Term
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Definition
The microscopic photoreceptors, rods and cones,
are found in the retina.
• Rods are sensitive to dim light but cannot sense
details or colors. They give the animal dim light
vision in shades of gray.
• Cones are sensitive to color and detail, but don’t
work well in dim light.
• Both rods and cones send messages to the brain
via the optic nerve. |
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Term
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Definition
Nerve fi bers from the retina converge to form the
optic nerve at the optic disc (Figures 12-16 and 12-
17). There are no rods or cones in the optic disc,
so this is a blind spot in the eye where no images
are formed.
When viewed in a live animal with an ophthalmoscope,
the optic disc looks like a large white circle
in the back of the eye. Large retinal blood vessels
converge at this spot, too.
• The optic nerve leaves the eye just caudal to the
optic disc. |
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Term
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Definition
The optic nerve (Figure 12-18) carries the visual
impulses from the eye to the optic chiasm (Figure
12-19). |
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Term
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Definition
The optic chiasm is where half the nerve fi bers
carrying visual impulses from the right eye cross
to go to the left side of the brain and vice versa
(Figure 12-19).
• This ensures that visual impulses from each eye
reach both sides of the brain.
Only half the nerve fi bers cross over, the other half
stay on the same side as the eye of origin.
• This means that if an animal suffered a right-sided
brain injury that affected vision pathways, it would
lose only half the vision from its right eye and half
from its left eye. It would still have partial vision in
both eyes. |
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Term
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Definition
The eye has two major fl uid-fi lled compartments
(Figures 12-20 and 12-21).
• The aqueous compartment located rostral to (in
front of) the lens is fi lled with watery aqueous
humor. This compartment is divided into two
chambers:
• The anterior chamber is between the iris and
the cornea
• The posterior chamber is between the iris and
the lens
• The vitreous compartment located caudal to
(behind) the lens is fi lled with gelatinous vitreous
humor (a.k.a. the vitreous body). |
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Term
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Definition
The medial junction of the eyelids is called the
medial canthus and the lateral junction is the
lateral canthus.
• The conjunctiva is a thin, transparent membrane
that lines the eyelids and covers the “front” part of
the eyeball (Figures 12-22 and 12-23).
• The conjunctiva is a mucous membrane
The eyelids are lined with palpebral conjunctiva.
(“Palpebral” refers to eyelid.)
• The bulbar conjunctiva covers the “front” part of
the eyeball.
• The conjunctival sac is the space between the
palpebral and bulbar layers of conjunctiva.
• Tiny tarsal glands (also known as the meibomian
glands) along each eyelid margin secrete a
waxy substance that helps keep tears from
overfl owing. |
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Term
| THIRD EYELID (NICTITATING MEMBRANE |
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Definition
The third eyelid is located at the medial canthus
of the eye behind the two eyelids (Figure 12-24).
• Contains a “T”-shaped plate of cartilage.
• Covered on both sides by palpebral conjunctiva.
• Not always seen when looking at the eyes, but
under certain conditions it may cover most of the
visible eye.
• Movement is passive and not controlled by muscular
actions. It will frequently come up when an
animal is anesthetized or if there is an irritation on
the cornea.
• On its inner surface (the surface next to the
eyeball), it has tear-producing glands and lymph
nodules. |
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Term
| EXTERNAL EYE–RELATED LANDMARKS |
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Definition
The medial canthus and lateral canthus are where
the upper and lower eyelids meet (Figures 12-24
through 12-27).
• The lacrimal puncta are openings into the nasolacrimal
system. This system of tubes carries tears
into the nasal passages where they can be eliminated
by swallowing. (Most animals don’t know
how to blow their noses.) |
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Term
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Definition
The extraocular muscles attach to the sclera of the
eye back inside the bony orbit.
• They hold the eyeball steady and produce delicate
and accurate eye movements.
