Managing diseases of the lens-clarification of a cloudy type (Proceedings)

The crystalline lens is a remarkable structure. As with the cornea, nature found a way to arrange living cells such into an optically transparent structure. The purpose of the lens is to serve as a final focus of light entering the eye onto the retina and to accommodate focus for near and far images. This discussion will focus on the normal anatomy and development of the canine lens as well as the most common clinical conditions affecting the lens.

The lens develops from surface ectoderm cells that grow inward to form the lens placode. The orientation of the lens depends on the proximity to the developing retina; cells nearest the retina become the primary lens fibers whereas the opposite cells form the mitotically active lens epithelial cells. The primary lens fibers become the embryonic nucleus, the most central cells in the lens. Lens epithelial cells produce new lens cells at the equator which become the lens cortex. These new cortex cells become very elongated and lose their nucleus and most of their organelles. They extend to join other lens fiber cells, forming the lens sutures where they meet. The sutures are visible on ophthalmic exam as a "Y" anteriorly and an inverted "Y" posteriorly. The fully developed lens contains several distinct layers. Starting from the outside, they are the lens capsule, the anterior lens epithelium, the cortex, and the nucleus. The nucleus can be subdivided into an adult, fetal, and embryonic nucleus. The ability to localize abnormalities to a specific layer of the lens can be helpful in determining cause and the potential for progression. The lens is suspended in place by tiny strands extending from the ciliary processes and attaching to the equatorial lens capsule. These strands, known as zonules, can degenerate as a hereditary condition in some breeds, most notably terriers, and result in a lens luxation.

As stated earlier, the function of the lens is to focus incoming light on to the retina. Despite being significantly thicker than the cornea, it is not as effective at bending light rays. Light rays change direction when they enter a medium with a different refractive index. The greater the difference in the refractive index between two media, the more light will bend as it passes through them. The refractive index difference between air and the pre-corneal tear film is significantly different, while the difference between lens cells and the surrounding aqueous and vitreous is relatively small. Thus, the lens exists to provide fine focus. It is also plays a critical role in changing the depth of focus, a process called accommodation. In many species, such as humans, primates, and birds, the lens is quite soft and changes shape easily to change the degree to which the light is bent. The light from near objects must be bent more than that from distant objects to focus on the retina, so the lens becomes more spherical. It then becomes more disc shaped for distance viewing. Dogs and cats have a much reduced ability to alter their lens curvature. It is thought that they accommodate primarily through anterior and posterior translocation of the lens.

Cataracts are easily the most common clinical problem seen with the lens. Simply, a cataract is any opacity of the lens or lens capsule that can block or distort light. There are multiple ways to classify cataracts: density, location, shape, age of onset, and etiology. Some cataracts, especially those confined to the nucleus or which have a classic hereditary shape, do not progress to cause vision defects. In these cases, no treatment or surgery is needed. Most cataracts, however, will progress at some point. Genetic cataracts are commonly seen. Although many breeds commonly develop hereditary cataracts, among the more common are Boston Terriers, Pugs, Cocker Spaniels, Shih Tzus, miniature Poodles, and Siberian Huskies.

The treatment option selected depends on many factors, including the level of vision compromise, presence of lens induced uveitis, age and health of the patient, and financial considerations. While surgery is the most effective option for most cataracts, medical options do exist. Topical anti-inflammatory therapy on a long term basis with steroids or NSAIDs is effective at reducing lens induced uveitis and limiting the risk of secondary glaucoma. While this option does not improve vision or treat the cataract, it is useful to help maintain patient comfort. In the last few years, medications containing the ingredient n-acetyl carnosine have appeared on the market. This compound has been shown to cause disaggregation of lens crystallins in select circumstances in-vitro. A preliminary study in 2006 showed there was in fact some reduction in the density of most cataracts, although it was only significant with immature cataracts and dogs with nuclear sclerosis from age. 80% of owners felt there was an improvement in visual acuity, however. While this sounds great, there has been little additional research on veterinary patients since then. Furthermore, my personal experience with these medications has not been encouraging. I have seen little, if any, reduction of the density of cataracts with any of the patients I have tried them on. Still, they are available and may be an alternative to surgery in some cases.

