What are the causes of Horner's (Horner) syndrome?

Causes of Horner's Syndrome

Horner's syndrome is most commonly caused by carotid artery dissection, iatrogenic procedures in the neck and chest, and tumors, with a significant portion of cases remaining idiopathic despite thorough investigation. 1, 2

Anatomical Understanding

Horner's syndrome results from interruption of the oculosympathetic pathway, characterized by the classic triad of:

• Ptosis (drooping eyelid)
• Miosis (constricted pupil)
• Anhidrosis (decreased sweating) on the affected side 1

The sympathetic pathway can be divided into three segments, each with distinct causes:

First-Order (Central) Neuron Lesions

• Brainstem stroke or infarction
• Demyelinating diseases
• Tumors of the brainstem or hypothalamus
• Syringomyelia

Second-Order (Preganglionic) Neuron Lesions

• Pancoast tumors (apical lung cancer)
• Trauma to neck or thorax
• Iatrogenic causes from surgical procedures
• Cervical spine injuries

Third-Order (Postganglionic) Neuron Lesions

• Carotid artery dissection (most common vascular cause) 3
• Cavernous sinus pathology
• Cluster headaches
• Middle ear infections

Common Etiologies by Frequency

1. Iatrogenic/Procedural Causes

   • Neck and chest surgical procedures
   • Interscalene nerve blocks 4
   • Central venous catheterization

2. Vascular Causes

   • Carotid artery dissection (accounts for approximately 15% of cases) 3
   • Vertebral artery dissection
   • Carotid aneurysms

3. Neoplastic Causes

   • Pancoast tumors (apical lung cancer)
   • Metastatic disease
   • Neuroblastoma (in children)
   • Neck and mediastinal tumors

4. Inflammatory/Infectious Causes

   • Cavernous sinus inflammation
   • Orbital infections
   • Middle ear infections

5. Other Causes

   • Fibromuscular dysplasia (FMD) 3
   • Trauma
   • Cluster headaches
   • Idiopathic (up to 40% of cases) 2

Diagnostic Approach

When Horner's syndrome is suspected:

1. Pharmacological Testing

   • Apraclonidine 0.5% (preferred) or cocaine testing to confirm diagnosis 1, 2
   • 1% phenylephrine to localize lesion (helps identify postganglionic lesions) 1

2. Imaging Based on Suspected Location

   • MRI for suspected central or preganglionic lesions
   • CT/CTA for suspected vascular causes
   • MRA for suspected carotid or vertebral artery dissection 1

Clinical Pearls and Pitfalls

• In an important minority of cases, Horner's syndrome may be the first sign of serious underlying pathology such as carotid dissection or malignancy 2
• Carotid dissection typically presents with pain on one side of the head or neck, accompanied by Horner's syndrome 3
• After these warning symptoms occur, cerebral or retinal ischemia develops in 50-95% of cases of carotid dissection 3
• New-onset Horner's syndrome should always prompt thorough investigation, especially in adults with risk factors for malignancy 1
• Anhidrosis is often minimal or absent in postganglionic Horner's syndrome 5
• In children, Horner's syndrome may be congenital or acquired and requires prompt recognition to prevent permanent damage 6

Treatment Considerations

Treatment is directed at the underlying cause:

• Anticoagulation or antiplatelet therapy for carotid or vertebral artery dissection 1
• Surgical resection, radiation, or chemotherapy for tumors
• Conservative management for iatrogenic causes, which often resolve spontaneously 4

The prognosis for Horner's syndrome depends entirely on the underlying cause, with iatrogenic or trauma-related cases often having a good prognosis with spontaneous resolution 1.