Ординатура / Офтальмология / Английские материалы / Small Animal Ophthalmology Secrets_Riis_2002

should be involved in the decision to proceed with cataract surgery and be made aware of the advantages and disadvantages of each option. The visual status of the dog before surgery will have an impact on the impression of degree of visual improvement after surgery (Fig. I).

Figure l. A l2-year-old chocolate Labrador retriever with nuclearsclerosis and multifocal cortical opacities. This Labrador is visual, and cataract surgery at this stagemay not result in clinically evidentvisualimprovement.

4. When a dog has a complete cataract in one eye and a small or no cataract in the other eye, should cataract surgery be performed?

In addition to the discussion above regarding the degree of preoperative visual deficit in a single eye, when considering the whole animal, regard must also be given to the degree of visual compensation that the fellow eye provides when considering surgery. As described above, much debate exists as to when cataract surgery is indicated in cases of asymmetrical cataract formation.

Before phacoemulsification techniques for cataract removal (see question 6) became widespread, older lens extraction techniques (extracapsular lens removal) required cataracts to be almost fully mature. This is the origin of the concept of waiting for a cataract to "ripen" before surgery. Now that newer lens removal techniques are widely available, cataracts of any degree of maturity (and even normal lenses) can be removed with similar degrees of success.

Arguments for the early bilateral removal of such asymmetrical cataracts include a potential reduction in preand postoperative complications and avoidance of a preoperative period of visual impairment. However, the early removal of a mostly normal lens would result in the loss of any accommodative ability that the normal parts of the lens might still be able to provide. The optical effects of aphakia may be reduced by the implantation of an intraocular lens (see question 10). Arguments that might support a plan to delay surgery (and monitor cataract progression, with possible lens removal later) include a greater improvement in post versus preoperative vision and the possibility of avoiding surgery altogether in the less affected eye.

Unilateral cataract surgery has been performed in dogs with a significant unilateral cataract and no or a small nonprogressive cataract in the fellow eye. Issues to be considered in unilateral versus bilateral cataract surgery include cost (often bilateral surgery is less than twice the cost of unilateral surgery), number of anesthetic episodes, number of postoperative/recuperation periods, the potential need to do surgery in the second eye at a later date, and the potential risks to the cataractous eye (lens-induced uveitis, glaucoma, or retinal detachment) while monitoring progression in the fellow eye.

As mentioned above, these issues should be discussed with the owner so that an informed decision can be made.

If monitoring for progression of the fellow eye is the chosen plan, the owner should be informed to watch for signs that might indicate that complications are developing. These include redness (conjunctival or scleral hyperemia), pain (blepharospasm, rubbing of the eye, depression),

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generalized whiteness to the cornea obscuring the iris (corneal edema), and globe enlargement (buphthalmos due to chronic glaucoma).

In any case, early referral for individualized discussion with a specialist is never inappropriate.

5.What type of patient is the ideal cataract surgery candidate? With the above caveats in mind, the ideal cataract surgery patient:

•Has vision loss solely due to the presence of cataracts

•Has lenses that are in place (nonluxated)

•Has no other ocular disease such as uveitis, keratoconjunctivitis sicca, glaucoma, or corneal opacities of any type

•Is tractable and will tolerate topical medication

•Is systemically as normal as possible (well-controlled diabetes, well-controlled hyperlipidemia) and can tolerate general anesthesia

•Has an owner or caretaker who is willing to administer multiple topical medications, potentially for long periods of time

•Has an owner or caretaker available to present the animal for recheck examinations

•Has an owner or caretaker who understands the risks and financial responsibilities required While some of these characteristics are more important than others, it should be emphasized that cataract surgery is an elective procedure.

6.What are the techniques used for cataract surgery?

Currently in both animals and humans, cataracts can generally be removed using three tech-

niques: intraand extracapsular extraction and phacoemulsification. Laser technology for initial cataract extraction is experimental at this time and is not routinely used in animals.

