The Aging Crystalline Lens: A Review of Cataracts

Authors:
Leonid Skorin Jr, DO, OD, MS
Ophthalmologist, Mayo Clinic Health System, Albert Lea, Minnesota

Stephanie Norberg, OD
Resident optometrist, Minneapolis VA Medical Center, Minneapolis, Minnesota

Citation:
Skorin L Jr, Norberg S. The aging crystalline lens: a review of cataracts. Consultant. 2019;59(1):13-17,19.

ABSTRACT: Difficulty seeing street signs and difficulty reading are common concerns among patients. In younger individuals, most cases require spectacle prescriptions that solve their blurry vision. In older adults, however, the cause may be age-related changes within the eye. Cataracts are crystalline lens opacities that cause light scatter, resulting in decreased vision. The 3 main types of age-related cataracts are nuclear sclerosis, cortical degeneration, and posterior subcapsular cataract. Treatment for a cataract includes updating the spectacle prescription, wearing sunglasses for glare, and eventually cataract surgery to remove the opacified crystalline lens. This article reviews the anatomy of the human lens, cataract symptoms, treatment of cataracts, and the role of the primary care provider.

KEYWORDS: Cataract, opacification, crystalline lens, nuclear sclerosis, posterior subcapsular cataract, cortical degeneration, monocular diplopia, presbyopia

Cataract is one of the leading causes of treatable blindness in the world and is the most common age-related eye disease in the United States.^1,2 

A cataract is a clouding or opacification of the natural lens within the eye.^1,3 According to the National Institutes of Health’s National Eye Institute, more than half of the population who have reached 80 years old develop clinically significant cataracts.³ The 2010 US Census data estimated that approximately 24.4 million Americans had a cataract, a dramatic increase of 19% from the 2000 US Census data, when 20.5 million Americans had a cataract.³ The prevalence of cataracts is projected to double by 2050.³ 

Cataract development is often the result of altered lens metabolism, altered lens structure, and environmentally induced changes.^1,4 Most cataracts arise due to aging of the crystalline lens, although other causes of cataracts include congenital causes, trauma, systemic disease, and toxicity.^5-7

Anatomy and Physiology of the Human Lens

The crystalline lens is located posterior to the iris and is suspended by thin zonular fibers near the ciliary body.¹ The lens is a transparent elliptical structure.⁸ The innermost part of the lens contains the oldest cells known as the nucleus.^4,8 Surrounding the nucleus is the lens cortex.^1,4 The clear envelope that surrounds the entire lens is called the lens capsule.^1,8

The primary function of the lens is to focus light rays onto the retina.⁸ The lens increases the refractive power of the eye while viewing near objects, known as accommodation.¹ The lens also absorbs ultraviolet radiation to protect the retina.¹ Lens clarity is essential for light to reach the retina.¹ Clarity of the lens is achieved by the absence of blood vessels, few cellular organelles, an orderly arrangement of lens fibers, and a gradient index of refraction throughout the lens.^5,8

The human lens is continually growing throughout life.^1,7 New lens fibers are created and laid down on the outside of existing fibers, while the existing fibers are not replaced.^7,8 The resulting lens has concentric layers of lens fibers similar in shape to an onion.¹ 

The lens comprises approximately 65% water, 35% protein, and a few minerals such as potassium and ascorbic acid.¹ Metabolic activity is required for cellular replication and to maintain crystalline lens transparency.^4,7 The nutrients for the lens are obtained from the surrounding aqueous humor and vitreous humor.^1,5 The capsule is permeable to water, ions, and proteins necessary for lens function and growth.⁸

