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A cataract is a clouding of the crystalline lens, resulting in vision loss. There are different types of cataracts, and they may be associated with underlying conditions. Understanding the differences between types of cataracts will improve clinical management of your patients.
Figure 1. Lens anatomyA cataract is a clouding of the crystalline lens, resulting in vision loss. There are different types of cataracts, and they may be associated with underlying conditions. Understanding the differences among types of cataracts will improve clinical management of your patients.
The crystalline lens has a biconvex shape with a central nucleus, an outer cortex, and a shell, called the capsule. The outmost layer is composed of epithelium. It is held in place by lens zonules that anchor the lens to the ciliary body. As we age, the lens grows around the nucleus, forming the cortex (Figure 1). The lens increases in size with age as the lens continues to produce lens fibers, and it is the only part of the eye that continues to grow in adulthood. The crystalline lens is made of water and protein. The protein fibers are precisely arranged in parallel such that the lens is clear, allowing light to pass through and land on the retina. As we age, degradation of proteins disrupts the array, causing a clouding of the lens.
Increased risk of cataract development is associated with ultraviolet light (UV) exposure, steroid use, diabetes, and smoking. This process cannot be reversed, but a healthy lifestyle may slow the progression. Avoiding UV is important, so UV protection in the form of sunglasses and hats is recommended. Not smoking or drinking excessive alcohol is also recommended, as well as maintaining tight blood sugar levels if diabetic. Antioxidant supplements have not been found conclusively to alter the progression of cataracts.
Symptoms associated with cataracts include cloudy or blurry vision, glare particularly at night, double vision, fading color vision, and a shift in the refractive error. As the proteins age, the refractive power of the lens changes, which may lead to a myopic shift even in the absence of other visual symptoms. This is often the first sign of cataracts.
Figure 2. Nuclear sclerosis initially presents as central yellowing with opacification. Cataracts are diagnosed based upon their anatomical location and appearance. If the clouding is limited to the center of the lens, it is termed nuclear sclerosis (Figure 2).
Figure 3. Peripheral cortical changes do not effect vision and are simply monitored over time. If the clouding is located in the cortex, it is called a cortical cataract (Figure 3).
Figure 4. Note the yellowing of the nucleus (nuclear sclerosis) and cortical spoking (cortical cataract) in this combined cataract. Cataracts may occur together, and they are then called a combined cataract (Figure 4). Cataracts adjacent to the capsule are called subcapsular cataracts. Anterior and posterior subcapsular cataracts may occur in younger people because they are associated with diabetes and steroid use.
Figure 5A. Congential cataracts may be small opacities along the Y suture of the nucleus. Patients may develop cataracts as babies (congenital), as children or young adults (presenile), or with typical aging (senile) (Figure 5). Cataracts may be secondary to trauma, or related to a systemic condition (see Table 1).
Figure 5B. Or larger, more visually significant opacification.
Next: Diabetes and cataracts
Diabetes and cataracts
Not only do diabetics develop cataracts earlier, they are more likely to suffer complications associated with cataract surgery. For this reason, considerable research has been performed to better understand why diabetics develop lens changes.
Several concerns with diabetics:
1. Diabetics do not process glucose normally. The enzyme aldose reductase changes glucose to sorbitol through the polyol pathway. Sorbitol should be changed to fructose by the enzyme sorbitol dehydrogenase, but the sorbitol is produced faster than it can be converted to fructose, causing a buildup of sorbitol in the lens. Accumulation of sorbitol leads to increased water within the lens, changing the lens fiber array and formation of sugar cataracts.1,2
2. Osmotic stress caused by the sorbitol accumulation3 causes death of lens epithelial cells4 leading to the development of cataract.5
3. Increased glucose levels in the aqueous humor may cause chemical changes of lens proteins. This has negative effects upon the lens.6
4. Lenses of diabetics show an impaired antioxidant capacity, increasing the effect oxidative stress.7
For this reason, a healthy lifestyle to reduce the risk of developing diabetes over one’s lifetime is highly recommended.
Smoking and cataracts
Smoking has been linked to cataract risk and is one reason eyecare practitioners have been more diligent to discuss smoking cessation with patients. One study reported that anyone with a history of smoking cigarettes was associated with an increased risk of age-related cataract. Current smokers had a higher risk of incidence. They found that former and current smokers were associated with nuclear and subscapular cataracts.8
Recently, another study found a significant dose–response relationship between smoking and the need for cataract extraction. Conversely, smoking cessation was associated with a decrease in risk that accumulated over time.9
Cataracts secondary to other disorders
Various types of cataracts occur related to systemic disease. Inflammatory disorders causing uveitis may require recurrent use of topical or oral steroids. Common inflammatory disorders include ankylosing spondylitis, Crohn's disease, ulcerative colitis, juvenile idiopathic arthritis, Behçet's syndrome, and sarcoidosis. Repeated treatment of allergies using oral steroids or chronic use of topical or inhaled steroids may also cause posterior subscapsular cataracts in a presenile patient.
Neurofibromatosis, a genetic disorder causing tumors to form on nerve tissue, is associated with cataract formation, as is Wilson disease, an inherited disorder associated with copper accumulation in the liver, ocular tissues, and other organs. Cataracts are now associated with syndromes such as Cohen syndrome, Degos disease, and Dubowitz syndrome. In some cases, the formation of cataracts may lead to initial diagnosis of a systemic condition.