Intracapsular cataract extraction (ICCE) or intracapsular lens extraction (ICLE) involves removal of the entire lens intact. The zonules holding the lens in place are broken down or dissolved before the lens can be removed using this technique. This is accomplished by the use of enzymatic zonulolysis (such as with a-chymotrypsin) or mechanical zonular rupture. Because the entire lens is extracted whole, a relatively large corneal incision must be made to accommodate the lens. This technique often disrupts the anterior vitreous face and may potentially dislocate the rest of the vitreous body and retina. The resulting aphakic eye ideally has vitreous behind the pupil, and the vitreous body is held in place by its peripheral attachments to the posterior ciliary body and retina, which are relatively weak.

Extracapsular cataract extraction (ECCE) or extracapsular lens extraction (ECLE) requires an initial removal of the central portion of the anterior lens capsule. This anterior capsulotomy is ideally a continuous curvilinear capsulorrhexis (CCC), formed by tearing ("-rhexis") of the capsule. A corneal incision is made large enough to accommodate the lens nucleus. The hard lens nucleus is then delivered using a broad instrument outside the eye to push the nucleus onto a second spatula-type instrument. Fibers of the lens cortex (which are adherent to the interior of the lens capsule) are then irrigated and aspirated out of the capsule. This technique ideally preserves the posterior capsule within the eye, leaving the vitreous undisturbed.

Phacoemulsification (PE) utilizes ultrasound energy delivered directly to the lens through a hollow needle-like probe. This probe is inserted into the lens through relatively small incisions in the cornea and lens capsule. Ultrasound energy is used to liquefy (or emulsify) the lens fibers, and the resulting liquified material is aspirated. A CCC is performed before or after phacoemulsification. After emulsification of the hard nucleus, removal of the adherent cortical lens fibers is performed using an automated irrigation/aspiration (IIA) handpiece. The posterior lens capsule is usually preserved with this technique, and the posterior segment of the eye remains untouched. Phacoemulsification is made possible by a machine that supplies electrical power to generate ultrasound energy, creates a vacuum enabling aspiration, and controls flow of irrigation fluids. This is currently the preferred technique for removal of most cataracts in both humans and animals.

"Phaco-" is the prefix denoting "lens" (from the Greek phakos, meaning "lentil" or "lentilshaped object").

7.What is viscoelastic material?

Viscoelastic (often referred to as "visco") is a clear, gel-like material used for many purposes

in intraocular surgery. It is used to maintain depth and normal anatomy of the anterior chamber, to provide a protective coating for intraocular structures, to atraumatically manipulate structures such as the iris and lens capsule, to hydrodissect the lens capsule away from the cortical lens fibers, and to tamponade hemorrhage. Its specific rheologic properties (viscosity, elasticity, pseudoplasticity, surface tension) make it ideal for these purposes. Various materials are used for visco, including hyaluronic acid, chondroitin sulfate, and hydroxypropylmethylcellulose. Different combinations of these substances in different concentrations provides a variety of visco products with a range or rheologic properties.

Visco is typically supplied in syringes and is injected through a blunt-tipped cannula.

8.Why is the anterior lens capsule removed in cataract surgery?

The interior surface of the anterior lens capsule is lined with a monolayer of viable lens ep-

ithelial cells. The lens epithelial cells located in the periphery (at the lens bow) normally undergo mitosis throughout the animal's life, with individual cells elongating and migrating centrally within the lens with maturation. Lens epithelial cells retain mitotic capability even after phacoemulsification. Ifleft behind, they can proliferate and form clumps oflens-like material (called lentoid). This lentoid can both optically obstruct the pupil, mimicking a cataract, and serve as a continuing source of lens-induced uveitis. By removing a large area of anterior lens capsule, most lens epithelial cells will be removed, reducing the possibility of the postoperative lentoid production.

Removal of the anterior lens capsule will also allow the placement of an intracapsular intraocular lens (IOL). In this case, the anterior capsulotomy should be large enough to enable passage of the IOL into the capsule, but small enough to keep it there. The circular shape of an anterior CCC allows for maximal flexibility and stretch of the opening without tearing of the capsule.

