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.

Table 11.1 Different classifications for cataracts in animals

Classification	Description
Age of onset	Congenital
	Developmental (juvenile or adult)
	Senile
Position within the lens	Anterior capsular
	Anterior cortical
	Nuclear
	Posterior cortical
	Posterior capsular
	Equatorial and axial
Degree of opacification and lens size	Incipient
	Immature
	Mature (subdivision intumescent)
	Hypermature (subdivision Morgagnian)
	Resorbing
Possible cause	Inherited
	Associated with other ocular anomalies
	Secondary to retinal degeneration
	Secondary to trauma (direct/indirect)
	Viral
	Post-inflammatory
	Metabolic
	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.

Evolution of cataract surgery in dogs

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.

Evolution of cataract surgery in horses

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).

Development of the intraocular lens for the dog, cat, and horse

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.

IOL in the dog

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.

IOL in the cat

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.

IOL in the horse

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.

Surgical anatomy

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.