Chapter 11 Surgical procedures of the lens and cataract
The major causes of blindness in small animals are corneal diseases, cataracts, the glaucomas, and retinal degenerations. Of these disorders, the treatment of cataracts is clearly a surgical condition, and has been the surgery that has characterized the specialty of human and veterinary ophthalmology since their beginning.
Embryologically, the lens originates from surface ectoderm. Congenital abnormalities of the lens may or may not be associated with other congenital intraocular abnormalities or cataract. Examples of congenital anomalies that may be associated with cataracts include persistent pupillary membranes, persistent hyaloid artery, persistent hyperplastic primary vitreous/tunica vasculosa lentis (PHPV/PHTVL), posterior lenticonus, microphakia, lens coloboma, and microphthalmos.
Cataracts in dogs parallel those in humans, and considering the large number of recognized breeds of dogs, the number of inherited cataracts in the dog probably exceeds those in humans. Primary and possibly inherited cataracts affect a large number (about 125) of breeds of purebred dogs. In about 20 breeds, the age of onset, mode of inheritance, and site of the lens initially involved in cataract formation have been documented. In other breeds of dogs, cataracts appear at a higher frequency than in the general population, but their characteristics have not yet been defined. Cataracts secondary to traumatic, inflammatory, and metabolic disorders also affect dogs, and are also treated by surgical removal.
Cataracts in cats occur less frequently. Primary cataracts occur infrequently in young cats, and most of the inherited cataracts appear in kittens rather than in adults. The most frequent cataracts in cats are secondary types, associated with anterior uveal inflammations. As many uveal inflammations are secondary to serious systemic diseases, cats presented for evaluation for cataract surgery require a complete physical examination and blood tests for feline viral leukemia, feline immune-deficiency virus, and other infectious diseases. Cats usually have less intense iridocyclitis than dogs after lens and cataract extraction, and higher success rates, although only limited numbers of reports are available.
Cataracts are not infrequent in horses, but are infrequent in cattle (and more rarely diagnosed). The majority of cataracts in the horse are secondary to inflammation and trauma, but inherited cataracts occur in the Morgan, Belgian, Quarter Horse, and Rocky Mountain horses. The cataracts in the latter three breeds also have additional concurrent ocular anomalies. Inherited cataracts may occur in other breeds of horses. When presented with a foal with congenital cataracts, a dilated examination of the mare and, if possible, the stallion is advised as they may also have incipient cataracts, supporting the possibility of inheritance. Breeding of foals with congenital cataracts should be discouraged and the mare, if re-bred, should not be bred back to the same stallion.
Cataracts in cattle are rarely reported but when herds of cattle are closely examined, the frequency of cataract formation is more common. Inherited cataracts occur in the Holstein–Friesian, Jersey, and Hereford breeds. Cataracts with other ocular anomalies occur in the Hereford, Holstein–Friesian, Jersey, and Shorthorn breeds. Cattle also exhibit a viral-induced congenital cataract, retinal detachment, and microphthalmia in calves secondary to bovine viral diarrhea and developed in the first part of pregnancy (76–150 days post-gestation).
Removal of displaced or luxated lenses in dogs, cats, and horses is often utilized either to prevent the onset of secondary glaucoma or to assist in the treatment of these glaucomas. The possible mechanisms by which these displaced lenses contribute to the onset of secondary glaucomas are presented in a later section. Loss of part-to-all of the zonulary attachments results in lens subluxation, posterior (intravitreal) luxation, and anterior chamber luxation.
The lens is normally held within the patellar fossa by 120–180 zonules attaching the lens equator to the ciliary body. In dogs, the zonules appear to be of two major types, resulting in a cruciate type of arrangement at the lens equatorial capsule. These zonules may degenerate with age, trauma, inflammation, malformation, and breed-associated and chronic glaucoma. With significant loss of zonules, the lens position changes, with tilting, decentralization, looseness within the patellar fossa (phacodonesis), possible tearing of the hyaloideocapsular ligaments (Wieger’s ligament), and tearing of the anterior hyaloid membrane, resulting in vitreous entering the posterior chamber, pupil, and eventually the anterior chamber. The iris normally touches the middle and central positions of the lens; however, with these lens positional changes, the iris becomes unstable and iridodonesis occurs. As a result, lensectomy or removal of the entire lens with its capsules (intracapsular extraction) is often performed in dogs and less frequently in cats.
Cataracts appear in many different forms and, as a result, classification of these lens changes is the basis for qualifying these opacifications. No single classification method accommodates all these variables. As a result, several different cataract classifications have evolved. Classification of cataracts ensures a common system to identify cataracts in all animals and provides a uniform language among veterinarians diagnosing and performing cataract surgery.
In small animals, the biochemical changes that initiate cataractogenesis have not been documented, except to some extent in the inherited and congenital cataract in the Miniature Schnauzer and sheep. The lens is highly cellular with no extracapsular spaces. The lens is also predominately proteins (soluble and insoluble) and cellular structures. In cataract formation, regardless of the inciting cause, the lens fibers eventually die, and lens epithelial cells may die or undergo transformation and proliferation. With continued lens fiber death, lens osmolarity is altered, resulting in an osmotic imbalance and the imbibition of water. With hydration of the lens, which occurs late in the genesis of progressive cataracts, the lens becomes translucent and then opaque.
