Eye AnatomyLearn more about normal eye anatomy and common disorders of the eye and surrounding areas in the cat.
Accessory structures
The eye has many accessory structures present to ensure that it is protected and clean. These structures include the eyelids, eyelashes, lacrimal (tear) glands, and nictitating membrane. Most animals have three eyelids: the upper lid, lower lid, and nictitating membrane (third eyelid). The three eyelids and the surrounding conjunctiva lubricate, nourish, and protect the eyeball. The conjunctiva is the delicate membrane that lines the inside of the upper and lower lids and some outer portions of the eyeball. The nictitating membrane is clear and affords the eye extra protection, still allowing the animal to see through it. Large eyelashes are attached to the upper eyelids in most animals and help to keep dust particles from getting in the eye.
The eye has many accessory structures present to ensure that it is protected and clean. These structures include the eyelids, eyelashes, lacrimal (tear) glands, and nictitating membrane. Most animals have three eyelids: the upper lid, lower lid, and nictitating membrane (third eyelid). The three eyelids and the surrounding conjunctiva lubricate, nourish, and protect the eyeball. The conjunctiva is the delicate membrane that lines the inside of the upper and lower lids and some outer portions of the eyeball. The nictitating membrane is clear and affords the eye extra protection, still allowing the animal to see through it. Large eyelashes are attached to the upper eyelids in most animals and help to keep dust particles from getting in the eye.
The eyelids serve much the same purpose, but they also have a blinking reflex to help spread tears and other lubricating oils over the cornea, plus cleanse it of dust and microscopic debris. Tears are produced by the lacrimal glands, and also contain lysozyme, an antibacterial enzyme. Tears exit the eye and its related structures through a small duct or opening at the inside corner of the eye, called the lacrimal or tear duct.
Eye anatomyThe eye has three main layers: the outer fibrous tunic, middle vascular tunic, and inner nervous tunic. The names are clues as to their basic structures and functions, but a closer look at the components of each layer will make understanding the mechanism of sight much easier.
Fibrous tunic: The fibrous tunic is the outermost layer of the eye. An opaque (not transparent) network of collagen (fibrous protein) and elastic fibers, called the "sclera," covers the posterior (back) three fourths of the eye. The sclera is tough and somewhat stretchy, like a thick balloon filled with the gelatinous contents of the eye. The rest of the fibrous tunic, the anterior (front) quarter of the eye, is a clear structure called the "cornea." It is made up of extremely thin layers of cells arranged in a unique fashion so the cornea is transparent. A normal cornea allows light to enter the eye.
Vascular tunic: The vascular tunic, as the name implies, is a network of blood vessels that supply oxygen and nutrients to the tissues of the eye. The actual area where this network is located is beneath the portion covered by the sclera and is called the "choroid." Anterior to the choroid is a circular structure called the "ciliary body." The ciliary body has muscles that act on suspensory ligaments called "zonules," which suspend the lens in the correct position. The ligaments are either taut or relaxed based on the action of the ciliary muscles. The tension on the ligaments changes the shape of the lens, depending on the distance of the object being viewed. This process is called "accommodation" and will be discussed in more detail in the following section. The iris is the colored portion of the eye. At its position in front of the ciliary body, it is the most anterior portion of the vascular tunic, and it divides the front portion of the eye into two chambers – the anterior and posterior chambers. The opening in the middle of the iris is called the "pupil," which appears as the dark center of the eye. The iris either dilates or constricts the pupil to regulate the amount of light entering the eye. In bright light the pupil will be small, but in dim light the pupil will be very large to let in as much light as possible.
Fibrous tunic: The fibrous tunic is the outermost layer of the eye. An opaque (not transparent) network of collagen (fibrous protein) and elastic fibers, called the "sclera," covers the posterior (back) three fourths of the eye. The sclera is tough and somewhat stretchy, like a thick balloon filled with the gelatinous contents of the eye. The rest of the fibrous tunic, the anterior (front) quarter of the eye, is a clear structure called the "cornea." It is made up of extremely thin layers of cells arranged in a unique fashion so the cornea is transparent. A normal cornea allows light to enter the eye.
Vascular tunic: The vascular tunic, as the name implies, is a network of blood vessels that supply oxygen and nutrients to the tissues of the eye. The actual area where this network is located is beneath the portion covered by the sclera and is called the "choroid." Anterior to the choroid is a circular structure called the "ciliary body." The ciliary body has muscles that act on suspensory ligaments called "zonules," which suspend the lens in the correct position. The ligaments are either taut or relaxed based on the action of the ciliary muscles. The tension on the ligaments changes the shape of the lens, depending on the distance of the object being viewed. This process is called "accommodation" and will be discussed in more detail in the following section. The iris is the colored portion of the eye. At its position in front of the ciliary body, it is the most anterior portion of the vascular tunic, and it divides the front portion of the eye into two chambers – the anterior and posterior chambers. The opening in the middle of the iris is called the "pupil," which appears as the dark center of the eye. The iris either dilates or constricts the pupil to regulate the amount of light entering the eye. In bright light the pupil will be small, but in dim light the pupil will be very large to let in as much light as possible.
