Small animal ophthalmology a problem oriented approach WWW VETBOOKSTORE COM

Invagination of the lens placode occurs concurrently with that of the optic vesicle to form a hollow lens vesicle within a bilayered optic cup Fig.. 1.1C,D, the inner layer of which will

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Contributors

Peter GC Bedford BVetMed PhD DVOphthal DipECVO FRCVS

GBDA Professor of Canine Medicine and Surgery

Royal Veterinary College

Hatfi eld, UK

Ellen Bjerkås DVM PhD DipECVO

Professor

Department of Companion Animal Clinical Sciences

Norwegian School of Veterinary Sciences

Professor of Clinical Neurophysiology

Department of Clinical Sciences

Swedish University of Agricultural Sciences

Uppsala, Sweden

Bruce H Grahn DVM Diplomate ABVP ACVO

Professor of Veterinary Ophthalmology

Department of Small Animal Clinical Sciences

Western College of Veterinary Medicine

University of Saskatchewan

Saskatoon, Saskatchewan, Canada

R Gareth Jones BVSc CertVOphthal MRCVS

The Park Veterinary Group

Leicester, UK

Olivier Jongh DMV

Clinique Vétérinaire du Val de Saône

Neuville sur Saône, France

Mary L Landis MS VMD

Resident in Ophthalmology

Bucks County Animal Ophthalmology

Doylestown, PA, USA

Sebastien Monclin DVM

Resident of Ophthalmology

University of Liège

Belgium

Domenico Multari DVM SCMPA PhD

Centro Veterinario Oculisto ‘Fontane’

Treviso, Italy

Kristina Narfström DVM PhD DipECVO

Professor of Veterinary Ophthalmology

Department of Veterinary Medicine & Surgery

University of Missouri

Columbia, MO, USA

Robert L Peiffer Jr DVM PhD DipACVO

Bucks County Animal Ophthalmology

Doylestown, PA, USA

Simon M Petersen-Jones DVetMed PhD DVOphthal DipECVO MRCVS

Assistant Professor of Comparative Ophthalmology

Department of Small Animal Clinical Sciences

Veterinary Medical Center

Michigan State University

East Lansing, MI, USA

Peter W Renwick MA VetMB DVOphthal MRCVS

Willows Referral Service

Shirley, Solihull, UK

Serge G Rosolen DVM PhD

Eye Veterinary Clinic

Asnières, France

Robin Stanley BVSc(Hons) FACVSc

Animal Eye Care

East Malvern, Victoria, Australia

Wendy M Townsend DVM MS DipACVO

Assistant Professor of Comparative Ophthalmology

Small Animal Clinical Sciences

Veterinary Teaching Hospital

Michigan State University

East Lansing, MI, USA

Joe Wolfer DVM DipACVO

Veterinary Ophthalmologist

Animal Eye Clinic

Toronto, Ontario, Canada

Mike Woods MVB CertVOphthal MRCVS

Practice Principal & Ophthalmologist

Primrose Hill Veterinary Hospital

Dun Laoghaire, Co Dublin, Ireland

Preface to First Edition

Ophthalmology has blossomed and matured as a recognized, valued specialty

of veterinary medicine and surgery; ophthalmic exposure is generally sized in the professional curriculum; the competency and sophistication of the general practitioner is continually improving; and several excellent contempo-rary comprehensive textbooks are available on the subject

empha-Then why this text? We have recognized a need by the general practitioner for an informative source that he or she can turn to as a guide to the manage-ment of a particular problem Appropriate management implies two insepar-able principles – accurate diagnosis and adequate therapy We have attempted

to address each with equal emphasis We perceive a need by the student for a text that condenses a large amount of information into a ‘friendly’ manual that emphasizes problem solving rather than memorization and that provides more usable information than lecture notes without the depth of a reference text We hope this manual meets these needs

Why these authors? The profession and the specialty are evolving and ing Although I am somewhat reluctant to classify myself as ‘mature’ as a clini-cal ophthalmologist, I cannot help but be impressed by the energy, enthusiasm, and ideas of a younger generation of amazingly well-trained ophthalmologists All of the contributors fi t this mold, and I hope that they and their colleagues who follow will continue to probingly question the established as well as addressing unsolved problems Experience is almost always tainted by dogma-tism, which in turn can cloud truth; I have encouraged Drs Cook, Leon, Cottrell, and Petersen-Jones to express their ideas and philosophies without unwarranted respect for sacred cows The product is exciting

chang-We have attempted not to reproduce a comprehensive text but to produce

a clinical manual; references are not included As conditions may present with more than one presenting sign, there is some repetition; conditions are discussed in detail under their most obvious or signifi cant sign We have discussed in detail only those surgical procedures that are likely to be routinely performed by the practitioner, and details of these procedures are described with their pictorial presentation rather than in the text Emphasis is placed

on techniques that have proven to be most valuable and effective for the authors, and readers should recognize that there may indeed be quite accept-able alternative approaches to clinical problems We do hope that this

handbook will prove a ready and valuable reference to the general practitioner

presenting with a challenging ophthalmic case and when reviewed in its entirety

will provide a practical overall approach to small animal ophthalmology

When the fi rst edition of Small Animal Ophthalmology: a Problem-Oriented Approach was published in 1989 I would not have foreseen struggling with the Preface to the Fourth Edition almost two decades later The children have grown and moved away, and a German Shorthair and Redbone Coon Hound have been replaced by a pair of Labrador Retrievers The cat, I suspect,

is reincarnate of his predecessors, and the Pennsylvania winters are a bit longer and colder than those in the South I have been fortunate to have Simon Petersen-Jones to share the labor from the second edition onward and both myself and the text have benefi ted from his diligence and insight While the world has changed, the scope and intent of the text remain constant – to provide the student or general practitioner with a practical reference that condenses an ever-expanding base of knowledge in small animal ophthal-mology into an affordable user-friendly clinical manual that emphasizes problem-solving in dealing with patients that present with ophthalmic signs This was a novel approach at the time, and the fact that the book has been translated into Japanese, Spanish, and French, and oft mimicked since, speaks

to its utility

We have maintained the theme of recruiting accomplished contributors who provide broad, contemporary, and international perspectives All share a com-mitment to excellence in the management of their patients that is refl ected in the quality of their work

As I compare their contributions to those in the fi rst edition I realize that progress is made in small steps; successful management of canine glaucoma is still largely an exercise in frustration in spite of new potent drugs and the con-temporary technologies of laser and implants Treatment of tear fi lm defi cien-cies still requires long-term management and a motivated and educated pet owner, although the lacrimostimulants have obviated the necessity of parotid duct transposition in many Technologies and methodologies in imaging, cata-ract surgery, and retinal detachment repair have remarkably enhanced out-comes for many of our patients The potential of molecular medicine beckons from a seemingly distant horizon Practicing ophthalmology during these times has been an adventure and a privilege indeed

We are grateful for the competence and professionalism of the Elsevier staff who have provided encouragement, guidance, and the occasional nudge that Preface to Fourth Edition

xiv

these projects seem to require The opportunity to include a CD-Rom allows

us to expand the visual impact of observation to formulate differential ses We will be content with our labors if readers emerge from their study more profi cient in the management of their ophthalmic cases

diagno-Bob PeifferDoylestown, Pennsylvania 2008

I am delighted to join with Bob again to help edit another edition of Small Animal Ophthalmology: a Problem-Oriented Approach I well remember over