The different cataract classifications in animals include: 1) age of onset; 2) position within the lens; 3) degree of opacification; and 4) possible cause (Table 11.1). Hence, for the clinical description of a cataract, all methods are usually combined. For instance, the breed-specific cataract in Golden Retrievers is a juvenile, inherited, axial, posterior cortical and capsular incipient cataract. The early cataract secondary to diabetes mellitus in dogs and cats is an equatorial cortical incipient cataract.
|Age of onset||Congenital|
|Developmental (juvenile or adult)|
|Position within the lens||Anterior capsular|
|Equatorial and axial|
|Degree of opacification and lens size||Incipient|
|Mature (subdivision intumescent)|
|Hypermature (subdivision Morgagnian)|
|Associated with other ocular anomalies|
|Secondary to retinal degeneration|
|Secondary to trauma (direct/indirect)|
|Toxic (infant formulas, radiation, drugs)|
Relative to surgery, the classification system that assesses the degree of opacification (or maturity) of cataracts is most useful, as this quantification of lens opacity correlates to clinical vision (Fig. 11.1). Incipient cataracts are the earliest detectable lens opacities. Often appearing as vacuoles and water clefts, these opacities do not significantly impair clinical vision in dogs, cats, or horses. Immature cataracts have noticeable opacification and a tapetal fundic reflection. The animal still has clinical vision, but some visual impairment may be demonstrable, especially during daytime with miosis. With an immature cataract the lens is still normal in size. By indirect ophthalmoscopy, the ocular fundus is visible but some details may be hazy from lens opacification.
Fig. 11.1 The different stages of cataract maturity in the dog. (a) Incipient cataract – the beginning of cataract formation. (b) Immature cataract with more advanced opacification. This is generally the best candidate for phacoemulsification. (c) Mature cataract. These cataracts generally also have concurrent lens-induced uveitis. (d) Hypermature cataract with secondary lens-induced anterior uveitis. Note the dense opacities scattered throughout the anterior cortex.
With mature cataracts, the entire lens is opaque, there is no tapetal fundic reflex, no demonstrable vision with or without mydriasis, and the ocular fundus cannot be observed with indirect ophthalmoscopy. With mydriasis, the lens periphery is opaque, and the peripheral ocular fundus cannot be viewed.
In hypermature cataracts, the entire lens is opaque and there is no demonstrable vision. The ocular fundus is not observable with indirect ophthalmoscopy; however, with mydriasis, some of the peripheral ocular fundus may be observed around the shrunken lens. The dog or cat is blind unless mydriasis is induced, and then visual impairment is present. However, the cataract is reduced in size and beginning to undergo dissolution, probably secondary to the death of the lens fibers and the release of intracellular proteases. As a result, the high molecular weight lens proteins break down into small proteins and even polypeptides that diffuse through the intact anterior lens capsule and perhaps even the posterior lens capsule. A subdivision of the hypermature cataract in small animals is the Morgagnian cataract, in which the lens cortex becomes liquefied, resulting in a nucleus that moves freely through the internal lens and settles in the dependent and most ventral area of the lens. A hypermature cataract often has associated secondary lens-induced uveitis (LIU). This may result in miosis, iris hyperemia, ectropion uvea, synechia, hypotony, aqueous flare, keratitic precipitates, glaucoma, zonulary and vitreous degeneration, and retinal detachment.
An additional subdivision of the hypermature cataract, the resorbing cataract, occurs not infrequently in dogs less than 3 years of age and frequently in dogs younger than 1 year old. In resorbing cataracts the overall size of the lens ranges from slightly smaller than normal to the complete loss of all intralenticular material except for the anterior and posterior lens capsules which now contact each other. As the cataractous material is lost, clinical vision may gradually return.
Variations in the techniques for canine cataract surgery, including discission, couching, and extracapsular extractions, were reported in the latter part of the nineteenth and first part of the twentieth century. During this same time, the different types of cataract surgery were being developed and refined in humans.
The first cataract extractions in dogs were performed by Möller in 1886 and later by Berlin in 1887. Nicolas reported discission, couching, and extracapsular cataract removals in dogs in 1908. Gray, describing the same procedures, was less impressed with the results in dogs. Muller and Glass in 1926 and Ratigan in 1928 reported good results with the discission technique in young animals. Intracapsular cataract extraction in the dog was described in 1936 by Bartholomew. In 1937, Überreiter published several reports on the different cataract surgeries, including removal of luxated lenses. Several additional veterinarians, such as Condemine (1939), Love (1940), Perry (1941), Means (1942), O’Connor (1942), Brumley (1943), Greaud (1950), and Morgan (1952) recommended specific types of cataract extraction, and debated the advisability of cataract surgery in small animals. In 1952, Formston cautioned that cataract surgery is ‘fraught with difficulties and is speculative’.
Nevertheless, the evolution of cataract surgery continued in small animals. In 1953, Magrane described aspiration of canine cataracts, and the following year defended the rationale for canine cataract surgery using extracapsular extraction. Knight, in 1957, reported fair results (29%) in 106 cases of cataract extraction in dogs, primarily using the intracapsular technique. Three years later, Knight reported a 34% success rate in 233 cases of cataract removal, primarily using the intracapsular technique.
