Nictitating Membrane


“Cherry Eye”
eye_structure_7The nictitating membrane is also called the “third eyelid”. It is composed of a T-shaped cartilage that is covered by conjunctiva. On its inner surface it contains a tear producing gland. This gland of the third eyelid can protrude over the free edge of the nictitating membrane when the gland undergoes hypertrophy or hyperplasia. Dog breeders commonly call this condition “cherry eye”. Surgery is usually necessary as the gland seldom returns to its normal space once it has prolapsed. In the past the gland was surgically excised but this caused subsequent keratoconjunctivitis sicca therefore this practice has been discouraged. Surgical tacking of the gland to the globe or orbital rim as well as infolding the gland back into the nictitating membrane are all possible surgeries.

Cornea

Corneal ulcers

The loss of the corneal epithelium or outer layer of cells constitutes a corneal ulcer. This condition can result from trauma, abnormal eyelashes, loss of tears or caustic substances. Corneal ulcers threaten the integrity of the eye and should be treated very aggressively. Every animal that presents with a painful eye or for rubbing at the eye should be checked for a corneal ulcer. A special stain is applied to the eye which will only stain areas that have a damaged epithelium. This stain will give an indication of how deep the ulcer is and how aggressively it needs to be treated. It is not uncommon to treat corneal ulcers on an hourly basis to promote rapid healing and reduce infection. Because the cornea has no direct blood supply, it is susceptible to rapid deterioration in the face of infection. Careful examination corneal ulcers will reveal blood vessel migration across the cornea to the area of ulceration. These blood vessels are bringing nutrition as well as the healing properties of the body to the area in need. Unfortunately these vessels also bring scar tissue and pigmentation that may make the cornea opaque after the healing process is complete. Once a corneal ulcer is healed and covered with epithelium, cortisone-containing medications are used to send the blood vessels back home. One treatment for corneal ulcers is to make a serum preparation from the dog’s own blood. Serum is made by separated blood cells from the plasma by spinning in a centrifuge. One or two drops of serum are applied to the cornea every hour or two hours. The serum nourishes the cornea and contains antibodies which help to fight infection.

Using a conjunctival flap to bring nutrients and healing properties directly to the cornea commonly treats severe corneal ulcers more aggressively. These flaps must be created surgically and sutured to the corneal ulcer area or placed so they cover the corneal ulcer. They have been shown to greatly improve the chances of healing of a severe “melting” corneal ulcer.

Glaucoma

Glaucoma is a very serious and common disease of our pet animals. Glaucoma denotes excessive pressure in the eye. This pressure has a fatal effect on the retina and optic nerve resulting in rapid blindness. The increased pressure occurs when the aqueous humor fluid produced by the ciliary body cannot drain from the eye fast enough. This fluid flows through the filtration angle between the cornea and the iris. The filtration angle is a fine trabecular network of fibers that filter the aqueous humor before allowing it out of the eye. If the filtration angle fails to allow fluid out of the eye, the fluid pressure increases and glaucoma occurs. This condition is congential in many breeds including cocker spaniels and beagles. Other causes of glaucoma include plugging of the filtration angle with inflammatory
debris, blood or white blood cells. Glaucoma is diagnosed by measuring the pressure in the eye. Measuring the force it takes to dent the anterior chamber a certain amount does this. This force is measured by an instrument called a Schiotz Tonometer or by a newer instrument called a Tonopen. Therapy includes medications that pull fluid from the eye by diuresis (increased urination) as well as surgical procedures to kill the ciliary body with cryotherapy or laser. Aggressive emergency therapy with intravenous mannitol is required to save vision in acute glaucoma.

Lenticular sclerosis or nuclear sclerosis is the normal increase in density of the lens with age. The lens of the eye appears slightly cloudy. This change does not affect the passage of light into the eye and vision is not affected. It is often mistaken for cataracts.

A cataract is a clouding that develops in the lens of the eye or in its capsule, varying in degree from slight to complete opacity and obstructing the passage of light. The condition affects both eyes but almost always one eye is affected first. Cataracts can be a genetically inherited trait in breeds listed below, they are secondary to diabetes, hypertension and anterior uveitis in cats. Cataract surgery has been known since the 6th century BC where a curved needle was used to move the lens out of the field of vision. Specialist veterinary ophthalmologists can perform cataract surgery but not all cataracts are good surgical candidates since uveitis and glaucoma are common complications.

Cataracts usually are classified by their age of onset (congenital, juvenile, senile), anatomic location, cause, degree of opacification (incipient, immature, mature, hypermature), and shape. Cataracts are best examined after the pupil has been dilated.

Breeds of dogs with inherited cataracts:

  • Afghan hound
  • Bichon
  • Boston terrier
  • Bichon frise
  • Chesapeake Bay retriever
  • Rottweiler
  • Siberian husky
  • Staffordshire bull terrier
  • German shepherd
  • Golden retriever
  • Labrador retriever
  • Min schnauzer
  • Old English sheepdog
  • Standard Poodle
  • Welsh springer spaniel
  • West Highland white terrier

Luxation of the lens

Luxation or displacement of the lens is an inherited condition in terrier breeds. In other circumstances it can be secondary to trauma or hypermature cataracts If it is displaced into the anterior chamber it is often associated with acute signs such as glaucoma and corneal edema. Treatment is surgical removal. Posterior displacement into the vitreous cavity is asymptomatic or associated with ocular inflammation or glaucoma.. Because procedures to remove the lens are associated with higher levels of postoperative complications of glaucoma and retinal detachment surgery is only done if the lens displacement is causing serious problems.

