Horner’s Syndrome
A classic neurological complex of four distinct eye signs caused by a disruption of the sympathetic nerve pathway to the eye.
💬 In Plain English
Horner’s Syndrome is a distinctive neurological condition that causes dramatic changes to one eye. It happens when the sympathetic nerves (the "fight-or-flight" wiring that controls eye posture and pupil dilation) are pinched, severed, or irritated anywhere along their path—from the brainstem, down the neck, through the chest, and up through the middle ear to the eye. It causes four classic signs: a constricted small pupil (miosis), an elevated third eyelid covering half the eye, an eyeball sunken back into its socket (enophthalmos), and a droopy upper eyelid (ptosis). While it looks alarming, in dogs over 50% of cases are idiopathic (meaning the nerves heal spontaneously over several months).
🔬 Clinical & Technical Depth
For Vets & TechniciansFormal Clinical Definition
Neuro-ophthalmic syndrome resulting from loss of ocular sympathetic innervation, characterized by the clinical tetrad of unilateral miosis, ptosis, enophthalmos, and prominent protrusion of the third eyelid.
Why Veterinarians Use It
Veterinarians use ophthalmic phenylephrine eye drop testing to pharmacologically localize whether the nerve injury is first-order, second-order, or third-order (middle ear).
Clinical Context Encountered
Seen in dogs or cats presenting with sudden unilateral droopy eye, elevated third eyelid, and unequal pupils (anisocoria).
Clinical Reference Guide (Medical Condition)
Typical Presentation
Unilateral miosis (small pupil), prominent protrusion of the third eyelid, ptosis (drooping upper lid), and enophthalmos (sunken eye).
Management Priorities
- Treat underlying middle ear or thoracic disease
- Monitor idiopathic cases; benign neglect and patience as nerves regenerate
- •Horner’s syndrome in cats is frequently caused by deep middle ear infections, bulla polyps, or thoracic mediastinal tumors—investigation of the middle ear and chest is mandatory.
❓ Questions to Ask Your Veterinarian
Take these actionable questions with you to your next clinic visit or appointment:
- 1Has a deep ear examination (otoscopy) been performed to check for middle ear infection (otitis media) damaging the tympanic bulla?
- 2Can we perform a topical phenylephrine drop test to pinpoint where the nerve pathway is disrupted?
- 3Could chest trauma, neck pull injuries, or bite wounds have damaged the vagosympathetic trunk in the neck?
Academic & Clinical References
- ACVIM Clinical Diagnostic Guide to Neuro-Ophthalmic Disorders and Horner’s SyndromeOrganization: American College of Veterinary Internal Medicine (ACVIM) • Accessed: 2026-01-10https://www.acvim.org