Throughout the 1960s the debate among veterinarians performing cataract surgeries in small animals no longer focused on the justification for the surgery, but on whether the extracapsular or the intracapsular technique was superior. At this same time, in humans, the intracapsular cataract extraction method with enzymatic zonulolysis (alpha chymotrypsin) was yielding the highest success rates.
In this decade (1960s) it gradually became apparent that the highest success rates for cataract surgery in dogs resulted from extracapsular lens removal, because of adherence of the vitreous to the posterior lens capsule, and the inability of alpha chymotrypsin to produce zonulolysis within a few minutes. In 1961, from a series of 104 cataract extractions in the dog, Magrane reported a 76% success rate in dogs undergoing operations for juvenile cataracts and a 37% success rate in dogs with senile cataracts. The difference between these two groups was believed to be associated with undetected accompanying degenerative changes, exaggerated intraocular inflammation, and postoperative complications. A larger series of cataract extractions by Magrane (429 cases) in 1969 revealed an 80% success rate. The higher success rate was believed to be associated with an improved extracapsular extraction procedure, and pre- and postoperative administration of corticosteroids. The more years of cataract surgery experience the veterinary ophthalmologist had, the higher the rate of surgical success. If the dog’s fellow eye underwent a subsequent operation, the success rate was 20% less than for the entire group. This lower success rate for second operated eyes appears related to the sensitization of both eyes during the first cataract surgery to the cataractous material, and the more intense uveitis that resulted in these second eyes postoperatively. The success rate was 15% higher for cataract extractions for congenital and juvenile cataracts than for diabetic and senile cataract extractions. The success rate for lensectomy decreased 18% when combined with iridectomy, which in the dog is often associated with intraocular hemorrhage and a more severe iridocyclitis.
In the late 1960s, Startup published several articles on cataract surgery, including cryoextraction, in dogs. In a more recent series of 113 unilateral and 77 bilateral extracapsular cataract extractions in dogs, restoration or improvement of functional vision was achieved in 79.6% of the eyes with unilateral extractions and 85.7% of the eyes with bilateral extractions at 4–6 weeks postoperatively. Success at 3–9 months postoperatively was 68.9% (unilateral extractions) and 69.4% (bilateral extractions not performed at the same time). In dogs with bilateral cataract extractions, 93.5% exhibited restoration or improvement of vision with successful surgery in one or both eyes.
In the late 1970s and early 1980s the new technique of phacoemulsification of cataracts was developed, with postoperative aphakia being treated intraoperatively with implantation of intraocular lenses (IOLs). With about the same postoperative results, shorter hospitalization, smaller corneal incision and astigmatism, and same-day surgery, phacoemulsification of cataracts became the preferred cataract surgery in humans in the 1980s and to date.
With the same surgical instrumentation, phacoemulsification of cataracts evolved for small animals. As a result, phacoemulsification has largely replaced the extracapsular cataract method in many veterinary ophthalmology clinics worldwide. Although limited in patient numbers, Miller and co-workers reported on the success of phacofragmentation and aspiration. In 82 cataracts removed by this method, vision was present immediately after surgery in 95% of the dogs. At 2 years after surgery, vision was still present in 85% of these patients. The reasons for this gradual decline were primarily related to postoperative anterior uveitis and capsular fibrosis.
In a later series by Nasisse and Davidson that included 158 dogs, phacoemulsification with and without IOL implantation resulted in good-to-excellent postoperative visual results in 90.3% of the eyes. Phacoemulsification time of the cataractous lens increased with patient age but hypermature cataracts required less fragmentation time (average times for immature, mature, and hypermature cataract were 180 s, 174 s, and 137 s, respectively). The endocapsular (‘in the capsular bag’) fragmentation technique was used and the main intraoperative complication was posterior lens capsular tears (19%).
Today, the majority of veterinary ophthalmologists in the US and other countries regard phacoemulsification, a refinement of the extracapsular technique, as the cataract surgery method of choice in dogs, cats, and horses. Extracapsular lens extraction is also used routinely by ophthalmologists with limited numbers of cataract surgeries, but with favorable results. The intracapsular lens technique is used primarily for subluxated lens, and anterior and posterior (intravitreal) lens luxations.
While there was keen interest in cataract surgery in horses, most of the early reports concentrated on the dog. With very low success rates for canine surgery in the 1940s and 1950s, there has been considerable discussion and debate as to whether cataract surgery in dogs could be recommended. Fortunately, with considerable progress in cataract surgery in both humans and dogs in the 1970s and 1980s, cataract surgery was finally advocated for other species, especially the horse.
In the veterinary literature, cataract surgery in horses is infrequently mentioned. Lack of adequate general anesthesia was often reported as a major impediment to equine cataract surgery; general anesthesia at this time consisted of a combination of chloroform, morphine, and local or regional anesthesia. Überreiter, in his chapter in Advances in Veterinary Science (1959), reported on the current progress in canine surgery, but barely mentioned the horse. He noted that Daviel (1753) reported cataract surgery in the horse, but gave no details. Überreiter noted that attempting cataract surgery in horses with recurrent uveitis invariably ended in atrophy of the globe (presumably from recurrence of the disease). Cataract surgeries were divided into: 1) discission; 2) linear extraction (similar to aspiration); 3) reclination (or couching); and 4) flap extraction (the traditional extracapsular and intracapsular methods). Other pioneers in veterinary ophthalmology, including Lanzillotti-Buonsanti, Röder and Bayer, reported no success in the horse using the discission method.
