Ординатура / Офтальмология / Английские материалы / Slatter's Fundemental of Vetrinary Ophthalmology 4th edition_Maggs, Miller, Ofri_2008

should be paid to the “approach angle” of the tip to the cornea so that the tip’s flat surface is parallel to the corneal surface (i.e., so that the Tono-Pen itself is perpendicular to the corneal surface). This is best achieved by viewing the interface between the cornea and the tip from the side. The cornea is “blotted” sufficient times to elicit an average reading. The digital display is in mm Hg. The reliability (coefficient of variance) of the result should be 5%, or tonometry should be repeated.

Rebound Tonometry

Rebound (or impact or dynamic) tonometry is a third mechanism by which IOP may be measured and which uses a different mechanical principle. Rebound tonometers eject a small probe (such as a metal pin with a rounded end) at a fixed distance from the cornea and assess the motion of the probe as it strikes the cornea and is returned (rebounds) to the instrument. Eyes with higher IOP cause a more rapid deceleration of the probe and shorter return time to the instrument. This technique is affected by ocular surface tension and so should be performed before application of any topical medications, including topical anesthetic. This feature raises some questions as to how readings by such tonometers might be affected by keratoconjunctivitis sicca (in which surface tension would be altered) and by the presence of corneal pathology as is frequently encountered in animals with glaucoma or uveitis. Additionally, the probe distance from the eye is likely to exert some influence on readings, and ensuring the proper distance can be difficult in uncooperative animals. The technique was developed more than 50 years ago and only recently has undergone a resurgence in popularity owing to the release of a new rebound tonometer, manufactured as TonoVet or iCare (Figure 5-36). This instrument has so far been calibrated for dogs, cats, and horses only, and one must select the correct software for each species before beginning IOP measurements. In a study using normotensive dog eyes, the TonoVet consistently reported lower IOP than the Tono-Pen did; however, the difference was only about 1 to 2 mm Hg. The reliability of the rebound tonometer in patients with diseased corneas or when measured through a contact lens has not yet been assessed.

The IOP should be measured in all red, inflamed, or painful eyes to diagnose or eliminate from consideration both glaucoma and uveitis.

FIGURE 5-36. The TonoVet rebound tonometer in use.

BASIC DIAGNOSTIC TECHNIQUES 97

Normal Intraocular Pressure

Across large populations, normal canine and feline IOP is reported as approximately 10 to 20 mm Hg. However, significant variation is noted among individuals as well as among techniques and the time of day at which IOP is measured. Therefore comparison of IOP between right and left eyes of the same animal is critical to interpretation of results. A good rule of thumb is that IOP should not vary between right and left eyes of the same patient by more than 20%. As with all other measured biologic values, one should not treat a low or high IOP but should use it as an indicator of uveitis or glaucoma, respectively, and should assess the patient for other signs consistent with those diagnoses. Additionally, IOP can vary with some exogenous and endogenous factors. Sedatives, tranquilizers, and anesthetic drugs can cause lowered IOP readings because of reduced extraocular and adnexal muscle tone. By the same mechanism, ketamine may cause a slightly elevated reading. Patient cooperation is an important factor, and care should be taken to minimize pressure applied around the neck or orbital area, or in retracting the eyelids.

Tonography

Tonography is the study of aqueous outflow facility in response to pressure applied to the eye. It is based on the observation that applied pressure softens an eye because fluid is forced out through the iridocorneal (or “drainage”) angle. This softening occurs to a lesser extent in a glaucomatous eye because aqueous is less able to pass out of the anterior chamber (i.e., outflow facility is decreased). In tonography, a Schiøtz-type weight and plunger are placed on the cornea of a still patient and the gradual reduction in IOP over 4 minutes is measured graphically on a strip chart recorder. The outflow facility coefficient (C) can be calculated from this graph. This technique is used predominantly in research settings.

