Horner syndrome is a defect in the oculosympathetic pathway that classically results in miosis, ptosis, and anhydrosis. There are three neurons that work together from the brain down the spinal cord to the upper chest and back to the eye and face. Horner syndrome occurs when there is an interruption in this nerve pathway. 

The first-order neuron extends from the hypothalamus down the spinal column and synapses at the ciliospinal bulge between C8 and T2. Pathology here may be due to vascular occlusion, tumors, or cervical spinal disease. 

The second-order neuron travels from the spinal cord to the superior cervical ganglion in the upper neck. Lesions here may be due to apical lung tumors, chest surgery, metastases, thoracic aortic aneurysms, or trauma. 

The postganglionic third-order neurons destined for the pupil dilator muscles follow the course of the internal carotid artery to cranial nerve (CN) VI and nasociliary fibers of CN V within the cavernous sinus. In the orbit, the postganglionic fibers pass through the ciliary ganglion without synapsing and travel with the ciliary nerves to the pupillary dilator muscles. Postganglionic lesions are usually caused by vasospasm or degenerative changes of the wall of the carotid artery. Internal carotid artery dissection and tumors invading the cavernous sinus are less common but significant causes of postganglionic Horner syndrome.

Patients may present with symptoms of neck pain or cough or may be asymptomatic, with detection of anisocoria (unequal pupil size) on eye examination. The anisocoria is more apparent in dim versus bright light. Light and near pupillary reactions remain intact. 

Patients also have a ptotic eyelid on the same side of the miotic pupil due to a paresis of the sympathetically innervated Müller's muscle. Classically, Horner syndrome is associated with facial anhydrosis, but this is only seen occasionally. 

Pediatric Patient Considerations: 
In cases of congenital Horner syndrome, the iris color may be lighter in the affected eye.