In America, cataract surgery was reported in adult horses using the intracapsular method by Van Krunigen in 1964. He demonstrated that these surgical techniques were possible, although the majority (18/19) of the operated horses had normal lenses. During this time, inhalational anesthesia with halothane become available, but positive-pressure ventilation and neuromuscular paralysis, as well as the delivery of topical medications via the subpalpebral system, were not introduced until later.
Gelatt reported discission and aspiration of congenital and soft cataracts in a foal in 1969, and in a larger series with Meyer and McClure in 1972 and 1974 (28 horses). Higher success rates were reported in foals less than 6 months old (77% with vision) versus older horses (60% visual). Riis first reported phacofragmentation in the horse in 1981 in the first edition of Veterinary Ophthalmology (Lea and Febiger).
Whitley, Moore, and Sloan (1983) reviewed the state of cataract surgery in the horse, and described aspiration surgery in eight foals with success in nine of the 16 eyes. In a subsequent report by Whitley and co-workers in 1990, both aspiration and phacofragmentation were described in six horses. Five of the six animals were less than 6 months old; a single 4-year-old stallion was operated. They noted the problem of postoperative enteritis in the horse, and its profound adverse effect on the success of surgery.
In the last 20 years or so, additional and more comprehensive reports of cataract surgery in the horse were published by Dziezyc and co-workers (1991, 1992, 1999), Brooks et al (1999, 2005, 2006), and Fife et al (2006). These studies used the current general anesthesia and neuromuscular paralysis methodologies, and provide the best results to date for the horse. Again, the higher success rates occurred in young foals (less than 6 months old). Success rates in older horses were lower, in part related to the possibility of recurrent uveitis, previous trauma, and lens displacement.
Phacoemulsification was preferred, although the current human ultrasonic tips of the phaco handpiece are a little short to access the ventral capsular bag in the adult horse. This has been corrected with the introduction of a longer phaco needle specifically designed for use in the equine eye (Acrivet, Hennigsdorf, Germany). In the last reported series of 39 horses with 55 cataracts removed by phacoemulsification (Fife et al 2006), similar results were obtained. The majority of patients were foals (25/39 animals); there were 14 adults with either traumatic cataracts (9/39) or cataracts secondary to uveitis (5/39).
Success rates varied by age group and duration of follow-up (46/47 sighted immediately after surgery; 23/29 eyes sighted at 4 weeks postoperatively). At last examination, 38/47 eyes (81%) were sighted; 2/47 eyes (4%) had poor vision, and 7/47 eyes (15% were blind). The success rate for congenital cataracts was 85% at 4 weeks, for the traumatic cataracts 100% at 6 weeks and 1 year (three horses lost to follow-up), and for cataracts secondary to anterior uveitis, 20% (five horses; one eye sighted at 1.5 years postoperatively).
The implantation of the modern IOL in humans was first reported by Ridley in 1951 in England. The plastic IOL was positioned between the iris and the anterior lens capsule. Hundreds of studies followed, with general consensus to place the IOL within the capsular bag, after phacoemulsification.
The first report of IOLs in dogs was by Simpson in 1956 in America. His study evaluated two different IOLs: 1) an 11 mm diameter IOL for intracapsular placement after an extracapsular lens extraction; and 2) a 14 mm diameter plastic IOL positioned in front of the posterior lens capsule and presumably the ciliary sulcus after extracapsular lens extraction through either a peripheral iridotomy or the pupil. In 1980 Olson and co-workers evaluated the Shearing IOL in the dog, because the canine ciliary body sulcus diameter is approximately equal to that of humans.
With improved success rates of extracapsular extractions and phacoemulsification in dogs in the early 1980s, and the common use of IOLs in humans following phacoemulsification, the routine implantation of IOLs was evaluated in dogs by many veterinary investigators, including Campbell, Davidson, Gaiddon, Nasisse, Peiffer, and co-workers. It quickly became apparent that the initial IOLs developed for humans (15–20 dioptric power) were not of sufficient strength for dogs. The IOLs used now in dogs have dioptric powers of 40–42 D.
The materials from which IOLs are constructed may influence the development of postoperative capsular opacification (PCO) in dogs. Both the hard polymethylmethacrylate (PMMA) and soft foldable (acrylic hydrophil) IOLs are now available for the dog, cat, and horse. Although the PMMA IOL, supported by two haptics, has been the most common IOL for the dog (Fig. 11.2), the foldable IOLs have recently also become popular. The 6–7 mm biconvex optical center of the IOL, or the optic, produces the refractive power of the IOL. The +41 diopter (D) canine IOL requires a fairly large optic (compared to humans); larger optics tolerate slight decentralization without significant optical aberrations. Currently, foldable acrylic IOLs are the most commonly implanted IOLs in dogs, cats, and horses. These allow for implantation through a smaller incision, resulting in less astigmatism, shortened surgical time, and possibly less PCO.
Fig. 11.2 An intraocular lens can restore the post-cataract eye to preoperative optics. (a) An example of a polymethylmethacrylate (PMMA) intraocular lens (IOL) implanted in a dog after phacoemulsification. (b) Foldable or soft IOLs (generally hydrophilic acrylic) are also available for the dog.