Gonioscopy

Gonioscopy describes examination of the iridocorneal or “drainage” angle (the junction between the iris and cornea). In the normal eye of most species light rays that are reflected from the drainage angle strike the posterior cornea and undergo total internal reflection as in a prism (Figure 5-37, A). This occurs because of the difference in refractive index between the cornea and the surrounding air, and the high angle of incidence of the light rays from the drainage angle. By replacing the air surrounding the cornea with a goniolens that has an index of refraction close to that of the cornea, total internal reflection is avoided and light rays from the drainage angle can be viewed directly through the goniolens. Additional modifications may be made to the goniolens to provide magnification. Magnifying instruments such as the biomicroscope, gonioscope, head loupe, and fundus camera may also be used. Direct goniolenses (e.g., Koeppe; Figure 5-37, B), through which the angle is viewed directly and indirect goniolenses (e.g., Goldmann; Figure 5-37, C), in which the image is viewed in a mirror, are both available. Low-vacuum goniolenses are very useful for veterinary use, as they are held in place by low-pressure vacuum applied by a 2-mL syringe and a column of saline held below the level of the eye. All goniolenses are bonded to the cornea with a liquid medium such as saline (for low-vacuum goniolenses) or methylcellulose solutions (for nonvacuum goniolenses).

A

B

C

FIGURE 5-37. Gonioscopy permits examination of the iridocorneal or “drainage” angle. A, Normally, light rays from the drainage angle undergo total internal reflection at the posterior cornea. B, A direct goniolens refracts light so that the drainage angle may be viewed directly. C, An indirect goniolens refracts light so that the image is viewed in a mirror.

Gonioscopic examination is a frequent part of the examination of patients in which suspected glaucoma or ocular hypertension is suspected and complements tonometry and tonography. In cooperative patients topical anesthesia is usually sufficient (Figure 5-38); however, tranquilization may be necessary in refractory animals. The normal structures of the canine drainage angle are shown in Figure 5-39. Gonioscopy is used primarily to determine whether the angle is open, narrow, closed, or obstructed by mesodermal remnants, and to check for the presence of foreign bodies, tumors, and inflammatory

FIGURE 5-38. A gonioscopic lens in place on the anesthetized cornea of an unsedated dog.

D

C

B

A

FIGURE 5-39. Gonioscopic view of a normal canine iridocorneal (drainage) angle. A, Outer pigment band; B, inner pigment band; C, pectinate ligaments; D, iris root.

exudates. The technique is applicable to all domestic species but is most commonly used in dogs. In cats and horses the anterior chamber is deeper than in dogs and parts of the drainage angle can be examined without a goniolens.

Vital Dyes

Vital dyes stain living tissues. Fluorescein and rose bengal are most commonly used in veterinary ophthalmology.

Fluorescein

Fluorescein is a water-soluble dye that is retained by all hydrophilic but not hydrophobic structures. The classic example of its use is in the identification of a corneal ulcer, in which the fluorescein is retained by the hydrophilic stroma wherever it is exposed by loss of the hydrophobic epithelium. Fluorescein should always be applied from an impregnated paper strip. Prepared solutions should not be used because they can become contaminated by bacteria. The strip is removed from the packet, moistened with a drop of sterile saline or eye rinse, and touched very briefly to the conjunctival and not the corneal surface. Direct application of the strip to the cornea can cause artifactual stain retention. Excess dye should always be rinsed with sterile saline, and the eye examined with magnification and a blue light from a cobalt filter attached to a Finoff transilluminator or from the direct ophthalmoscope. A Wood’s light also may be used.

Defects in the corneal epithelium appear as bright green areas. However, various ulcer types demonstrate different and characteristic staining patterns. Recognizing these patterns will greatly assist in differentiating simple (superficial) ulcers from complicated (deep, infected, or indolent) ulcers. In superficial ulcers the stain adheres only to the ulcer floor and has distinct margins (Figure 5-40). In deeper stromal ulcers both the walls and floor of the ulcer will stain and there may be some diffusion of fluorescein into the neighboring stroma, producing less distinct margins (Figure 5-41). With descemetoceles the center of the ulcer will fail to take up stain and will appear black because Descemet’s membrane, which does not stain

FIGURE 5-40. Characteristic staining pattern of a superficial ulcer. Note that the fluorescein stain adheres only to the floor of the ulcer and has distinct margins.

