Neurology: Horner's syndrome in Horses (Equis) | Vetlexicon
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Neurology: Horner’s syndrome

ISSN 2398-2977

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  • Loss of sympathetic innervation to head and specifically to the eye; often unilateral.
  • Cause: secondary to other disease process, eg perivascular injections, periorbital masses, cervical neck trauma, thoracic masses and guttural pouch abnormalities   Guttural pouch: empyema    Guttural pouch: mycosis  .
  • Signs: ptosis , miosis, enophthalmos   Eyeball: enophthalmos - prolapsed third eyelid  , patchy head/neck sweating.
  • Diagnosis: history, clinical signs.
  • Treatment: directed at primary problem.
  • Prognosis: guarded - related to primary problem.

Presenting signs

  • History determined by primary problem.
  • Onset often sudden.
  • Clinical signs include:
    • Ptosis.
    • Miosis.
    • Enophthalmos   Eyeball: enophthalmos - prolapsed third eyelid  .
    • Sweating   Skin: perspiration - patchy  .
    • Increased skin temperature on head/neck of affected side.
    • Dilation of facial vessels.
    • Hyperemia of the nasal and conjunctival mucous membranes.

Cost considerations

  • Treatment: depending upon cause.
  • Loss of animal subsequent to primary cause.




  • The sympathetic tract arises at T1-T3, it passes cranially through the cervicothoracic ganglion. The sympathetic fibers join the vagosympathetic tract following the path of the jugular vein to ascent up the neck to the cranial cervical ganglion situated just beneath the guttural pouch. These anatomical associations explain why just a diverse range of diseases can result in Horner's syndrome.
  • Loss of sympathetic innervation to the head results in a combination of lesions, including abnormalities of the eye.
  • Damage to the sympathetic innervation of the head   →   denervation of the dilator iris muscles, tarsal and orbital muscles and increased sensitivity of the skin blood vessels and neck sweat glands to circulating catecholamines.


Presenting problems

  • Anisocoria   Pupil: anisocoria  .
  • Enophthalmos   Eyeball: enophthalmos - prolapsed third eyelid  /protrusion of third eyelid   Third eyelid: prolapse  .
  • Patchy swelling.

Client history

  • Depends upon primary disease entity.
  • Usually acute onset.

Clinical signs

  • Ptosis (drooping of upper eyelid) on affected side.
  • Miosis - subtle.
  • Enophthalmos and protrusion of third eyelid - variable degrees   Eyeball: enophthalmos - prolapsed third eyelid  .
  • Sweating and increased skin temperature (vascular dilation) at base of ear, side of face and neck of affected side:
    • Sweating often subsides.
    • Exacerbated by excitement or an injection of epinephrine   Epinephrine  .
  • Mild dilation of conjunctival and nasal mucosal vessels.

Diagnostic investigation

Confirmation of diagnosis

Discriminatory diagnostic features

  • Clinical signs and ancillary tests specific to primary disease(s).
  • Post-ganglionic disease can be distinguished from pre-ganglionic disease using application of dilute (1:1000) adrenaline to the eye. Mydriasis occurs within approximately 20 min in post-ganglionic lesions but is delayed (up to 50 min) in pre-ganglionic lesions.

Definitive diagnostic features

  • Clinical signs and ancillary tests specific to primary disease(s).

Gross autopsy findings

  • Depends upon primary disease.

Differential diagnosis

Constricted, unresponsive pupil
  • Parasympathetic response to intraocular inflammation and pain, eg uveitis   Uveitis: anterior - overview  .
  • Posterior synechia   Synechiae  .
  • Posterior opacitiy.
  • Central nervous system stimulation of parasympathetic nerves to iris, eg systemic diseases.
  • Drug stimulation, eg pilocarpine , organophosphates.


Initial symptomatic treatment

  • Treatment of primary disease.
  • Topical sympathomimetics, eg phenylephrine   Phenylephrine  may provide temporary relief from ocular signs.


  • Depends upon primary disease but gradual resolution of Horner's syndrome can occur spontaneously.
  • Assessment of ocular signs requires ophthalmic examination on a daily basis.




  • Depends upon primary problem.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Furr M (2011) Horner's syndrome. Equine Vet Educ 23 (9), 453-454 Wiley Online Library.
  • Palumbo M I P, Moreira J J, Olivo G et al (2011) Right-sided laryngeal hemiplegia and Horner's syndrome in a horse. Equine Vet Educ 23 (9), 448-452 VetMedResource.
  • Firshman A M, Hayden D W, Valberg S J & McKenzie E C (2003) Horner's syndrome associated with fungal mediastinitis in a horse. Equine Vet Educ 15 (2), 82-85 VetMedResource.
  • Hahn C N (2003) Horner's syndrome in horses. Equine Vet Educ 15 (2), 86-90 VetMedResource.
  • Bacon C L et al (1996) Bilateral Horner's syndrome secondary to metastatic squamous cell carcinoma in a horse. Equine Vet J 28 (6), 500-503 PubMed.
  • Simoens P et al (1990) Horner's syndrome in the horse - a clinical, experimental and morphologic study. Equine Vet J Suppl 10, 62-65 PubMed.
  • Milne J C (1986) Malignant melanomas causing Horner's syndrome in a horse. Equine Vet J 18 (1), 74-75 PubMed.