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Horner's syndrome is a collection of signs that have 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 diagnosis.

What is Horner's Syndrome?

Horner's syndrome consists of five signs:

• Constricted pupil
• Elevated 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)

These signs appear on the side of the face (and eye) with damaged sympathetic nerves. Sometimes, the cause of Horner's syndrome in pets can't be found (this is called idiopathic Horner's), and it might go away on its own. Still, it's important to check if other health issues are causing it.

Image Courtesy of Christian N. Young .jpeg

Image Courtesy of Christian N. Young 11/2022

What is the Sympathetic Nervous System?

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 (enlargement), and constriction (shrinking) 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.

Sympathetic vs Parasympathetic

The autonomic nervous system is divided into two parts: one part for day to day regular life and one part for emergency situations. The part for day to day regular life is called the "parasympathetic nervous system" and the emergency part is called the"sympathetic nervous system." What this means is that you have day to day automatic functions and emergency automatic functions, each set controlled by its respective part of the autonomic nervous system.

In the eyes, the parasympathetic nerves constrict the pupil in response to light, raise the third eyelid and retract the eye for protection as needed and generally assist the eyes in doing their job. The sympathetic nerves dilate the pupil, widen the eyelids, keep the eyes forward and drop the third eyelid for maximum vision in a fight or flight situation. Both systems are at work simultaneously to some extent (sort of like driving with one foot on the accelerator and one foot on the brake.) Which system dominates is determined by hormones of excitement, anxiety and fear.

All these signs are caused by damage to sympathetic nervous system as it supplies the eye on the affected side of the head. At least three signs must be present to qualify as Horner's Syndrome.

How can the Sympathetic Damage Occur?

The nerve carrying the tiny nerve fibers that provide sympathetic control to the eye has a long path, and damage may occur anywhere along it. Also, some types of injuries are more likely to occur in certain areas along the path. 

The path begins in the brain (technically the hypothalamus but let's just keep it simple and say the brain). A group of nerves exit the brain and travel down the brainstem continuing down the spinal cord, down the length of the neck and just into the chest. Brain, all the way down the neck, and just into the chest cavity, following the spinal cord.  This segment is called the central segment and is shown in blue in the illustration above.

At this point, the nerve bundle splits into right and left bundles (one for each eye) and make a U-turn from the chest, back the neck and towards the eyes. They closely pass by the middle ears on their way back towards the eyes. These bundles are shown in red in the illustration and are called the right and left sympathetic trunks. (They are also sometimes called the "preganglionic segments" in case you hear that term.)

From there, the nerves connect to the last segment of nerves (the postganglionic segments - one on the right and one on the left), as shown in yellow. This segment starts just below the ear and travels 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 knowing which segment is involved gives us a good idea of what caused the damage.

Locating the Damage

Simplified Cervical Sympathetic Trunk Illustration

Illustration courtesy of MarVista Vet

So we have three nerve areas and each are is vulnerable to different types of injury. This means that localizing the nerve damage will help figure out the nature of the damage. 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)

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 Horner's syndrome. Third order lesions are associated with vestibular disease, the imbalance and dizziness of the middle ear infection, in many cases. When Horner's syndrome localizes as third order, the ears should be thoroughly investigated as the source.

Treatment

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