FIGURE 5-41. Characteristic staining pattern of a deeper stromal ulcer. Note that the fluorescein stain adheres to the walls and floor of the ulcer and that there is some diffusion of fluorescein into the neighboring stroma, producing less distinct margins.

with fluorescein, is exposed (Figure 5-42). Indolent ulcers (see Chapter 10) have a characteristic fluorescein staining pattern because they are superficial and therefore have a stained floor (like other superficial ulcers) surrounded by a halo of less distinct stain seen through their nonadherent epithelial lip (Figure 5-43). A penetrating corneal wound will also produce a characteristic stain pattern. In addition to all areas of exposed corneal stroma retaining fluorescein dye, the egress of aqueous resulting from the globe rupture will produce tiny rivulets in the fluorescein dye as they dilute it at the corneal surface. Evaluating for this feature is called a Seidel test. This test is performed by applying a concentrated solution of fluorescein and not rinsing it off but rather allowing the aqueous humor to do so, while viewing with a blue light source and a source of magnification.

Nonulcerative lesions sometimes stain and can cause confusion unless they are recognized. For example, the surface of a vascularized or roughened corneal lesion may show diffuse, faint fluorescein staining because of pooling of stain due to surface tension. Fibrovascular (“granulation”) tissue will also retain stain owing to its hydrophilic nature. Finally, an epithelialized stromal defect (a facet) pools stain and must be differentiated from a corneal ulcer. One can make this distinction by examining the

FIGURE 5-42. Characteristic staining pattern of a descemetocele. Note that the fluorescein stain adheres only to the walls of the ulcer; the center (“floor”) of the ulcer fails to take up stain and appears black because the exposed regions of Descemet’s membrane do not retain fluorescein stain.

FIGURE 5-43. Characteristic staining pattern of an indolent ulcer. Note that fluorescein stains the floor of the ulcer but that this area does not have distinct margins. Rather, it is surrounded by a halo of less distinct stain seen through the nonadherent epithelial lip.

eye with a blue light and magnification while an assistant rinses the cornea. Fluorescein stain cannot be rinsed from an ulcer, whereas stain pooling in a facet can easily be rinsed.

All red, inflamed, or painful eyes should be stained with fluorescein to diagnose or eliminate from consideration corneal ulcers.

Fluorescein dye may also be used for other ocular diagnostic tests. Topical ophthalmic application of fluorescein dye and observation for its appearance at the nares confirms patency of the nasolacrimal duct on that side and is referred to as the Jones or fluorescein passage test (Figure 5-44). The interval required for fluorescein to appear is variable (up to 5 to 10 minutes in some normal dogs). In some dogs and cats, especially brachycephalic breeds, drainage from the nasolacrimal duct may occur into the posterior nasal cavity, resulting in false-

FIGURE 5-44. A positive Jones or fluorescein passage test result, as evidenced by the appearance of fluorescein stain at the nostril following its application to the ipsilateral corneal surface.

FIGURE 5-45. In some dogs the nasolacrimal duct opens caudally within the nasopharynx, and fluorescein stain is found in the mouth rather than the nostril after application of the stain to either corneal surface.

negative result of the Jones test unless the mouth is also examined (Figure 5-45).

The tear film break-up time (TFBUT) is an assessment of the stability of the precorneal tear film. The clinician applies a drop of fluorescein stain to the cornea and immediately closes the lids until a pre-prepared source of magnification and a blue light are moved into the viewing position. The lids are then opened, and the dorsolateral quadrant of the precorneal tear film is observed closely while the lids are held apart. The time is recorded from lid opening until the tear film “breaks up” as evidenced by dark spots appearing in the dorsolateral quadrant of the otherwise green fluorescein stain. In the presence of a normal lipid layer of the tear film the TFBUT is a quantitative measure of mucin quantity and quality. Decreased mucin quantity or quality causes tear film instability and a shortening of the TFBUT. Average TFBUT in dogs is approximately 20 seconds, and in cats is about 17 seconds. In cats with tear film disturbances TFBUTs as short as 1 second have been recorded.

Rose Bengal

Rose bengal stains dead and devitalized cells and therefore is retained by corneas in which the epithelium is eroded to less than its full thickness. Therefore there is no exposure of corneal stroma, and fluorescein stain would not be retained. It is even retained by surface squamous cells that have altered surface characteristics or altered mucin coating. As such, it is very useful for the diagnosis of keratoconjunctivitis sicca, qualitative tear film deficiencies, or early dendritic corneal ulcers associated with the herpesviruses, in which there is necrosis and desquamation of corneal and conjunctival epithelium but not exposure of the underlying stroma.