Uveitis

The uvea is the middle pigmented layer of the eye – it consists of the iris, ciliary body and the layer between the retina and sclera. It is divided up into anterior uvea (iris and ciliary body) and posterior uvea. The blood vessels in this layer provide nutrition directly to the iris and ciliary body and indirectly to the cornea, lens and outer retina. It also stops light entering the eye via the sclera. Uveitis is inflammation of the uvea.
Anterior uveitis is more common in the cat than the dog. It is caused by the following infections: FIP, FeLV, VIV, toxoplasma, canine hepatitis, canine brucellosis, leptospirosis and also by immune mediated disease. Anterior uveitis in one eye is often secondary to penetrating injury or foreign bodies.

Retinal Disease

The retina may be affected by various disease processed. Because the eye is a direct extension of the brain, many diseases that affect the brain affect the retina similarly. Examination of the retina for plaques or granuloma formation can help make an early diagnosis of internal fungal infections, FIP, Toxoplasmosis, cancer or other disease. The optic disc is in the center of the retina and is the nerve bundle that carries the visual impulses out of the eye to the brain. This disc is white and has a characteristic shape in every species. The blood vessels that feed the retina share this common point of entry and exit from the eye. Therefore, the optic disc on examination contains a characteristic shape as well as a characteristic vascular pattern. Increased pressure in the brain from hydrocephalus for example will change the shape of the optic disc. Other disease similarly affects the optic disc as well as the retinal blood vessels causing visual changes in their appearance. These changes can be seen on ophthalmoscopic examination and can reveal disease not only of the eye but of the entire body. Becoming proficient in this examination process is very important in clinical veterinary practice and effort is well spent in this endeavor. Retinal degeneration is genetically preprogrammed in certain breeds and can be diagnosed by seeing fewer and thinner retinal blood vessels as well as a brighter than normal retinal tapetum. A common disease seen in many species is Acute Retinal Degeneration Syndrome (ARDS) which causes acute onset of blindness in otherwise healthy pets. The cause is unknown but certain breeds are affected more than others. As you can see there is much need for a good retinoscopic examination.

Proptosis or prolapse of the eye

Acute prolapse or proptosis of the eye occurs as a result of trauma. It is more common in dogs than in cats. In cats it is usually secondary to severe head trauma. In small brachiocephalic breeds of dogs the eye socket is relatively shallow compared to the size of the eye and the eye can prolapse if the dog is held roughly by the scruff. Often when this happens the eye is not hanging out but is stuck in front of the eyelids. The amount of damage to vision depends on how much trauma there has been, how long the eye has been out of place, the depth of the eye socket and the breed. The eye should be replaced as soon as possible. It may be necessary to incise the lateral canthus of the eye and the eyelids may be sutured closed afterwards to prevent relapse. Systemic antibiotics and sometime steroids are given, topical antibiotics and mydriatics (dilate the pupil) are also used. The prognosis is fair with vision restored about 50% of the time.

The eye – Definitions
♥ Anterior chamber Anterior chamber
♥ Blepharospasm Involuntary closure of the eyelid due to pain
♥ Cataract Clouding of the lens
♥ Cherry eye Hypertrophy of the gland in the third eyelid
♥ Ciliary body Sits between the iris and lens and produces fluid which maintains eye pressure
♥ Cones Specialized nerves for seeing color
♥ Conjunctiva Layer of tissue that surrounds the eye and lines the eyelids
♥ Conjunctivitis Inflammation of the conjunctiva
♥ Cornea Clear part of the eye
♥ Corneal ulcer Loss of the corneal epithelium
♥ Distichiasis Fine hairs that grow at the edge of the eyelid
♥ Ectopic cilia Eyelashes growing on the inner part of the eyelid
♥ Ectropion The eyelid is turned out
♥ Entropion The eyelid is turned in and the eyelashes rub the cornea
♥ Epiphora Excessive production of tears
♥ Epithelium Cells covering the cornea
♥ Exophthalmos Eyes that stick out – buggy eyes like a pug
♥ Eye tack A procedure to correct entropion
♥ Filtration angle Area where fluid drains at the junction of the cornea and sclera
♥ Glaucoma Excessive pressure inside the eye
♥ Iris The colored part of the eye
♥ Keratoconjunctivitis sicca Conjunctivitis caused by lack of tear production
♥ Lens Part of the eye that focuses light
♥ Nictitating membrane The third eyelid
♥ Optic disc The centre of the retina
♥ Pupil The hole in the centre of the iris
♥ Retina A layer of nerve cells at the back of the eye
♥ Retinal degeneration Degeneration of the layer of cells at the back of the eye leading to blindness
♥ Rods Specialized nerves for seeing light
♥ Schirmer tear test Test for measuring tear production
♥ Sclera The white part of the eye
♥ Stroma Non-cellular cornea
♥ Sty Infected or impacted gland in the eyelid
♥ Tapetum The reflective part of the retina
♥ Tears Clear liquid that lubricates and nourishes the eye
♥ Tear gland Glands that produce tears
♥ Tonopen A device to measure eye pressure
♥ Uveitis Inflammation of the interior structures of the eye
♥ Vision One of the five senses
♥ Vitreous humor Gelatinous mass between the back of the eye and the lens