Cataract surgery in veterinary medicine today (Proceedings)
It is important to recognize qualities that affect candidacy for cataract surgery in our small animal patients.
It is important to recognize qualities that affect candidacy for cataract surgery in our small animal patients. Although dogs more commonly have hereditary and diabetic cataracts than others domestic species, and so benefit from phacoemulsification lens extraction more often, potentially any species can benefit from cataract surgery. To warrant cataract extraction surgery, animal patients must have sufficient visual compromise to warrant taking a small risk of blindness from complications. However, it is a myth that cataracts need to "mature" before extraction. In fact, with today's technology, postoperative vision (with intraocular lens placement) can be expected to closely approximate "normal" barring significant complications and early cataract removal actually warrants an improved prognosis compared to removal of chronic cataracts in dogs.
Stable small (incipient) and early immature cataracts may not warrant removal, especially if the contralateral lens is clear, due to their less significant effect on vision. Late immature and mature cataracts are typically good candidates for extraction. It is highly beneficial for vision to remove hypermature cataracts, but postoperative risks are greater with removal of these advanced chronic cataracts. Nuclear lenticular sclerosis is a normal age-related cloudiness of the nuclear lens caused by continued deposition of cortical lens fibers over time, and which begins to become clinically noticeable around 8 years of age in the dog. Nuclear sclerosis is most easily diagnosed following pharmacological dilation as bilaterally symmetrical nuclear cloudiness surrounded by clear peripheral cortex. A fundic exam can be performed the through lens nucleus, which is cloudy, but not opaque. Only rarely do very advanced cases of nuclear sclerosis cause secondary changes (senile cataract) and visual compromise sufficient to warrant cataract surgery if the patient is healthy enough to safely undergo the necessary general anesthesia.
Although older and/or ill patients are not precluded from candidacy for surgery, it is important to remember that this is elective surgery. Any significant underlying systemic condition (diabetes mellitus, hypertension, Cushing's disease, renal insufficiency, heart disease, etc.), should be as accurately diagnosed and as well-controlled as possible prior to surgery and potential risks of general anesthesia discussed at length with the client. In fact, one of the most common indications for cataract extraction in canine patients is for the bilateral blinding mature cataracts that develop secondary to diabetes mellitus. In most cases, demonstration of adequate diabetic control (blood glucose curve or fructosamine) prior to surgery is indicated. That being said, some diabetic patients develop cataracts so rapidly that severe progressive lens swelling develops, which without rapid surgical intervention can lead to lens capsule rupture and/or development of glaucoma. In these cases, the luxury of waiting is lost and "emergency cataract surgery" may be indicated if there is to be hope of maintaining vision long-term.
Positive clinical signs suggesting that cataract surgery will provide visual benefit include the presence of a strong dazzle reflex, strong direct and indirect pupillary responses, and presence of good vision before cataract development. Most veterinary ophthalmologists will require preoperative scotopic electroretinogram (ERG) and ocular ultrasound to confirm retinal function, often in combination with gonioscopy to evaluate iridocorneal (ICA) angle conformation. Animals that have low amplitude or "flat" ERGs, or retinal detachment usually are not candidates for cataract surgery. Those with significantly abnormally formed ICAs are at increased risk of postoperative glaucoma development, and although foreknowledge of this is useful, further client education is required before pursuing surgery.
In the lecturer's opinion, all small animal patients with advanced cataracts (but not those with lenticular sclerosis or incipient cataracts) should receive ongoing topical ant-inflammatory therapy even if anterior uveitis is not clinically evident. All significant cataracts have associated lens-induced uveitis (LIU) caused by leakage of lens protein through the lens capsule. Lens proteins are normally sequestered from the immune system, but cataract formation results in their leakage into the eye, and exposure to the immune system, which reacts to them similarly as to a foreign object. Uncontrolled LIU results in decreased surgical prognosis, so it is important to start treatment for LIU right away, even if surgery will be delayed. In fact, even low-grade uncontrolled LIU probably results in a cumulative risk of developing secondary glaucoma, so even if the clients would never consider cataract surgery, you should still treat. Aggressive treatment may be needed initially, followed by a taper to the lowest effective dose.