Tests of Lacrimal Patency

Blockage of the nasolacrimal ducts causes overflow of tears (epiphora) at the lid margin near the medial canthus, with staining of the surrounding hair. For more detailed discussion of the lacrimal system, see Chapter 9. Blockage of the nasolacrimal ducts may be evaluated by:

•The fluorescein passage (or “Jones”) test as described previously

•Flushing the upper and lower puncta in dogs or cats or the lower (nasal) punctum in horses

•Dacryocystorhinography with radiographs or computed tomography (see later)

Neuroophthalmic Testing

Many cranial nerves are involved to varying degrees with ocular function (see also Chapter 16), as follows:

•CN II: vision and PLRs

•CN III: globe movement via the medial, ventral, and dorsal rectus and inferior oblique muscles; eyelid opening; pupil constriction (via parasympathetic fibers carried with CN III)

•CN IV: globe movement via the superior oblique (extraocular) muscle

•CN V: facial and ocular sensation; lacrimation (via parasympathetic innervation of the lacrimal gland); pupil dilation (via sympathetic innervation of the dilator muscle)

•CN VI: globe movement via the retractor bulbi and lateral rectus muscles

•CN VII: eyelid closure

Basic neuroophthalmic tests therefore form a critical part of the complete ophthalmic examination. In most cases they are extremely simply performed. Commonly employed neuroophthalmic tests include the following:

•PLRs (described previously)

•Swinging flashlight test

•Dazzle reflex

•Palpebral reflex

•Menace response

•Behavioral testing of vision

Swinging Flashlight Test

The swinging flashlight test is a modification of PLR testing. Owing to incomplete decussation in most mammals, the pupil under direct illumination usually constricts slightly more than

the contralateral pupil. Therefore when a light source is directed rapidly from one eye to the other (as would be done to test and verify the consensual PLR), the newly illuminated pupil should constrict a little farther than it had already constricted due to the consensual PLR. In patients with a unilateral prechiasmal lesion (i.e., a lesion of the optic nerve between the chiasm and the retina, or of the retina itself), the pupil on the affected side will constrict when stimulated via the contralateral, normal retina (i.e., will have a normal consensual PLR) but will dilate as the light is swung from the normal eye to the affected eye. This is a called a positive swinging flashlight test result or a Marcus Gunn pupil and is pathognomonic for a prechiasmal lesion on the side on which the pupil dilates when illuminated.

Dazzle Reflex

The dazzle reflex is manifested as partial or complete eyelid closure on the illuminated side (and sometimes on both sides) when a bright light is directed at the eye. The reflex follows the same afferent pathway as the PLR but synapses with fibers of the facial nerve in the nucleus of CN VII in the midbrain (presumptively at the rostral colliculus). Therefore it can be used in association with the PLR to further localize some lesions. The reflex is absent in the presence of severe retinal, optic nerve, optic tract, or facial nerve lesions.

Palpebral Reflex

The palpebral reflex consists of a partial or complete closure of the eyelids in response to touching the eyelid skin. Interpretation of the palpebral reflex requires an understanding of the neurologic reflex as well as other potential confounding factors. The afferent arm of the neural arc being tested includes the sensory fibers of the trigeminal nerve; the efferent pathway uses the motor fibers of the facial nerve and the muscles of eyelid closure (principally the orbicularis oculi). In addition to pathology at any point along this neurologic pathway, lagophthalmos due to a physical obstruction of eyelid closure, such as severe buphthalmos or exophthalmos, can cause decrease or absence of the palpebral reflex. The reflex can be overridden in very fearful animals, especially birds and exotic species. The palpebral reflex should be tested through stimulation of the skin at both the medial canthus and lateral canthus. Most animals respond with complete eyelid closure to stimulation at the medial canthus. Fearful animals and animals with breed-related lagophthalmos may not completely close the eyelids in response to stimulation at the lateral canthus.

Menace Response

A normal menace response is evident as eyelid closure when the examiner stimulates the eye in a visually “threatening” way, usually by waving a hand in front of it. However, this response has a number of limitations that must be understood, as follows:

•The stimulus must be visual only. No direct contact with the patient’s periocular tissues or creation of air currents, odors, or noise can be associated with the gesture.

