Ocular examination techniques for private practitioners (Proceedings)

Anatomic and physiologic considerations are the basis for proper diagnostic techniques. We will discuss basic diagnostic procedures and relative pharmacological consideration to enhance the ophthalmic examination.

Normal anatomy and physiology

The eye is a sensitive neurological tissue and is considered the anterior extension of the brain. It functions to translate light waves into a nerve impulse, which can then be interpreted by the brain as sight. The eye is extremely delicate and sensitive, and has special anatomic and physiologic adaptations. A basic understanding of ocular anatomy and physiology are necessary to appreciate abnormalities and understand their significance. The eyes and their associated structures are bilateral and symmetrical in most species.

Eyelids

Eyelids protect, lubricate and meter out excessive light. The shape of the palpebral fissure is species and breed dependent. A wide variation among the breeds results in a particular facial expression. Two main sets of muscles control eyelid position. The orbicularis muscles, innervated by the facial nerve, close the eyelid. The levator palpebral muscle innervated by the oculomotor nerve raises the upper eyelid. The margins of the eyelids contain specialized cilia known as eyelashes, and openings to the sebaceous meibomian glands provide oily secretions. The glands of Zeis and Moll are sebaceous and sweat glands that are associated with the follicles of the eyelash.

Conjunctiva/third eyelid

The conjunctiva is the mucous membrane lining inside of the eyelids (palpebral conjunctiva) and is continuous onto the anterior sclera (bulbar conjunctiva). The palpebral conjunctiva is thicker and pink-red in color, whereas the bulbar conjunctiva is very thin, colorless and nearly transparent. Both contain blood vessels which when irritated will become congested and red. Jaundice contrition will give the conjunctiva a yellowish color. Anemia will give it a blanched, pale appearance. The third eyelid (nictitating membrane) is a specialized conjunctival structure of non-primate animals. It serves to protect the cornea, secrete part of the tear film and fight infection via lymph follicles. This is supported by a T-shaped piece of cartilage and at the base of this cartilage is the gland of the third eyelid, which produces a significant quantity of tears. The excursion of the nictitating membrane, or third eyelid, is passive in dogs and due to the retraction of the globe.

Lacrimal apparatus

Lacrimation is the production of tears. Tears are a composite of secretions of several glands. The two main glands in small animals are their orbital lacrimal gland located in the dorso-temporal orbit and the gland of the third eyelid. Tear secretion provides a protective function (antibacterial activity) and a source of nutrition and wastes product removal for the cornea. The tear film is comprosed of the trilaminar layer: the out oily layer (from the meibomian glands), the middle aqueous layer (from the lacrimal gland and gland of the third eyelid), and an inner mucin layer (conjunctival glands). A lack of watery, aqueous tear secretion results in keratoconjuctivitis sicca, while a lack of mucin secretion results in abnormalities of the tear film and focal areas of dryness. Chronic blepharitis may compromise meibomian gland secretions. This will decrease the outer, oily layer, resulting in more rapid evaporation of the aqueous layer.

Tears exit the palpebral fissure by evaporation or through the nasolacrimal system. The openings of the apparatus are located at the medial canthi and are comprosed of a small opening (punctum) in the upper and lower eyelids. The puncta, via canaliculi (lacrimal ducts), join to forma a lacrimal sac. The nasolacrimal drainage duct exists from the lacrimal sac, passes through the bones of the nose, and exits trough the nasal punctum; generally, a few millimeters from the external nares.

Globe

The eye proper is composed of three layers or tunics: (1) the outer fibrous tunic is composed of the corneo-scleral shell; (2) the middle vascular tunic is comprosed of the choroids, Ciliary body, and iris; and (3) the inner nervous tunic, which is of neural tissue origin and is comprosed of the neurosensory retina, retinal pigment epithelium of the Ciliary body, and the posterior iris surfaces.

Cornea –sclera

The outer fibrous layer serves to support the more delicate intraocular structures. The posterior five-sixth of the globe is a clear structure known as the cornea. The cornea is comprosed of three basic layers the most important of which is the outer epithelial layer, the central stroma and the inner endothelial layer with a basement membrane known as Descemet's membrane. The cornea being avascular needs to maintain its hydration and provide oxygen through trilaminar tear film comprosed of an outer oily layer from the meibomian glands, a central serous layer from the lacrimal glands and an inner lucent layer with secretion from the conjunctival goblet cells.

A break in the outer epithelium is known as a superficial ulcer. Ulcers may penetrate into deeper structures resulting in stromal defects down to Descemet's membrane resulting in a descemetocele. The cornea's function is to provide the majority of the focusing power to the eye apparatus.