9. Why is the posterior capsule left in place in cataract surgery?

Following routine extracapsular cataract procedures, the posterior capsule is left in place as a physical barrier between anterior and posterior segments. The presence of this capsular "diaphragm" can provide stability to the posterior segment of the eye, specifically the vitreous and the retina. An opening in the posterior capsule may allow vitreous to herniate into the anterior chamber, especially if the vitreous is degenerate and liquified. Anterior vitreal displacement may facilitate retinal detachment due to traction on the anatomic adhesions between the vitreous and retina. An intact posterior capsule also helps keep an intracapsular IOL in place.

Despite these advantages, in some cases an opening in the axial posterior lens capsule is made on purpose at the end of cataract surgery. This is generally done to treat or prevent axial posterior capsular opacities (PCOs), or opaque plaque-like areas in the center of the posterior lens capsule. These PCOs, if dense enough, can effectively mimic a cataract. They may be present at the time of cataract surgery or develop at any time after surgery. If severe enough at the time of primary surgery, removal of PCOs is performed at that time in order to improve optical clarity of the eye.

10. Why are IOLs used?

Prosthetic IOLs are used to improve the optics of an aphakic eye. An eye without a lens is severely hyperopic, or farsighted, since the lens is not present to focus incoming rays of light onto the retina. The image formed in an aphakic eye is focused at a plane behind the retina. In general, aphakic canine eyes are approximately +14 diopters (D) hyperopic. When an intraocular lens is used, incoming light rays are refracted such that this focal point is at the retinal plane (a condition called emmetropia), producing sharper images and a corresponding improvement in vision. In humans, preoperative measurements are made of the globe and cornea, and the proper optical strength required for an IOL for that particular eye is computed. Refractive studies have shown that most dogs require an IOL of approximately 41 D to establish emmetropia or near-

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emmetropia. Emmetropia may be particularly important for a working dog that requires excellent vision for its job.

Another advantage to using IOLs is that they may reduce the degree of postoperative PCO formation. By directly contacting the axial posterior capsule, IOLs can reduce the amount of axial fibrous pseudometaplasia from migrating residual lens epithelial cells, thereby decreasing subsequent capsular folding and opacification.

Eyes with implanted IOLs are termed pseudophakic.

11.Are there any disadvantages to using IOLs?

Despite their great theoretical benefits, IOL use may be associated with some drawbacks. Im-

planting IOLs increases the cost and duration of cataract surgery. If an IOL eventually becomes displaced (decentrated) within the eye or remains mobile within the eye, any optical advantage the IOL once provided may be considerably reduced because the IOL may then produce distorted images on the retina. Current IOL technology in dogs requires that the initial corneal incision be significantly enlarged to place an IOL; a wider corneal incision can increase risk of both intraand postoperative complications. Intraocular lenses that are placed in the anterior chamber (used sometimes in humans) or those that luxate from the posterior into the anterior chamber directly contact the anterior iris face, potentially leading to irritation and mechanical chafing of the iris. In addition, some ophthalmologists believe that the use of IOLs may lead to chronic postoperative uveitis. This uveitis may be so severe as to negate any benefit from the original cataract surgery. As a foreign body implanted within the eye, IOLs may induce chronic inflammation that may only be partially responsive to medical therapy (Fig. 2). Occasionally, explantation ofIOLs are necessary when the IOLs become either excessively irritating or displaced.

Figure 2. An IOL with a cloudy appearance. Cloudiness may be due to the presence of cells, proteins, and minerals on the IOL surface and opacity and wrinkling of the lens capsule. In this case, the fundus can still be visualized despite the opacity.

12.What do IOLs look like and what are they made of?