Surgical removal of the lens is the best way to remove the cataract and restore vision. Phacoemulsification is the current standard for this procedure. This procedure allows removal of the lens through an incision less than 5 mm. The handpiece and attached needle performs 3 functions simultaneously: irrigation, phacoemulsification, and aspiration. The irrigation fluids maintain the intraocular pressure and help to cool the needle tip and prevent thermal injury. Phacoemulsification is the use of ultrasound energy to break up the cataract. This is achieved by rapid vibration of the needle tip. Finally, the lens fragments are aspirated through the lumen of the needle. Following removal of the lens, insertion of an artificial lens into the lens capsule allows for more natural post-operative vision. There are multiple lens designs; however all are made with the same dioptric strength: 41.5 D for dogs and 53 D for cats. These are considerably more powerful than is normal for human intraocular lenses. Acrylic foldable lenses are used by most ophthalmologists currently as they allow the incision size to remain small. Post-operative vision is considered normal by owners of patients that received a lens implant (pseudophakic). Aphakic animals (no lens implant) have adequate vision for maneuvering through their environment; however they are extremely hyperopic (far sighted).

Improved techniques and technology have contributed to greatly increased success rates over the years. Currently, it is estimated that 90-95% of patients will be comfortable and visual at 1 year after surgery. Several complications can occur, both in the short and long term. Glaucoma is the most common problem and can be blinding and uncomfortable for the patient if not aggressively treated. Retinal detachments are also seen occasionally. Other complications include corneal ulcers, persistent uveitis, capsular fibrosis, and endophthalmitis (rare). Regrowth of the cataract is not a concern following this surgery, however capsular fibrosis can become quite dense in some patients and possibly begin to restrict vision again. To minimize the risk for these complications, I treat with long term topical anti-inflammatory medications, either steroids or NSAIDs. This is true for cataract patients that do not opt for surgery as well as long term lens induced uveitis can lead to the same debilitating complications as are seen with surgery.

Lens luxation is another condition affecting the lens commonly seen by the ophthalmologist. This is almost exclusively a hereditary condition, although severe trauma or an intraocular neoplasm may also cause it to occur. It is seen most frequently in terrier breeds, shar peis, and border collies. It is caused by a progressive degeneration of the lens zonules that suspend the lens in place. Early warning signs of lens instability include vitreous herniation into the anterior chamber (visible as wispy, gray material) or phacodenesis (visible movement of the lens whenever the eye moves). An aphakic crescent is the classic sign of a posterior lens luxation. While not every patient with early warning signs will develop a complete luxation of the lens, it is important to advise the owner of the risks and consider treatment or referral to an ophthalmogist for evaluation. When a lens does luxate, it will either fall to the ventral posterior segment or move forward into the anterior chamber. Both conditions result in a high risk of vision loss through glaucoma and/or retinal detachment, however anterior luxations are much more concerning in the acute stage. These patients often develop a pupillary block glaucoma from the lens obstructing normal flow of aqueous humor through the pupil into the anterior chamber. These patients are acutely painful and typically present with a cloudy eye and very injected episcleral vessels. Occasionally, the luxated lens can be difficult to identify, especially if it is clear. It is helpful to look at the eye from different angles, ideally with a narrow beam of light. Although often unsuccessful, an attempt can be made to dilate the iris and apply pressure to the cornea in the hopes of pushing the lens back behind the iris. In most cases, surgical removal of the lens is needed to prevent or control glaucoma before vision is lost. This procedure is called an intracapsular lens extraction and involves a large corneal incision just inside the dorsal limbus. The lens and its capsule are removed together in one piece through the incision. Most patients are left aphakic and can function well as discussed earlier. A technique has been described to suture artificial lenses in place after this procedure, however there is a significant risk of bleeding and I have not attempted this yet. Glaucoma and retinal detachments are the main complications reported with this surgery, as with phacoemulsification. A diode laser retinopexy is often recommended to limit the risk of retinal detachment. To have the best chance at avoiding complications, surgery should be performed before acute glaucoma and inflammation are present, thus the importance of early referral.