Horner’s syndrome is a common neurological disorder of the eye and facial muscles, and involves loss of sympathetic nerve supply to the eye, eyelids, and muscles that dilate the pupil.
To understand Horner's syndrome, one must first understand how the nervous system operates. There are two major divisions of the nervous system: the somatic and the autonomic nervous systems. The somatic nervous system is responsible for conscious actions, such as taking off a jacket when one is overheated. The autonomic nervous system, controls functions that are unconscious, such as breathing and the heartbeat. It is responsible for causing a human to sweat, or a dog to pant, when overheated.
The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems. The parasympathetic nervous system governs the “feed and breed” actions, such as digestion and sexual arousal. With regards to the eye, the parasympathetic nervous system causes a small pupil when activated. The sympathetic nervous system is responsible for the “fight or flight” actions and prepares the body for physical exertion. This branch of the nervous system causes pupil dilation.
The eye, as well as the rest of the body, is connected to both sympathetic and parasympathetic nerves. Under normal conditions, there is a fine balance between sympathetic and parasympathetic stimulation. Horner’s syndrome is characterized by a lack of sympathetic impulses into the eye, leading to an over balance of parasympathetic supply to the eye.
On its journey to the eye, the sympathetic nerves fibers begin in the brain, travel down the spinal cord to the shoulders and exit from the spinal cord, where they joins an artery and vein and travels back toward the head. At the base of the ear, a nerve junction (ganglion) occurs. This portion of the nerve (from the brain to the shoulders and back to the base of the ear) is called the pre-ganglionic nerve.
After the ganglion, the nerve fibers travel towards to the eye. This portion is referred to as the post-ganglionic nerve. Movement of the eyeball is mediated by several muscles, which are under voluntary control. Surrounding these muscles is a smooth muscle cone controlled by the sympathetic nerve. The nerve causes the smooth muscle cone to become constricted and this in turn maintains the eye in its normal position within the bony orbit. If the sympathetic nerve is not working well, the eye will sink into the orbit. When the eye sinks deeper into the orbit, this allows the third eyelid to passively elevate, and will appear more prominent. The sympathetic nervous system also controls the normal muscle tension of the upper eyelid – this eyelid will become droopy in Horner’s syndrome.
The size of the pupil is mediated by a balance between the sphincter muscle (controlled by the parasympathetic nervous system), and radially arranged dilator muscles (controlled by the sympathetic nervous system). If the sympathetic nerve is not working well, the pupil will be smaller than normal because the dilator muscles cannot work against the sphincter muscle.
Whenever your pet is showing signs of a health issue your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from Horner’s syndrome or another ocular or neurologic condition, a veterinary specialist is available at an ExpertVet certified hospital.