(Photograph courtesy of l-MED Animal Health, a division of l-MED Pharma Inc., Dollard des Ormeaux, Qc, Canada.)
The haptics are the arms of the IOL and serve to center the optic within the capsular bag. In the one-piece IOL the haptics are part of the IOL; in the three-piece IOL the haptics are usually polypropylene (prolene) and much more flexible. The stiffer haptics and one-piece IOL are recommended for the dog, and range from about 13.5 to 17 mm in diameter. The IOL design angles or vaults the haptics about 3–10° posteriorly to reduce chaffing and increase contact between the optic and the posterior lens capsule. The direct contact between the optic and the posterior lens capsule is thought to reduce the development of posterior capsular opacities.
Dialing holes (one or two) are often part of the canine IOL. Rotation of the IOL into the capsular bag or once within the capsular bag may also use IOL forceps grasping the junction (or base) of the haptic to the optic. Forceps contact with the optic is avoided as scratches may result. The placement of IOLs in humans has been variable. IOLs have been implanted in the anterior chamber, pupil supported, posterior chamber, and within the lens capsular bag. The anterior chamber IOLs are conveniently inserted, and do not require an intact posterior lens capsule, but are associated with damage to the iridocorneal angle and corneal endothelia. The pupil-supported IOLs have fewer complications than the anterior chamber IOLs, but unacceptable rates of corneal edema. Direct contact of posterior chamber IOLs, placed in front of the anterior lens capsule, causes iridal problems. Currently, the most frequent IOLs in humans are those placed in the posterior chamber, usually in the capsular bag. Different placements of IOLs have not been compared experimentally in small animals, as the early results on IOL placement in humans were accepted; however, almost all IOLs used clinically in dogs, cats, and horses are implanted in the capsular bag. When capsular bag integrity is compromised, the IOL may be placed in the ciliary sulcus using various suture techniques.
The foldable IOL, made of hydrophilic acrylate and ultraviolet blocking material, has also become available for the dog, and allows a shorter corneal incision through which to introduce the IOL into the anterior chamber. Many veterinary ophthalmologists now prefer these IOLs. These foldable IOLs require a holding/folder forceps or cartridge injector to introduce the IOL through the corneal incision and into the capsular bag.
Current research in canine IOLs is investigating the role of the IOL in the genesis of PCO. Early studies suggest that some IOLs can significantly reduce these opacities, which adversely affect vision in the dog and reduce the success of cataract surgery long term. Also, lens instability and use of IOLs in these eyes has attracted attention. As glaucoma and retinal detachments are the most frequent complications after the removal of lens luxations in the dog, it is hoped that earlier surgical intervention and implantation of an IOL will improve vision postoperatively and significantly reduce these complications.
Studies have recently been reported on IOLs in the cat. The mean lens thickness is 7.77 ± 0.23 mm. IOL studies in vitro and in vivo in normal cat eyes suggest that the IOL power should be 53–55 D, with the cornea curvature, as measured by keratotomy, to be 38.93 ± 0.73 D. Experimental implantation of IOLs ranging in power from 48 to 60 D suggested that a 52–53 D IOL would be most appropriate. The apparent difference between the cat and the dog is related to the anterior chamber depth (cat, 5.0 mm; dog, 3.5 mm) and lens axial length (cat, 7.9 mm; dog, 7.6 mm). Cat IOLs are commercially available.
Experimental IOLs placed in the horse eye range in size from +14 D (resulted in 6 and 12 D postoperative refractive error of +2.5 D) to +30 D (resulted in overcorrection in equine cadaver eyes of +2.96 D), and +25 D IOLs in clinical patients resulted in +3.94 D at 30 days post-surgery.
The current equine IOL is made of a foldable acrylic material (Acrivet 90®). There are three different equine IOLs commercially available. The optic size ranges from 12 to 13 mm and the haptic to haptic length from 21 to 24 mm. The diopter power of the current equine IOLs is either 14 D or 21 D, and there remains some discussion regarding the optimum IOL power to achieve emmetropia in the horse. One of the difficulties in determining the optimal IOL power for the horse is the limited number of IOLs that are implanted clinically and the even smaller number of horses that are refracted once the IOL is in place. While calculations of IOL power based on globe measurements are valuable, the final position of the IOL in the eye with respect to the retina has a significant effect on final refraction. It would appear that the current one-piece, plate haptic acrylic equine IOLs, once implanted in the equine eye, tend to sit more anteriorly in the lens capsule than expected. As a result, recent reports suggest an 18 D IOL may be required to achieve emmetropia (McMullen R, personal communication). An 18 D IOL is currently in production (13 mm optic, 24 mm haptic to haptic) and should be available at the time of publication (Acrivet). Currently, no data are available on this IOL in vivo. Equine IOLs are commercially available.
To perform cataract surgery in animals, the anatomy of the peripheral cornea and limbus, iris and ciliary body, lens, and anterior vitreous is important (Fig. 11.3). Cataract surgeries gain access to the anterior chamber, pupil, and anterior lens capsule through corneal or limbal incisions. Sclerotomies and removal of cataracts through the pars plana ciliaris and equatorial or posterior lens capsule are technically difficult and not used. The corneal incision is performed in the most peripheral cornea, consisting of a combination of beveled and perpendicular incisions, or only a perpendicular incision. The combination method provides a larger tissue surface for closure of the cornea and is less likely to leak aqueous humor postoperatively.
