	Preganglionic Horner	Ciliospinal center to superior cervical ganglion	Pancoast tumor, thyroid/neck surgery, trauma, neuroblastoma (children), cervical rib	Ipsilateral arm/shoulder pain (brachial plexus). Anhidrosis of the ipsilateral face. History of thoracic surgery or lung mass
Third-order (Postganglionic)	Postganglionic Horner	Superior cervical ganglion to the eye (via internal carotid artery)	Carotid artery dissection, cavernous sinus lesion, cluster headache, middle ear infection (children)	Neck pain or headache. Minimal or absent anhidrosis. May present with signs of cerebral ischemia if dissection is the cause

By Onset

Category	Features	Key Associations
Congenital	Present from birth; iris heterochromia (affected iris is lighter due to failed melanocyte stimulation); often discovered incidentally	Birth trauma (brachial plexus injury), neuroblastoma, congenital vascular anomalies
Acquired	New onset in a previously unaffected individual; symptoms develop over hours to weeks depending on cause	Pancoast tumor, carotid dissection, iatrogenic (surgery), stroke

Pediatric Considerations

Etiology	Age Group	Additional Features
Neuroblastoma	Infants and toddlers	Abdominal mass, opsoclonus-myoclonus, elevated urine catecholamines (VMA, HVA)
Birth trauma	Neonates	Breech delivery, Klumpke palsy (lower brachial plexus), iris heterochromia

3. Diagnostic Workup

Test	Purpose	Mechanism	Expected Result in Horner Syndrome
Cocaine drops (4 to 10%)	Confirms the diagnosis	Blocks norepinephrine reuptake at the synapse; depends on intact sympathetic NE release	Normal pupil dilates. Horner's pupil fails to dilate (no NE being released to accumulate)
Apraclonidine drops (0.5 to 1%)	Confirms the diagnosis (preferred alternative to cocaine)	Weak alpha-1 agonist; works via denervation supersensitivity of the affected iris dilator	Horner's pupil dilates (reversal of anisocoria). Normal pupil may constrict slightly from alpha-2 effect
Hydroxyamphetamine drops (1%)	Localizes the lesion (third-order vs. first/second-order)	Stimulates release of stored NE from intact postganglionic nerve terminals	Pupil dilates = first or second-order lesion (intact postganglionic neuron). Pupil fails to dilate = third-order lesion (damaged postganglionic neuron with depleted NE stores)
MRI brain and brainstem	Evaluate first-order causes	Imaging	Brainstem infarction, demyelination, tumor
CT or MRI chest	Evaluate second-order causes	Imaging	Pancoast tumor, mediastinal mass, lymphadenopathy
CTA or MRA of the neck	Evaluate third-order causes (urgent)	Vascular imaging	Carotid artery dissection (intimal flap, mural hematoma, "flame-shaped" tapering of the vessel)
Urine VMA and HVA	Screen for neuroblastoma in children	Catecholamine metabolites	Elevated levels support neuroblastoma

The diagnostic approach has two phases: confirm the syndrome and then localize and identify the cause.

Phase 1: Confirming Horner Syndrome. The best initial pharmacological test is apraclonidine (0.5 to 1% eye drops), which has largely replaced cocaine in clinical practice because cocaine is a controlled substance and difficult to obtain in many settings. Apraclonidine works through the principle of denervation supersensitivity: when the sympathetic pathway is disrupted, the postsynaptic alpha-1 receptors on the iris dilator muscle upregulate and become hypersensitive to even weak agonists. In Horner syndrome, the affected pupil dilates while the normal pupil may constrict slightly, producing a reversal of anisocoria. This reversal pattern is diagnostic.

If cocaine drops are used instead, the mechanism is different: cocaine blocks norepinephrine (NE) reuptake at the synaptic cleft. A normal eye has active sympathetic NE release, so blocking reuptake causes NE to accumulate and the pupil dilates. In Horner syndrome, the disrupted pathway means no NE is being released; there is nothing to accumulate, and the pupil fails to dilate.