Herpesvirus Infection of the Feline Eye
Veterinary & Aquatic Services Department, Drs. Foster & Smith, Inc.
Dr. Keith Collins, a board-certified veterinary ophthalmologist, has treated many kittens and cats with eye disease caused by feline herpesvirus type 1 (FHV-1) infections. FHV-1 is the virus that causes rhinotracheitis in cats. FHV-1 usually infects young cats, but can remain dormant for years, and reappear if the cat is stressed, is on high doses of corticosteroids, has feline leukemia virus (FeLV) or feline immunodeficiency virus infection (FIV), or there is local irritation or bacterial infection of the eye. The most common eye conditions caused by FHV-1 are conjunctivitis and keratitis (inflammation of the cornea).
FHV-1 is one of the most common causes of conjunctivitis in cats. Young kittens are most commonly affected, with both eyes having considerable clear or cloudy discharge. In almost all cases, the kitten also has signs of a respiratory infection, such as nasal discharge. Oral ulcers are commonly present. In adult cats, often only one eye is affected, and there may be no respiratory signs. For treatment, antibiotic ointments are often used to prevent secondary bacterial infections. If the infection is severe, especially in a kitten, antiviral drugs are used.
FHV-1 is the most common cause of corneal ulcers in cats. Any cat with a corneal ulcer should be considered to have an FHV-1 infection until proven otherwise, and antiviral medication should begin immediately. Ulcers resulting from FHV-1 are generally not deep unless they become secondarily infected with bacteria, so antibiotics are generally included in the treatment. Ulcers of the eye are very painful, so affected cats are often treated with oral or injectable pain relievers, and atropine ointment, which dilates the eye and reduces pain.
Idoxuridine is a common antiviral agent used in cats. L-lysine, which is an amino acid, has been found to interfere with the replication of FHV-1. L-lysine is administered orally and its use in combination with idoxuridine is recommended as the first choice of therapies. If this combination is not effective, other drugs, alone or in combination, may be used. Some of these are very expensive, but may be necessary to save the cat's sight.
References and Further Reading
Collins, BK. Diagnosis and management of feline ocular herpesvirus. Presented at the 2002 Wisconsin Veterinary Medical Association Convention, Milwaukee, WI. October 13, 2002.
Veterinary & Aquatic Services Department, Drs. Foster & Smith, Inc.
Dr. Keith Collins, a board-certified veterinary ophthalmologist, has treated many kittens and cats with eye disease caused by feline herpesvirus type 1 (FHV-1) infections. FHV-1 is the virus that causes rhinotracheitis in cats. FHV-1 usually infects young cats, but can remain dormant for years, and reappear if the cat is stressed, is on high doses of corticosteroids, has feline leukemia virus (FeLV) or feline immunodeficiency virus infection (FIV), or there is local irritation or bacterial infection of the eye. The most common eye conditions caused by FHV-1 are conjunctivitis and keratitis (inflammation of the cornea).
FHV-1 is one of the most common causes of conjunctivitis in cats. Young kittens are most commonly affected, with both eyes having considerable clear or cloudy discharge. In almost all cases, the kitten also has signs of a respiratory infection, such as nasal discharge. Oral ulcers are commonly present. In adult cats, often only one eye is affected, and there may be no respiratory signs. For treatment, antibiotic ointments are often used to prevent secondary bacterial infections. If the infection is severe, especially in a kitten, antiviral drugs are used.
FHV-1 is the most common cause of corneal ulcers in cats. Any cat with a corneal ulcer should be considered to have an FHV-1 infection until proven otherwise, and antiviral medication should begin immediately. Ulcers resulting from FHV-1 are generally not deep unless they become secondarily infected with bacteria, so antibiotics are generally included in the treatment. Ulcers of the eye are very painful, so affected cats are often treated with oral or injectable pain relievers, and atropine ointment, which dilates the eye and reduces pain.
Idoxuridine is a common antiviral agent used in cats. L-lysine, which is an amino acid, has been found to interfere with the replication of FHV-1. L-lysine is administered orally and its use in combination with idoxuridine is recommended as the first choice of therapies. If this combination is not effective, other drugs, alone or in combination, may be used. Some of these are very expensive, but may be necessary to save the cat's sight.
References and Further Reading
Collins, BK. Diagnosis and management of feline ocular herpesvirus. Presented at the 2002 Wisconsin Veterinary Medical Association Convention, Milwaukee, WI. October 13, 2002.
Microphthalmia (Small Eyes)
Race Foster, DVMMarty Smith, DVMDrs. Foster & Smith, Inc.
The eyeball is comprised of several structures including the cornea, iris, lens, chambers, and the retina. The eyeball is located within the bony socket of the head and is partially protected by the three eyelids.
Microphthalmia is a condition in which the eyeball is smaller than normal. Usually, the internal structures of the eyeball are abnormal as well. Microphthalmia can occur in newborns whose mothers received certain medications during pregnancy. This is seen, for instance, in pregnant queens who were treated for ringworm with griseofulvin.
What are the symptoms?
The eyeball will appear smaller than normal for the breed. Signs of visual impairment will also be noticed.
What are the risks?
Many cats become visually impaired or completely blind.
What is the management?
There is no treatment, and affected individuals should not be bred.
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