Disease and surgery of the lens (Proceedings)

The lens is composed of crystalline fibers specifically arranged to allow light rays to transilluminate through the lens to the retina. The lens focuses light rays on the retina. The lens is enclosed in a capsule composed of a basement membrane, epithelium, and differentiated lens fibers. The lens is avascular and is suspended in the globe by zonular fibers that insert in the ciliary body processes. Specific biochemical processes must be maintained in the lens to maintain clarity; most diseases that affect the lens will lead to opacity. Many different processes may occur in the eye that require surgery.

The most common disease that affects the lens is opacification of the lens or cataract formation. Any opacity no matter how small is technically termed a cataract. There are multiple classification schemes for cataract description including degree of opacification, location of the opacity in the lens, appearance of the cataract, etiology of the cataract, and age of onset of the opacity. The stage or degree of opacity provides a visual picture of the progression and may be most helpful in communication with the owner. Cataract formation must be differentiated from nuclear or lenticular sclerosis, the normal aging change in the lens. The easiest way to do this is to dilate the pupil and then transilluminate the lens. If a fundic reflex is present throughout the whole lens then the lens change, no matter how cloudy, is sclerosis.

Cats have few reports of primary cataract formation and the majority of cataracts in cats develop secondary to persistent uveitis. Uveitic cataracts usually develop slowly with opacification beginning in the cortex. Posterior synechiae, rubeosis iridis, and preiridial and pupillary inflammatory membranes may be present.

Cataract formation may be considered a developmental abnormality when opacification is present at a very young age, however most cataracts would be classified as acquired. Aphakia, microphakia, and lens colobomas are true congenital abnormalities that are vision impairing; other ocular abnormalities may be present and cataract formation may be present as well. Other congenital abnormalities are lenticonus and lentiglobus; lenticonus leads to protrusion of the front or back of the lens and lentiglobus results in a more spherical lens than normal. Visual acuity will be affected and other ocular abnormalities may be present as well. Vascular abnormalities of embryonic origin may also lead to visual defects and other complications. Persistent hyperplastic primary vitreous (PHPV) develops when the embryonic blood supply to the lens during development does not recede and atrophy normally. The pupillary membrane, extending from the iris, and the intravitreal hyaloid vascular system begin to atrophy by day 45 of gestation in the dog. If they do not completely regress then the remnants lead to persistent pupillary membranes (PPM) anteriorly and PHPV posteriorly. Six grades of PHPV have been described consisting of 1) Grade 1: retrolental fibrovascular dots, 2) Grade 2: dots and proliferation of retrolental tissue on the posterior lens capsule, 3) Grade 3: retrolental plaque and persistent parts of the hyaloid vascular system, 4) Grade 4: plaque and posterior lenticonus, 5) Grade 5: combination of grades 3 and 4, 6) Grade 6: combinations of above grades and lens coloboma, microphakia, and retrolental clots of pigment or free blood. The Doberman Pinscher, Staffordshire Bull Terrier, and Bouvier des Flandres are predisposed breeds. Anomalies in the Doberman originate from the tunica vasculosa lentis (TVL) and posterior lens capsule; microphakia, cataract formation, and PPM's are associated lesions. In the Staffordshire Bull Terrier the disease does not usually involve PPM's or PTVL and secondary cataracts are uncommon. An autosomal recessive inheritance is most likely. Cataract formation may be part of other multifactorial ocular abnormalities such as merle ocular dysgenesis. The Australian Shepherd as well as any dog with the merle gene is predisposed. Double merle offspring are at higher risk. Cataract formation may also be present with retinal dysplasia. The Labrador Retriever and the Samoyed have a syndrome of dwarfism with retinal dysplasia; these dogs are predisposed.

Cataract formation occurs when the metabolism and stringent biochemistry that occur in the lens is disrupted. Clarity of the lens is maintained by a lack of cell organelles and nuclei in the lens fibers, relative consistency of the refractive index of the lens fiber cytoplasm with only small spatial fluctuations, and a highly organized lattice arrangement of the lens fiber cells. Fluctuations in the refractive index of the cytoplasm must be small to minimize light scattering. The molecular weight of lens crystallins, the concentrations and volume fractions of intracellular proteins, and the organization of proteins within the cytoplasm all affect spatial fluctuations and transmission of light. In humans with incipient senile cataracts small changes are present in lens protein conformation and then proteolysis occurs leading to the development of a lens opacity and light scattering. Studies of cataract formation indicate that in many cataracts an increase in high-molecular weight proteins (albuminoids) occurs with a decrease in low molecular weight proteins (crystalloids). Metabolic pumps, such as the NaK-ATPase pump may be affected, ionic concentration alterations may play a role, and antioxidant activity may change all leading to lens opacity. These changes may start a domino effect in which cell proteolytic activity increases and cell membrane rupture occurs, water content and associated cellular osmotic pressure changes, and the products of proteolysis leak into the lens creating further opacification.

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 are 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.

Cataract formation, regardless of the etiology, leads to uveitis. Before a patient is a candidate for surgery the lens-induce uveitis (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. An 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 that 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 be 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 aspirates the lens fragments from the eye. 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 remain 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 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 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 necessitates 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 outcome.

The other common indications for lens surgery are lens luxation and aqueous misdirection glaucoma (malignant glaucoma) that occurs mostly in cats. Aqueous misdirection glaucoma is addressed in the cat by performing a standard cataract surgery as described above. After the lens has been removed a posterior capsulotomy is performed followed by an anterior vitrectomy. This addresses the obstruction to normal circulation of the aqueous humor by removing the anterior entrapping vitreous and a portion of the posterior lens capsule. I do not usually implant an IOL because it may obstruct the flow of aqueous humor and lead to IOP increases. Typically adhesions develop between the lens capsule and IOL that maintain stability of the IOL; in this case the adhesions may lead to post-operative complications. Intracapsular lens extraction (ICLE) is the technique used to remove luxated lenses. Treatment of uveitis pre-operatively will increase post-op success. A 160-180 degree corneal incision must be made (much longer than for routine cataract surgery). The anterior chamber is maintained with viscoelastic and the lens may be removed using a cryoprobe or lens loupe. If degenerated vitreous is present an anterior vitrectomy may be performed. The corneal incision is closed or if a sulcus IOL is being placed then that proceeds. Sulcus IOL's may lead to a higher glaucoma risk post-op.

Overall, lens surgery has a high success rate and advances in technique and equipment allow 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.