The basic design of IOLs includes a central optic, or lens, and peripheral supporting devices

called haptics. The optic is a lensor lentil-shaped clear structure that is to be placed in the pupillary path of light. This is held in place with any of a variety of supporting haptics, from curved (J- or C-shaped) arms to angled (V-shaped) arms to flat trapezoidal plates. Both haptics and optics often have small circles or loops cut into them to aid in proper IOL placement. The eventual location of the IOL (anterior chamber, intracapsular or "in-the-bag," "ciliary sulcus" or posterior chamber) and fixation method (sutured or sutureless) all determine the haptic shape and orientation (Fig. 3).

Various clear synthetic materials have been used to make IOLs, including polymethylmethacrylate (PMMA), acrylic and silicone. Lenses made of PMMA are rigid. Only thin structures made of PMMA (e.g., thin J-shaped hapties) can bend to any degree, but these will break if they are excessively stressed. Acrylic and silicone lenses are softer and potentially bendable. Some IOLs are made of two different materials (the optic of one material and the hapties of another) to take advantage the benefits of each.

Intraocular lenses with optics made of PMMA require corneal incisions large enough to accommodate them, usually 5-7 mm, about twice the size of the incision required to do phacoemulsification. Both silieone and acrylic are flexible, allowing bending or folding of the optic. These increasingly popular foldable lenses allow for smaller (3--4 mm) corneal incisions to be used in cataract surgery. The technology for such lenses is widely used in humans but is in an early stage of development in veterinary ophthalmology, and may gain more widespread use as techniques for their use advance.

Figure 3. An IOL 19 months after surgery. The IOL is nicely centered in the lens capsule. There is no reaction to the IOL, and the owners claim that the dog has very good vision.

13.Do all eyes that have cataract surgery have an IOL placed?

Not necessarily. All the considerations discussed above should be weighed for the individ-

ual case. Surgeon preference is also very important. In addition, the question of which eyes are mechanically able to properly hold an IOL must be addressed.

The majority of IOLs placed in animals are intracapsular IOLs, so conditions of the lens capsule must be appropriate for ideal IOL placement. Secure and proper placement of intracapsular IOLs requires that the anterior capsular opening is the correct size and shape to both allow insertion of an IOL and retain the IOL after surgery. Because the haptics hold the lens in the capsule and keep the optic in the center of the capsule, the integrity of the lens zonules is important for keeping the capsule and IOL in an axial location. If an IOL is placed in a subluxated lens capsule (i.e, lens zonules are torn), the IOL will be correspondingly subluxated as well. The posterior lens capsule should be clear (or removed axially) to derive the full visual benefit of the IOL.

Removal of a lens via ICLE results in an eye with no lens capsule. In this case, a sutured ciliary sulcus IOL may be placed. This type of IOL is secured with haptics that are sutured to the sclera in two sites 1800 apart just behind the iris and in front of the ciliary body. Risks of this procedure include hemorrhage, shifting/nonaxial optic positioning, and retinal detachment. Other methods of improving visual acuity in aphakic dogs (e.g., laser in situ keratomileusis [LASIK]) have been used only experimentally in dogs and are not in widespread use.

14.What is the typical postoperative medical regimen following cataract surgery?

The specific medical regimens used after cataract surgery are as varied as the surgeons do-

ing the procedure.

•Anti-inflammatories, usually topical steroids, and less frequently topical nonsteroidal anti-inflammatory drugs (NSAIDs), are given after cataract surgery. These can be supplemented with systemic or subconjunctival anti-inflammatories. Anti-inflammatory drugs are given to counteract the anterior uveitis that invariably results from canine cataract surgery. Sutured surgical corneal wounds generally appear to heal well in the face of topical or systemic anti-inflammatory drug administration. However, these drugs should be temporarily discontinued if a corneal ulcer develops postoperatively and resumed as soon as possible after the ulcer heals.

•Antibiotics. Broad-spectrum topical or systemic antibiotics are usually given routinely during and after cataract surgery because of the invasive nature of cataract surgery. Though the risk of postoperative infection is remote, antibiotics may help to further reduce this risk.

•Mydriatics/cycloplegics. Topical mydriatics such as atropine or tropic amide are sometimes prescribed to treat postoperative uveitis. These may help to stabilize the blood-aqueous barrier, relieve painful ciliary muscle spasm, and reduce the possibility of posterior synechia development, particularly small-pupil posterior synechiae.

