Outcomes of Dogs Presented for Cataract Evaluation: A Retrospective Study
Two hundred and forty-four dogs were evaluated for cataracts at the University of Tennessee from January 2001 to December 2002. Fifty-four canine breeds were affected. Odds ratios for cataracts were significantly higher for six purebred dogs (including the cocker spaniel, miniature schnauzer, toy poodle, Boston terrier, miniature poodle, and bichon frise) compared with mixed-breed dogs. One hundred fifty-nine dogs did not have cataract surgery. The most common reason for dogs not having surgery was retinal degeneration (n=66).
Introduction
Cataracts are the most common cause of treatable blindness in dogs.1 Focal cataracts may not affect vision, but severe cataracts cause blindness by disrupting the ability of the lens to focus light rays onto the retina. Proven or suspected primary cataracts are common in purebred dogs including the toy poodle, miniature and standard poodle, English cocker spaniel, Boston terrier, golden retriever, rottweiler, Labrador retriever, miniature schnauzer, bichon frise, and chow chow.2–10 Cataracts can occur at any age; however, the prevalence of cataracts increases with age in dogs.11 Genetic abnormalities are the most common cause of cataracts.12 Diabetes mellitus is the second most common cause of cataracts in the dog.13,14 Other, less common causes include other metabolic diseases, toxins, drugs, congenital abnormalities, dietary deficiencies, trauma, radiation therapy, and aging.15–21
A commonly used classification system divides cataracts into four categories based on the degree of opacification of the lens.12 The opacification with incipient cataracts involves <10% to 15% of the lens [Figure 1].12 Immature cataracts vary widely in appearance and may affect only a small portion of the lens to almost the entire lens [Figures 2, 3]. By definition, a tapetal reflection is apparent with immature cataracts, although vision may be diminished if the cataract is well developed. A mature cataract affects the entire lens.12 A mature cataract appears as a solid white opacity of the lens, and affected dogs are severely impaired visually [Figure 4]. Hypermature cataracts have undergone liquefaction and resorption.21 Hypermature cataracts often have a crystalline appearance with wrinkling of the lens capsule [Figure 5]. A tapetal reflection may or may not be present. Vision with hypermature cataracts is variable depending upon the degree of liquefaction.
Lens-induced uveitis may arise with cataracts from the release of antigenic lens proteins through the lens capsule.21,22 As canine cataracts progress, there is a reduction in the amount of antilens crystalline antibody detected in serum.23 This negative association is surprising, but it may reflect altered lens proteins not detected by the enzyme-linked immunosorbent assay used.23 Lens-induced uveitis occurs most commonly in the miniature poodle, toy poodle, and cocker spaniel, although it can be seen in any breed.22 Surgery may be difficult or impossible in cases of severe lens-induced uveitis because of accompanying miosis, posterior synechia, intense inflammation, phthisis bulbi, or secondary glaucoma.22 Additionally, the prognosis for long-term vision after surgery is lower in dogs with preoperative lens-induced uveitis, owing to an increased risk for retinal detachment and glaucoma postoperatively.22
The most common treatment for cataracts is surgical removal by phacoemulsification.1 At the University of Tennessee, the preferred stage of cataract submitted for phacoemulsification is the immature cataract. Tests indicated prior to surgery include a complete ophthalmic examination, a minimum laboratory database (i.e., complete blood count, biochemical panel, urinalysis), electroretinography (ERG), and ocular ultrasonography.24 The ERG assesses retinal function and is important because long-term visual rehabilitation is not possible in dogs with concurrent retinal degeneration. Many dog breeds are genetically predisposed to both cataracts and progressive retinal atrophy.25 Funduscopic signs of retinal degeneration include tapetal hyperreflectivity, optic nerve pallor, retinal vascular attenuation, and depigmentation of the nontapetum.26 Funduscopic signs of retinal degeneration may be visible in dogs with only mild cataract formation. Early referral of a dog with cataracts for evaluation by an ophthalmologist helps detect these changes without an ERG. Retinal detachments are common in dogs with hypermature cataracts and can be diagnosed by ocular ultrasonography.24 Ocular ultrasonography may detect retinal detachments that are not extensive enough to alter ERG results.27,28
The purposes of this retrospective study were to identify and quantify the reasons that dogs referred for cataract evaluation did not proceed to surgery.
