Horner’s Syndrome in Dogs & Cats – A Unique Eye Condition
A syndrome is a collection of symptoms 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.
Horner syndrome is a condition that affects the face and eye on one side of the body. It is caused by the disruption of a nerve pathway from the brain to the head and neck.
Typically, signs and symptoms of Horner syndrome include decreased pupil size, a drooping eyelid and decreased sweating on the affected side of the face.
Horner syndrome may be the result of another medical problem, such as a stroke, tumor or spinal cord injury. In some cases, no underlying cause can be found. There’s no specific treatment for Horner syndrome, but treatment for the underlying cause may restore nerve function.
Horner syndrome is also known as Bernard-Horner syndrome or oculosympathetic palsy.
Horner’s syndrome is a common neurological disorder of the eye and facial muscles. The condition usually occurs suddenly and typically affects one side of the head but can be bilateral (affect both sides of the head) in rare cases.
What are the clinical signs of Horner’s syndrome?
The most common clinical signs of Horner’s syndrome are:
- drooping of the upper eyelid on the affected side (ptosis)
- the pupil of the eye on the affected will be constricted (miosis)
- the eye on the affected side often appears sunken (enophthalmos)
- the third eyelid of the affected side may appear red and raised (prolapse of the third eyelid, conjunctival hyperemia)
What causes Horner’s syndrome?
Horner’s syndrome is due to a dysfunction of the sympathetic nerves of the eyes and surrounding facial muscles. This is part of the autonomic nervous system, which helps to control normal functions such as blinking, muscle tone, etc.
There are many reasons for Horner’s syndrome. The dysfunction may be caused by damage to the sympathetic pathway as it runs through the neck or chest. This may be due to an injury such as a bite wound or blunt trauma, a tumor, or intervertebral disc disease. Middle or inner ear disease (otitis media or otitis interna) can also cause Horner’s syndrome. Other causes for an elevated or protruding third eyelid gland include: tetanus, facial nerve paralysis, facial muscle atrophy, and dehydration. However, Horner’s syndrome is often classified as idiopathic, which means it is without known cause.
The onset of Horner’s syndrome can be sudden and without warning. In some cases, the dog may have eye symptoms, as well as excessive salivation and/or difficulty eating on the affected side.
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)
All these signs are caused by damage to sympathetic nervous system as it supplies the eye on the affected side of the head.
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 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, a 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.
How can the Sympathetic Damage Occur?
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 it. 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.
Sorting it Out
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. 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.
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. The Horner’s syndrome itself probably does not need treatment, but its underlying cause very well might.
Does it affect any particular breed or age of dog?
Any dog can develop Horner’s syndrome although Golden Retrievers and Collies have a somewhat higher incidence.
What is the treatment?
Most cases of Horner’s syndrome will resolve spontaneously; however, it is important to treat any underlying disease. There are several diagnostic tests that will be performed to determine if there is an underlying cause in your pet including an eye and ear exam, radiographs (X-rays) of the skull and chest, and possibly advanced imaging such at CT scans or MRIs. Pharmacologic tests may include phenylephrine drops placed in the affected eye to help localize the source of the problem.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development) Image-Courtesy-Google Reference-On Request.
