Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




A “syndrome” is a collection of symptoms that have special significance when they go together. It is important to realize that having a syndrome is not the same as having a diagnosis. A syndrome, however, often has a limited number of causes such that recognizing a specific syndrome brings one substantially closer to a specific diagnosis. 



Horner’s syndrome consists of five signs:

  • Constricted Pupil
  • Elevation of the Third Eyelid
  • Retraction of the eyeball into the head
  • Slight drooping of the Eyelid
  • Increased pink color and warmth of the ear and nose on the affected side (very hard to detect in small animals)

All these signs are caused by damage to sympathetic nervous system as it supplies the eye on the affected side of the head.

Normal eye on the left; Horner's syndrome on the right
(original graphic by


Our bodies have numerous functions that are controlled by our nervous systems yet we are completely unaware of them. Our heart and respiratory rates, the amount of sweat and other secretions we produce, circulation to different body areas, pupil dilation and constriction are all regulated by our nervous systems automatically and without our knowledge or control. The part of our nervous system dedicated to these automatic systems is called the “Autonomic nervous system.”

The autonomic nervous system is divided into the “sympathetic nervous system” and the “parasympathetic nervous system.” The parasympathetic system maintains a status quo, normal, “business as usual” state; the sympathetic system prepares the body for a “fight or flight” situation. Some changes that might be stimulated by the sympathetic system include: increased sweating, dilated pupils, increased heart rate, and increased blood flow to muscles. Both systems coexist in balance in the healthy body.

In the eye, the sympathetic nerve fibers dilate the pupil, widen the eye lids, drop the third eyelid, and keep the eye in a forward position in the socket. The parasympathetic nerves constrict the pupil, raise the third eyelid and retract the eye for protection. Both systems are working at the same time, one system slightly dominating the other depending on what is happening.

When the sympathetic nerves controlling one of the eyes is damaged, only the parasympathetic nerves work and Horner’s syndrome is created.



Horners Brain in dog
(original graphic by

The nerve carrying the tiny nerve fibers that provide sympathetic control to the eye have a long path and the damage may have occurred anywhere along this path. Not only that but certain types of injuries are more likely to occur at certain areas along the path. So, let us now describe the path.

The path begins in the brain's hypothalamus. Nerves exit the brain and travel through the brainstem and down the spinal cord in the neck area, exiting just inside the chest at the level of the second thoracic vertebra as shown in blue in the illustration. (This pathway is called the central segment.)

The nerves then form the "cervical sympathetic trunk," a bundle of nerves that travels back up the neck, this time outside the spinal cord, to the vicinity of the middle ear. This segment is shown in red and is called the "preganglionic segment." There are actually two sympathetic trunk nerve bundles, one on the right and one on the left.

From there, the nerves connect to the last segment of nerves (the "postganglionic segments" - one on the right and one on the left) shown in yellow. This segment starts just below the ear and travels all the way to the eye.


The damage can occur in the neck or spinal cord area, the ear area or the eye area. Damage can occur in the form of trauma, tumor involvement, infarction (abnormal blood clot), middle ear infection, or diseases of the eye itself. Each segment of the nerve pathway is vulnerable to different types of damage so that knowing which segment is involved gives us a good idea of what caused the damage.



As mentioned, localizing which area of the sympathetic nervous system is affected goes a long way in determining the nature of the damage as different areas of the system are prone to different types of injury. Special eye drops can be used to stimulate different areas of the above pathway and determine which area is damaged. Damage is described as being first order, second order, or third order. Most lesions turn out to be third order.

First Order Lesions (involving the blue segment above): Diseases that hit nerve fibers in the brain, brainstem, or spinal cord include tumors of the brain, vascular accidents (such as "stroke") in the nerve tissue, fibrocartilaginous embolism in the spinal cord where disk material sprays into the spinal cord, or even a herniated intervertebral disk in the area of the neck. Horner's syndrome stemming from any injury such as one of these might prompt a search for other neurologic issues. Advanced imaging such as an MRI might be a good idea.

Second Order Lesions (involving the preganglionic red segment): Diseases that strike the sympathetic trunk include foreleg injuries especially if the foreleg is pulled and the nerves that exit the spinal cord in the armpit area become over-stretched. Sometimes a mass in the chest such as a tumor or fungal granuloma will damage the sympathetic trunk. Neck trauma such as pulling very hard on a leash could be severe enough to cause a second order lesion. If there is no obvious history to suggest injury, it might be a good idea to radiograph the chest to see if there are masses in the lung that might be involved in a second order lesion.

Third Order Lesions (involving the postganglionic yellow segment): These are the most common causes of Horner's syndrome because ear infections are so common for small animals. Inflammation in the middle ear can easily lead to a Horner's syndrome. Third order lesions are associated with vestibular disease, the imbalance and dizziness of the middle ear infection, in many cases. When a Horner's syndrome localizes as third order, the ears should be thoroughly investigated as the source.



It is not necessary to treat Horner’s syndrome. The syndrome is not painful and does not interfere with vision. The significance of the syndrome is that it indicates nerve damage and it is important to determine the nature of the nerve damage. Most patients will resolve their Horner's syndrome without treatment within an 8 week period, though occasionally a patient will require much longer. Horner's syndrome virtually never occurs in both eyes at once.

If one wishes to treat the syndrome for cosmetic reasons, phenylephrine eye drops can be prescribed to relieve clinical signs but the most important thing is to determine what caused the Horner's syndrome. The Horner's syndrome itself probably does not need treatment but it's underlying cause very well might.


Page last updated: 10/9/2021