Uveal layer

The middle layer of the eye is known as the uveal tract and is rich in blood vessels. (Uvea is derived from the Latin word for grape.) The anterior extension of this layer, the iris, forms the colored part of the eye. An opening in the center of the iris, the pupil, permits light to enter the posterior segment of the glove. Pupil shape is determined by the constrictor muscles of the eye and their fibrous orientation. Pupil size is controlled by sympathetic and parasympathetic nerve impulses. The size of the pupil determines the amount of light reaching the retina. Conditions where the pupil action is incomplete may indicated decreased vision. Older dogs with iris sphincter atrophy and a resulting dilated pupil may wink and blink in bright sunlight. Persistence of papillary membranes during the post-natal period can result in adhesions and synechia. This may result in cataract changes or corneal opacities. The posterior extension of the uvea is the Ciliary body. The Ciliary body supports the lens and functions in accommodation of the lens through a muscular action. The epithelium of the Ciliary body also secretes the aqueous humor, which is the clear fluid of the anterior chamber. The fluid circulates through the pupil and exits through the iridocorneal angel. Abnormalities sin the aqueous humor can be caused by inflammation. Glaucoma is the abnormal increase in intraocular pressure due to reduced outflow of aqueous. This can be a primary disease or secondary to inflammation or trauma. The most posterior extent of the uveal tract is the choroids. The choroid serves to nourish the retina. A specialized layer of the choroid, known as the tapetum, results in a unique iridescent eye shine for animals. The tapetum has a wide variation in appearance and may be absent in certain individuals.

The retina-neurosensory layer

The inner layer of the eye is comprised of the the retina and the posterior segment, the Ciliary epithelium over the Ciliary body area and the pigment epithelial layer if the iris anteriorly. The retina is divided into the neurosensory retina and the retinal pigment epithelial layer. The neurosensory layer is a multilayer neurological tissue starting with the photoreceptor outer segments proceeding to the outer nuclear layer and outer plexiform layer, bipolar cell layer and inner plexiform layer, ganglion cell layer and a nerve fiber layer. The optic nerve transmits information from the neurosensory retina to the brain for interpretation. The anterior extension of the neurosensory retina is responsible of the pigmented layer of the epithelium of the Ciliary body on the posterior surface of the iris. The anterior extension of the retinal pigment epithelial layer is responsible of the secretory cells of the ciliary body and muscular system of the iris. Part of this layer gives rise to the dilator muscles and the sphincter muscles of the iris. The retina is nourished passively by choroidal circulation and actively by retinal circulation. Retinal vascular patterns vary widely mound different species and among different breeds within the same species and individuals within the same species.

Lens

The crystalline lens is a biconvex structure located behind the retina and is supported by the zonules of Zinn, which attach to the Ciliary body. When muscles of the Ciliary body contract a certain amount of accommodation is possible. The muscles of accommodation are weakly developed in most domestic animals and there is little focusing power. Cataracts are opacities in the crystalline lens and may result from nutritional, metabolic, toxic, and inherited causes.

History taking

History is a very important part of the ophthalmic examination. History taking starts when a client initially calls for an appointment. At that point, the receptionist must be able to discern those ophthalmic problems that are of an emergency nature and require immediate attention from those, which may be schedule at a later time. If the history includes rapid onset of pain, discharge or discoloration, the patient should be seen as soon as possible. In the absence of pain and excessive discharge, slow discolorations or changes can be generally scheduled at the client and veterinarian's convenience. With history taking, the client should not be lid into making assumptions about the state of vision. Thorough history taking may provide keys to a diagnosis of the presence of ocular disease. History taking should include the period of ocular changes, the type of ocular change, history of ocular disease within the pedigree, recent changes in general health, previous and concurrent therapy, and information on other animals in the household.

Observation

When the animal is presented, the technician should observe the animal as it is being brought to the examination room. The veterinary hospital provides a unique opportunity in a new location to evaluate vision. If the floor changes color and the animal are reluctant to step across this, this may indicate something about the vision of the patient.

Examination

When the patient arrives in the examination room, a penlight can be used to assess papillary light reflexes. This is best evaluated in a dark room. The penlight should be shined from a position of 1-2 feet away from the animal. Each pupil should be noted individually. As the penlight is moved closer, individual papillary light reflex can be assessed. Both a direct response in the pupil being stimulated and an indirect response as the fellow pupil is stimulated should be noted. A painful eye should receive minimal attention until the veterinarian has had a chance to assess the patient. Ocular discharge should not be cleaned away prior to the veterinarian's initial examination. Minimal equipment that should be prepared for the ophthalmic examination is a transilluminator, a direct ophthalmoscope, fluorescein dyes, Schirmer's Tear Test, Schiotz' tonometer and magnification loupes.

Ophthalmic diagnostic instruments

     Finoff transilluminator

This is a small beam of light that can be directed through the various tunics of the eye and used to evaluate intraocular structures and papillary light reflexes.