• Most animals have seven extraocular muscles:
• Four rectus muscles—medial, lateral, dorsal,
and ventral
• Two oblique muscles—dorsal and ventral
• One retractor bulbi muscle |
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Term
| The basic structure of tears includes: |
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Definition
oil (shown in yellow) floating near the air interface
- aqueous mixed with proteins and soluble mucins (blue) beneath the oil
- dense membrane bound mucin layer (the glyocalyx) on the cornea surface with its numerous microvillous processes. |
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Term
| The eye has three main layers |
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Definition
| The eye has three main layers: the outer fibrous tunic, middle vascular tunic, and inner nervous tunic. |
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Term
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Definition
| 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. |
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Term
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Definition
| The vascular tunic, as the name implies, is a network of blood vessels that supply oxygen and nutrientsto 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 |
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Term
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Definition
| The nervous tunic is a layer of photoreceptor cells called the "retina." These cells are able to change light into electrochemical signals, which are transmitted to the nervous system. There is a roughly circular opening where the optic nerve and blood vessels exit, called the "optic disc." Often the optic disc is called the "blind spot," because there are no photoreceptor cells there, so no images can actually be perceived at that position. There are two types of photoreceptor cells which perform different functions and are named for the shape of the cell. These are the rods and cones. The rods are very light sensitive, so they are most abundant in nocturnal species. The cones need bright light, and they are for sharp image formation and perception of color. Domestic mammals have mostly rods, and are unable to distinguish colors well. Some reptiles and most birds can see color, though, since they have many cones. There is a centrally located indentation at the back of the retina. It is called the "fovea centralis," and it is much more pronounced in larger animals. Surrounding the fovea is a slightly raised ring of cells called the "macula lutea." Because most of the light is focused on this region, the concentration of photoreceptor cells is increased greatly. The anterior edge of the retina is non-visual and does not contain any photoreceptors, as light does not come into contact with that surface. A line called the "ora serrata" demarcates the division between the visual and non-visual retina. This name was given because the line appears jagged, or serrated, in humans. In domestic animals, however, the division is not serrated, and it is sometimes referred to as the "ora ciliaris retinae." Most often the term "ora serrata" is still used to describe the structure in animals as well as humans |
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Term
| 1. What structures compose the lacrimal apparatus of the dog? |
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Definition
| Includes the structures that produce and secrete tears and the structures that drain them away from the surface of the eye. The lacrimal glands and the accessory lacrimal glands of the 3rd eyelids. |
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Term
| 2. Compare the visual fields of predators and prey. |
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Definition
| Predators and prey have differences in their visual fields. Predators have large, forward-facing eyes that allow them to see best in the downward and forward direction. As they are often looking down and forward on their intended prey, this visual field is best suited for their hunting lifestyle. Prey, on the other hand, generally have eyes situated more to the sides and top of the head. This position allows them excellent peripheral and upward vision. Because prey animals are often attacked from above, behind, or the side, it is advantageous for them to have this wide visual field, even if they cannot see well in front of their faces. |
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Term
| What are the 3 layers of tears, where is each layer produced, and name the functions of each |
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Definition
Outer oil layer – from the tarsal (meibomian) glands, helps reduce evaporation of the underlying tear layer and prevents tears from flowing over the lid margin
Middle tear layer – from the lacrimal glands and the accessory lacrimal glands of the 3rd eyelids, serves to keep the cornea moist
Inner mucus layer – comes from cells in the conjuctiva, contains antibacterial substances that help protect the eye from infxn |
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Term
| 5. What are the 3 main layers of the eye and what are their structures and functions? |
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Definition
| : the outer fibrous tunic, middle vascular tunic, and inner nervous tunic. |
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Term
| what causes most animals' eyes to shine in the dark? |
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Definition
| Most animals have a reflective layer in the choroid called the "tapetum lucidum." This is what causes their eyes to shine in the dark. |
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Term
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Definition
| The tapetum is a reflective structure that lies beneath the retina. It acts like a mirror; reflecting light back through the retina so the retina gets two chances to catch the light. Animals that are active at night have a tapetum. Dogs, Cats, Horses, and Cows all have tapetums. It causes the yellow or green glow you see when light hits an animal's eyes. Humans do not have a tapetum. |
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Term
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Definition
| The clear front part of the eye that covers the iris, pupil and anterior chamber. It is covered with a transparent epithelium. |
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Term
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Definition
| Colored part of the eye. Brown irises have pigment. Blue irises have much less pigment. The iris controls the amount of light that enters the eye, by varying the size of the pupil opening. |
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Term
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Definition
| The hole in the iris. The pupil gets smaller in bright light, larger in dim light conditions. |