Achieving this goal is very difficult in any animal in which these senses are heightened, especially in cats and in most visually impaired animals. To prevent air currents, the

BASIC DIAGNOSTIC TECHNIQUES 101

threatening motion can be performed behind a transparent sheet of Plexiglas.

•Each eye must be tested individually (with the nontested eye closed) because unilaterally blind animals are adept at protecting both eyes when the sighted eye is stimulated.

•Placid or fearful animals may show little response, preferring to watch the menacing gesture.

•The eye should be menaced from the nasal (medial) and temporal (lateral) directions.

•The menace response is a learned response that is absent for the first 10 to 14 weeks of life in puppies and kittens and the first 10 to 14 days in foals.

Even when the test is performed correctly, however, the menace response is a particularly coarse assessment of vision. To put it in perspective, the equivalent test in humans—the ability to simply see hand motions—is considered one grade better than “light perception,” after which total blindness is diagnosed. The afferent arm of the menace response includes all components of the eye necessary for vision, especially the retina and optic nerve; and the efferent pathway requires normal facial nerve (CN VII) and eyelid function. However, the arc also passes through the cerebellum, so absence of the menace response is also associated with degenerative lesions of the cerebellar cortex. Whenever the menace response is absent, the palpebral reflex (see earlier) should be tested as a second method of evaluating the efferent pathway shared by the two reflexes.

Behavioral Testing of Vision

Evaluation of vision in veterinary patients remains one of the most challenging parts of the ophthalmic examination (see Chapter 1). Because vision is a cortical function and many neuroophthalmic tests are actually tests of the visual pathways (or segments of them) but not truly tests of vision, vision is best assessed with various behavioral tests. However, results of these tests are affected by some subjectivity and must be very critically analyzed. Each test must be interpreted by the clinician, with consideration given to the animal’s personality, emotional state, state of consciousness, and cognitive function. Given the subjective nature of visual testing, visual deficits can be described at best as mild, moderate, severe, or total.

Much can be gained from an accurate history. In particular, information should be sought about the extent and rapidity of vision loss, whether both night vision and day vision are affected, and whether the owner believes the visual impairment is unilateral or bilateral. When judging apparent unilateral visual loss, one must recall the physiologic effects of optic nerve fiber decussation at the optic chiasm. Objects approaching from the left of the patient are perceived by the medial (or nasal) half of the left retina and the lateral (temporal) half of the right retina. The inverse is true for the right visual field. Therefore patients blind in only one eye may still retain vision in the visual field on the side of the affected eye; from the lateral retina of the opposite, functioning eye (see Figure 1-15 in Chapter 1).

During any behavioral tests of vision, individual eyes should ideally be tested independently by “patching” of the remaining eye if the animal will tolerate it.

Various visual “tracking” tests form the basis of vision testing in small animals. A small cotton ball is dropped 20 to 30 cm in front and to each side (i.e., in each visual field) of the patient. Most dogs and cats follow the object to the floor, especially on the first one or two attempts. Alternatively, some animals “track” a silent toy or a laser pointer or other light source directed onto the wall or floor in front of them. A maze test is an excellent method of assessing vision in dogs and horses but is of little value in most cats. In this test, a variety of test obstacles of different sizes and shapes (e.g., chairs, buckets) are placed around a room or pen. With a small animal, the owner is placed on one side of the obstacles and calls the animal from the clinician, once only. Both the client and clinician remain still and quiet during the test. With horses, the patient is led through the maze on a long (3 to 4 m) lead rope. Horses, cattle, and sheep that are unused to being led can be released in a pen or barn that they are not used to, and their movements watched. If the result of such a passive test is negative, large animal patients may be driven through the obstacles.

Regardless of style used, maze testing should be performed in the dark (to test scotopic vision) and in lighted conditions (to test photopic vision). Scotopic maze tests assess rod dysfunction (e.g., early retinal degeneration in dogs or cats, vitamin A deficiency in cattle) and should be performed before photopic testing or the obstacles should be changed between scotopic and photopic testing because animals with decreased visual function often become very adept at memorizing their way through tight spaces.