20 years ago writing a chapter for the fi rst edition I was an ophthalmology resident visiting Dr Peiffer (as have many aspiring young ophthalmologists before and after me) when he asked if I would be interested to write a chapter for the book he was developing I jumped at the opportunity, never suspecting that I would join Bob to edit the subsequent editions

Veterinary ophthalmology has a rapidly expanding knowledge base but the problem-oriented approach still works well Our patients present to us with certain clinical signs that fall into the broad categories of the chapters in the book, rather than with a diagnosis of, for example, retinal detachment or dis-tichiasis It is our job to identify the clinical signs and through a systematic and thorough eye examination reach a diagnosis The aim of the book is to help practitioners achieve this goal

In this latest edition we have added a CD-Rom that allows for case tions – we hope that this will be useful and educational for our readers

presenta-Simon Petersen-JonesEast Lansing, Michigan 2008

Clinical basic science

Cynthia S Cook, Robert L Peiffer, Jr

and Mary L Landis

1

OCULAR EMBRYOLOGY

The ocular primordia appear during the fi rst weeks of gestation as bilateral evaginations of the neural ectoderm of the forebrain These optic sulci gradu-ally enlarge and approach the surface ectoderm as optic vesicles connected to the forebrain by the optic stalks Thickening of the overlying surface ectoderm

to form the lens placode (Fig 1.1A,B) occurs as a result of inductive infl uences

by the optic vesicle Invagination of the lens placode occurs concurrently with that of the optic vesicle to form a hollow lens vesicle within a bilayered optic cup (Fig 1.1C,D), the inner layer of which will form the stratifi ed layers of the neural retina and the inner epithelial layer of the iris and ciliary body; the outer layer becomes the cuboidal monolayered retinal pigment epithelium, the outer pigmented epithelial layer of the iris and ciliary body, and, in the dog and cat, the pupillary sphincter and dilator muscles (the only muscles in the body of neural ectodermal origin) The potential space between the two apposed layers becomes formed and fl uid-fi lled in retinal detachment and uveal cysts The stalk attaching the lens vesicle to the surface ectoderm atrophies through a combination of cell death and active migration of cells out of the stalk (Fig 1.1E,F)

Invagination to form the optic cup occurs eccentrically, with formation of a slit-like opening called the optic (choroid) fi ssure located inferiorly (Fig 1.1F) The vascular supply to the embryonic eye, the hyaloid artery (or primary vitre-ous), enters the optic cup through this opening and arborizes extensively around the lens to form the tunica vasculosa lentis Embryonic remnants of this vas-cular structure may persist as insignifi cant posterior capsular opacities (includ-ing Mittendorf’s dot, located inferior to the suture junction), persistent tunica vasculosa lentis, or, more signifi cant clinically, persistent hyperplastic primary vitreous (PHPV) The term persistent embryonic vasculature, or PEV, encom-passes the entire spectrum Failure of the optic fi ssure to close normally may result in congenital defects anteriorly (iridial coloboma) or posteriorly (chorio-retinal or optic nerve coloboma) Microphthalmos or anophthalmos may occur

as a result of defi ciencies in the early formation of the optic sulcus or vesicle,

or from incomplete closure of the optic fi ssure with failure to establish early intraocular pressure (Fig 1.2)

Thickening of the future neural retina occurs with segregation into inner and outer neuroblastic layers Cellular proliferation takes place in the outer neuro-blastic layer, with migration to form the inner layer The ganglion cells are the

fi rst to achieve fi nal differentiation, extending axons that form the nerve fi ber layer and collectively form the optic nerve The horizontal, amacrine, and

Fig 1.1 Sequential development of ocular structures These scanning electron

micrographs are of mouse embryos on days 10 and 11 of gestation, corresponding to days

17–24 of gestation in the dog The sequence in most mammals is quite similar (A) On

external examination the invaginating lens placode can be seen (arrow) Note its position relative to the maxillary (Mx) and mandibular (Mn) prominences of the fi rst visceral arch

(B) Embryo of the same age as that in (A) Frontal fracture through the lens placode

(arrow) illustrates the associated thickening of the surface ectoderm (E) Mesenchyme

(M) of neural crest origin is present adjacent to the lens placode The distal portion of the optic vesicle concurrently thickens as the precursor of the neural retina (NR), while the proximal optic vesicle becomes a shorter, cuboidal layer which is the anlage of the retinal pigment epithelium (PE) The cavity of the optic vesicle (V) becomes progressively

smaller (C) The epithelium of the lens placode continues to invaginate (L) There is an

abrupt transition between the thicker epithelium of the placode and the adjacent surface ectoderm, which is not unlike the transition between the future neural retina (NR) and the

future pigmented epithelium (PE) (periodic acid–Schiff) (D) As the lens vesicle enlarges,

the external opening, or lens pore (arrow), becomes progressively smaller The lens

epithelial cells at the posterior pole of the lens elongate to form the primary lens fi bers (L) NR = anlage of the neural retina; PE = anlage of the pigmented epithelium (now a

very short cuboidal layer) (magnifi cation ×221) (E) External view of the lens pore

(arrowhead) and its relationship to the maxillary prominence (Mx) (F) Frontal fracture

reveals the optic fi ssure (*) where the two sides of the invaginating optic cup meet This forms an opening in the cup allowing access to the hyaloid artery (H), which ramifi es

around the invaginating lens vesicle (L) The former cavity of the optic vesicle is

obliterated except in the marginal sinus (S), at the transition between the neural retina (NR) and the pigmented epithelium E = surface ectoderm Arrowhead = stalk of

separating lens vesicle (Reprinted with permission from Vet Comp Ophthalmol (1995) 5:

109–123.)

Fig 1.2 Microphthalmia in a merle Australian Shepherd pup This genetic syndrome

(merle ocular dysgenesis) occurs in dogs with a predominantly white coat color

Microphthalmia occurs through multiple mechanisms including hypoplasia of the optic vesicle

Following detachment of the lens vesicle from the surface ectoderm, ment of the anterior chamber structures progresses A specialized population

develop-of the neural ectoderm called the neural crest cells migrate between the surface

ectoderm and lens vesicle to form the corneal endothelium, which secretes its basement membrane, Descemet’s membrane Additional neural crest cells form the corneal stroma between the surface epithelium and endothelium The pupil-lary membrane and anterior iris stroma develop from neural crest cells migrat-ing onto the anterior surface of the optic cup; persistence or dysplasia of the pupillary membrane results in uveal attachments between the iris and lens and/or cornea (Figs 1.3 & 1.4) Neural crest cells also form the outer two coats

of the posterior globe, the choroid (including the tapetum) and sclera

OCULAR ANATOMY, PHYSIOLOGY, AND BIOCHEMISTRY Orbit

The orbit in the cat and dog is formed by contributions of the frontal, tine, lacrimal, maxillary, zygomatic, and presphenoid bones The bony orbit

pala-is incomplete superotemporally, where it pala-is bridged by the dense orbital ment spanning the frontal process of the zygomatic bone and the zygomatic process of the frontal bone The lacrimal gland lies superiorly, under this orbital ligament The orbital contents are covered by a connective tissue layer, the periorbita, which is fi rmly attached to the orbital margins rostrally Seven extraocular muscles innervated by the third, fourth, and sixth cranial nerves

liga-Fig 1.3 Peter’s anomaly

in a cat Note the persistent pupillary membranes attached to the anterior lens capsule with associated anterior subcapsular opacity