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Term
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Definition
| Clear and as thick as a stack of 5 dimes. It is suspended behind the iris by hundreds of microscopic fibers. The lens is biconvex (ie it is shaped like a lentil bean). The lens helps to bring rays of light to a focus on the retina. |
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Term
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Definition
| A thin layer of light sensitive nerve tissue in the back of the eye that allows us to see, by conversion of optical images to electrical impulses that are sent by the optic nerve to the brain. |
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Term
| 1. Describe what a Schirmer Tear test is for, the technique and list the normal values for dogs and cats. |
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Definition
Schirmer Tear test
Indications
1. Assessment of normal tear production
2. Chronic mucoid epiphora
3. Chronic pigmentary keratitis
4. Epiphora
The prepackaged, sterile strips are removed and the notched end is placed in the lower conjunctival fornix. Do not touch this end. The eye is held closed and the strip allowed to remain in place for exactly 1 minute. If convenient both eyes may be tested at the same time. The strip is then removed and using the standard measurement on the package the tear production is measured and recorded. Normal dogs should secrete 15 mm or more in one minute. Topical anesthetic is not used for this test as we are measuring the response of the eye to an irritant. This requires the ophthalmic branch of cranial nerve 5 as the afferent arm and the parasympathetic fibers in cranial nerve 7 as the efferent arm. |
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Term
| 2. Would you instill a topical anesthetic before you perform a Schirmer tear test? Why |
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Definition
| Topical anesthetic is not used for this test as we are measuring the response of the eye to an irritant. |
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Term
| 3. Define and describe fluorescein staining. What are the indications for this test? |
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Definition
Fluorescein is an orange stain that is applied to the cornea to reveal corneal lesions. Once the stain is applied to the eye, the excess is rinsed off, and the remainder turns fluorescent green. The stain adheres to any areas where the surface layer of the cornea (the epithelium) is missing and where the underlying layer (corneal collagen or stroma) has been exposed. This test outlines the extent of any ulceration and permits more accurate assessment of the size, depth and type of ulcer. A fluorescein stain is indicated anytime a pet has a red or painful eye, or if any corneal irregularities or trauma to the eye are noted. Fluorescein staining is also used to determine if the duct that allows passage of tears from the corner of the eye to the nose is open and working. (This duct is why our nose runs when we cry). There are no real contraindications to performing this test in an animal with eye problems. What Does a Fluorescein Stain Reveal? Fluorescein staining of the cornea identifies abrasions, scratches, ulcerations and lacerations present in the cornea on the surface of the eye. Early treatment is crucial in promoting repair and healing of corneal ulcers and injuries, and in preventing rupture of the eye. Fluorescein is also important in revealing if the duct from the eye to the nose is open and draining properly. How Is a Fluorescein Stain Done? Initially, the surface of the eye is cleaned of any mucous or discharge. The eyelids are opened and a drop of fluorescein stain is dropped on the surface of the eye. The eyelids are then closed to allow the stain to flow over the entire surface of the eye. If the duct from the eye to nose is being evaluated, the opening of the nostril is examined for evidence of fluorescent green stain showing up at the tip of the nose.
Fluorescein Stain Page 2 of 2
If the cornea and surface of the eye are to be examined, the fluorescein is flushed out of the eye with eyewash. The eye is then examined with either a penlight or special cobalt blue filtered light to detect the presence of the green stain on the eye. In a normal undamaged cornea, no stain remains on the surface of the eye. If the surface layer of the cornea is damaged, stain remains and outlines the damage. This test usually takes less than five minutes. |
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Term
| 5. What is the most commonly used topical ophthalmic anesthetic? |
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Definition
Proparacaine – 2 drops last up to 25 min in dogs, 1 drop = 15 min, duration of onset is < 1 min
1 drop lasts 5 min in cats |
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Term
| 4. Define and describe Rose Bengal staining and what it is used for. |
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Definition
Rose bengal
An iodine derivative of fluorescein. Rose bengal stains dead or degenerated epithelial cells of the cornea and conjunctiva (including nuclei and cell walls) a red color. The mucus of the precorneal tear film is also stained. Cells will stain when they are not covered by mucin as is seen in deficiency of the precorneal tear film. The presence of mucin will block the staining of live or damaged cells. Can be associated with irritation. Rose bengal is not a vital dye, but is associated with a loss of cell vitality resulting in loss of cellular motility, cell detachment, and cell death. This effect is augmented by light exposure.
Rose bengal has been shown to have anti-viral activity against ocular herpes virus.
Indications
Keratoconjunctivitis sicca
Herpes
Examination for corneal abrasions |
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Term
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Definition
Prevent generation and conduction of nerve impulses by reducing sodium permeability, increasing electrical excitation threshold, slowing nerve impulse propagation and reducing the rise of action potential. Action is reversible with no evidence of structural damage. Rapid onset of action, 15-20 seconds with 15-20 minute duration. If greater anesthesia is desired for more painful procedures repeated instillation of several drops over several minutes will increase the effect. Topical anesthetics are esters of para-aminobenzoic acid. Local tissue pH effects anesthetic efficacy. Anesthetic exist in nonionized and ionized states with the former responsible for absorption and the later for anesthesia. The higher the pH the greater the ionization. In inflammation, the pH is lowered making the anesthetic more deionized and less available to penetrate tissue. The most common topical anesthetic is proparacaine 0.5%. It has less side effects and is better tolerated.