Changes occur within the lens as we age.⁸ New fibers are continuously being produced, allowing the lens to slowly become larger and less elastic throughout life.^1,2,4 The earliest change in the crystalline lens is the decrease of refractive power to focus light rays while viewing near objects, known as presbyopia.¹ As the eye continues to age, there is also a reduction of nutrients and antioxidants being transported, while there is an increase of water-insoluble and protein aggregates.¹ Disruptions from ultraviolet radiation, fluid and ion imbalance, oxidative damage, and protein modification of the lens result in opacification of part or all of the lens.^1,2,8 Crystalline lens opacification is known as a cataract.^1,4 Most cataracts are due to gradual, progressive changes in lenticular physiology.^1,9 Aging is a major risk factor for developing cataracts.^4,10 Modifiable risk factors include nutrition, ultraviolet light exposure, poor diabetic control, medications such as corticosteroids, smoking, alcohol consumption, and increased body mass index.^2,11 Most individuals develop some extent of lens opacification by age 75.² Cataracts tend to occur in both eyes, although the rate of progression between either eye may vary.²

The most common forms of age-related cataracts include nuclear sclerosis, cortical degeneration, and posterior subcapsular lens opacities.^7,12 Cataracts are named based on their location within the lens and graded by a numbering system based on the severity of opacification.^1,2

NEXT: Nuclear Sclerosis

Nuclear Sclerosis

An opacity located within the nucleus at the center of the lens is called a nuclear sclerosis cataract.¹ The discoloration of the nucleus appears yellow-green or brown and steadily increases in color with age (Figures 1 and 2).⁹ Nuclear sclerosis is thought to be the end-product of insoluble lens protein aggregation resulting in the nucleus becoming more rigid.¹⁰ Maturation of a nuclear sclerosis cataract can increase the refractive power of the lens, allowing the patient to gain near vision without reading glasses again.^1,6,13 This change is called “second sight of the aged.”^6,13 The improvement of near vision is temporary, since the nuclear zone becomes more opaque over time.^2,6 As the nuclear sclerosis advances, the lens appears brown and opaque.² The most common visual complaint of a nuclear sclerosis cataract is distance blur, due to the refractive shift in the lens.^6,12

NEXT: Cortical Degeneration

Cortical Degeneration

A cortical cataract has a radial, spoke-like shape extending from the periphery toward the center of the lens.¹ These spoke-like opacities are located within the cortex of the lens (Figures 3 and 4).^1,9 Disruption of the fiber cell membranes and cortical hydration allow cortical cataracts to form.¹³ The cortical lens changes begin in small clusters and occur where alterations develop in the cortical fibers.¹⁰ Typically, the inferonasal quadrant is the initial location of a cortical cataract.¹³ The spoke width expands to adjacent fibers slowly over time, causing the cortical cataract to progress throughout the entire lens.^1,2 When the entire cortex appears opaque or white, the cortical cataract is mature.² The most common visual complaint of a cortical cataract is glare.¹² Nevertheless, the patient may remain asymptomatic until the central portion of the lens is affected.^11,12

NEXT: Posterior Subcapsular Cataract

Posterior Subcapsular Cataract

A posterior subcapsular (PSC) cataract is an asymmetric granular opacity at the posterior surface of the lens.⁹ It typically is white in color, similar to a frosted-glass appearance (Figure 5).⁹ This type of cataract impacts vision significantly due to the central location within the line of sight.^1,13 PSC cataracts are common in patients with diabetes, corticosteroid use, and a history of intraocular inflammation.⁶ The most common visual complaints of a PSC cataract are glare and reading difficulties.^12,13

Symptoms of Cataract Formation

As cataracts form within the crystalline lens, the patient experiences painless, gradual vision loss and increased problems with glare.⁹ The visual acuity is directly related to cataract severity; thus, mild changes within the crystalline lens may not affect the vision significantly.^2,6 Cataracts also result in decreased contrast sensitivity and changes in glasses prescription and rarely cause double vision in one eye (monocular diplopia).⁹ Common complaints and their related symptoms are listed in the Table.

