Horner syndrome

Horner pic & pathway

Fig. 1: Horner Syndrome OS in a cat with otitis media (top). The sympathetic pathway to the eye is shown at bottom.

Horner syndrome is not a disease, but rather a constellation of clinical signs, including:

  1. Miosis – small pupil
  2. Enophthalmos – eyeball retracted in orbit
  3. Ptosis – droopy eyelid
  4. Elevated nictitating membrane (“third eyelid”)

Horner Syndrome occurs when there is disruption of sympathetic innervation to the affected eye.  This is one of the neuroanatomic pathways that is helpful to understand and remember. Sympathetic innervation to the eye is a three-neuron pathway.  The 1st order neuron cell bodies are located in the hypothalamus. Their axons project caudally through the brainstem and cervical spinal cord to synapse on 2nd order (preganglionic) neurons located in the T1-T3 segments of the spinal cord. Preganglionic axons exit the spinal cord, passing cranially through the thoracic cavity, brachial plexus and vagosympathic trunk in the neck. The final synapse occurs in the cranial cervical ganglion located near the tympanic bulla. From there, axons of the 3rd order (postganglionic) neurons pass near (dog) or through (cat) the tympanic bullae, enter the calvaria and continue rostrally to the eye.

Differential diagnoses

Disease anywhere along the pathway (fig. 1) can lead to Horner Syndrome. Listed below are the common differential diagnoses causing Horner Syndrome.

Post-ganglionic dysfunction:

  • Otitis media
  • Trauma or injury (e.g., head trauma, bite wound)

Pre-ganglionic dysfunction:

  • Idiopathic – most commonly reported in Golden Retrievers & Collies
  • Brachial plexus injury / avulsion
  • Neoplasia (e.g., malignant peripheral nerve sheath tumor, thyroid/parathyroid tumor, mediastinal tumor)
  • Trauma or injury
  • Cervical abscess

1st order neuron dysfunction:

  • Cervical spinal cord disease (e.g., IVDD, FCE, neoplasia)
  • Disease in the brainstem or hypothalamus

Diagnostic testing

Fig. 2: Phenylephrine testing may help localize the cause of Horner Syndrome. *In some cases, the miosis may not resolve, but the others signs (ptosis, enophthalos, elevated third eyelid) often resolve.

Pharmacologic testing may help localize the lesion.  Both eyes should be tested so that the normal eye serves as a control. Instill 1-2 drops of 1% phenylephrine (or 0.01% epinephrine) into each eye and watch for resolution of clinical signs. Instillation of phenylephrine leads to temporary resolution of clinical signs at specific times that may help localize the lesions (Fig. 2).  In many cases, miosis does not fully resolve so monitor for improvement in the other abnormalities. Pharmacologic testing of patients with Idiopathic Horner Syndrome usually reflects a preganglionic (2nd order neuron) lesion.

Why the difference in times?  There are a certain number of receptors on any given target organ to receive neurotransmitter. Postganglionic neuron dysfunction causes significantly less neurotransmitter being released. This, in turn, causes the body to upregulate target receptors, in this case, an increased number of receptors on the dilator pupillae muscle and smooth muscles of the eyelids and orbit. Phenylephrine is a direct-acting sympathomimetic agent, meaning that it acts directly on the receptors of the target organs with the same function as the natural neurotransmitter. Since there are an increased number of target receptors in patients with a postganglionic lesion, there will be a rapid improvement in clinical signs.  With a preganglionic lesion, the post-ganglionic neurons are intact and there is still spontaneous release of neurotransmitter from the post-ganglionic neurons. As a result, there is no upregulation of receptors on the target organs. Phenylephrine still causes improvement in clinical signs by acting on the normal numbers of receptors present, but takes longer since there are fewer receptors available than in patients with postganglionic lesions.

Otoscopy should be performed to look for evidence of otitis media or ruptured tympanic membrane. Routine testing (CBC, biochemical profile, T4) is usually normal, but should be obtained to look for evidence of infection or hypothyroidism. Radiographs of the neck and thorax are recommended to rule out cervical abscesses and neoplasia in the soft tissues of the neck or cranial mediastinum. Skull radiographs to evaluate the tympanic bullae may be helpful, but advanced imaging should be considered for improved visualization and to better evaluate the patient for intracranial and cervical diseases. Otitis media can be easily diagnosed with CT imaging of the head. Although CT is excellent for viewing bone of the middle and inner ear, this author prefers MRI because it provides superior resolution of soft tissue structures of the brain and cervical spinal cord, while still providing visualization of the inner and middle ear structures.


There is no specific treatment for Horner Syndrome. Repeat treatment with topical phenylephrine is not recommended because it is partially absorbed and may cause adverse systemic effects.


Full recovery from Horner Syndrome may or may not occur, even with successful treatment of the underlying cause. Partial recovery is relatively common. Idiopathic Horner Syndrome may resolve in 3-4 weeks. Horner Syndrome is a non-painful and generally cosmetic disorder. Vision may be disturbed in the affected eye if the third eyelid covers the pupil.

Further reading

  1. Boydell P. Idiopathic Horner’s syndrome in the golden retriever. J Small Anim Pract 1995;36:382-4.
  2. Jones BR, Studdert VP. Horner’s syndrome in the dog and cat as an aid to diagnosis. Aust Vet J 1975;51:329-32.
  3. Kern TJ, Aromando MC, Erb HN. Horner’s syndrome in dogs and cats: 100 cases (1975-1985). J Am Vet Med Assoc 1989;195:369-73.
  4. Morgan RV, Zanotti SW. Horner’s syndrome in dogs and cats: 49 cases (1980-1986). J Am Vet Med Assoc 1989;194:1096-9.
Last updated by NeuroPetVet on August 21, 2016.