Prolonged use will diminish duration of anesthesia, retard wound healing (interfere with actin and myosin cytoskeleton) and result in keratitis and corneal epithelial erosions. In addition, a rare severe immediate hyperallergic corneal reaction can occur with epithelial keratitis and sloughing. Proparacaine is the third most common topical ocular agent causing contact dermatitis after neomycin and atropine.
Preinstillation of anesthetic will increase the effect of subsequently applied mydriatic/cycloplegics. Also will enhance the non uniform penetration of fluorescein into the epithelium.
Local anesthetics are used for periocular anesthesia. Inadvertent intraocular injection of lidocaine results in pupil dilation and a decrease in the b-wave of the electroretinogram. |
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Term
| 6. Define nasolacrimal irrigation, list the indications for performing this, list the supplies needed, and describe the technique. |
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Definition
Nasolacrimal Irrigation
Indications
1. Epiphora without an obvious etiology
2. Failure of passage of fluorescein stain
3. Mucopurulent ocular discharge
This procedure can be performed on a minimally restrained dog with only topical anesthesia and causes minimal discomfort. It usually requires sedation or general anesthesia in cats.
Topical anesthetic is applied. A 23 or 25 gauge nasolacrimal cannula is connected to a 3 cc syringe filled with eyewash. An assistant restrains the head and the clinician elevates and rolls the upper eyelid in the medial canthus to expose the superior punctum. Place your thumb or index finger on the plunger of the syringe so that you are ready to inject. Hold the syringe loosely such that no injury will result if the patient jerks its head. Gently insert the cannula in the punctum and without forcing it allow it to seat itself in the duct. Apply gentle pressure to the plunger and observe fluid emerging from the inferior punctum. Occlude the lower punctum, tip the nose down and continue to flush. The fluid should now appear at the nostril.
Do not use excessive force in the placement of the catheter or when flushing. If you cannot establish a patent duct and the epiphora is severe, consider medical therapy such as topical antibiotics or anti-inflammatories and repeat flushing or general anesthesia to further pursue the problem. |
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Term
| 7. List the indications and technique for ophthalmic cytology. |
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Definition
Cytology
Indications
1. To obtain a sample for Gram staining, especially important in horses
2. To characterize the type of inflammation ie. neutrophilic, lymphocytic, eosinophilic
3. To aid in the diagnosis of feline conjunctivitis including Chlamydia and Mycoplasma
4. To obtain a sample for fluorescent antibody testing
Cytology is a simple, fast, and inexpensive method for characterizing the type of inflammatory process and in many cases to obtain a diagnosis. Topical anesthetic is placed in the patients conjunctival sac. The globe is gently retropulsed and the lower eyelid retracted. This will expose the nictitans. A spatula, or other suitable instrument, is used to scrape the palpebral surface of the nictitans and the adjacent palpebral conjunctiva. While you don't want to damage the surface you must use enough force to exfoliate cells. These cells are then placed on a glass slide and streaked to form a monolayer. The slide can then be Gram-stained to characterize the type of bacteria, examined for cell type using Giemsa, Wright's, or Diff-Quik stains, or submitted for fluorescent antibody testing for Chlamydia or Herpes. |
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Term
| 8. List the 2 instruments used to measure IOP. |
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Definition
There are 2 specific ways to determine intraocular pressure: indentation tonometry and applanation tonometry.
Applanation Tonometry
This is a more expensive, but more accurate method of intraocular pressure determination in the dog and cat. It is also the only method for use in the horse. The most commonly used applanation tonometer in veterinary ophthalmology today is the Tonopen®. Despite its initial increased cost, it will likely result in increased income and better patient care. This is a result of the increased use it receives due to the ease of use and the increased reliability of the results obtained. I would encourage every practice to own a Tonopen®.
Schiotz Tonometry
The eye is first anesthetized with topical proparacaine. The tonometer is assembled, placing the shaft in the housing and attaching the 5.5 gm weight. The instrument is held vertically and the plunger should slide freely within the hollow sleeve. Using the metal calibration standard supplied the tonometer is tested for accuracy. When the instrument is placed on the standard it should read 0 on the scale. This means that the 5.5 gm weight does not indent the metal standard. An assistant gently elevates the patient's nose. The upper eyelid is elevated and the tonometer foot plate placed on the cornea. Keep the tonometer vertical and on the center of the cornea. The tonometer should rest on the eye and does not need to be pushed down. Read the scale. Three readings should be taken and averaged. A conversion chart is used to convert these scale readings to an approximation of the intraocular pressure matching the weight used with the reading obtained. Normal values for the dog are 15-27 mmHg with no more than 5 mmHg difference between eyes.