Fig 1.4 Schematic of components of Peter’s anomaly (anterior segment dysgenesis)

which result from incomplete or delayed separation of the lens vesicle from the surface

ectoderm (A) Persistent pupillary membranes; (B) corneal opacity with absence of

endothelium and Descemet’s membrane; (C) iris hypoplasia; (D) anterior lenticonus and

anterior polar cataract associated with anterior capsular defects (Courtesy of Farid

Mogannam.)

control movement of the globe There is a variable amount of fat between the periorbita and the bony wall and surrounding the extraocular muscles The zygomatic salivary gland is located inferotemporally, deep to the zygo-matic arch, and may be a site of infection or mucocele formation

The wall of the bony orbital wall is thinner medially and may allow extension

of infectious or neoplastic processes originating in the nasal cavity or bital sinuses Infectious processes involving the roots of the molar teeth may also extend to involve the orbit

perior-Space-occupying orbital lesions include both infl ammatory and neoplastic etiologies Due to the incomplete nature of the bony orbit, both inferiorly and superotemporally, a space-occupying process may become quite advanced before exophthalmos and/or deviation of the globe is noted Diagnosis and management of such conditions are discussed in subsequent chapters

Eyelids

The eyelids form the initial barrier to mechanical damage to the eye They also serve to distribute the tear fi lm and, through the meibomian glands, provide

an oily secretion to slow tear evaporation The eyelids consist of:

1 An outer layer of thin, pliable skin

2 A small amount of loose connective tissue containing modifi ed sweat

glands and the circumferential fi bers of the orbicularis oculi muscle

(innervated by branches of the facial nerve)

3 The more rigid fi brous connective tissue of the tarsal plate

4 The radial fi bers of the levator palpebrae superioris (innervated by the oculomotor nerve) and Müller’s (sympathetic innervation via branches of the trigeminal nerve) muscles

5 The palpebral conjunctiva containing goblet cells

of these glands results in formation of aberrant hair follicles (distichia or ectopic cilia), which may contact the cornea and result in epiphora and, rarely, keratitis.

Surgical manipulations of the eyelids require delicate handling to minimize swelling and careful apposition of surgical or traumatic wound margins Par-ticular attention should be paid to maintenance of a smooth eyelid margin Closure of full-thickness defects should utilize a two-layer pattern; the tarsal plate has the greatest strength and should be included in the subcutaneous layer

Lacrimal system

The precorneal tear fi lm consists of three distinct layers:

1 A mucous layer located closest to the cornea and produced by the conjunctival goblet cells

2 A thick aqueous layer

3 An outer oily layer produced by the meibomian glands of the eyelids.The aqueous portion of the tear fi lm is the combined product of the orbital lacrimal gland and a gland located at the base of the third eyelid The major lacrimal gland is located in the superotemporal area of the orbit beneath the orbital ligament and supraorbital process of the frontal bone; its secretions gain access to the conjunctival sac from numerous small ducts in the superior fornix The tears are distributed over the surface of the cornea through the action of the eyelids and exit through the nasolacrimal puncta These two openings are located nasally, superior, and inferior to the medial canthus, just inside the eyelid margin (see Fig 1.5) The puncta open into two canaliculi joining to form the nasolacrimal duct, which passes through a bony canal in the maxilla

to open ventrolaterally in the nasal cavity

Dorsal (superior)

punctum

LimbusLateral(temporal)canthus

Fig 1.5 External appearance of the canine eye depicting the adnexal structures With

the exception of the pupillary shape, the feline eye is identical

Myelinatedfibers

Endothelium

Inner limiting membraneGanglion cellGanglion cell axonsforming optic nerveBipolar cellOuter limiting membraneNuclei of photoreceptors

Descemet’smembrane

OpticdiskCiliary body

Müller’s m

Levator palpebraesuperioris m

Nerve fiber layer

Ganglion cell layer

Inner plexiform layer

Inner nuclei layer

Outer plexiform layer

Outer nuclei layer

Rods and cones

Conjunctiva and third eyelid

The conjunctiva is a mucous membrane that covers the globe between the fornix

and the cornea, the third eyelid, and the inner surface of the eyelids (see Fig 1.6) Over the surface of the globe, the conjunctiva blends with Tenon’s capsule, which attaches fi rmly to the limbus The conjunctiva is a highly vascular, deli-cate tissue containing many mucus-secreting goblet cells The vascularity and mobility of the conjunctiva can be used to the surgeon’s advantage to act as a graft for corneal defects The stroma is rich in lymphatics and the conjunctiva

is a site of localization of lymphocytes, and provides a reservoir of competent cells for the globe, playing an important role in the infl ammatory responses of the avascular cornea

immuno-The third eyelid is a mobile, semi-rigid structure located inferonasal to the

globe (see Fig 1.5) It is covered on both palpebral and bulbar surfaces by conjunctiva The third eyelid owes its rigidity to a T-shaped piece of hyaline cartilage located within its substantia propria At the base of the cartilage is a seromucoid lacrimal gland that produces approximately one third of the pre-corneal tear fi lm Poorly defi ned connective tissue attaches the gland and base

of the cartilage to the sclera and periorbita inferiorly Inadequacy of these attachments with prolapse of the gland occurs not uncommonly, particularly

in the American Cocker Spaniel and English Bulldog breeds Removal of the gland in such cases is contraindicated as it may predispose to future develop-ment of keratoconjunctivitis sicca; the gland should be repositioned and fi xated

as described in Chapter 4 (pp 88–90)

Cornea

The cornea is the transparent, avascular, anterior portion of the outer fi brous

coat of the eye (see Fig 1.6A) The cornea consists of surface epithelium, lagenous stroma, and Descemet’s membrane, which is the basement membrane produced by the inner endothelial monolayer As the cornea is avascular, its oxygen and nutritional needs are met by diffusion externally from the precor-neal tear fi lm and internally from the aqueous humor; the peripheral cornea is also oxygenated by the limbal capillary plexus Corneal transparency is a product of several factors unique to corneal physiology Relative dehydration

col-of the cornea is maintained by an active Na+-K+ ATPase-associated pump mechanism within the endothelial monolayer The regular arrangement of the collagen fi brils in the corneal stroma minimizes scattered light and thus enhances transparency The normal absence of pigment and blood vessels in the stroma

is also a requirement for optical transparency

The cornea has remarkable healing capabilities Simple epithelial defects are covered by a combination of sliding of adjacent cells and mitosis to restore normal architecture Wounds that extend into the stroma heal fi rst by re-epithelialization, with a longer period of time required to fi ll the stromal defect Corneal scarring is a result of the irregular pattern created by replace-ment collagen fi brils Vascularization is expected to accompany any corneal injury or infl ammatory condition that persists longer than 7–10 days and contributes to the granulation tissue that initially fi lls a deep corneal wound Descemet’s membrane is elastic and tends to resist tearing during an injury Wounds extending to Descemet’s membrane (descemetocele) and full-thickness lacerations are indications for immediate surgical management Some regen-