NEXT: Treatment

Treatment

The definitive treatment of cataracts is to surgically remove the crystalline lens and replace it with an artificial lens implant.^7,11 However, the presence of a cataract does not necessarily require the lens to be removed.^2,6 A patient without visual symptoms would not require a surgical referral.⁷ The initial treatment of a symptomatic patient includes updating a spectacle or contact lens prescription, decreasing glare with sunglasses or a broad-rimed hat, and increasing light sources while reading.^2,14 Counseling the patient about cataract-related visual symptoms and reducing their exposure to risk factors, such as smoking cessation and diabetes control, is advised.²

Cataract surgery is indicated when the lens opacity is significant and interfering with the patient’s ability to perform daily activities.^11,13 Cataract surgery also can be justified if there is a reduction in vision with glare testing, decreased contrast sensitivity, and medical indications when the cataract is affecting the health of the eye.^13,15 Cataract surgery is considered a safe and effective procedure with an excellent success rate.^7,9,16

In one study that looked at people aged 85 years or older after cataract surgery, approximately 85% of patients had improved vision objectively, and 87% perceived a benefit.¹⁶ Several studies report that after patients have cataract surgery, their visual function improves, leading to a better quality of life.^2,17 Visual impairment, low contrast sensitivity, and poor depth perception caused by a cataract are contributing factors to increased fall rates, fractures, and motor vehicle accidents (MVAs).^2,17,18 In a study of Medicare patients aged 65 years or older, the risk of hip fractures decreased by 23% following cataract surgery.¹⁷ In another study, an improvement of visual acuity after cataract surgery resulted in a 43% decreased risk of falls over 12 months.¹⁸ Studies also indicate that patients who elected for cataract surgery were less likely to be in an MVA compared with patients who decided against cataract surgery.^2,19 One study found the risk of an MVA in cataract surgery patients decreased by 50% within a 4 to 6 year period after surgery.¹⁹ Furthermore, another study indicated that mortality risk is reduced by 30% in patients who undergo cataract surgery.²⁰ Numerous studies show that visual function, safety, and overall quality of life are better when a cataract is removed.^2,17,18

Surgical techniques have greatly improved in recent years, allowing for shorter operating times, more efficient anesthesia, and outpatient procedures.^7,11,16 The standard method of cataract extraction is phacoemulsification, which utilizes high-frequency ultrasound to break the lens into fragments and then suction them out of the eye (Figure 6).^2,9 Cataract surgery is typically performed under local anesthesia with monitored intravenous sedation at a hospital or surgical center.^11,13,21 Local anesthesia minimizes the risk of the incision to open, a complication frequently associated with postoperative nausea and vomiting after general anesthesia.²²

Patients should be advised of the risks involved with cataract surgery.¹⁵ Postoperative complications are possible, although cataract surgery is generally a safe procedure.^7,9 Increased eye pressure (intraocular pressure spikes) within the first 1 to 14 days is common.²³ Thickening of the retina around the central area of vision (cystoid macular edema) can occur in patients with diabetes, resulting in vision loss (Figures 7-9).¹⁵ Cystoid macular edema is relatively uncommon; however, the peak incidence is 6 to 10 weeks after surgery.^13,15 The most feared complication following cataract surgery is a bacterial infection, frequently caused by Staphylococcus epidermidis within the eye (infectious endophthalmitis).^2,15,23 Infectious endophthalmitis can quickly diminish vision and can present within the first few days after surgery.^14,15 The incidence of endophthalmitis is between 0.02% and 0.5% of patients undergoing cataract surgery.²³

Studies of potential ways to delay cataract progression or prevent formation are ongoing.^2,4 Currently, there are no conclusive clinically significant and reliable data from human trials that cataract formation can be slowed or prevented.⁴ Recent studies of cataract formation do suggest that high intake of dietary antioxidants may have a protective effect to slow the progression, especially foods rich in lutein and zeaxanthin.^4,23 The most effective strategies to delay the progression of cataracts include improving nutrition, avoiding illness, limiting sun exposure, increasing physical activity, and quitting smoking.^2,4,14,23

NEXT: Systemic Considerations and Primary Care Roles

Systemic Considerations and Primary Care Roles

A cataract is best diagnosed with a dilated pupil during a slit lamp examination.¹⁴ When a slit lamp is not available, identifying a cataract in an undilated pupil during a primary medical examination is simple using a direct ophthalmoscope.^11,14 Setting the direct ophthalmoscope to +4 diopters and holding the light approximately 30 cm from the patient allows a quick method to detect a cataract change in the lens.¹⁴ If the lens is clear (without opacities), the reflex will appear uniformly red.¹⁴ Mild cataracts appear as dark defects in the red reflex.²³ Moderate to severe cataracts will appear as larger dark defects, opacities within the red reflex, or without a red reflex entirely.¹⁴ If a cataract is suspected, the patient should be referred for a complete ophthalmic examination.¹¹

