Published by Rachel (Mathes) Davis, DVM, MS, DACVO November 2016
Publication: Veterinary Information Network (VIN)
Cataract (or cataracts) refers to any opacification of the intraocular crystalline lens.1 The lens is composed of an external capsule made mostly of collagen and internal precisely organized lens fibers made of approximately 60% protein and 40% water. While the biomechanical and physiologic processes of the lens are very complex, the lens is composed of basically one substance, protein. Thus, the lens’s response to a variety of insults results in essentially one basic outcome, that of cataract formation. Cataract formation is, therefore, not specific to any underlying pathologic process, but merely denotes an abnormality of the lens.
Cataracts are typically characterized based on overall involvement of the lens and severity. Incipient cataracts affect <15% of the lens, immature cataracts involve 15-100% of the lens and are not dense enough to abolish the tapetal reflection, mature cataracts involve the entire lens and prevent the tapetal reflection and hypermature cataracts involve the entire lens and have advanced to resorption seen as hyperreflective, “sparkly” area within the lens.
Cataracts may be caused by a variety of underlying pathologic processes in cats including nutrition,2-3 genetic factors,4-6 trauma,7-9 metabolic disorders,10,11 radiation12 and intraocular inflammation.13-16 The most common cause of cataracts in cats is uveitis secondary to systemic disease.1 Uveitis may cause cataract formation or lens luxation, which also leads to cataract formation. Thus, cats with chronic uveitis disease may present with luxated, cataractous lenses.
Cataract formation secondary to diabetes mellitus is rare in cats.17,18 Cats have, overall, low levels of aldose reductase, an enzyme that promotes conversion of glucose to sorbitol, the cause of diabetic cataracts.19-21 Studies have shown that aldose reductase levels are even lower in cats older than four years compared to younger cats.20 Feline diabetic cataracts are rare likely due to the overall low levels of aldose reductase in cats versus dogs and the low incidence of diabetes in cats younger than 4 years.
Ophthalmic Examination Findings: Cataracts are seen as an opacification of the lens. This may be observed easily with retroillumination of the eye after pharmacologic pupil dilation (i.e. tropicamide or atropine administration topically). Pharmacologic mydriasis is contraindicated if the intraocular pressure is elevated. Evaluation of intraocular pressure prior to dilation in cats is strongly recommended, especially because this species is typically affected with cataracts secondary to other intraocular disease. Retroillumination is performed by aiming a focal light source into the eye at arm’s distance away and at the observer’s eye level. Small opacifications of the lens may be observed as dark or light areas within the lens (i.e. areas of opacification that are impeding passage of light back) (figure). Retroillumination will also allow the observer to evaluate for a tapetal reflection (or fundic reflection in atapetal cats), thereby allowing differentiation between immature and mature cataracts if the entire lens is affected. Direct focal illumination with the light source close to the eye and the observer in front of the eye may also be performed and incipient or immature cataracts will appear as “white” areas within the lens or, if the entire lens is involved, the whole lens to appear as white.
Ophthalmic evaluation with retroillumination and direct focal illumination will help differentiate cataracts from nuclear sclerosis, a normal aging change of the lens caused by increased density of nuclear lens fibers. The increased lens fiber density imparts a gray or milky color to the lens. Unlike cataracts, nuclear sclerosis does not cause impedance of light passage through the lens. Thus, the tapetal reflection will not be altered nor will the lens appear as white using retroillumination and direct focal illumination respectively.
Identification of the location of the cataract within the lens and further characterization of the cataracts typically require advanced training and slit-lamp biomicroscopy performed by a veterinary ophthalmologist.
Physical Examination Findings: Because most cases of feline cataracts are secondary to uveitis related to systemic disease (e.g. infectious, neoplastic, autoimmune), systemic work up and diagnostics are warranted.1 A full physical examination, clinical history and, when warranted, a comprehensive hematologic and blood chemistry, imaging and retroviral testing should be pursued for feline patients presenting with cataracts.
Disease Description in this Species
Any breed, sex or age of cat may be affected by cataracts.
White static or progressive opacification of the lens. See ophthalmic examination.
Ophthalmic examination – See ophthalmic examination
Slit-lamp biomicroscopy – Full ophthalmic evaluation using slit-lamp biomicroscopy with a veterinary ophthalmologist is warranted for some patients affected by cataracts, especially if the owner is pursuing or interested in cataract surgery or if concurrent ocular disease is present.
Ocular ultrasound – Evaluation for cataracts is best performed by using multiple types of illumination during ophthalmic examination with pharmacologic mydriasis; however, ocular ultrasound may be used to evaluate for cataracts or lens integrity if anterior ocular pathology precludes evaluation of the lens partially or entirely. Cataracts are seen ultrasonographically as hyperechoic uniform or irregular opacification within the lens. High-resolution ocular ultrasound may also be used to identify lens capsular ruptures or tears prior to cataract surgery or as a cause of fulminate uveitis.
Systemic Evaluation – A full physical examination, clinical history and, when warranted, a comprehensive hematologic and blood chemistry, imaging and retroviral testing should be pursued for feline patients presenting with cataracts.
The standard-of-care for complete cataracts causing vision loss is lens removal using phacoemulsification with intraocular artificial lens implantation.7 Often, in cats, concurrent and potentially serious intraocular disease precludes cataract surgery. Thus, studies reporting case series of cats undergoing phacoemulsification and intraocular lens implantation are lacking. Experimental studies have shown that a stronger intraocular lens implant is needed to bring cats to emmetropia after cataract surgery compared to dogs.22,23
Identification of underlying systemic and concurrent ocular disease is important in cats that are diagnosed with cataracts. Often, systemic work up is warranted. Topical steroidal medication (e.g. dexamethasone, prednisolone acetate) and topical non-steroidal medication (e.g. flurbiprofen, diclofenac, ketorolac) may be used BID-TID together or individually to treat chronic or acute uveitis in cats. This is important to help prevent secondary glaucoma. If glaucoma is concurrently present, caution should be used with high-frequency topical anti-inflammatory therapy as both topical non-steroidal and steroidal medications may exacerbate intraocular pressure elevation in cats.24-26 Topical steroids may also cause feline herpes virus-1 (FHV-1) recrudescence.27 Because most cats carry this virus in a latent form, the clients should be educated about signs of FHV-1 activity if their cat is placed on topical steroidal therapy and cessation of these medications should be pursued if FHV-1 recrudescence is suspected. Prior to institution of long-term topical steroid therapy, fluorescein staining and evaluation of the pre-ocular tear film should be performed as steroids may worsen corneal ulceration or promote corneal malacia. For feline patients with multiple ocular diseases, referral to a veterinary ophthalmologist should be considered for management.
Monitoring and Prognosis
Because cataracts may cause vision loss or blindness, client education regarding monitoring for progression of incipient or small immature cataracts is warranted. In addition, because feline cataracts are most often caused by other underlying systemic or ocular disease, further evaluation systemically is warranted. Referral to a veterinary ophthalmologist should be considered for feline patients affected by cataracts, especially if there is other ocular pathology.
Evaluation of intraocular pressure Q4-6mo is warranted to monitor for glaucoma. Glaucoma secondary to chronic uveitis tends to be poorly responsive to topical anti-tensive therapy, thus enucleation should be considered for blind, painful globes with advanced disease.
- Nuclear sclerosis
- Lens luxation
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