Iris and ciliary body

The iris and ciliary body comprise the anterior portion of the middle,

vas-cular coat of the eye, called the uvea (see Fig 1.6) The iris creates a

pupil-lary opening of variable diameter to adjust the quantity of light that is able

to pass through the lens to reach the photosensitive retina This variable aperture is maintained by the sympathetically supplied radial dilator muscle and the parasympathetically supplied circumferential sphincter muscle Both muscles are located on the posterior side of the iris, adjacent to the pig-mented epithelial layer The iris anterior to these muscles consists of a loose, vascular connective tissue that is variably pigmented Full-thickness corneal wounds often seal with prolapsed iris tissue, which must be replaced into the anterior chamber (if viable) or excised Surgical manipulations of the iris are frequently accompanied by hemorrhage that may complicate post-operative healing

The ciliary body is the posterior continuation of the iris and consists of an anterior portion called the pars plicata (with the ciliary processes) and a poste- rior portion called the pars plana The ciliary body is lined by a bilayered epi-

thelium of which only the inner layer is pigmented Aqueous humor is produced

by the ciliary epithelium through a combination of passive ultrafi ltration and active secretion involving carbonic anhydrase The passive production of aqueous humor is infl uenced by mean arterial blood pressure Infl ammation of the anterior uvea will result in reduced active aqueous secretion and thus lowered intraocular pressure The stroma of the ciliary body contains the smooth fi bers of the parasympathetically innervated ciliary muscle, which is important in accommodation of the lens for near vision

Aqueous humor circulates from the ciliary processes into the posterior chamber of the eye, through the pupil, to exit via the trabecular meshwork within the iridocorneal angle During this process, metabolites are exchanged with the avascular lens and cornea Morphologic or physiologic barriers to aqueous circulation and outfl ow are responsible for elevations in intraocular pressure (glaucoma)

Lens

The lens is a transparent, biconvex structure anchored equatorially to the

ciliary body by collagenous zonular fi bers (see Fig 1.6) Contraction of the ciliary muscle alters the degree of curvature of the lens, thereby changing its optical power The lens is surrounded by an outer capsule; deep to the anterior portion of the capsule is a monolayer of cuboidal epithelium These epithelial cells are metabolically active and undergo mitosis throughout life As the cells multiply they migrate to the equator of the lens where they elongate and gradu-ally lose their nucleus and other organelles to form the lens fi bers These fi bers are added in a circumferential arrangement so that older fi bers are within the deeper portion of the lens The fi ber ends meet anteriorly at the upright Y suture and posteriorly at the inverted Y suture

The anterior epithelial cells utilize glucose, which diffuses into the lens from the circulating aqueous humor and is broken down anaerobically to lactic acid

The retina (see Fig 1.6) is a complex photosensory structure consisting of ten

layers:

1 Pigment epithelium

2 Photoreceptors (rod and cone outer segments)

3 External limiting membrane (Müller cell processes)

4 Outer nuclear layer (photoreceptor nuclei)

5 Outer plexiform layer

6 Inner nuclear layer (nuclei of Müller; amacrine, horizontal, and bipolar cells)

7 Inner plexiform layer

8 Ganglion cell layer

9 Nerve fi ber layer (axons of ganglion cells)

10 Inner limiting membrane (Müller cell processes)

The principal neuronal connections of the retina involve the photoreceptors, which synapse with the bipolar cells that then synapse with the ganglion cells

in the inner plexiform layer The axons of the ganglion cells form the nerve

fi ber layer and join to make up the optic nerve at the posterior pole The crine and horizontal cells form internal connections between bipolar cells and may thus exert a regulatory infl uence Müller cells are a non-neuronal constitu-ent that forms a supporting matrix and the barriers of the inner and outer limiting membranes

ama-Inherited retinal degenerative processes and sudden acquired retinal eration (SARD) initially involve the photoreceptors, either rods or cones, or both With time the condition usually progresses to involve the other retinal layers, and diffuse thinning and blindness results

degen-Tapetum

The tapetum is a modifi cation of the choroid located deep to the pigment

epi-thelium and choriocapillaris It is composed of a highly organized arrangement

of cells containing zinc and ribofl avin, which results in a refl ective appearance The color of the tapetum ranges from green to blue to yellow and varies with the species, breed, and age Thinning of the overlying retina (as occurs in retinal degeneration) results in a hyper-refl ective appearance of the tapetum

Optic nerve and central visual pathways

The optic nerve consists of combined axons of the ganglion cells and is rounded by all three meningeal layers of the central nervous system The optic disk is the origin of the optic nerve within the globe; its irregular triangular appearance in the dog is a result of the variable quantity of myelin surrounding the nerve fi bers of the optic disk (see Fig 1.6) The optic nerve exits the orbit

sur-at the optic foramen The right and left optic nerves meet sur-at the optic chiasm, located rostral to the pituitary gland In cats and dogs, the majority (65–75%)

contralat-The majority of axons in the optic tracts terminate in the lateral geniculate nucleus, synapsing on neurons whose axons form the optic radiations and ter-minate in the occipital cortex This pathway is responsible for conscious visual perception.

The remaining optic tract axons bypass the lateral geniculate nucleus and terminate in the rostral colliculus of the pretectal area Parasympathetic axons originating here synapse in the oculomotor nucleus of the midbrain, origin of the oculomotor nerves, whose axons synapse in the ciliary ganglion prior to entering the globe as the short ciliary nerves to the pupillary sphincter muscles This pathway is responsible for the direct and consensual pupillary light responses The cat has two short ciliary nerves whereas the dog has several

ConstrictorOptic nn

Cervicalsympathetictrunk

Middleear

LVisual fields:

Cervical spinal cord

Thoracic spinal cord T1–T3

hypo-of the upper lid), miosis (pupillary constriction), and protrusion hypo-of the third eyelid.

OCULAR PATHOLOGY

The systematic examination of surgical and necropsy-obtained ocular tissue

is essential for optimal patient management, the career-long educational process, and enhancing understanding of ocular disease in animals Maximal benefi t is obtained from optimally fi xed tissues; in almost all cases, immersion

fi xation in 10% formalin is adequate Fixation should be expedient as the retina, especially, undergoes rapid autolysis; trimming of periocular tissues enhances penetration of fi xatives, and injection of 0.5 ml of the fi xative into the vitreous cavity with a 27-gauge needle at the equator will minimize neu-rosensory retinal separation artifact Otherwise, submit globes intact so that the pathologist can appreciate the intertissue relationships Use adequate volumes of fi xative (at least 100 ml for dog and cat eyes), and allow 72 h for

fi xation to occur

Ocular response to disease

A detailed discussion of ocular pathology would fi ll a text of its own; principles and concepts of importance to clinicians are discussed with particular disease processes throughout the following chapters Three related features warrant note:

1 The propensity of the ocular tissues (especially the epithelium of lens, uvea, and retina, but also the corneal endothelium and uveal vasculature)

to undergo reactive changes of hypertrophy, hyperplasia, and metaplasia (in the case of feline ocular sarcomas, perhaps neoplasia as well)

2 In contrast to the above, the fact that many of the specialized ocular tissues are post-mitotic, with limited regenerative potential

3 Because of the dependence of the ocular tissues on tissue transparency and intertissue relationships for normal function, the devastating effect that these changes can have on vision A focus of hepatitis may resolve with scarring and minimal, if any, functional signifi cance, while a

comparable process in the eye may lead to blindness

Fibroplasia in the cornea, for example, will result in scarring and opacifi tion In the anterior chamber, peripheral anterior and posterior synechia and membranes are associated with secondary glaucoma Iris neovascularization, also known as rubeosis irides or pre-iridal fi brovascular membrane, is a common cause of intraocular hemorrhage and secondary angle closure glaucoma

a relentless pharmacologic battle against these processes with anti-infl tories and antimetabolites, and new approaches will likely play an important role in the future management of ocular disease.