Phase 2: Localizing the Lesion. Once Horner syndrome is confirmed, the next question is where along the three-neuron chain the lesion sits. Hydroxyamphetamine (1%) drops are used for this purpose. This drug stimulates the release of NE from intact postganglionic (third-order) nerve terminals. If the third-order neuron is intact (meaning the lesion is at the first or second level), stored NE will be released and the pupil dilates. If the third-order neuron is damaged, the depleted terminals cannot release NE, and the pupil fails to dilate. This test must be performed at least 48 hours after cocaine drops if cocaine was used in Phase 1.

Phase 3: Imaging. Once the lesion is localized, imaging is directed accordingly. For a suspected first-order lesion, order MRI of the brain and brainstem. For second-order, CT or MRI of the chest and neck (looking for Pancoast tumor or mediastinal pathology). For third-order, CTA or MRA of the head and neck is the priority because the most dangerous cause, carotid artery dissection, demands urgent identification. In children, urine catecholamine metabolites (vanillylmandelic acid and homovanillic acid) should be checked to rule out neuroblastoma regardless of the localization result.

A clinical shortcut often tested: if a patient presents with acute Horner syndrome plus ipsilateral neck/head pain, assume carotid dissection until proven otherwise and proceed directly to vascular imaging without waiting for pharmacological localization.

4. Management and Treatment

Clinical Scenario	Management Priority	Treatment
Carotid artery dissection	Urgent	Anticoagulation with heparin followed by warfarin for 3 to 6 months (to prevent thromboembolic stroke); some centers use antiplatelet therapy instead. Endovascular intervention if there is flow-limiting stenosis or failed medical therapy
Pancoast tumor	Oncologic	Neoadjuvant chemoradiation followed by surgical resection (if resectable). Staging workup before treatment
Brainstem stroke (Wallenberg)	Acute stroke protocol	Thrombolytics (alteplase) if within the treatment window; otherwise supportive care, swallowing evaluation, and secondary stroke prevention
Neuroblastoma (pediatric)	Oncologic referral	Staging with MIBG scan; treatment ranges from observation (low-risk) to chemotherapy, surgery, and radiation (high-risk)
Cluster headache	Acute and prophylactic	Acute: 100% oxygen via non-rebreather mask, sumatriptan subcutaneous injection. Prophylaxis: verapamil
Iatrogenic / post-surgical	Observation	Spontaneous recovery may occur. Cosmetic ptosis repair (Muller's muscle conjunctival resection) if persistent and bothersome
Idiopathic	Monitoring	Follow-up with serial examinations. Repeat imaging if new symptoms develop

Horner syndrome itself does not require direct treatment. The ptosis is mild, the miosis does not impair vision, and the anhidrosis is rarely symptomatic. The entire management effort is directed at identifying and treating the underlying etiology, because some causes are life-threatening.

Acute stabilization applies primarily when carotid dissection is the cause. In this setting, the risk of ischemic stroke from thrombus formation at the dissection site is the immediate concern. Anticoagulation with intravenous unfractionated heparin bridged to oral warfarin (target INR 2.0 to 3.0) for 3 to 6 months is the traditional approach. Recent evidence also supports antiplatelet therapy (aspirin 325 mg daily) as a noninferior alternative, and practice varies by institution. The choice between anticoagulation and antiplatelet therapy should weigh the risk of hemorrhagic conversion, especially if the dissection is intracranial rather than extracranial.

For Pancoast tumor, the standard approach is neoadjuvant concurrent chemoradiation (typically cisplatin-based chemotherapy with thoracic radiation) followed by surgical en bloc resection if the tumor is deemed resectable. The key contraindication to surgery is vertebral body invasion or extensive brachial plexus involvement above T1.

For Wallenberg syndrome, follow standard acute ischemic stroke protocols. Posterior circulation strokes are often missed or delayed in diagnosis because the symptoms (vertigo, dysphagia, ataxia) overlap with benign conditions. Horner syndrome in this context is a localizing clue, not the primary management target.