Electroretinography and Visual Evoked Potentials

Electroretinography (ERG) is the study of electrical potentials produced by the retina when light strikes it. Light of varying intensity, wavelength, and flash duration is directed onto the retina and the resulting potential differences are detected by electrodes placed around the eye (Figure 5-46). These are then amplified and form a characteristically shaped wave that can be recorded on paper or stored electronically and assessed for amplitudes and implicit times (Figure 5-47). For accurate results the ERG is performed with the animal under general anesthesia or deep sedation to minimize periocular muscle

FIGURE 5-46. An electroretinogram being performed on a dog. Note the ground and reference (subcutaneous) electrodes as well as the corneal contact lens electrode.

FIGURE 5-47. A normal electroretinogram.

movements. It is useful in all species. Electroretinography is a test of retinal but not optic nerve or visual function. It is usually available only at specialty ophthalmology practices. The ERG may be used for the following purposes:

•Preoperative evaluation of retinal function before cataract extraction when fundic examination is not possible

•Diagnosis and differentiation of inherited retinal disorders (e.g., rod-cone dysplasias, progressive retinal degeneration, hemeralopia)

•Investigation of unexplained visual loss (amaurosis) in which retinal lesions are not visible ophthalmoscopically (e.g., sudden acquired retinal degeneration [SARD], optic neuritis, CNS disease)

These diagnoses are discussed more fully in Chapter 15. Following placement of additional electrodes in the skin over

the visual cortex and with some alterations in the stimulatory and analytical protocols, electrical potentials at points in the visual pathways central to the retina can be recorded in response to a flash of light into each eye. These responses, called visual evoked potentials (VEPs), are rarely used clinically.

Retinoscopy

Retinoscopy is a technique for objective evaluation of the refractive state of the eye and allows determination of refractive errors such as hyperopia (“farsightedness”), myopia (“nearsightedness”), and astigmatism. It is used to evaluate refractive errors after cataract extraction and for evaluation of the visual state of animals with apparent visual problems but in which no abnormalities are found on ophthalmoscopy or electroretinography.

Imaging Techniques

Radiography

Dorsoventral, lateral, anterior-posterior, and oblique plain film radiographs of the orbit and surrounding skull may reveal disease processes in and around the orbit, maxillary dental arcade, and paranasal sinuses. Sometimes a radiopaque ring of stainless steel wire is placed around the limbus to serve as an anatomic reference point. Occasionally contrast techniques are also used, although these are now used less commonly because cross-sectional imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) have become more widely

5-48. A lateral dacryocystorhinogram in a dog. A, Lacrimal canaliculus; B, lacrimal sac; C, nasolacrimal duct. (Courtesy Dr. R. Wyburn.)

available and provide more information with less risk to the patient. Plain films are always assessed before contrast techniques are commenced. All radiographic techniques for ocular disease require general anesthesia for maximal diagnostic yield and radiologic safety, except perhaps a simple lateral or DV radiograph to assess for a radioopaque foreign body. Contrast techniques still in reasonably common use include the following:

•Dacryocystorhinography: injection of contrast medium into the lacrimal canaliculi, lacrimal sac, and nasolacrimal duct for assessment of nasolacrimal obstruction/dysfunction

BASIC DIAGNOSTIC TECHNIQUES 103

(Figure 5-48). Lateral and dorsoventral views are both useful. The technique is equally applicable to large and small animals. CT dacryocystorhinography combines the opportunity for two-dimensional cross-sectional images with the contrast-assisted outlining of the nasolacrimal apparatus and is particularly useful.

•Contrast zygomatic sialogram: injection of contrast medium into the zygomatic salivary gland duct within the mouth to outline the gland in the ventral orbit

Orbital venography (injection of contrast medium into the angularis oculi vein) and contrast orbitography (injection of air or radiopaque contrast agents into the orbit) have now been replaced by CT and MRI.

Ultrasonography

In ultrasonography, high-frequency sound waves above the audible range are directed posteriorly through the eye from the cornea, and the echoes are detected, amplified, and displayed on an oscilloscope screen. In B-scan ultrasonography a twodimensional cross section of the eye and orbit is obtained (Figure 5-49). A-scan ultrasonography is less commonly used. In this technique the echoes are viewed as a series of peaks (Figure 5-50) and permit measurements of various anteriorposterior distances within the eye (biometry). Ultrasonography is used to examine the contents of eyes in which opacity of one of the usually clear ocular media (cornea, aqueous humor, lens, or vitreous) prevents visualization of the structures caudal to it. It is also useful to assess orbital structures and to guide fineneedle aspiration of intraocular and orbital structures.

