Chapter 4 Diseases of the Eye
Special structures exist in all animals that help them to survive in their environment. The special senses sight, hearing, smell, and taste are extensions of the central nervous system and are different from each other in their form and function. Problems involving sight and hearing are frequently seen in the dog and cat. This chapter focuses on ocular problems commonly seen in small-animal practice. The topic of deafness is discussed in Chapter 8.
The eye, made up of the globe and its accessory structures (Fig. 4-1), is perhaps the most highly developed of all the special senses. Its structure converts light into electrical impulses that travel to the brain and are interpreted as visual pictures.
(From McBride DF: Learning veterinary terminology, ed 2, St Louis, 2002, Mosby, by permission.)
The function of the eye depends on all components of the visual system functioning properly. Disruption of any of these components can result in abnormal vision for the animal. Although most pets can live quality lives with a loss of vision, proper diagnosis and quick treatment of eye problems is essential if sight is to be preserved.
Diseases that involve the eyelids, conjunctiva, tear ducts, third eyelid, and the lacrimal glands may be included in the group of diseases of the accessory structures. Trauma to or infection of these tissues is a common reason for small animals to be presented to the veterinary hospital. Typical presenting signs include red eyes, blepharospasm (squinting), and ocular discharge. Many eye problems present with similar signs; a thorough clinical examination is needed before a treatment plan can be formulated.
Canine conjunctivitis, or inflammation of the conjunctiva, is rarely a primary disease process; therefore it is important for the veterinarian to discover the underlying cause to treat this condition effectively.
The conjunctiva is a highly vascular tissue. When injured, it responds by developing hyperemia (redness), chemosis (swelling), and ocular discharge. Dogs typically develop noninfectious conjunctivitis. Causes of noninfectious conjunctivitis in dogs include immune-mediated follicular conjunctivitis, allergic conjunctivitis (atopy), and anatomic conjunctivitis (ectropion/entropion). Bacterial conjunctivitis can develop in the dog as a result of the disruption of normal tear production, injury, or foreign bodies.
Feline conjunctivitis is primarily infectious. Feline herpes virus (FHV) is the most common cause of bilateral conjunctivitis in young kittens and is typically seen in conjunction with upper respiratory tract symptoms. FHV-1 virus replicates best in epithelial tissue that is slightly cooler than body temperature, and therefore tends to infect the superficial epithelial tissues of the nasal, oral, and conjunctival regions.
Calicivirus may also cause a mild conjunctivitis in cats. Chlamydia psittaci infection may present as a unilateral problem with marked chemosis in some cats. Mycoplasmas have also been isolated from cases of feline conjunctivitis.
Epiphora, an overflow of tears, may be the result of overproduction of tears or faulty drainage by the lacrimal system. Overproduction of tears is always the result of ocular pain or irritation. Faulty functioning of the lacrimal drainage system may occur for several reasons, including blockage of the lacrimal duct by swelling or inflammatory cells, imperforate puncta, or trauma.
Brachycephalic dogs and cats have large globes in shallow orbits, leaving little room for the accumulation of tears. Subsequently, tears spill out onto the face. Accumulations of hair or face folds may wick the tears onto the face in some animals. An entropion or ectropion may also result in faulty drainage of tears.
Surgical correction of lid position is the treatment of choice in animals with entropion or ectropion. Keeping the facial hair cut shorter may also be beneficial. Obstruction of the lacrimal puncta may occur in animals as a result of inflammation, the presence of foreign bodies, or accumulation of debris. Cocker spaniels and poodles typically have imperforate puncta (no opening to provide drainage). Many times the obstruction can be removed by flushing the nasolacrimal ducts or surgically removing the tissue covering the puncta.
Facial hair or cilia originating from the meibomian glands of the lid may rub against the cornea, creating irritation and often corneal ulceration. Epiphora then results as a reflex against the pain created by the irritation. Treatment includes removal of the cilia or shortening of the facial hair and topical therapy.
The eyelids are important for ocular health. They protect the globe, help to remove debris from the eye, shade the eye during sleep, and spread lubricating secretions over the eye. Eyelashes project from the border of each lid. At the base of each lash is a sebaceous gland, which produces a lubricating fluid for the hair follicles (glands of Zeis or meibomian glands).
An abscess of the sebaceous gland is called a hordeolum, and it is usually the result of a staphylococcal infection. When the inflammation involves the meibomian glands and granuloma formation occurs, it is called a chalazion. Therapy for both of these swellings includes warm compresses, manual expression, topical antibiotic ointment, and possibly surgical curettage.
When the eyelids themselves become inflamed, blepharitis results. Causative factors include bacterial infections (Staphylococcus), parasitic infections (Demodex, Notoedres), and mycotic infections (dermatophytes). Atopy may frequently present with inflamed and pruritic eyelids.
Eyelid neoplasms are frequently seen in older animals. Most tumors of the eyelids are benign and can be treated by surgical resection. Eyelid neoplasms in the cat are usually malignant. Squamous cell carcinomas are the most common type of tumor.
Blepharitis is defined as a swelling of the eyelids. The causes include exposure to allergens, nutritional deficiencies, viral infections, or dermatitis from any cause. The symptoms include edema of the lids with redness, discharge and spasms of the lids.
Entropion and ectropion defects involve eyelids that either roll in against the cornea (entropion) or roll outward, exposing the cornea (ectropion). In either case, the lids are incapable of performing their protective functions for the eye and disease may result.
Entropion is common in dogs, but less common in cats. Entropion exists in three main forms: congenital (inherited), acquired nonspastic, and acquired spastic. The congenital form includes those breeds predisposed to entropion because of large orbits with deep-set eyes, which provide inadequate lid support. The lid droops over the lower orbital rim and inverts. Collies, Great Danes, Irish setters, Doberman pinschers, golden retrievers, Rottweilers, and Weimaraners are breeds that exhibit congenital entropion.
Several breeds are predisposed to poor muscular development that involves the ocular muscles. Chesapeake Bay retrievers, Labrador retrievers, Chow Chows, and Samoyeds may exhibit this condition, although it is not well documented. A large number of breeds are predisposed to entropion from primary lid deformities.
The cause of acquired nonspastic entropion is usually surgical or traumatic, resulting in scarring of the lid with contraction. This causes the lid to turn inward toward the globe. The third form of entropion, acquired spastic, is the most commonly observed form in cats. This form of entropion usually occurs secondary to painful corneal lesions, conjunctival inflammation, or both.
Ectropion is the reverse of entropion. In this condition, the lid is excessive and droops outward. Ectropion is a natural breed characteristic in basset hounds, bloodhounds, cocker spaniels, Clumber spaniels, English bulldogs, and Saint Bernards. In these animals, it is usually asymptomatic. In any breed, however (even the ones listed), ectropion can develop clinical symptoms. Acquired ectropion can form secondary to muscular disease in senile dogs that lose muscle tone and in dogs that have had overcorrection of an entropion.