     Ophthalmoscope

A direct ophthalmoscope is an instrument used to evaluate the intraocular structures and is primarily used to evaluate the fundus of the eye.

     Magnification loupes

A system of lenses used to provide 2 to 5x magnification of the eye.

     Biomicroscopy

There are five basic techniques of biomicroscopy including diffuse illumination, retro illumination, direct illumination, optical section and specular reflection. Quality instruments can be provided from a number of different sources and this is the basis for a complete examination of the anterior segment of the eye. A slit lamp provides a slit beam of light that can be viewed at various degrees of magnification from an angle and permits a thorough examination of tear film, cornea, iris, lens and anterior vitreous.

     Indirect ophthalmoscope

A system of light and lenses used to evaluate the posterior segment of the eye (fundus); gives a panoramic view but sacrifices degree of magnification (See section on fundus examination)

     Tonometry

Various methods used to evaluate intraocular pressure. Schiotz' tonometry involves the use of an indenting device that correlates between the degree of corneal indentability and intraocular pressure. Applanation tonometers of various types plane the cornea and then the signal is electronically translated into millimeters of mercury. Newer rebound tonometers provide a reliable means of evaluating intraocular pressures in horses, cats and dogs.

     Gonioscopy

This is the examination of the filtration angle and is done in patients with glaucoma. Special goniolens is fitted on the cornea and magnification and a light source enable visualization of the angle.

     Electroretinography (ERG)

A special test that records the electrical activity of the eye when the retina is stimulated by light. It is a valuable aid in determining retinal function when direct visualization of the retina is not possible due to an opaque cornea or lens. It can detect the presence of some retinal diseases before physical changes can be noted. (See section on retinopathies)

Ophthalmic diagnostic tests

Schirmer Tear Test: A method to quantitate the secretions of aqueous tear. This test must be done at the beginning of the examination because other procedures may alter the results. The test strip should be places in the lower cul-de-sac for sixty seconds. The normal range for the doge is 15 to 25mm. A Schirmer II tear test uses the same technique as the Schirmer Tear test, but follows the application of topical anesthetic. It estimates basal tear secretion.

     Cultures and antibiotic sensitivities

Bacterial cultures need to be obtained before any external solutions are placed within the cul-de-sacs of the eye. Care should be taken to take the culture from the area of primary interest. In the case of generalized conjunctival infections, cultures may be obtained from the cul-de-sac; if it a primary corneal ulcer care should be taken to take the culture from the ulcer. Studies have shown that moistening the tip of the applicator can enhance recovery. The specimen should be sent to a diagnostic laboratory that is familiar with ocular cultures. Enrichment broth should be used to enhance recovery. Antibiotic sensitivity dust should be appropriate for the medicines used to treat ophthalmic diseases and special protocols can be established with reference laboratories. Approximately 85% of normal eyes do have bacterial growth and these are largely Gram-positive in nature. The cultures should be plated on blood agar and MacConkey agar and the swab placed in thioglycolate for enrichment. If initial growth is not obtained, from the original agar plates, then the broth should be plated. If a significant growth is obtained, antibiotic sensitivity tests should be run.

     Cytology

Can be obtained by either aspiration of intraocular fluids or smears from external membranes. The slide should be allowed to air dry for 10-15 minutes and then fixed in methyl alcohol for 10 seconds and then allowed to air dry for another 10-20 minutes. The slides can then be processed by a variety of techniques. One technique is the use of Diff Quick® method. The evaluation of cytology should include the presence of cornified or non-cornified epithelial cells, populations of white blood cells and intracellular inclusions. A second slide can be evaluated by using the Gram-staining technique for bacterial components. Potassium hydroxide can be used to evaluate for fungal inclusions, as can the methylene blue stain. Extra slides should be retained for special staining procedures as may be indicated.

     Corneal stains

Fluorescein stain is applied to evaluate the extent of corneal ulcers. Fluorescein stain is a water-soluble dye that will not adhere to the corneal epithelium. Breaks in the epithelium result in retention of the dye through the exposed stroma. The dye is not retained by Descemet's membrane, so in deep ulcers there may be a defect in stain retention centrally due to the depth of the ulcer to the level of Descemet's membrane. Rose Bengal is a vital stain used to evaluate the integrity of the corneal epithelium. A positive retention is seen frequently in cases of keratoconjunctivitis sicca.

     Topical anesthetic agents

Examination of the third eyelid and fornix for disease or foreign body requires topical anesthetic. The third eyelid is then grasped with smooth forceps and examined closely.

     Nasolacrimal drainage system

Patency of the lacrimal system can be checked by placing a drop of fluorescein dye in the conjunctival sac and waiting for the dye to appear in the nose: or a topical anesthetic can be placed in the eye and the palpebral punctum cannulated and flushed.