Serge G Rosolen, Domenico Multari,

Mike Woods and Olivier Jongh

2

INTRODUCTION

The ophthalmic examination, combined with history and signalment, provides the foundation for obtaining an accurate diagnosis Ophthalmic diagnosis is achieved by a combination of basic knowledge, the mastering of simple instru-mentation, and critical observation The former includes an understanding of anatomy, physiology, and disease mechanisms Instrumentation facilitates critical observation Basic equipment and simple techniques, including a mag-nifying loupe, bright focal illumination, Schirmer tear test strips, diagnostic dyes, cytology, direct ophthalmoscopy, and Schiøtz tonometry should be readily available in any practice, and in experienced hands will be adequate to manage the great majority of ophthalmic cases More expensive and sophisti-cated instrumentation and technologies, including the slit-lamp biomicroscope, indirect ophthalmoscope, applanation tonometry, electrophysiology, gonios-copy, ultrasonography, and other imaging modalities, fl uorescein angiography, keratoscopy, and retinoscopy represent the next level of diagnostics and are available to specialists or to those with a particular interest in the fi eld A sys-tematic approach to examination should be followed and modifi ed for each individual case based upon the history and signs Technical competency in diagnostics is achieved simply by practice; making an ophthalmic examination

a part of every routine physical examination will hone skills for the occasion upon which they are more urgently required

INSTRUMENTS AND BASIC DIAGNOSTIC TECHNIQUES Magnifying loupe

A binocular magnifying loupe of ×2 to ×4 magnifi cation and a focal length of 15–25 cm is useful not only for diagnostics but also for surgery; it allows freedom of both hands for manipulation and a loupe-mounted diffuse illumina-tor facilitates observation

Focal illumination

A transilluminator provides an excellent light source for external eye tion and to evaluate the pupillary light refl exes (PLRs) For the latter, it is

Schirmer tear test (STT)

This test is used quantitatively to evaluate the aqueous component of the tear

fi lm and thus aid in the diagnosis of keratoconjunctivitis sicca (KCS) The STT

is indicated in all patients with external ocular disease Individually wrapped sterile fi lter paper test strips may be dye impregnated to facilitate reading; these strips are typically 5 mm wide and 50 mm in length If performing a STT, it should be undertaken before any other procedures or tests; if there is discharge

in or around the eye, dry cotton swabs should be used gently to clean the area, avoiding irritation and refl ex lacrimation The strips have a notch near one end where they are folded prior to use; fold the strip without touching it with fi ngers while it is still in the overwrap Then open the package and, grasping the strip from the end opposite the notch with fi ngers or forceps, place it into the lower conjunctival sac approximately midway between the medial and lateral canthus with the short folded end in the fornix and the notch on the eyelid margin (Fig 2.1) The lower lid can be rolled outward with the thumb to facilitate insertion, but care should be applied not to compress the eye, which may likewise elicit refl ex lacrimation The lids may be maintained in an open position, or closed

by gentle pressure on the upper lid if blinking and retention of the strip becomes

a problem After 1 min, the moistened distance from the notch in the longer part is measured Normal values in the dog are 15–25 mm/min; values lower than 10 mm/min are suggestive of a defi cit in aqueous tear production Most clinical cases of KCS have a wetting of less than 5 mm; cats have slightly lower and more variable normal values There is a wide range of normal readings, and results should be interpreted in association with clinical signs Increased aqueous tear production may occur if conditions causing ocular irritation are present

Fig 2.1 Schirmer tear test being performed in a feline patient.

to penetration in the healthy eye resides in the outermost cells of the corneal epithelium As Descemet’s membrane does not retain fl uorescein, descemeto-celes will not stain Fluorescein is available as impregnated paper strips or as

a solution; the solution may become contaminated with multiple usage, and individually wrapped strips are preferred

Fluorescein staining is indicated in all patients with ocular pain or able corneal lesions The tip of the fl uorescein-impregnated strip is moistened with a drop of sterile saline and gently applied to the superior bulbar con-junctiva If the patient exhibits severe blepharospasm, local anesthetic can be instilled but may result in a mild diffuse positivity that is usually readily discernible from signifi cant retention Blinking will distribute the dye over the corneal surface The excess dye is immediately fl ushed with a sterile saline rinse and the eye is then examined with a focal light and magnifi cation (Fig 2.2) A cobalt blue fi lter will facilitate detection of subtle lesions To evaluate nasolacrimal patency, apply the fl uorescein as described above, but do not rinse the eye If the ipsilateral nostril shows dye within 5 to 10 min, the nasolacrimal drainage system on that side is patent; the absence of dye passage, however, does not necessarily mean the contrary, and negative passage is followed by cannulation and irrigation Dye may be seen in the nasopharynx related to alternative duct openings

observ-Biomicroscopic observation of the fl uorescein-stained tear fi lm while holding the lids open enables evaluation of the tear break-up time (BUT) as an indirect method of evaluating the non-aqueous components of the tear fi lm; mucus defi ciency will result in shortening of the BUT from the 20–30 s normally encountered

Rose bengal and lissamine green

These dyes stain cells of the cornea and conjunctiva that are not covered

by mucin; usually these are degenerating cells The stains are taken up by plastic cells as well and may be useful in defi ning the extent of epithelial neo-

neo-Fig 2.2 Fluorescein

uptake by the corneal stroma associated with a boxer ulcer

fl uorescein.

Cytology, culture, and additional diagnostic procedures

Cytologic examination is increasingly utilized in small animal ophthalmology; over the last decades, it has emerged as a reliable tool in facilitating diagnosis

in a minimally invasive way Techniques of sampling of a smear are outlined for each of the ocular structures On the other hand, the microscopic interpreta-tion of a smear is beyond the scope of this chapter Routine dermatologic techniques can be utilized to obtain scrapings from the eyelid skin for parasitic and fungal detection Fine needle aspiration may prove useful for characteriz-ing proliferative lesions (by using a 23-G needle and a 5 ml syringe) Impression smears can be obtained from ulcerated lesions (Figs 2.4 & 2.5), notably in cats with suspected squamous cell carcinoma If necessary, biopsy of skin lesions is performed to evaluate tissue architecture with histopathology

Conjunctival cytologic evaluation is useful:

1 In the differential diagnosis of acute conjunctivitis (the cellular response associated with specifi c conjunctivitis is helpful when performed early in infl amed conjunctivas, and Gram stain can provide guidelines for

antibiotic selection)

2 In the identifi cation of inclusion bodies (chlamydial, mycoplasma, canine distemper, and leishmania inclusions)

3 To facilitate the diagnosis of conjunctival tumors including

lymphosarcoma, mast cell tumor, melanoma, and squamous cell

carcinoma

Fig 2.3 Rose bengal positivity in a punctate pattern was evident in this 6-year-old Shih Tzu with

a vascularized cornea due

to keratoconjunctivitis sicca

Fig 2.4 A middle-aged mixed breed presented with ulcerated lesions of both eyelids.