Cataract surgery is the most common surgical ophthalmic procedure requiring anesthetic care.²¹ Before the patient elects to have surgery, several aspects of medical health and medications should be considered.^6,13 A routine preoperative general medical examination is necessary in order to proceed with surgery.^2,13 Multiple medical comorbidities increase the risk of any procedure.¹¹ Patients with diabetes, myotonic dystrophy, neurofibromatosis type 2, and HIV/AIDS often develop cataracts at an earlier age.^11,13 Uncontrolled diabetes or systemic hypertension, recent myocardial infarction or stroke, or upper respiratory infections will postpone the surgery until the medical conditions are in better control.^2,11,13

Respiratory function should be optimal, since the patient will be lying flat on their back during the procedure.^2,13 Drug allergies and medications should be documented and may need to be discussed with the operating surgeon.¹³ Medications such as anticoagulants and systemic α₁ adrenergic blockers can complicate the surgical procedure.¹¹ Anticoagulants including aspirin and warfarin increase the risk of bleeding within the eye during surgery.¹¹ Depending on the surgeon, anticoagulants may be continued or stopped prior to cataract surgery.^11,13 The use of α₁ adrenergic blockers, such as tamsulosin, can cause intraoperative floppy iris syndrome and pupillary constriction.^6,11 The pupil abnormalities exist even when the patient discontinues tamsulosin 4 weeks prior to cataract surgery.⁶

Summary

Cataract can be the underlying cause of blurry vision. It is important to properly diagnose and potentially rule out other pathologies that also may result in decreased vision. Cataracts are frequently diagnosed during a routine eye examination. Treatment involves updating the spectacle prescription, reducing sources of glare, and eventually cataract surgery to replace the crystalline lens with an artificial lens implant.

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3. National Eye Institute. Cataracts. https://nei.nih.gov/eyedata/cataract. Accessed December 18, 2018.

4. Taylor A, Jacques PF, Chylack LT Jr, et al. Long-term intake of vitamins and carotenoids and odds of early age-related cortical and posterior subcapsular lens opacities. Am J Clin Nutr. 2002;75(3):540-549.

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merckmanuals.com/professional/eye-disorders/cataract/cataract. Revised November 2017. Accessed December 18, 2018.

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16. Wong TY. Effect of increasing age on cataract surgery outcomes in very elderly patients. BMJ. 2001;322(7294):1104-1106.

17. Tseng VL, Yu F, Lum F, Coleman AL. Risk of fractures following cataract surgery in Medicare beneficiaries. JAMA. 2012;308(5):493-501.

18. Coleman AL, Stone K, Ewing SK, et al. Higher risk of multiple falls among elderly women who lose visual acuity. Ophthalmology. 2004;111(5):857-862.

19. Owsley C, McGwin G Jr, Sloane M, Wells J, Stalvey BT, Gauthreaux S. Impact of cataract surgery on motor vehicle crash involvement by older adults. JAMA. 2002;288(7):841-849.

20. Tseng VL, Yu F, Lum F, Coleman AL. Cataract surgery and mortality in the United States Medicare population. Ophthalmology. 2016;123(5):1019-1026.

21. Macias AA, Bayes J, McGoldrick KE. Anesthesia for elective eye surgery. UpToDate. http://www.uptodate.com/contents/anesthesia-for-elective-eye-surgery. Updated July 6, 2018. Accessed December 13, 2018.

22. Garg A. Anesthesia in cataract surgery. In: Agarwal A, Agarwal A, Jacob S, eds. Phacoemulsification. 4th ed. New Delhi, India: Jaypee Brothers Medical Publishers Ltd: 2012: 124-127.

23. Duncan G, Wormstone IM, Davies PD. The aging human lens: structure, growth and physiological behaviour. Br J Ophthalmol. 1997;81(10):818-823.