Horner’s syndrome in dogs: aetiology and diagnostic protocol
Horner’s syndrome in dogs occurs when the sympathetic innervation to the eye is interrupted and it may affect one or both eyes.1
Horner’s syndrome is a that can affect dogs of any age. Breeds including .
Horner’s syndrome :
- First-order neurons are found in the section connecting the brain to the first thoracic segments of the spinal cord;
- run from the end of the first-order pathway to the cranial cervical ganglion; and
- connect the cranial cervical ganglion to the sympathetic nerves of the eye and its adnexa.
As such, , with postganglionic Horner’s syndrome the most frequent clinical presentation. In any case, the regardless of the lesion’s location.
Clinical manifestations of Horner’s syndrome in dogs include:
- , due to interruption of innervation of the dilator muscle of the iris.
- , due to a lack of activity of the periorbital muscles that work antagonistically to the retractor bulbi.
- , secondary to enophthalmos.
- , due to a lack of tone of the palpebral muscles and enophthalmos.
In addition, loss of sympathetic innervation may cause and the auricular pavilion may feel warmer on the affected side compared to the contralateral ear. can also be observed. These latter signs seem to be rare in small animals.
Approximately of Horner’s syndrome in dogs are thought to be .
Other possible causes include craniocervical trauma (falls, traffic accidents, bites), media/interna, anaesthetic blockades, ear surgeries (bulla osteotomy and ear canal ablation), thoracic surgery or trauma, brachial plexus avulsion, infectious diseases (tick paralysis, neosporosis), fibrocartilaginous embolism, disease, diabetes and thoracic or intracranial neoplasms.
In general, the clinical diagnosis of Horner’s syndrome is straightforward and is . In any case, , that is, that , and to rule out other possible causes of miosis (uveitis, endophthalmitis, panophthalmitis or ulcerative keratitis).
- The application of . In healthy dogs, .
In unilateral Horner’s syndrome, there is more marked anisocoria, because the affected pupil does not dilate or only minimally, and the healthy pupil shows full mydriasis. However, this test does not reveal the anatomical location of the lesion. In addition, the legal requirements for obtaining cocaine and the fact it must be applied separately from parasympatholytics mean that it is not used routinely.
- is used in to diagnose Horner’s syndrome. In a healthy eye, it has virtually no effect on pupil size, while in affected eyes, it causes pupil dilation, which in patients with unilateral disease manifests as a reduction in anisocoria within 30–45 minutes of administration.
- is administered to .
- The application of causes the affected pupil to dilate in the first 20 minutes for postganglionic cases, whereas it takes 30–40 minutes in healthy animals or animals with preganglionic Horner’s syndrome.
- If the test is performed with , mydriasis is evident after 5–8 minutes with a postganglionic lesion, but there is no effect on normal eyes or eyes with a preganglionic lesion. In patients with postganglionic Horner’s syndrome, the application of a drop of 1% phenylephrine to the affected eye resolves ocular signs in under 20 minutes, but it does not cause any effect in animals with central or preganglionic disease. It is important to apply phenylephrine in both eyes at the same time. Both pupils will dilate in animals with bilateral disease.
- The use of has been proposed as a (the pupil dilates within 45 minutes) (no effect); however, this test appears to have a higher percentage of false negatives and false positives than phenylephrine.
Although the clinical signs of Horner’s syndrome in dogs are easily recognised, it is important to rule out other possible diagnoses first in a patient with anisocoria. The next step is to determine the location of the lesion and establish the causal disease. Either way, owners must be warned that even with an appropriate diagnostic protocol, there is only approximately a 50% chance of identifying the cause.
- VETERINARY CLINICAL HOSPITAL
- CEU CARDENAL HERRERA UNIVERSITY