Fig 2.5 Impression smears obtained from these ulcerated lesions revealed a

neoplastic population formed by lymphoid cells (Giemsa, original magnifi cation ×400) Mycosis fungoides was confi rmed with partial-thickness biopsy of the eyelid lesions

Cells from the conjunctival surface may be harvested using a sterilized spatula (i.e Kimura spatula), the blunt end of a sterile surgical blade, or a small nylon cytology brush; moist sterile cotton swabs are less likely to capture as many cells Sterile swabs may be utilized for culture of potentially pathogenic microorganisms (bacterial and fungal), and a cytology brush is preferred for

the detection of the presence of herpesvirus, calicivirus, or Chlamydia dophila) by polymerase chain reaction (PCR) Unfi xed and unstained slides may be submitted for immunofl uorescent antibody (IFA) studies The scraping should be made from the area most severely involved

(Chlamy-A topical anesthetic may not be necessary in most cases if the animal’s head

is fi rmly restrained, but greatly facilitates the procedure and is unlikely to affect culture results Mucus and exudate are removed prior to scraping It requires gentle but fi rm manipulation to collect an adequate sample (the conjunctiva should blanch but should not bleed) Collected cells are immediately trans-ferred to a glass microscope slide; gentle spreading will avoid fracturing nuclear

Corneal cytology is often a rewarding diagnostic method for characterization

of exudative lesions (keratomalacia, keratomycosis) and may aid in the entiation of proliferative lesions (eosinophilic keratitis or nodular episclero-keratitis) Topical anesthesia is usually applied prior to obtaining the sample Care must be taken not to rupture deep ulcers with pressure Special stains for fungi (periodic acid–Schiff) may be useful Cells can also be harvested by using a cellulose strip that is gently applied on the corneal surface (‘impression cytology’)

differ-Aspiration of aqueous humor may be undertaken with topical or general anesthesia, with a 25–27-G hypodermic needle, inserted just anterior to the limbus, bevel up and parallel to the iris surface (Fig 2.6) Approximately 0.1–0.2 ml of fl uid can be collected and is generally safe; remove the barrel from the tuberculin syringe and allow the fl uid to collect by pressure differ-ential rather than aspiration Centrifugal cytology (‘cytospin’) is particularly well suited for the preparation of small sample volumes and dilute cell suspen-sions Aqueous humor cytology may allow distinction between non-granulomatous and granulomatous uveitis, and protein-laden macrophages are encountered with phacolytic or phacoclastic uveitis Lymphosarcoma and feline melanoma cells may exfoliate into the aqueous; if cells are seen with the biomicroscope, aqueocentesis with cytology may confi rm the diagnosis Aqueous humor samples can also be collected for culture, PCR, and antibody level determination

Tumors of the anterior uvea are candidates for trans-corneal fi ne needle aspiration, best performed by those familiar with the technique Aspiration is accomplished as described above for aqueous humor as a microsurgical proce-dure with the needle positioned over the tumor and an attempt made to aspirate surface cells Alternatively the needle may be directed into the tumor Technical challenges include obtaining an adequate sample and interpretation may be problematic Potential complications of hemorrhage, lens trauma, and tumor seeding temper the decision to pursue this modality

Fig 2.6 Aspiration of aqueous humor with a 25-G hypodermic needle in a cat.

is performed transsclerally, usually with general anesthesia and a 25-G needle For vitreous aspiration, the needle penetrates the eye 5–6 mm posterior to the limbus (entering through the pars plana but avoiding the lens) and is directed toward the posterior pole; a volume of 0.5 ml liquefi ed vitreous can be aspi-rated For withdrawal of subretinal fl uid, the needle is gently introduced through the sclera overlying the bullous retinal detachment.

Assessment of orbital disorders using fi ne needle (23 or 24 G) aspiration remains a reliable method that can facilitate diagnosis for the clinician, pro-vided that localization of the lesion allows for accurate sampling Exophthal-mos results from a space-occupying lesion in the orbit (benign or malignant neoplasm, orbital infl ammation, cystic disease) Palpation, imaging, and the direction of globe displacement may be used to determine the site of the orbital lesion Ultrasound guidance provides optimal assurance of a representative sample Several routes for fi ne needle aspiration biopsy are available, depend-ent upon location: through the eyelids, conjunctiva, the mucosa caudal to the last upper molar, or, in the case of posterior orbital lesions, transdermally at the posterior junction of the orbital ligament and zygomatic arch (Fig 2.7)

Fig 2.7 Fine needle aspiration biopsy through the mucosa caudal to the last upper molar in a dog

Evaluation of the lacrimal drainage system

To evaluate the structure and function of the lacrimal puncta, lacrimal liculi, lacrimal sac, and nasolacrimal duct, topical anesthesia, sedation, or general anesthesia may be required in dogs, dependent on the nature of the patient In cats, general anesthesia is usually required; the lacrimal puncta are smaller and less accessible A curved stainless steel lacrimal cannula may be utilized; 22–23 G works well in dogs, 26 G in cats Its rigidity allows the opera-tor easily to identify and enter the opening of the nasolacrimal duct in the lac-rimal bone after having entered the lower punctum and nasolacrimal sac The disadvantage of a rigid cannula is that of possible damage to the mucous mem-branes if the animal is not adequately restrained and anesthetized, or if the procedure is not performed gently; alternatively a Tefl on intravenous catheter works almost as well The cannula should be mounted on a 2.5–3.0 ml syringe

cana-fi lled with sterile saline, or a small saline-cana-fi lled compressible bottle

The cannula is inserted into the upper punctum, located 4–5 mm from the medial canthus, stretching the upper lid superiorly with the index fi nger to immobilize and straighten the canaliculus and facilitate cannula penetration After the lacrimal punctum is entered, the system is fl ushed; saline will exit from the lower punctum Smooth movements are then used to pass through the lacrimal sac and locate and enter the opening of the nasolacrimal duct At this point, the lower punctum is closed with fi nger pressure on the adjacent lid The nasolacrimal duct is fl ushed and keeping the nose of the animal angled downward, the fl uid should fl ow from the ipsilateral nostril Cannulation with monofi lament nylon suture can be used to localize and attempt to dislodge obstructions Radiographs can be helpful in diagnosing nasolacrimal cysts

or obstructions occurring secondary to sinus disorders Contrast media may

be injected through the upper puncta (dacryocystorhinography) to localize obstructive lesions

