Which nerves are involved in Horner syndrome?

Nerves Involved in Horner Syndrome

Horner syndrome results from interruption of the three-neuron oculosympathetic pathway that extends from the hypothalamus to the eye.

Anatomical Pathway of the Oculosympathetic Innervation

The sympathetic pathway involved in Horner syndrome consists of three neurons:

1. First-order (Central) Neuron:

   • Originates in the hypothalamus
   • Descends through the brainstem
   • Continues down the spinal cord to synapse at the C8-T2 level (intermediolateral gray matter)

2. Second-order (Preganglionic) Neuron:

   • Originates from the first three thoracic spinal cord segments (T1-T3)
   • Exits the spinal cord
   • Travels through the thorax and cervical region
   • Ascends to synapse at the superior cervical ganglion

3. Third-order (Postganglionic) Neuron:

   • Originates from the superior cervical ganglion
   • Travels along the internal carotid artery
   • Enters the skull and eventually reaches the orbit
   • Innervates the iris dilator muscle and Müller's muscle

Clinical Manifestations

Interruption of this pathway results in the classic triad of Horner syndrome 1:

• Miosis (pupillary constriction)
• Ptosis (drooping of the upper eyelid)
• Anhidrosis (decreased sweating on the affected side of the face)

Localization Based on Associated Findings

The location of the lesion can often be determined by associated symptoms:

• Central (First-order) Lesions:

  • Often accompanied by other neurological symptoms
  • May be seen with brainstem stroke, demyelination, or tumors 2

• Preganglionic (Second-order) Lesions:

  • Commonly caused by trauma, tumors (especially Pancoast tumors), or thoracic pathology 3
  • Superior sulcus (Pancoast) tumors frequently cause Horner syndrome by invading the sympathetic chain and stellate ganglion 2

• Postganglionic (Third-order) Lesions:

  • Often associated with pain/headache
  • Frequently caused by carotid artery dissection or cluster headache 4
  • Anhidrosis is minimal or absent in this type 4

Diagnostic Approach

Pharmacological testing can help localize the lesion:

• Cocaine test confirms the presence of Horner syndrome
• 1% phenylephrine test can identify postganglionic lesions 1
• Apraclonidine is an alternative to cocaine for confirming Horner syndrome 4

Clinical Significance

Horner syndrome may be the first sign of serious underlying pathology:

• In Pancoast tumors, it indicates invasion of the sympathetic chain 2
• In aortic dissection, it may indicate extension into the carotid artery 2
• In the setting of head and neck pain, it may indicate carotid dissection requiring urgent evaluation 5

Understanding the complete oculosympathetic pathway is essential for proper localization of the lesion and guiding appropriate diagnostic workup, which may include MRI of the brain, neck, and thorax depending on clinical suspicion.