Update on cataract surgery (Proceedings)

A cataract is technically any opacity in the lens; it may be a small focal spot or, at the other end of the spectrum, the whole lens may be involved. The most common causes of cataracts in dogs are hereditary and diabetes mellitus. In cats the most common cause is uveitis. All cataracts lead to intraocular inflammation or lens-induced uveitis (LIU). This occurs from leakage of altered lens proteins through the lens capsule. Normally, the lens proteins are sequestered from the intraocular environment, but with cataract formation they may leak through the capsule and the ocular immune system "sees" them as a foreign object leading to inflammation.

Cataract surgery has evolved and progressed since it was first developed in the 1960's. Today many techniques and improvements in equipment and intraocular lens implants allow more patients to be surgical candidates. Overall, cataract surgery is considered to have a 90-95% success rate.

A complete ophthalmic examination, including visualization of the cataract following dilation, is the initial step in evaluating a patient for cataract surgery. Good prognostic factors include a positive menace response, a brisk dazzle reflex and good PLR's. Additionally, notation in the client history of previous good vision may also help in evaluating the patient for surgery. If the cataract formation is minimal then surgery may not be recommended or re-evaluation of the cataract at a specific time point may be recommended. It is important to differentiate advanced lenticular sclerosis from cataract formation. Lenticular sclerosis may lead to depth perception issues and increased glare but does not generally necessitate surgery. Very advanced lenticular sclerosis may progress to a senile cataract and visual disturbance; in those cases surgery may improve vision, however there may be anesthetic risks associated with systemic disease as many of these patients are very advanced in age. Lenticular sclerosis may be diagnosed after inducing mydriasis. Central or lens nucleus cloudiness with a clear peripheral lens cortex is usually noted and importantly a fundic reflex is noted through the whole lens.

Before a patient is a candidate for surgery the LIU must be controlled. Additionally any systemic diseases or issues should also be addressed to make the patient the best anesthetic candidate possible. The age of a patient is usually not a limiting issue, however systemic disease, especially diabetes mellitus, hypertension, and Cushing's disease, if not controlled, may lead to significant post-operative intraocular complications. Pre-surgical testing includes blood glucose curves, fructosamine levels, normal ACTH stimulation responses, normal systolic blood pressure, etc... as well as routine CBC and Chemistry submission. As mentioned above, diabetes mellitus should optimally be controlled prior to surgery unless the cataract has formed so quickly that the cataract itself is creating blinding intraocular inflammation and making the eye a non-surgical candidate. In those cases the owner must be counseled on the necessity as well as the risks of immediate surgery before the eye becomes a non-surgical candidate.

Lens-induced uveitis is controlled by using topical steroidal and/or non-steroidal anti-inflammatories. Prior to cataract surgery the frequency of the anti-inflammatory drops is increased. Glaucoma medications may also be instituted to address post-surgical glaucoma risk. Long term topical anti-inflammatory medication is necessary in all patients with LIU in the lecturer's opinion. Lack of uveitic control may lead to glaucoma or zonular degeneration and lens luxation. Uncontrolled LIU may decrease the surgical prognosis so treatment should be started in a timely manner.

Most veterinary ophthalmologists perform a scotopic electroretinogram (ERG) and ocular ultrasound, after controlling the lens-induced uveitis, for cataracts deemed surgical candidates. Low amplitude or "flat" ERG's are consistent with retinal dysfunction and surgery would not be recommended. Similarly, retinal detachments diagnosed on ocular ultrasound would preclude surgery for the cataract. Gonioscopy is also routinely used to evaluate the eye and the risk of glaucoma prior to surgery. A wide open or normal iridocorneal angle (ICA) with normal pectinate ligament architecture is ideal. Narrowing of the ICA or goniodysgenesis is a potential complication that needs to be addressed with the owners prior to surgery. Approximately 5%-10% of patients develop post-operative glaucoma so in predisposed candidates the eye may be treated with additional medication or alternatively prophylactic glaucoma surgery is an option at the same time as cataract surgery.

Surgery is recommended on immature and newly mature vision-compromising cataracts as it results in a shorter surgery time, fewer chronic changes associated with LIU and a faster healing time. The newer techniques and equipment have eliminated the need to wait until the cataract is mature or "ripe".

Cataract surgery is an out-patient procedure although some patients may benefit from overnight hospitalization to monitor intraocular pressures (IOP). The morning of surgery the pupil is dilated and additional pre-operative medications are administered. After inducing anesthesia the patient is placed in dorsal recumbency and positioned under the operating microscope. The extraocular muscles are paralyzed using iv atracurium and the patient is maintained on a mechanical ventilator. An approximately 3mm dorsal limbal corneal groove is created. The anterior chamber (AC) is entered and trypan blue is used to stain the anterior lens capsule. The AC entry is enlarged using a 2.8 mm keratome. The trypan blue is irrigated out of the AC and the AC is then maintained with viscoelastic which also protects the corneal endothelium. A capsulotomy is created using a 22 gauge needle or special scissors to facilitate a continuous curvilinear capsulorrhexis (a circular hole in the anterior lens capsule). The cataractous lens may loosened from the surrounding capsule using saline irrigation through a special curved cannula; this is called hydrodissection. A special beveled needle is attached to the phacoemulsification handpiece and the needle and handpiece are inserted through the corneal incision and through the capsulorrhexis opening to fragment the lens. Vacuum and ultrasound energy are used to break up the cataractous lens. The same handpiece aspirated the lens fragments from the eye. As mentioned previously a second instrument in the AC may facilitate phacoemulsification and shorten surgery time. The residual lens cortex is then removed from the eye via irrigation and aspiration which utilizes a second handpiece on the phacoemulsification machine. An artificial lens or IOL is injected into the lens capsule. 42D strength lenses are used for dogs and 53D for cats. In some cases an IOL may not be utilized due to lens capsule instability or lens capsule tears; in those cases the eye may left aphakic or a sulcus lens may be placed. In the case of aphakic canines the dog will be visual, but hyperopic or far-sighted by 14D.