Direct ophthalmoscopy

The ophthalmoscope has a light source which is directed into the patient’s eye

so that the beam is nearly parallel with the line of sight of the examiner A rheostat controls the light intensity while the dimension and the characteristics

of the beam may be varied with a series of colored fi lters (blue to excite fl rescein, green to help differentiate pigment from retinal hemorrhage), a slit (to help evaluate the elevation of lesions), and a grid (to project onto the fundus

uo-in order to measure lesions) A selection of lenses ranguo-ing from + (black) 40 D

to – (red) 25 D (diopters) is assembled on a rotating wheel which adjusts the depth of focus into the eye (Table 2.1)

Thorough examination of the fundus of the eye can be performed only in a dark room through a well-dilated pupil; 1–2 drops of 1% tropicamide should

be applied 15–20 min prior to examination Observation with a setting of around 0 to +1 or +2 D and the instrument held 30–60 cm from the eye allows critical evaluation of the fundus refl ex The fundus is then observed from a distance of 2–5 cm and starting with a setting of 0, altered to achieve optimal focus Direct ophthalmoscopy provides magnifi cation of fundic features by 14

to 15 times The disk is located and evaluated initially, the major vessels are traced to the periphery, and each quadrant is evaluated systematically to obtain

a mental panorama The main disadvantages of direct compared with indirect

There are notable intra- and inter-species differences in the ophthalmoscopic anatomy of the fundus including color and extent of the tapetum, intensity of retinal pigment epithelium (RPE) pigment in the non-tapetum, degree of myelination of the optic disk, location of the disk in relation to the tapetal/non-tapetal junction, and vascular patterns (Fig 2.8).

Hand lens monocular indirect ophthalmoscopy

The most economical way to perform indirect ophthalmoscopy is by using a 14–30 D hand-held lens and a focal light source such as the Finoff transillumi-

A

Fig 2.8 Variations in normal fundus appearance

(A) The fundus of a

sandcoated retriever with

a richly myelinated optic disk

Table 2.1 Ophthalmoscope settings for examination of

normal canine eyes

(diopters)

Anterior capsule of lens +12 to +15

Posterior capsule of lens +8 to +12

Optic disk and fundus +2 to −2

inexpen-of the animal at arm’s length, holding the transilluminator in front inexpen-of the observer’s own nose and the lens 5–6 cm in front of the patient’s cornea (an assistant is required to restrain the patient’s head and retract the lids) The fundus image is made to fi ll the entire lens by moving the lens toward or away from the cornea.

B

C

Fig 2.8 Variations in normal fundus appearance

(B) An Australian

Shepherd fundus: the tapetum is aplastic, which allows visualization of the underlying choroidal vessels and sclera

(C) This Siamese cat

fundus exemplifi es the non-myelinated optic disk characteristic of felines

There is absence of pigment in the non-tapetal retinal pigment epithelium and choroid to reveal the radial choroidal vasculature

Tonometry is the assessment of intraocular pressure (IOP) Digital tonometry

is a very crude technique; two-fi nger ballottement of the globe through the upper lids can detect a discrepancy between the two eyes, but should not be used without additional objective measurement Instrumental tonometry may

be performed by indentation or applanation methods Indentation tonometry

is based on the measurement of the extent of indentation of the cornea obtained with a Schiøtz tonometer It is a relatively inexpensive instrument which con-sists of a plunger that glides through a cylindrical chamber stabilized by a bracket handle, and a footplate that conforms with and is placed on the anes-thetized corneal surface The plunger can be charged with different weights (5.5 g, 7.5 g, 10.0 g) The depth of indentation is refl ected by movement of a lever, which allows a scale reading to be converted to an estimation of IOP Theoretically, the curvature and rigidity of the human cornea differ from those

of the small animal cornea, so species-specifi c conversion tables are required for optimal quantitative accuracy; practically, indentation tonometry only esti-mates IOP and it is not critical whether the table with human data that comes with the instrument or a veterinary scale is used Normal values should be less than 25 mmHg As a rule of thumb, with the 5.5 g weight, scale values between

3 and 7 on the Schiøtz scale represent normal pressure in the dog; normal values

in the cat are 2–6 Readings of less than 2–3 suggest an elevated IOP and those greater than 7 a hypotensive eye Size of the eye (smaller eyes give values higher than actual IOP), age-related differences in scleral rigidity (young eyes are more elastic and give higher values as well), and corneal lesions (edematous corneas will indent more, scarred corneas less) can affect accuracy of results

Before each patient evaluation, the instrument should be calibrated on the convex steel test block; the indicator should read 0 The patient is given a few drops of topical anesthetic and the instrument is applied to the eye Because the plunger is gravity driven, it is essential that the tonometer be held as close

to perpendicular as possible and that its components are well cleaned and free moving The footplate is applied to the cornea, positioning the head of the dog

by elevating the nose toward the ceiling It is important not to occlude the jugular veins in order not to artifactually increase the IOP, or to compress the globe while retracting the eyelids, for the same reason Occasionally it is easier

to restrain the dog on its back, with the head held perpendicular to the body axis with the cornea in the horizontal plane The measurement should be repeated several times in order to obtain three readings within 1–2 scale units

of each other The instrument should be placed as centrally as possible on the corneal surface, as the sclera has a different rigidity The curved surface of the footplate should be in perfect and complete contact with the cornea No force should be applied on the handle, which should be held gently to allow the instrument to rest freely on the corneal surface Readings should be regarded

as estimations of IOP rather than precise determinations The main tage is that the technique is demanding and requires practice to master.Suggestions for reliable use include:

disadvan-• Calibrate the tonometer before each use

• Ensure that the cornea is well anesthetized; most systemic anesthetics and sedatives alter blood pressure and thus IOP, and are ideally avoided

• Do not compress the jugular veins or the globe

• Keep the cornea horizontal, the tonometer vertical and in the center of

the cornea; avoid the limbus and the sclera as well as the third eyelid (you can slip the footplate beneath the third eyelid if it protrudes)

• Make several measurements (3–5)

• Always evaluate both eyes

• Interpret readings in conjunction with other clinical signs

• After each use, disassemble and clean the instrument

• Make tonometry a part of your routine physical/ophthalmic examination

to build confi dence in your technique

ADVANCED DIAGNOSTICS

To appreciate fully the anatomic details and pathologic changes of the eye, special examination techniques and more sophisticated equipment may be necessary to refi ne preliminary observation and pursue differential diagnoses

to appear diffuse, pinpoint (to detect subtle fl are and cells), or a slit, and may

be colored by inserting various fi lters Observations of refl ected and/or mitted light provide a magnifi ed three-dimensional view of the various ocular structures

trans-Indirect ophthalmoscope

The monocular hand lens method, already described, can be replaced by a more sophisticated and expensive instrument, the binocular indirect ophthalmo-scope, which emits a bright light from a unit on the examiner’s head that is directed into the eye of the animal; the emergent rays are converged by a 14–

30 D biconvex condensing lens placed in the same fashion as described for monocular indirect ophthalmoscopy The image is inverted and magnifi ed less than with direct ophthalmoscopy, dependent upon the dioptric power of the lens utilized The indirect ophthalmoscope has three major advantages: both hands can be used to manipulate the patient’s head and eyelids while the exam-iner is at arm’s length from the animal; it is possible to obtain a panoramic (although inverted) view of the ocular fundus; and bright illumination can penetrate translucent ocular media The technique is easily mastered with practice