Phacoemulsification technology provides many advantages over other more antiquated cataract extraction techniques and is routinely used today for cataract surgery in both humans and dogs. It uses ultrasound technology to break up cataract (laser cataract surgery is a myth). Distinct advantages include rapid lens removal through a small incision while keeping the globe continuously inflated. For this procedure, my patients are placed in dorsal recumbency under general anesthesia. The eye is positioned under an operating microscope and routinely prepared with dilute betadine. The extraocular muscles are paralyzed via IV atracurium for optimal globe positioning and the patient placed on a mechanical ventilator. A small dorsal perilimbal corneal groove incision is placed, the anterior chamber entered with a 23 guage needle, which is also used to puncture the anterior lens capsule in a J-shaped pattern at about 11:00. Viscoelastic agent is used to refill the anterior chamber and protect the endothelium. A 2.8 mm disposable keratome is used to enter AC and special forceps are used to grasp the anterior lens capsule flap and perform a controlled continuous circular tear (capsullorhexis),which results in a round anterior capsular defect.The lens may then be loosened from its overlying capsule by injection of fluid (hydrodissection) through a special cannula. A bevel-tipped phacoemulsification handpiece is then smoothly inserted through the corneal incision and lens capsular window. Phacoemulsification power is used to break up the lens nucleus and sculpt it away.The same handpiece aspirates out the lens fragments. An irigation/aspiration handpiece may be used to simply, and more safely, aspirate out the remaining soft peripheral lens cortex. It is important to take great care when approaching the posterior lens capsule, because it is exceptionally thin. Providing the lens capsule remains intact, an artificial intraocular lens (IOL) can be placed into the lens "capsular bag." Foldable IOLs are commonly used now and are advantageous in that they can be injected through a small corneal incision. Standard strength IOLs are placed (41 Diopters for dog, 52 D for cat) which make an average of the species emmetropic. A dog left aphakic can see,but is far-sighted by 14 D.
As with any surgical procedure, complications can occur. Intraoperative complications may include lens capsule tears and hyphema. Spontaneous or iatrogenic lens capsule tears may preclude placement of an IOL. A posterior one may allow liquified vitreous to move anteriorly. Since vitreal strands are integrally attached via vitreous body to the retina, a special attachment (vitrectomy handpiece) may be used to cut vitreal strands and remove them, reducing the risk of retinal detachment. Small amounts of hemorrhage in anterior chamber often can be flushed out or self-resolve post-operatively. If a large clot develops, drainage through the iridocorneal angle cannot occur. Tissue-plasminogen activator (TPA) can be injected intracamerally a day or so later to dissolve an established clot. Vitreal bleeds are less common, may be associated with retinal detachment, and don't resolve as readily. A common short-term complication is post-operative ocular hypertension (POH). This term describes a well-documented elevation of IOP within the first 24 hours or so of intraocular surgery. It is related to multiple factors, including the viscoelastic agent blocking iridocorneal angle. It offers the same immediate danger to vision as true glaucoma, it has no relation to true (chronic, progressive) glaucoma, so if it is successfully treated before significant vision loss occurs, there should be no residual consequence. Glaucoma and retinal detachment are the most severe postoperative complications and can occur at any time. Glaucoma risk is <10% for at least the first 3 years. Boston Terriers and Cocker Spaniels are at increased risk. Dogs with hypermature cataracts are more likely to develop glaucoma. It may occur less often with IOL placement. Although medical and/or surgical treatment may be successful short-term, it is ultimately a progressive, blinding, painful condition. Retinal detachment is fortunately an uncommon (<=1-2%) and painless occurrence, but is usually visually devastating unless reattachment surgery is pursued promptly (J Am Vet Med Assoc. 2006 Jan 1;228(1):74-9). A retrospective of canine cataract surgery revealed that clients that did not present their dogs for follow-up exams were more likely to be unhappy with the outcome, suggesting that prompt detection and treatment of complications may be key to extending postoperative success (J Am Vet Med Assoc. 2006 Mar 15;228(6):870-5). Although regrowth of cataract is a myth, posterior capsular opacification (PCO) or "after cataract" is a common long-term complication. This occurs due to fibrosis of the lens capsule causing mild to moderate visual obstruction. Young age at time of cataract surgery is a risk factor associated with increased severity of this complication, whereas placement of an IOL decreases it. Only the most severe cases of PCO may warrant treatment by surgery or NdYAG laser.
Vision-threatening cataracts are considered a surgical disease by the ophthalmology community, although complicating factors can be treated medically. Recently, there has been heavy marketing of N-acetyl-carnosine eye drops to veterinarians and pet-owners. The theory assumes that the antioxidants can enter the eye through topical application and act there to repair DNA and realign lens fibers more normally. In fact, a recent study of its use in dogs (Veterinary Ophthalmology 9 (5), 311-316) demonstrated only about a 5% decrease in opacity of immature cataracts and nuclear sclerosis over 8 weeks given TID. No significant improvement was seen with mature cataracts. Interestingly, 78% clients perceived improvement in vision, I presume due to a placebo affect in a biased population (the people using it were motivated to avoid surgery).