Pathophysiology

Central

Anatomical trajectory: Hypothalamus (first-order oculosympathetic neuron) → brainstem, cervical, and thoracic spinal cord → ciliospinal center (C8–T2)
Typical lesions:
- Brainstem stroke (e.g., Wallenberg syndrome)
- Cervical spinal cord injury (e.g., Brown-Séquard syndrome, syringomyelia)
- Brain tumors
- Brainstem (pontine) hemorrhage
- Demyelinating disease (e.g., multiple sclerosis)
- Malformations (e.g., Arnold-Chiari)
- Meningitis

Anatomical trajectory: Ciliospinal center (second-order oculosympathetic neuron) → pulmonary apex → stellate ganglion → superior cervical ganglion
Typical lesions:
- Tumors (e.g., breast or lung cancer compressing stellate ganglion, esp. Pancoast tumor)
- Iatrogenic (e.g., birth trauma, central venous catheterization)
- Lymphadenopathy
- Arterial lesions/dissection: aorta, subclavian, common carotid
- Cervical rib

Anatomical trajectory: Superior cervical ganglion (third-order oculosympathetic neuron) → internal carotid artery and ophthalmic nerve → iris dilator muscle
Typical lesions:
- Dissection of the internal carotid artery
- Cluster headache
- Cavernous sinus
- Tumor
- Herpes zoster infection

Triad of Horner syndrome
- Miosis (constriction of the pupil)
  - Occurs because the sympathetically controlled iris dilator muscle fails to contract. See Pupillary light reflex.
  - Leads to anisocoria and a dilation lag on exam
    - More noticeable in the dark when the sympathetic tone is increased
- Partial ptosis (drooping of the upper eyelid)
  - Occurs because the sympathetically controlled superior tarsal muscle fails to keep the upper eyelid raised
  - It is milder than ptosis associated with oculomotor nerve or levator palpebrae muscle lesions.
- Anhidrosis (absence of sweating) or reduced sweating on the face and arm, depending on the location of the lesion
  - Occurs because the sympathetic innervation of the facial sweat glands is impaired
  - Seen in central and preganglionic lesions
Facial flushing due to vasodilatation
- Occurs because the vasoconstrictive effect of the sympathetic nervous system is lost.
(Apparent) enophthalmos
Associated symptoms depending on the etiology:
- Atrophy of arm and hand muscles
  - Infiltration of the brachial plexus causes pain (plexus neuralgia), as well as motor and sensory deficits in the arm.
- Pain in the neck or face