Applanation tonometry

In contrast to Schiøtz indentation tonometry, applanation tonometry enables measurement of the variable force necessary to fl atten a constant small area of the cornea The ‘Tono Pen’® and the ‘Tono Vet’® are hand-held tonometers with several advantages over the Schiøtz tonometer, but are much more expen-sive Readings are not as subject to the infl uences of rigidity and other tissue characteristics as with indentation tonometry although readings are sensitive

a number on the digital display indicating the IOP expressed in mmHg Every four valid readings the device sounds a prolonged beep and the mean IOP is displayed The tip of the instrument has to be covered with a disposable latex protective membrane to ensure sterility and prevent exposure to the preocular

fl uids The device is light and fi ts comfortably in the user’s hand and can be used regardless of the position of the animal’s cornea, so minimal restraint is needed

Gonioscopy

The iridocorneal angle and outfl ow pathways are not directly visible without using a refractory lens placed on the corneal surface In most cases, gonioscopy can be performed with topical anesthesia Many different lenses are used in small animal ophthalmology, with the Franklin, Barkan, and Koeppe lenses the most popular direct goniolenses; an indirect (mirror) lens facilitates 360° examination simply by rotation of the lens The interface between lens and cornea is maintained with saline or 1.0% methylcellulose solutions A coaxial light source and some magnifi cation are needed for optimal observation (the biomicroscope is ideal); an otoscope can be satisfactorily used

The technique is indicated to evaluate glaucoma patients; when the glaucoma

is unilateral, the presence of goniodysgenesis in the contralateral eye is an important risk factor, as well as suggesting the pathogenesis of the glaucoma

in the involved eye Cross-sectional and gonioscopic anatomy of the outfl ow pathway is depicted in Figure 2.9 Parameters of interpretation are summarized

in Table 2.2

Retinoscopy

Retinoscopy, also called skiascopy, is a technique by which the refractive state

of the eye can be determined objectively by observing characteristic light refl tions that are created by illuminating the retina with a band or circular beam

ec-of light emitted from the retinoscope The nature ec-of these refl exes and how they are infl uenced both by the properties of incoming light and by refractive lenses placed between the eye and the retinoscope indicates the refractive power of the eye This technique has been used to defi ne the normal, pathologic, and surgically induced refractive state of the eye in dogs

It is necessary to provide basic defi nitions of refractive properties of the eye and refraction

• Emmetropia is an eye without refractive error where the plus power of

the cornea and lens refracts light to a point source on the plane of the retina

• Ametropia is an eye with a refractive error, generally from variations in

axial length of the eye, astigmatism, or a shift in position or absence of the crystalline lens

• Myopia is ametropia due to relatively excessive refractive power, generally

due to a longer than normal axial length; images are formed in front of the plane of the retina

• Hyperopia is a refractive error caused by relatively inadequate refractive

power, generally due to a shorter than normal axial length; images are

formed behind the plane of the retina

• Astigmatism is an eye with aspherical ametropia caused when the

refractive surfaces of the eye have different radii of curvature in different meridians, generally caused by differences in corneal curvature, such that

an eye has two focal points

• A meridian is an imaginary line on the surface of a spherical body; a

corneal meridian is a line formed by the intersection with the corneal

surface of an anteroposterior plane passing through the apex of the

cornea and can be horizontal or vertical

a Corneal dome

b Superficial band of pigment zone – varies in density

c Deep band of pigment zone

d Individual fibers of the pectinate ligament

e Ciliary cleft (space of Fontana) containing the

uveal trabecular meshwork

f Iris

g Pupil

gfedcba

B

cbil

cboltmccpl

i

svpc

A

Fig 2.9 Anatomy of the

outfl ow pathways is depicted schematically in

cross-section in (A) and gonioscopically in (B) The

normal gonioscopic appearance of the canine

(C) and feline (D) outfl ow

pathway is shown Key for

(A): c: cornea; i: iris; pl:

pectinate ligment; cc:

ciliary cleft containing uveal trabecular meshwork; cbil: inner leafl et of ciliary body; cbol: outer leafl et of ciliary body; tm: scleral trabecular meshwork; svp: scleral venous plexus (Courtesy

of R.L Peiffer.)

• Refraction is the bending of light rays; minus lenses (concave) diverge

light rays and plus lenses (convex) converge light rays

• Diopters are a measure of lens power, defi ned by the inverse of the focal

length in meters

• Optical infi nity is any distance greater than 6 m.

A retinoscope is characterized by a light projection system and an examiner observation system The projection system has a bulb that projects a linear band or streak of light into the patient’s eye The observation system is an aperture that allows the examiner to view emergent light rays from the eye When performing retinoscopy, refracting lenses from a trial lens may be used;

C

D

Fig 2.9 For caption see previous.

Computerized topography of the cornea (keratoscopy)

This examination of the curvature of the corneal surface involves projecting onto it concentric rings of light (Placido’s rings), the refl ected image of which

is analyzed by a computer which measures the distance between these rings Optical measurement of corneal curvature is termed keratometry The results are reported in millimeters or diopters; for the dog eye mean corneal curvature

in diopters is 39.94 ± 2.61; mean radius of curvature in mm is 8.46 ± 0.55.3Mean curvature for large breed dogs is less than that for dogs of medium size

or small breeds, indicating a fl atter cornea in larger dogs This technique allows evaluation of astigmatism and is requisite for refractive procedures on the cornea.3

Table 2.2 Scheme for classifi cation of gonioscopic observations.

Iridocorneal angle

• Open (approximately 2 mm)

• Narrow

• Closed (pectinate ligament, ciliary cleft, inner and outer pigment zones not visible

with iris root in contact with peripheral cornea)

Pectinate ligament

• Normal

• Goniodysgenesis (pectinate fi bers shortened/thickened to imperforate; fl ow holes

reduced in number and size; anterior insertion displaced axially)

Ciliary cleft and trabecular meshwork

• Normal

• Compressed

• Collapsed (iris root apposed to inner pigment zone; pectinate ligament not visible)

• Obstructed (with infl ammatory or neoplastic infi ltrates)

The specular microscope allows in vivo observation of the corneal endothelium;

endothelial cells of the dog and cat form a regular monolayered mosaic of hexagonal cells at a normal density of about 2000 cells/mm2 (Fig 2.10).4,5

Ultrasonography

Ocular ultrasonography in two-dimensional B-mode with a 10 MHz probe is ideal but very adequate studies can be performed with a 7.5 MHz probe, which

is generally more readily available to practitioners (Fig 2.11A) The technique

Fig 2.10 Specular microscopic appearance of the normal canine corneal endothelium

demonstrates a monolayer of regular hexagonal cells

A

Fig 2.11 Ultrasonography.

(A) Normal eye (7.5 MHz probe) (B) Pathology I Exophthalmos/orbital abscess/cellulitis (10 MHz probe) II Orbital abscess/cellulitis (7.5 MHz probe) (Courtesy of Dr A Bertoldi.)