Horner's syndrome usually presents as unilateral miosis, ptosis, enophthalmos and third eyelid protrusion on the affected side. In some cases, anisocoria is the only presenting ocular sign. Horner's syndrome arises from damage to the sympathetic innervation on the affected side and 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 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. anterior mediastinal 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).
In cats, second-order Horner's syndrome may present with concurrent ipsilateral laryngeal hemiplegia.