The limbal incision is performed in the ‘blue zone’ just before the bulbar conjunctiva attaches to the cornea. The incision can also combine corneal and limbal incisions, starting with a beveled partial (about 50–75%) thickness in the limbus beneath the beginning of the bulbar conjunctiva, and once in the clear peripheral cornea, entering the anterior chamber through a perpendicular incision. Whether peripheral corneal or limbal incisions are used, entry into the anterior chamber should be in front of the insertions of the pectinate ligaments, and at or near the termination of Descemet’s membrane.
Access to the anterior lens capsule and entire lens requires a maximally dilated pupil. Small and irregular pupils limit access to the lens and physically impair delivery of the lens. The dog iris is friable and highly vascular. Contact with the iris and ciliary body with instrumentation should be avoided as it may stimulate miosis (probably from the release of prostaglandins from the iris). The first ‘valley’ between the peripheral posterior iris and the anterior boundary of the ciliary body is termed the ciliary sulcus. This area was the initial site for securing posterior chamber IOLs and is currently used for sutured IOLs. However, posterior chamber IOLs were eventually replaced with IOLs fixed within the capsular bag because of several complications including decentration, pupil capture, chronic iridocyclitis, and the formation of posterior synechiae. Placement of sulcus-fixated IOLs is reserved for cases of lens luxation and lens capsule insufficiency.
The lens volume in dogs and cats is about 0.5 mL. The feline lens is slightly larger than that of the dog. The diameter of the canine lens at its equators is 10–11 mm; its anteroposterior length is 7–7.7 mm. The lens is surrounded by two capsules, usually identified as the anterior lens capsule (ALC) and the posterior lens capsule (PLC). These lens capsules are the basement membrane for the lens. The ALC and PLC are highly elastic and stain deeply with periodic acid–Schiff stain. The thickness of the ALC in the dog and cat appears to vary with age, with older animals possessing thicker capsules. The ALC is decidedly thicker (50–70 μm) than the PLC (2–4 μm), and their equatorial junction is about 8–12 μm. The ALC also varies in thickness by region. It is thickest in its axial portion and becomes noticeably thinner approximately 2 mm from the equator. For capsulotomies and capsulectomies performed by tearing the ALC with special forceps, the irregular and thick ALC can present problems. Radial tears of the ALC can readily enter the thinner equatorial lens capsule and PLC, and result in IOL instability.
The surface of the PLC totally contacts the anterior vitreous. The concave front of the vitreous is referred to as the patellar fossa. The PLC is nearly inseparable from the anterior hyaloid membrane, and often the two structures appear as one, held together by the lenticulohyaloid or hyaloideocapsular (Wieger’s) ligament. Tears or holes in the PLC invariably result in defects in the anterior hyaloid membrane.
The ALC encloses the lens cortex and the central nucleus. Beneath the ALC is a single layer of epithelia that, at the equator, turns inward (‘the lens bow’) to form lens fibers throughout life. These elongated cells span 180° of the lens. The ends of these lens fibers contact each other in the anterior and posterior cortices, forming the anterior and posterior lens sutures. The outer cortex consists of the youngest lens fibers; these cells become more compact as they travel centrally, forming the central adult lens nucleus. As these central fibers compact, first noticeable in dogs and cats at about 3 years of age, the nucleus becomes a noticeable blue or gray in small animals over about 6 years of age. This is termed lenticular or nuclear sclerosis. Lenticular sclerosis is bilateral and symmetrical, affecting the central nuclear portion of the lens. It must be differentiated from a cataract and is a normal aging change. As the lens nucleus ages, it becomes progressively dense and more resistant to fragmentation by instruments and phacoemulsification.
The lens anatomy in cats is poorly defined. Slightly larger than that of the dog, the feline lens has a steeper anterior curvature, a 12–13 mm diameter, and a thickness of 8 mm anteroposteriorly. The large cornea of the cat, in spite of the more rounded anterior lens shape, accommodates a fairly deep anterior chamber. The anterior lens capsule measures 40–50 microns thick, with the equatorial capsule about 10 microns and the posterior lens capsule about 3–7 microns.
Experimental IOLs for the cat are currently available at 53 D while the average IOL for the dog is 41 D. Cataract surgery is less common in the cat as cataracts are often secondary to anterior uveitis and systemic disease.
The normal equine lens measures 20 mm in diameter, and has an axial length of 11–13.5 mm. Its volume is about 2.5–3.2 mL, and its optical power is about 14.88 D. The thickness of the lens capsule is similar to other species, with the anterior lens capsule the thickest (91 μm), the posterior capsule the thinnest (14 μm), and the equatorial capsule in the middle range (20 μm thick). Like other animal species, the thickness of the anterior lens capsule increases with age. Cataracts in horses are described as being congenital or juvenile-onset, inherited, post-traumatic, and post-inflammatory, with the last being the most common cause of cataract in the horse.