Figure 6. Traumatic cataracts may develop anytime after an injury. Trauma may result in clouding of the lens (Figure 6). Injury may be directly to the eye or indirectly, as seen with head trauma. It may be blunt or penetrating trauma, or it may be related to radiation exposure. Changes may occur years or decades after the injury, and diagnosis is often made based upon appearance even if the patient is unable to report a traumatic event. A monocular, presenile cataract is a telltale sign of trauma. Blunt trauma is associated with a characteristic rosette or stellae-shaped opacification and often involves the posterior capsule.10 Penetrating injuries typically directly compromise the lens capsule leading to cortical opacification at the injury site.
Figure 7. Phakic IOL patients are at higher risk for development of an anterior subcapsular cataract. Ocular surgery may also induce cataract formation. Retinal surgery, such as scleral buckling and vitrectomy, may result in presenile cataracts. Phakic lens implants, such as the Visian lens (Staar Surgical Company), which rests in the space between the anterior lens and the iris, may cause friction and cause an anterior subscapular cataract (Figures 7 and 8).
Figure 8. The ASC is easily seen in the Scheimpflug image.
Next: Treatment of cataracts
Treatment of cataracts
When a cataract is first noted clinically upon slit lamp biomicroscopy, it may not affect vision, and it is simply monitored over time. When a significant loss of vision is noted subjectively and objectively (on the Snellen chart), cataract surgery is typically recommended. Visual effect of cataracts is typically documented using Snellen acuity at distance, and may include glare testing. The patient is asked to look at the distance chart while a bright light is directed into the eye. Cataracts increase light scatter, reducing the vision below the best-corrected vision. For example, the patient may read 20/25 with his glasses, but may “glare down” to 20/40.
Modern cataract surgery is typically “no stitch” outpatient surgery with light anesthesia. A small incision is made for instruments to enter the eye, and a process of phacoemulsification is performed. Ultrasound is used to break apart the lens cortex and nucleus while a vacuum removes the debris from the eye. The capsule is left in place to hold the new intraocular lens. Recently, femtosecond lasers have been approved to perform incisions for cataract surgery and to break up the lens to reduce or eliminate the need for ultrasound inside the eye.
Certain cataracts are associated with greater surgical risks. Posterior subscapsular cataracts are more difficult to remove due to adhesion of the cataract to the lens capsule and increase risk of capsule rupture during removal. Anterior subscapsular cataracts are also more problematic because adhesions create problems with creation of the capsulorhexis, the opening of the lens capsule during surgery.
Figure 9. Dense, white cataracts are more difficult to remove.Long-standing, mature cataracts are typically extremely dense centrally (Figure 9) and are associated with smaller pupils, shallow anterior chambers, and shaky zonules. All of these physical problems complicate cataract removal. White cataracts, characterized by a golden center and cortical spoking, clefting or cracking, adhesions to the capsule, and severe cortical opacification. A “Morgagnian“ cataract, is an extremely difficult case because the center is liquefied, increasing risk of dropping the nucleus into the vitreous during surgery. Traumatic cataracts may be difficult to remove if the trauma affected the lens zonules, or the cataract is very dense.
Capsular haze or ‘secondary cataracts’
Figure 10. Capsular haze, also referred to as a secondary cataract, occurs after phacoemulsification, and is easily corrected using a YAG laser capsulotomy.
After removal of cataracts, lens fibers continue to be produced, and they may cause clumping on the inside of the capsular surface. Should the clumping of fibers significantly reduce vision, a diagnosis of capsular haze is made (Figure 10). The patient is said to have a secondary cataract. There is no actual cataract because the lens has been removed. A YAG laser capsulotomy is performed to correct the problem, typically in the surgeon’s office. The laser is aimed at the clumping and fired to open the back of the capsule to improve the vision.ODT
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2. Kinoshita JH, Fukushi S, Kador P, et al. Aldose reductase in diabetic complications of the eye. Metabolism. 1979 Apr;28(4 Suppl 1):462-9.
3. Srivastava SK, Ramana KV, Bhatnagar A. Role of aldose reductase and oxidative damage in diabetes and the consequent potential for therapeutic options. Endocr Rev. 2005 May;26(3):380-92.
4. Takamura Y, Sugimoto Y, Kubo E, et al. Immunohistochemical study of apoptosis of lens epithelial cells in human and diabetic rat cataracts. Jpn J Ophthalmol. 2001 Nov-Dec;45(6):559-63.
5. Li WC, Kuszak JR, Dunn K, et al. Lens epithelial cell apoptosis appears to be a common cellular basis for non-congenital cataract development in humans and animals. J Cell Biol. 1995 Jul;130(1):169-81.
6. Hong SB, Lee KW, Handa JT, et al. Effect of advanced glycation end products on lens epithelial cells in vitro. Biochem Biophysl Res Commun. 2000 Aug 18;275(1):53-9.
7. Olofsson EM, Marklund SL, Behndig A. Enhanced diabetes-induced cataract in copper-zinc superoxide dismutase-null mice. Invest Ophthalmol Vis Sci. 2009 Jun;50(6):2913-8.
8. Ye J, He J, Wang C, et al. Smoking and Risk of Age-related Cataract: A Meta-analysis. Invest Ophthalmol Vis Sci. 2012 Jun 22;53(7):3885-95.
9. Lindblad BE, Håkansson N, Wolk A. Smoking Cessation and the Risk of Cataract A Prospective Cohort Study of Cataract Extraction Among Men. JAMA Ophthalmol. 2014 Mar;132(3):253-7.
10. Johns KJ, Feder RS, Bowes Hamill M, et al. Lens and Cataract: AAO Basic and Clincial Science Course Series. San Franscisco: The Foundation for the American Academy of Ophthalmology. 2001.