The sympathetic innervation to the eye is a 3-neuron pathway.

1. First order neuron – originates in the hypothalamus with the axons extending through the brainstem and cranial cervical spinal cord to synapse at the level of T1-T3 as part of the lateral tectotegmentospinal tract.
2.  Second order neuron (pre-ganglionic) – axons leave the spinal cord and travel cranially to synapse in the cranial cervical ganglion (near tympanic bulla)
3. Third order neuron (post-ganglionic) – axons travel very near (dog) or through (cat) the tympanic bulla into the calvaria before proceeding to the globe

Pharmacologic testing to determine lesion location:

Instill 1 drop of 2% phenylephrine into both eyes and monitor for improvement in clinical signs (see table below).  In my experience, there is often reduced degree of enophthalmos, ptosis, and elevated 3rd eyelid, but the miosis may persist. This test is relatively accurate, but results should be interpreted in light of other historical and clinical findings.

See the Horner Syndrome disease page for additional information.