The pathophysiology of cataract surgery includes the formation of cataracts, the effects of surgical entry into the eye, the development of lens-induced iridocyclitis (immune-mediated) secondary to lens and cataractous lens material, and lens-induced iridocyclitis (physical effects) secondary to lens displacement and instability. Cataract formation in animals is incompletely understood. In congenital and inherited cataracts in the Miniature Schnauzer, the cataractogenesis has been partially defined, including early development, biochemical, and electron microscopic studies. In cataract formation, the lens fibers die probably secondary to abnormalities involving specific intracellular enzymes or structural defects of the lens membranes. Once lens fibers begin to die, clinical ‘vacuoles’ develop that eventually coalesce into ‘water clefts’. Once significant numbers of lens fibers die, the intracellular debris probably causes further lens fiber loss. As the lens functions as an osmometer, the breakdown of the large molecular weight proteins results in increased lens osmolarity, and water is imbibed into the lens causing translucency and eventually opacification. This process can occur rather quickly, and explains why some cataracts seem to progress slowly by slit-lamp biomicroscopy only to become opaque in a few weeks.
Surgical entry into the anterior chamber is usually through the peripheral cornea or the limbus under a limbal- or fornix-based conjunctival flap. The most frequent entry for cataract surgery in small animals throughout the early 1980s was the limbal approach. However, the peripheral corneal entry has gradually increased in popularity so that now it is the most common approach for cataract surgery in small animals. Once the anterior chamber is entered, IOP rapidly decreases. This causes release of endogenous prostaglandins from the iris and perhaps other substances (primarily from the anterior uveal tissues) that may result in miosis, breakdown in the blood–aqueous barrier (BAB), increased levels of proteins and fibrin in the aqueous humor, and once the surgical wound has been apposed by sutures, a transient increase in IOP. In the 1960s and early 1970s, this cascade of events after surgical entry of the anterior chamber was thought to be associated with release of histamines; however, pretreatment with antihistamines did not prevent the miosis and other effects. Fortunately, topical and systemic non-steroidal anti-inflammatory drugs (NSAIDs) inhibit the release of anterior uveal prostaglandins, delaying or decreasing these tissue events, and limit the miosis.
Miosis during cataract surgery can markedly impair and even prevent cataract or lens extraction. The small pupil prevents instrument and lens manipulations, limits the size of lens material delivered through the pupil, prevents visualization of the majority of the cataract, and increases the likelihood of direct surgical trauma to the iris, ciliary body, and even the anterior vitreous. As a result, the intensity of postoperative iridocyclitis is greater, control of the pupil is more difficult, and the overall surgical success rate is decreased. Use of preoperative mydriatics and NSAIDs, as well as intraoperative use of adrenaline (epinephrine), lidocaine, and viscoelastic agents, has resulted in miosis becoming a rare intraoperative complication.
Perhaps the most important single event that affects the intraoperative and long-term postoperative cataract surgery success rates in small animals is iridocyclitis. Lens-induced uveitis (LIU) is the most frequent type of anterior uveitis in the dog, and has been associated with all stages of cataract formation by fluorophotometer measurements of the blood–aqueous barrier. Hypermature cataracts in dogs are the most frequent type of cataract extracted. Unfortunately, the release of lens materials during this phase of cataract development and maturity results in a variable and sometimes intense pre- and postoperative LIU.
In the prenatal development of the lens, the embryonic lens is already well developed and is surrounded with anterior and posterior lens capsules before the embryo’s immune system becomes organized. Consequently, release of lens material (e.g., following a traumatic or surgical tear in the anterior lens capsule, or a spontaneous lens capsule rupture, most common in diabetes mellitus) or a hypermature cataract results in iridocyclitis. In cataractous canine eyes, the presence of cataractous lens material in the posterior and anterior chambers incites a progressively intense iridocyclitis. This disparity in the intensity of lens-induced iridocyclitis between a normal lens and cataractous material may be associated with the gradual loss of cataractous material, rather than acute exposure to normal lens material, the progressive sensitization of the uveal tissues to the cataractous material, and/or the greater antigenicity of cataractous versus normal lens proteins. A recent study using fluorophotometry in cataractous dogs indicated that the blood–aqueous barrier is changed in all types of cataract maturity, suggesting that LIU occurs very early in the development of lens opacification.
From the 1950s through the late 1970s, cataract surgery was recommended once the dog became blind from bilateral cataracts becoming mature and opaque. The rationale, with the prevailing 70–80% success rates for cataract surgery, was that the dog was blind from cataracts, and if the surgery was unsuccessful, the dog would still be blind. Unfortunately, in these same patients, the most advanced cataract had usually become hypermature, sensitizing the uveal tracts of both eyes to lens materials. Some of this lens material may remain even after the best cataract surgery. Unfortunately, dogs with hypermature cataracts, as well as the second eye cataract surgeries, have lower success rates than dogs selected with immature cataracts that still have some vision, a low likelihood of LIU preoperatively, and less intense iridocyclitis postoperatively. As a result, selection of dogs for cataract surgery continues to change, choosing dogs with immature cataracts and a higher possibility of successful restoration of vision.
Monitoring of IOP by periodic tonometry in cataractous dogs appears to be a convenient diagnostic procedure to detect early LIU. Tonometry in dogs with LIU usually reveals an IOP of less than 10 or 12 mmHg. Some conjunctival hyperemia may also be associated with iridocyclitis. Intravenous fluorescein will indicate that the blood–aqueous barrier is impaired, as the dye rapidly diffuses into the pupil from the ciliary body and into the anterior chamber from the anterior surface of the iris. These pupils often dilate slowly and incompletely to 1% tropicamide.