Ultrasonography is easy to perform, and gives immediate results with excellent definition. However, differentiation

FIGURE 5-50. Schematic of A-mode ultrasonography of a normal eye, showing the transmitter pulse (A), anterior lens capsule echo (B), posterior lens capsule echo (C), posterior globe wall echo (D), and retrobulbar tissue echoes (E). (From Rubin LF, Koch SA [1968]: Ocular diagnostic ultrasonography. J Am Vet Med Assoc 153:1706.)

between neoplasia and inflammation is not reliable. The patient is best examined without general anesthesia or sedation, which causes enophthalmos and reduces the clarity of the image obtained. A drop of topical anesthetic is applied to the ocular surface, and the ultrasound probe with sterile coupling gel is applied directly to the cornea or eyelids as the patient permits. Direct corneal contact yields slightly superior images. A transcutaneous temporal technique also has been described and is useful for visualization of retrobulbar structures.

Ultrasonography is particularly useful for the following:

•Detection of retinal detachment (see Figure 5-49, B)

•Detection of lens dislocation or rupture (see Figure 5-49, C)

•Detection of vitreous degeneration

•Detection of intraocular tumors or foreign bodies

•Characterization of retrobulbar disease

•Guidance of fine-needle aspirates of orbital and ocular lesions

Computed Tomography and Magnetic Resonance Imaging

CT and MRI provide superb detail for localization of orbital lesions (Figure 5-51) and, with increasing availability, have largely replaced skull radiography. Contrast sialography and dacryocystorhinography are as applicable to CT as they are to radiography and provide superior detail. The normal CT and MRI appearances of canine, feline, and equine orbital, ocular, and periocular structures have now been well described, and case reports and case series are expanding knowledge of the CT and MRI appearances of various pathologic conditions. The superior detail shown by these cross-sectional imaging techniques not only greatly assists surgical planning but also may be used to help differentiate individual tumor types on the basis of differing invasion patterns. In cats, for example, indentation of the globe is more frequently seen with lymphomas, whereas squamous cell carcinomas are more likely to produce lysis of orbital bones. In dogs adenocarcinomas were associated with diffuse bony lysis. However, there are exceptions to these trends, and histopathologic or cytologic confirmation of retrobulbar

FIGURE 5-51. T1-weighted, post–contrast injection, frontal magnetic resonance image of a cat with a space-occupying mass behind the right eye. Cytology and culture testing performed on material aspirated from this mass permitted diagnosis of a bacterial retrobulbar abscess/cellulitis that responded well to antibiotic therapy. (Courtesy Dr. Winnie Lo.)

mass type remains essential. Therefore one or sometimes both of these imaging techniques are now performed almost routinely before biopsy or surgical excision of orbital masses in animals.

Fluorescein Angiography

Fluorescein angiography is used to investigate retinal and choroidal vascular patency, vessel-wall permeability, and pigmentary abnormalities of the fundus. For this technique the patient is sedated or anesthetized, fluorescein is injected intravenously, and the fundus is illuminated with a light of a specific wavelength that stimulates fluorescence. The emitted light is photographed in a series of approximately 30 photographs taken in the first 30 seconds, followed by single photographs 20 and 30 minutes later (Figure 5-52). Circulation of the fluorescein proceeds through the following phases:

•Choroidal phase: Choroidal vasculature has filled.

•Arteriolar phase: Retinal arterioles have filled.

•Arteriovenous phase: Retinal arterioles and venules have filled.

•Venous phase: Retinal arterioles have emptied and veins have begun to fill.

•Late phase: Certain tissues (e.g., optic nerve head) stain with fluorescein.

In the normal eye fluorescein does not penetrate the endothelium of retinal or choroidal vessels but does pass the choriocapillaris. In disease states these relationships are altered. For example, with neovascularization, hypertension, vasculitis, or chorioretinitis, retinal and/or choroidal blood vessels may show increased permeability to the dye. Fluorescence of the tapetum in domestic animals and the lower frequency of retinal vascular disease in domestic animals than in humans decrease the utility of the technique in veterinary medicine.

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