Materials and Methods
The medical records of dogs presented to the University of Tennessee College of Veterinary Medicine ophthalmology service for cataract evaluation from January 2001 to December 2002 were reviewed. Data collected included breed, age, sex, presence and stage of cataract, the bilaterality of stage of the cataracts, the presence of glaucoma, presence of anterior uveitis (as determined by episcleral injection, ocular hypotension, aqueous flare, or resistance to mydriasis), and presence of any concurrent systemic diseases. The ultrasound and ERG results were recorded if these tests were performed. In addition, if the dog did not have surgery, the reasons were also recorded.
Two methods were used to diagnose retinal degeneration. The first method was a clinical diagnosis based on indirect ophthalmoscopic findings of tapetal hyperreflectivity, optic nerve atrophy, retinal vascular attenuation, and nontapetal depigmentation.26 This method was not used if the fundus could not be visualized through the cataractous lens. The second method of diagnosing retinal degeneration was based on scotopic ERG results.a According to the standard protocol at the University of Tennessee, the scotopic ERG results were considered normal if the b-wave amplitude was ≥100 microvolts. The results were considered questionable if amplitude was <100 microvolts, and retinal degeneration was presumed if amplitude results were <70 microvolts. If the cataract did not preclude visualization of the fundus by indirect ophthalmoscopy and clinical signs of retinal degeneration were present, an ERG usually was not done.
Statistical evaluations were performed with a commercially available software program.b Breeds with >30 dogs in the hospital population during the review period and with at least two dogs evaluated for cataracts were compared with mixed-breed dogs by using a chi-square test.29 Odds ratios were calculated, and confidence intervals were adjusted to the level of 99.9995% for 18 individual comparisons by the Bonferroni method.29 Odds ratios for each breed were considered significantly greater or less than the reference population if the adjusted confidence interval did not include 1.00. Exact confidence limits around the odds ratios were reported when one or more expected cell counts in the 2 ×2 table were <5; otherwise, asymptotic confidence limits were reported.
Results
Two hundred forty-four dogs were presented to the University of Tennessee for a cataract evaluation from January 2001 to December 2002. The average age was 8 years (range 6 months to 16 years). There were 25 intact males, 84 neutered males, 11 intact females, and 124 spayed females. Of the 488 eyes examined, 443 had cataracts, and 39 eyes had no cataracts. Four eyes had been enucleated, and two eyes were phthisical. Of the 39 eyes that did not have cataracts, 17 had moderate to severe nuclear sclerosis that may have been misinterpreted as cataracts.
Eleven of the 54 affected breeds were represented by five or more dogs [see Table]. Labrador retrievers on average were the youngest dogs evaluated (average age 5 years). Miniature poodles, toy poodles, and shih tzus were the oldest dogs, with an average age at evaluation of 10, 11, and 11 years, respectively. Odds ratios for cataracts were significantly higher for six breeds, including the cocker spaniel (2.6), miniature schnauzer (3.7), toy poodle (6.1), Boston terrier (4.2), miniature poodle (4.3), and bichon frise (5.2), compared with mixed-breed dogs.
All stages of cataracts were represented, including incipient (n=42), immature (n=200), mature (n=86), and hypermature (n=115) cataracts. Lens-induced uveitis was present in five (0.06%) of the eyes with mature cataracts and in 49 (43%) eyes with hypermature cataracts. The stage of cataract was bilaterally symmetrical in 125 dogs.
Forty-two dogs had diabetes mellitus, with 19 breeds represented. Four of the miniature pinschers evaluated for cataracts were diabetic. Additional systemic conditions included cardiac, metabolic, and dental diseases. No detectable systemic diseases were found (based on physical examination and routine blood work) in 135 dogs.
Sixty-four (26%) dogs underwent bilateral phacoemulsification surgery; 21 (9%) dogs had unilateral surgery; and 159 (65%) dogs did not have cataract surgery. The unoperated eyes in the dogs having unilateral surgery were diagnosed with retinal detachments (n=5), incipient cataracts (n=4), hypermature cataracts (n=2), retinal degeneration (n=1), primary glaucoma (n=1), or secondary glaucoma (n=1). Four of these unoperated eyes were normal, and one had been enucleated. One dog had anesthetic complications, and phacoemulsification was stopped after the first eye. Surgery was also aborted in one dog because of a malfunction of the phacoemulsification machine.
