What Is Horner's Syndrome in Animals?

Horner's syndrome is a painless neurological condition that can affect a dog's or cat's eyes. This disorder can appear spontaneously and causes one of the animal's eyes to develop a drooping eyelid and constricted pupil, although it does not affect the pet's vision. The condition may resolve itself over time, but a veterinarian should properly diagnose it and treat any underlying illness that may be the cause.

  1. Identification

    • The autonomic nervous system in animals controls their involuntary body functions, such as heart rate, pupil dilation, blinking and digestion. The system has two parts: one to control the status quo, called the sympathetic nervous system, and one to control "fight or flight" reactions, called the parasympathetic system. When the series of nerves that connects an animal's eye to his sympathetic nervous system becomes damaged, his parasympathetic system takes over. This leads to Horner's syndrome. In addition to a drooping eyelid and constricted pupil, the affected animal's eye appears sunken, his third (inner) eyelid appears elevated, and the ear and nose on the affected side have an increased pink color.

    Diagnosis

    • A veterinarian must diagnose Horner's syndrome to rule out conditions such as facial paralysis and to determine where the damage to the nerves has occurred. Damage happens in three areas: central, pre-ganglionic and post-ganglionic, each indicative of a possible cause. First-order Horner's syndrome results from central nerve damage between the brain and spinal cord; second-order occurs in the pre-ganglionic area between the base of the skull and chest cavity; and third-order indicates post-ganglionic damage between the base of the skull and the eye itself. A veterinarian determines the location of the damage by applying eye drops to stimulate parts of the nerve and see how they react. Determining the area of damage helps to zero in on a possible underlying condition.

    Treatment

    • For Horner's syndrome with no evident or underlying cause, no treatment is given. Symptoms are alleviated with special eye drops to prevent the development of corneal ulcers if necessary, and your veterinarian may recommend the use of phenylephrine drops to dilate the eye. Underlying illnesses, such as hypothyroidism and certain cancers or tumors, require treatment or surgery to help resolve them. Patients with second-order Horner's syndrome are more likely have suffered neck trauma, and third-order can indicate a disease of the middle ear, which helps to determine treatment. Once an underlying illness is under control, Horner's syndrome usually resolves itself within a month or two.

    Considerations

    • When consulting with your veterinarian, let him know if your cat or dog has experienced a recent trauma. Pets who have been hit by a car or who have been involved in a fight with another animal may have incurred central or pre-ganglionic nerve damage. For pets with chest trauma, the recovery may be quicker, within days or weeks, than with other types of injuries. In certain cases, your veterinarian may refer you to a veterinary ophthalmologist for diagnosis and treatment, if necessary.