Current phacoemulsification technology utilizes bursts or pulses of ultrasound energy to break up the cataractous lens. In many cases the pulses of energy are short and intense enough to disrupt the cataract without creating any thermal change. The lens material is then aspirated from the lens capsule and an artificial lens or IOL (intraocular lens) is implanted into the lens capsule. The newer phacoemulsification machines create less thermal damage by utilizing pulses of energy and are more efficient in the use of ultrasound energy to disrupt the cataract. This translates to less post-operative uveitis, less stress on the corneal endothelium, decreased incisional scarring, and potentially a decreased risk of post-operative glaucoma.

The evolution of 2-handed cataract surgery has also led to increased success rates. A second instrument is often used to help disrupt the cataract. Especially with mature cataracts, this decreases surgery time and associated intra-operative trauma to the eye. The use of a second instrument also decreases stress on the lenticular zonules and allows successful surgery to be performed on eyes that may have been otherwise marginal candidates. Lenses that are subluxated with less than 50% zonular degeneration may now be addressed via standard phacoemulsification with IOL's. Additionally, the development of capsular tension rings is another advance in the treatment of subluxated lenses and often allows successful implantation of an IOL when the lens is subluxated.

Intraocular lenses have progressed from hard polymethylmethacrylate lenses to soft silicon lenses. The silicon lenses may be rolled or folded and injected into the eye without significantly enlarging the phacoemulsification incision. The newer soft lenses also decrease scarring of the posterior lens capsule. People report significant visual disturbance with the development of posterior capsular opacity (scarring) over time after surgery.

Although cataract surgery has a high success rate complications include glaucoma as previously mentioned. Other complications include retinal tears/detachments, hyphema, infection, corneal ulcers, incisional dehiscence, and iatrogenic capsular tears. All cataract surgery will lead to some post-operative uveitis. Some surgeons address this by injecting tissue plasminogen activator intracamerally at the end of surgery. The post-operative uveitis as well as any residual viscoelastic material in the eye may lead to post-operative ocular hypertension (POH). Typically the IOP begins to rise within 3 hours post-surgery and usually resolves within 24 hours. Although this is not considered a true glaucomatous episode damage may still occur to the retina and optic nerve. The pressure spike, if more than 25mm Hg, must be addressed immediately to prevent long term visual sequelae.

Intraoperative complications such as capsular tears may preclude placing an IOL. Additionally, excessive stress on the lens zonules during phacoemulsification may lead to anterior chamber hemorrhage if zonules are actually torn from their insertion on the ciliary body. This small amount of hemorrhage may be irrigated out of the AC and/or treated with TPA 48 hours post-surgery. If vitreous enters the AC during surgery associated with zonular breakdown or a capsular tear a vitrectomy is performed. This prevents the vitreous from clogging the phacoemulsification handpiece and prevents traction on the retina that could lead to a spontaneous retinal tear and detachment. If the retina is torn hemorrhage may occur in the vitreous as well. Immediate post-operative complications such as corneal ulcers generally resolve quickly with appropriate treatment.

Long term post-surgical complications most commonly include glaucoma. The risk remains low (less than 10%) for more than the first 3 years. Boston terriers and cocker spaniels are at increased risk. Dogs with hypermature cataracts have a greater risk of developing a late onset of glaucoma. Placement of an IOL at surgery may decrease this risk. Retinal tears and detachments occur at a low very low percentage, however are blinding unless retinal re-attachment surgery is pursued immediately. Retinal detachment is not painful in comparison to glaucoma, but both complications have devastating visual effects. The risk of these sequelae necessitate long term monitoring of cataract surgery patients. Non-compliance by owners in the use of medications and scheduling routine re-examinations may lead to a late complication. Non-compliance by owners was associated with decreased satisfaction in the surgical outcome, suggesting that prompt detection and treatment of complications is important in maintaining a good visual outcome.

Overall, cataract surgery has a high success rate and advances in technique and equipment allows more patients to be successful candidates for surgery. To ensure the best surgical outcome it is important that both the referring veterinarian and the owner understand the surgery and the potential complications, as well as how to treat them. Good communication between the referring veterinarian, the owner, and the veterinary ophthalmologist will only serve to increase the overall success rate and owner satisfaction.

References

1. Veterinary Ophthalmology, 4th ed., Gelatt KN, editor, Blackwell Publishing, Ames IA 2007.

2. Ophthalmic Disease in Veterinary Medicine, Martin CL, Manson Publishing, 2005.

3. Virtual Cataract Surgery Lecture, Gilger B, Wilkie D, Wolfer J, Colitz C, American College of Veterinary Ophthalmologists annual conference, Boston, MA 2008