The reasons surgery was not done at all in 159 dogs included retinal degeneration (n=65), incipient stage of cataracts (n=17), very immature stage of cataracts (n=17), keratoconjunctivitis sicca (n=6), retinal detachment (n=6), anterior uveitis with extensive posterior synechia (n=4), corneal endothelial dystrophy (n=2), and glaucoma (n=8). Other, less common reasons dogs did not proceed to surgery were unregulated diabetes mellitus (n=3), systemic neoplasia (n=3), heart failure (n=2), severe dental disease (n=4), aggressive temperament (n=3), and heartworm disease (n=1). Nuclear sclerosis, rather than cataracts, was diagnosed in 17 dogs.
Twenty-eight dogs were diagnosed with retinal degeneration by ERG, and 33 dogs were diagnosed based on the presence of retinal vascular attenuation and diffuse tapetal hyperreflectivity. Two dogs with a clinical diagnosis of retinal degeneration also had retinal degeneration based on ERG results. One of these dogs had an immature cataract in one eye and a normal lens in the other eye. The other dog had a history consistent with sudden acquired retinal degeneration. Over 35% of the cocker spaniels, toy poodles, Yorkshire terriers, and shih tzus had retinal degeneration as determined by indirect ophthalmoscopy or ERG. In dogs rejected for surgery because of retinal degeneration, the average b-wave amplitude was 40 microvolts. A total of 11 retinal detachments were diagnosed. Ten detachments were diagnosed in eyes with hypermature cataracts, and the detachments occurred in nine different breeds.
Nineteen of 42 diabetic dogs had surgery. Reasons for not performing surgery included poor glycemic control (n=3), retinal degeneration (n=5), retinal detachment (n=2), keratoconjunctivitis sicca (n=3), chronic heart disease (n=2), and monetary concerns (n=3).
While many dogs with incipient or early immature cataracts were lost to follow-up, nine dogs were followed for an average of 12 months with no evidence of progression. Four dogs with incipient cataracts seen early in the study had surgery later because of progression of the cataracts.
Discussion
The signalment of the dogs in this study was similar to prior reports.14,27 The majority of the dogs were middle-aged, but there was a wide range of ages at the time of evaluation. The significantly higher odds ratios for cataracts in the cocker spaniel, miniature schnauzer, toy poodle, miniature poodle, Boston terrier, and bichon frise, compared with mixedbreed dogs were similar to other studies that demonstrated cataracts more often in the poodle, cocker spaniel, and bichon frise.7,14,27 More affected dogs were neutered than intact in the present study, which reflected the general trend at the study institution.
The majority of cataracts in this study were immature. At the study institution, phacoemulsification is usually performed on immature cataracts. Incipient cataracts and cataracts that are not likely to progress are periodically monitored rather than removed, especially when they are not causing visual deficits. The second-largest group of cataracts was the hypermature group. Some of the dogs with hypermature cataracts were probably evaluated for ocular discomfort from their anterior uveitis. Some owners had known of the cataracts for years but had received recommendations to wait until the cataracts were mature before having their dog evaluated by an ophthalmologist. Many of these dogs may have been good surgical candidates had they been referred earlier in the course of their disease, but their severe anterior uveitis and/or glaucoma made them poor candidates for phacoemulsification at the time of evaluation. Historically, it was common to delay extracapsular cataract extractions until cataracts were mature; but with the advent of phacoemulsification, surgery is now performed much earlier, and it is preferable to refer dogs for evaluation when they have incipient or immature cataracts.
Typically, lens-induced uveitis is associated with hypermature cataracts that are leaking lens crystalline proteins, but lens-induced uveitis can occur with all stages of cataracts.30 The diagnosis of lens-induced uveitis is based on the presence of episcleral injection, aqueous flare, miosis, and decreased intraocular pressure in an eye with a cataract and no evidence of another cause of the uveitis.30 In this study, lens-induced uveitis was diagnosed in five (0.06%) of the eyes with mature cataracts and in 49 (43%) of the eyes with hypermature cataracts. In a prior study, lens-induced uveitis was present in 70% of eyes with hypermature cataracts, and it was also noted in some eyes with immature, mature, and intumescent cataracts.22 Although lens-induced uveitis does not preclude cataract surgery, it must be treated and controlled prior to surgery. Occasionally, lens-induced uveitis is so chronic or severe as to cause ocular changes that preclude surgery, such as extensive posterior synechia, secondary glaucoma, and phthisis bulbi.22 Eyes with lens-induced uveitis have a poorer prognosis for vision after cataract surgery because of pupillary occlusion, retinal detachment, and secondary glaucoma.22
At the time of cataract diagnosis, 17% of the dogs in this study had diabetes mellitus. One study showed that 75% of diabetic dogs develop cataracts within a year of diagnosis.14 While only five miniature pinschers were evaluated for cataracts in the study reported here, four of them were diabetic. In a previous study of breed distribution of dogs with diabetes mellitus, none of the dogs were miniature pinschers.31 The reasons diabetic dogs did not have cataract surgery were similar to the rest of the dogs in the study.
