Horner's syndrome reflects an underlying disease process, rather than a primary disease state. The important causes of Horner's syndrome in dogs are trauma and ocular disease, but 50% of cases are idiopathic (cause unknown).
It is an oculosympathetic dysfunction that results from disruption of the sympathetic innervations to the eye and adnexa, often due to damage to the sympathetic innervation on the affected side, although damage may occur to the sympathetic fibres within the brain or spinal cord. It is more common for the lesion to occur outside the spinal cord.
Horner's syndrome can be classified according to the site of involvement; as central (first order), preganglionic (second order) and post-ganglionic (third order). Pharmacological testing for lesion localisation, based on the principle of denervation hypersensitivity, is possible using a directly acting agent such as topical 1% phenylephrine; the results obtained will vary according to the time after the insult, the completeness of the lesion and its distance from the iris.
The normal eye will not respond to the weak concentration of the drug. If the lesion is post-ganglionic, the normally unresponsive pupil dilates within twenty minutes, whereas if the lesion is preganglionic the pupil will take 30-40 min to dilate.
In some cases of Horner's syndrome, the etiology cannot be identified, but common identifiable causes include head or neck trauma (e.g. damage to the vagosympathetic trunk by brachial plexus avulsion), anterior mediastinal disease (e.g. thymic lymphoma), brachial plexus and chest trauma, otitis media or otitis interna (may be concurrent vestibular syndrome) and iatrogenic damage (during surgery of the neck or bulla osteotomy).
Second-order Horner's syndrome may present with concurrent ipsilateral laryngeal paralysis.
- Tick paralysis
- Brain tumors
- Spinal tumors - e.g. osteosarcoma
- Mediastinal tumors - e.g. cranial mediastinal carcinoma, lymphoma
- Post-hemilaminectomy complication
- Post-epidural anesthetic complication
A diagnosis of idiopathic Horner's syndrome is based on ophthalmologic, neurological, and radiological examination and exclusion of other medical conditions.
Localization of a Horner’s syndrome is initiated by topical application of dilute topical adrenergics (0.1% epinephrine or phenylepinephrine) to both eyes to determine if a hyper-response is detectable in the miotic pupil within 20 minutes. No response is expected in the normal pupil, and it is important to ensure that the dilute adrenergic agent does not stimulate the normal iris dilator.
If the Horner’s pupil dilates within 20 minutes, while the nonaffected fails to dilate, the lesion is postganglionic, somewhere between the sympathetic ganglion (near the base of ear) and the iris on the ipsilateral side.
If the affected and the nonaffected pupils fail to dilate within 20 minutes, concentrated adrenergic (10% phenylephrine) should be applied to the corneas, and if dilatation of both pupils occurs within 20 – 40 minutes, the lesion is most likely preganglionic (somewhere between the base of the ear down the vagosympathetic trunk to the cranial mediastum).
Horner’s syndrome localized in a preganglionic location should have cervical and thoracic radiographs completed to rule out mediastinal and cervical tumors, which are the most common etiology in dogs.
Most cases of idiopathic Horner's syndrome resolve spontaneously in 4 - 8 weeks and acupuncture has shown to be an effective remedial therapy for this condition.
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