Preoperative treatment with topical corticosteroids and NSAIDs, and, if the anterior uveitis is intense, supplemented with these drugs systemically, usually controls the inflammation and results in a gradual increase in IOP to normal levels. Topical mydriatics, 1% tropicamide or 1% atropine, are instilled concurrently to dilate the pupil and prevent the formation of posterior synechiae.
With stimulation or reactivation of an existing iridocyclitis by cataract surgery, the plasmoid or secondary aqueous humor contains high levels of globulin, albumin, and fibrin. These proteins coat the posterior surface of the cornea, anterior and posterior surfaces of the iris, and aqueous humor outflow pathways. As a result, temporary or permanent iridal adhesions to the lens (posterior synechiae) and peripheral posterior cornea (peripheral anterior synechiae) are common in postoperative iridocyclitis in dogs and cats. The fibrin can also attach to remnants of the anterior lens capsule, posterior lens capsule, and anterior vitreous membrane, and form the scaffolding for other lens epithelia, fibrocytes, and iridal pigment cells to migrate on and establish permanent fibropupillary and capsular opacities. These inflammatory membranes can also crisscross the pupil, resulting in small and irregular pupils that limit vision.
The other important effect of postoperative iridocyclitis is miosis, secondary in part to the release of endogenous prostaglandins. Miosis starts during lens and cataract surgery, and continues thereafter until the anterior uveal inflammation from the entry of the anterior chamber and any remaining lens material has been resorbed or isolated. Mydriatics to achieve a moderately dilated and continuously moving pupil are started preoperatively and continued postoperatively.
Control and establishment of a moderately dilated pupil are usually obtained within 4–7 days postoperatively. After about 2 weeks, pupil changes (usually dilatation) are more difficult to obtain because of the formation of posterior synechiae. Frequency of daily instillations of a single or a combination of mydriatics is determined after periodic inspection of the eye and pupil size for the first 5–10 days postoperatively. Failure to control and obtain a reasonably sized pupil after cataract surgery usually contributes directly to most surgical failures. Topical atropine can reduce the rate of tear production to levels which result in acute keratoconjunctivitis sicca, and may affect both eyes. If corneal lesions develop soon after topical atropine instillations and low Schirmer tear test values are measured, cataract surgery should be delayed for several days. For an alternative mydriatic, 1% tropicamide may be used with apparently less effect on tear production.
The period of postoperative iridocyclitis after cataract surgery varies, but apparently spans several months. Continuing clinical investigations suggest that postoperative cataract patients should be treated topically (and occasionally systemically) with mydriatics, NSAIDs, and corticosteroids for 6 months or longer. Premature cessation of these cataract treatments contributes to smaller pupils, progressive fibropupillary membrane formation, and capsular fibrosis that cause a decline in the long-term success rates in small animals.
The relationship between the stability of the lens and its direct contact with the posterior surface of the iris and anterior hyaloid membrane is poorly understood. With the loss of 90–180° of the zonulary attachments to the lens, instability results. This lack of lens stability may be determined by close examination of the iris and its base during ocular movements to detect partial movement of the lens and the basal iris. Alternatively, after mydriasis, the periphery of the lens can be examined by slit-lamp biomicroscopy, and any instability ascertained. With the loss of additional zonules, more lens instability results. The role of the hyaloideocapsular (Wieger’s) ligament between the posterior lens capsule and the anterior hyaloid membrane in stabilizing the lens is unknown, but may be important in younger dogs and cats.
With instability of the lens, microtrauma of the iris results in iridocyclitis. Increased aqueous humor levels of fibrin, proteins, and inflammatory cells occur. The inflamed iris may adhere with formation of posterior synechiae to the unstable lens. Aqueous humor dynamics can also be impacted by lens instability, temporarily impairing pupillary passage of aqueous humor, and balloon the peripheral iris to embarrass the iridocorneal angle outflow pathways and contribute to the development of peripheral anterior synechiae.
With the loss of all zonulary attachments, the hyaloideocapsular attachments may tear, resulting in presentation of anterior vitreous through the rent, often partially adhering to the posterior lens capsule. The loose lens can remain in the patellar fossa, luxate into the anterior chamber or, through the torn anterior vitreal face, displace posteriorly into the vitreous.
With displacement of the lens from the patellar fossa and tearing of the anterior hyaloid membrane, vitreous can complicate these cascading events. Vitreous adhering to the posterior lens capsule with an anterior lens luxation may occlude the pupil, preventing pupillary flow of aqueous humor, and displace the base of the iris forward to cause iridocorneal angle and sclerociliary cleft closure. The complete effect on aqueous humor flow of this type of iris bombé is often missed as the peripheral lens masks the basal iris changes.
With posterior or vitreal luxation of the lens, the torn anterior vitreal membrane allows both liquid and formed (gel) vitreous access into the pupil and anterior chamber. Formed vitreous may cause pupillary blockage and secondary glaucoma. It can also adhere to the posterior cornea and iridocorneal angle. Blockage of the iridocorneal angle with formed vitreous sufficient to increase IOP is infrequent, and pupillary blockage is more common. Vitreous loss also seems associated with the development of retinal detachment; a major difference between extracapsular and intracapsular cataract extractions is the higher postoperative frequency of retinal detachment after the intracapsular procedure.