•Antiglaucoma medications. Topical or systemic antiglaucoma medications are occasionally prescribed to relieve an elevated postoperative lOP that results from postsurgical uveitis. If lOP elevation is not a primary condition (i.e., the animal does not have primary glaucoma), antiglaucoma medications may not need to be administered indefinitely. However, the decision as to when to discontinue antiglaucoma medications after cataract surgery should be left to the surgeon.

15.How soon after cataract surgery can an improvement in vision be expected?

Many dogs have a rapid improvement in vision after surgery (2-5 days). The degree of per-

ceived improvement often depends on the degree of preoperative visual impairment, whether the dog is aphakic or pseudophakic, the individual dog's personality, activities, and lifestyle, and the dog's use of other senses. Then ability to judge a dog's vision is (by necessity) subjective at best and can be strongly influenced by external circumstances.

BIBLIOGRAPHY

I. Glover TO, Constantinescu GM: Surgery for cataracts. Vet Clin North Am Small An Pract 27: 1143-1173. 1997.

2.Jaffe NS. Jaffe MS. Jaffe GF: Cataract Surgery and Its Complications. 5th ed. St. Louis, Mosby. 1990.

3.Miller TR. Whitley RD. Meek LA. et al: Phacofragmentation and aspiration for cataract extraction in dogs:

56 cases (1980-1984). J Am Vet Med Assoc 190: 1577-1580, 1987.

4. Nasisse MP. Davidson MG. Jamieson VE: Phacoemulsification and intraocular lens implantation: A study of technique in 182 dogs. Prog Vet Comp Ophthalmol 1:225-232. 1991.

into the vitreous. Most cataract surgeons operate with the patient in dorsal recumbency, with the iris plane approximately parallel to the floor. Thus, if the vitreous is liquefied, these fragments may settle to the dependent position at the posterior-most aspect of the globe, on the retina. Such fragments can be very difficult or impossible to remove safely through the typical approaches used for cataract surgery and with standard cataract surgery instruments. When this occurs in humans, referral is made to a vitreoretinal subspecialist for vitrectomy and fragment removal. Unfortunately, few in veterinary ophthalmology are equipped or experienced in performing such procedures. Large fragments that are left in the vitreous may be a source of chronic, refractory inflammation and may lead to retinal detachment and other sequelae of posterior uveitis. Sometimes, small lens fragments isolated in formed vitreous remain inactive and uninflamed for long periods of time.

Expulsive choroidal hemorrhage is a very rare, but for obvious reasons, very serious intraoperative complication. This sudden, unpredictable, massive bleeding from the posterior segment makes further surgery on the lens impossible and may lead to blindness.

Fortunately, serious intraoperative complications are very unusual in routine cataract surgery.

4. How can the incidence of intraoperative complications be minimized?

Proper choice of surgical candidates can reduce the incidence of surgical complications. Dogs without active uveitis, luxated lenses, or corneal disease are better candidates than those with these conditions (see Chapter 21); these animals should be treated for these complicating conditions first. Adequate preand intraoperative treatment with anti-inflammatories and mydriatic agents can help control against excessive intraoperative inflammation and miosis.

Control of patient positioning can also reduce the incidence of surgical complications. Proper positioning of the patient under the operating microscope is critical to a good outcome. Position of the head must be changed as surgery is performed on each eye, and an experienced surgical nursing staff can facilitate this intraoperative adjustment. Adequate skeletal muscle paralysis (with assisted ventilation) is also very important to positioning. Ideal globe position, with no external pressure on the opened eye, is most easily achieved with full paralysis (using agents such as intravenous atracurium or pancuronium). Canthotomy, stay sutures, and other globe immobilization techniques may also be helpful.

Proper surgical equipment and an experienced surgeon and staff can make a considerable difference in the prevention and management of complications. Adequate lighting, an operating microscope with excellent optics that can be adjusted by the surgeon, high-quality instrumentation (including the ability to perform vitrectomy), appropriate viscoelastic materials, and most of all meticulous surgical technique will help to reduce problems.