The indentation tonometer measures the amount of corneal indentation that occurs when a given weight is placed on the cornea. The result is inversely proportional to the intraocular pressure and the actual pressure must be obtained from a table of values. The Schiotz tonometer is an indentation tonometer. The Schiotz tonometer requires assembly, disassembly and cleaning in order to ensure its accuracy. The foot plate is large and the patient must be cooperative in order to place the foot plate on the cornea in a vertical position. If the animal is fractious or the eye painful then it is unlikely that accurate placement will be obtained and erroneous values will result. My clinical experience is that practices with a Schiotz tonometer either do not use it or do not believe the results obtained. In many instances, pressures obtained by this method are not confirmed on referral to a specialist. The result is that glaucoma is not diagnosed or monitored accurately and IOP determination is not performed at the frequency indicated by the breed or clinical signs.
Applanation tonometry determines IOP by evaluating the force require to applanate or flatten a given surface area of cornea. Typically in veterinary ophthalmology these are electronic or battery powered tonometers. They have been shown to be highly accurate across species and a wide range of IOP's. In recent years, the Tonopen® (Dan Scott & Associates 614-890-0370, 1-800-TONOPEN) has been evaluated and shown to be similar to other applanation tonometers in accuracy. It is light weight, portable, accurate, self calibrating and averages several readings and gives a % error to ensure accuracy. In addition, the small foot plate allows this tonometer to be used on painful eyes in less cooperative patients as only a small area of cornea is required to obtain a reading and the position of the patients head is not related to obtaining the reading. Finally, this is the only tonometer that is portable enough to allow routine IOP determination in the equine patient. Equine glaucoma in recent years has received attention and has increased in prevalence solely due to the availability of the Tonopen® and the increase in the number of equine eyes that are evaluated.
The ease of use, accuracy and comfort level this tonometer gives your practice will ensure its frequent use, increase your hospitals awareness of glaucoma, allow early and prompt referral of glaucoma patients to a specialist if indicated and subsequent referral of your glaucoma patients back to your hospital for monitoring following laser or other glaucoma surgery by a specialist. In addition, the Tonopen® will allow you to incorporate IOP determination as a routine part of the physical examination in those breeds predisposed to primary glaucoma |
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Term
| 9. What are the indications for measuring IOP? |
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Definition
Intraocular pressure determination
Indications
1. Any red or painful eye
2. Breeds that are predisposed to glaucoma
3. Predisposed breeds with a history of glaucoma in the opposite eye
4. Follow up in animals with medically controlled glaucoma
Determination of intraocular pressure (IOP) is indicated in all eyes with diffuse corneal edema, anisocoria, fixed and dilated pupils, episcleral congestion, blindness, buphthalmos and anterior uveitis. In addition, animals with medically or surgically controlled glaucoma require sequential determination of IOP to ensure adequate control and animals with unilateral primary glaucoma and breeds predisposed to primary glaucoma require monitoring of the IOP in the unaffected at risk eyes.
Determination of intraocular pressure requires an instrument that is fast, accurate, portable and user friendly. As clinicians we must feel we can rely on the accuracy of the test and make treatment decisions based on the results. In my opinion, most of us do not feel this way about the Schiotz tonometer and consequentially, this instrument remains in its case in a drawer. |
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Term
| 10. Define CERF and what this is for. |
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Definition
The Canine Eye Registration Foundation (CERF) is an organization that was founded by a group of concerned, purebred owner/breeders who recognized that the quality of their dog's lives were being affected by heritable eye disease. CERF was then established in conjunction with cooperating, board certified, veterinary ophthalmologists, as a means to accomplish the goal of elimination of heritable eye disease in all purebred and recently hybrid dogs by forming a centralized, national registry.
The CERF Registry not only registers those dog's certified free of heritable eye disease by board certified Veterinary Ophthalmologists (A.C.V.O. ), but also collects data on all dogs examined by A.C.V.O. Diplomates. These data are used to form the CERF research database which is useful in researching trends in eye disease and breed susceptibility. Not only are these data useful to clinicians and students of ophthalmology, but to interested breed clubs and individual breeders and owners of specific breeds. |
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