Evidence of retinal degeneration as diagnosed by indirect ophthalmoscopy or ERG was the most common reason that a dog did not have cataract surgery. While retinal degeneration is a nonspecific diagnosis, a large percentage of the dogs with retinal degeneration in this study were cocker spaniels, toy poodles, Yorkshire terriers, and shih tzus, suggesting that inherited progressive retinal atrophy was the most likely cause of retinal degeneration in this study.32,33
Only 11 (4%) eyes did not have surgery because of the presence of a retinal detachment. This was in contrast to a previous study of dogs with cataracts, where 11% of eyes examined by ultrasonography had a retinal detachment.27 This discrepancy may have arisen from the different order in which diagnostic tests were done. The dogs in the study reported here had ERGs done prior to ultrasonography, so if a dog had any retinal disease that altered the ERG, an ultrasound was not performed. A retinal detachment would have been detected in the study by Van der Woerdt et al., because the dogs underwent ultrasonography prior to ERG.27 Both studies found that most retinal detachments occurred in eyes with hypermature cataracts, and no breed predilections were noted.27
It was interesting to note that only 85 of 244 (35%) dogs underwent phacoemulsification to remove their cataracts, which was a low percentage of dogs evaluated for cataracts. Unfortunately, the most common contraindication for cataract surgery (i.e., retinal degeneration) usually cannot be diagnosed in general practice. Referring dogs early in the course of their disease and educating owners that the referral is for a complete ocular examination may better define which dogs are candidates for surgery and the optimal time for surgery.
Conclusion
Mixed-breed dogs were the most common type of dog presented for cataract evaluation in the present study, followed by the cocker spaniel, miniature schnauzer, and toy poodle. Only 35% of the dogs evaluated for cataracts underwent cataract surgery. The presence of retinal degeneration was the most common reason that dogs did not have cataract surgery.
Nicolet Viking IV; Nicolet Biomedical, Inc., Madison, WI 53744-4451
SAS statistical software, version 8.02; SAS Institute, Inc., Cary, NC 27513-2414
Citation: Journal of the American Animal Hospital Association 41, 4; 10.5326/0410235
Citation: Journal of the American Animal Hospital Association 41, 4; 10.5326/0410235
Citation: Journal of the American Animal Hospital Association 41, 4; 10.5326/0410235
Citation: Journal of the American Animal Hospital Association 41, 4; 10.5326/0410235
Citation: Journal of the American Animal Hospital Association 41, 4; 10.5326/0410235
Photograph of the left eye of an adult, mixed-breed dog with an irregular opacity in the anterior axial cortex of the lens (arrowhead). This incipient cataract involves <10% of the lens, and there is no attenuation of the tapetal reflection.
Photograph of an early, immature cataract of the right eye in an adult, castrated male cocker spaniel. A triangular-shaped opacity is present in the dorsal anterior lens cortex (large arrowhead), and a radial spoke-like opacity is visible at the four o’clock position (small arrowhead). In the background, a slightly out-of-focus posterior cortical opacity can be seen, and there is very mild attenuation of the tapetal reflection.
Photograph of a late, immature cataract of the right eye in a 4-year-old golden retriever. The nucleus of the lens (arrowhead) is more opaque than the cortex. A gold, tapetal reflection can be seen around the nucleus.
Photograph of a mature cataract of the left eye in a 12-year-old, castrated male Australian shepherd. The entire lens is opaque, and there is no tapetal reflection.
Photograph of a hypermature cataract of the left eye in a 13-year-old West Highland white terrier. Liquefied cortical material (small arrowhead) is present dorsal to the lens nucleus (large arrowhead). The dorsal aspect of the nucleus is irregular and pitted. Posterior synechiae and pigment deposition on the anterior lens capsule are visible in the four to eight o’clock position.