As with any surgery, preparation for the most likely complications is prudent. Items such as injectable epinephrine, mannitol, and cautery are useful to keep in stock in case of emergency.

5. What are the most common immediate postoperative complications of cataract surgery? How are they treated?

Some of the more common problems seen in the first 12-72 hours following cataract surgery include uveitis, corneal edema, postoperative hypertension, corneal erosions or ulcers, hyphema, and hypotony or surgical wound leakage.

Virtually every dog that undergoes intraocular surgery experiences some degree of postoperative anterior uveitis. This develops for several reasons. One of the most commonly cited is the predisposition of the canine anterior segment to irritability, particularly when compared with the human anterior segment. Very little intraocular disturbance is required to incite anterior uveitis in dogs. Infact, one model of experimentally induced canine uveitis involves only the simple paracentesis of a small amount (0.1 ml) of aqueous. In addition to the dog's tendency for uveitis. the mechanical trauma of surgery - the insertion and removal of instruments into and out of the eye, the application of ultrasound, and the large amount of fluid circulated into the eye by the phacoemulsitication handpiece (when this technique is used)-is a considerable stimulus for

uveitis. Good surgical technique minimizes uveitis. Postoperative uveitis is treated routinely by the use of topical, subconjunctival, and systemic anti-inflammatory drugs.

Corneal edema may develop around the surgical wound as fluid is imbibed by the stromal lamellae that have been exposed by incision. Mild diffuse corneal edema may also develop due to the generalized mechanical trauma of cataract surgery to the corneal endothelium. These forms of corneal edema are common following cataract surgery. They are typically transient and require no specific treatment.

Postoperative hypertension (acutely increased intraocular pressure [lOP] after surgery) is theorized to develop secondary to (I) a temporary alteration in the anatomy of the drainage angle due to surgery and (2) postoperative inflammatory products or viscoelastic material that "clogs" the angle. Depending on the lOP, this condition mayor may not be treated. Topical and systemic antiglaucoma medications, intracameral tissue plasminogen activator to break up fibrin that may be blocking aqueous drainage, intracameral miotics to increase aqueous outflow, and sometimes the mechanical removal of fluid from the anterior chamber have been used to treat extreme postoperative pressure spikes. Judicious monitoring of lOP after surgery is always warranted.

Corneal erosions or ulcers may develop as an unwanted result of general anesthesia and exposure, the effects of which are exacerbated by the use of muscle paralytics. Self-trauma and the use of presurgical topical medications (which may be locally irritating or inhibit corneal healing) may also contribute. Surgeons must be mindful of this possibility and take preventative action wherever practical. Fortunately, these erosions tend to heal relatively quickly when treated appropriately (i.e., avoidance of topical steroids).

Hyphema (bleeding into the anterior chamber) may result from excessive activity, barking, self-trauma (rubbing, head shaking), neck trauma, uveitis, or retinal detachment. Postoperatively, cataract surgery patients should wear Elizabethan collars that are fitted appropriately, and should be kept quiet.

Hypotony (excessively low lOP) may be due to surgical wound leakage. This situation is best prevented by proper wound closure, but occasionally it is difficult to avoid. Flattening of the anterior chamber, a visibly leaking wound, or a positive Seidel test are telltale signs of wound leakage. Identification of wound leakage is facilitated by the use of dry cellulose sponges placed at the sutured wound edge. If leakage is detected, another attempt at suture placement or a conjunctival flap placed is necessary to remedy the problem. These procedures may require a second anesthetic episode. Alternatively, some have advocated the injection of a very small amount of saline or other fluid into the corneal stroma at the wound site in an effort to create enough swelling and corneal edema to seal the wound.

Figure I. American cocker spaniel presenting several months after phacoemulsification and IOL placement surgery. Apparently uveitis resulted in fibrin formation, which luxated the IOL (lateral view).