For idiopathic Horner syndrome (no cause identified after thorough workup), close follow-up is recommended with repeat imaging at 3 to 6 months, because occult malignancy or dissection can be initially invisible on imaging.

If the ptosis is cosmetically bothersome and the underlying cause has been resolved or is benign, Muller's muscle conjunctival resection is a surgical option for correction.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Physiological anisocoria	Also presents with unequal pupil size	Pupil size difference is less than 1 mm and remains constant in all lighting conditions. No ptosis, no anhidrosis. The difference does not change with cocaine or apraclonidine drops
Third cranial nerve (oculomotor) palsy	Also causes ptosis	Produces the opposite pupil finding: mydriasis (dilated pupil) instead of miosis. Ptosis is complete (not partial). Often accompanied by a "down and out" eye position and diplopia
Adie tonic pupil	Also involves anisocoria	The affected pupil is dilated (not constricted), reacts sluggishly to light but constricts with prolonged near accommodation ("light-near dissociation"). Responds to dilute pilocarpine (0.1%) due to denervation supersensitivity of the sphincter muscle
Mechanical ptosis (e.g., dermatochalasis, myasthenia gravis)	Also presents with droopy eyelid	No pupil abnormality. Myasthenia gravis shows fatigable ptosis that worsens with sustained upgaze (ice pack test and anti-AChR antibodies help distinguish). Dermatochalasis is bilateral redundant skin
Argyll Robertson pupil	Also involves a small pupil	Bilateral miotic pupils that do not react to light but constrict with accommodation ("prostitute's pupil": accommodates but does not react). Associated with neurosyphilis. No ptosis
Cluster headache (isolated)	Horner features can appear during an attack	Transient autonomic features (miosis, ptosis, lacrimation, rhinorrhea) occur during the headache and resolve between attacks. Severe unilateral periorbital pain with restlessness and agitation is the dominant feature

6. Traps and High-Yield Pearls

The most common way students get Horner syndrome questions wrong is by confusing it with third cranial nerve palsy. Both conditions involve the eye, both cause ptosis, and both produce anisocoria. The critical distinction is the direction of the pupil abnormality: Horner syndrome produces a small pupil (miosis from sympathetic loss), while oculomotor palsy produces a large pupil (mydriasis from parasympathetic loss). When a vignette describes ptosis with a constricted pupil, the answer is Horner. When it describes ptosis with a dilated pupil and ophthalmoplegia, the answer is CN III palsy. Mixing these up is the single most frequent error.

A second trap involves the Pancoast tumor vignette that does not mention the eye at all. The stem will describe a smoker with shoulder pain radiating down the medial forearm (C8/T1 dermatome) and hand weakness. Many students select brachial plexus injury or cervical radiculopathy without recognizing the combination of upper extremity neurological symptoms plus a lung apex mass. The question writer adds Horner findings (miosis, ptosis, anhidrosis) as the discriminator that points to Pancoast, and students who do not actively look for the triad will miss it.

Third, students often forget that acute-onset Horner syndrome with neck or head pain is carotid dissection until proven otherwise. This is a time-sensitive diagnosis. The vignette may describe a young patient with recent neck trauma (chiropractic manipulation, sports injury, minor car accident) who develops sudden unilateral ptosis, miosis, and ipsilateral neck pain. The next best step is CTA or MRA of the neck, not pharmacological eye drop testing. Spending time on cocaine drops in this scenario is a management error because the priority is ruling out a vascular emergency.

Finally, in pediatric Horner syndrome, always consider neuroblastoma. A child with new Horner syndrome and an abdominal mass, or with opsoclonus-myoclonus, should trigger urine catecholamine testing. Congenital Horner with iris heterochromia is another classic visual clue: the affected eye has a lighter iris because sympathetic innervation is required for normal melanocyte development in the iris stroma.

The core competency being tested is pattern recognition of the sympathetic triad (miosis, partial ptosis, anhidrosis), the ability to localize the lesion using clinical context and pharmacological testing, and the judgment to prioritize urgent etiologies (carotid dissection in adults, neuroblastoma in children) over routine workup.

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