Print this Page

The crystalline lens is a remarkable structure that in its normal state, functions to bring images into focus on the retina. It is positioned just posterior to the iris, and is supported by zonular fibers arising from the ciliary body. These fibers insert onto the equatorial region of the lens capsule. The lens capsule is a basement membrane that surrounds the lens substance. Epithelial cells at the lens equator continue to be produced throughout life, so that older lens fibers are compressed into a central nucleus; younger, less compact fibers around the nucleus make up the cortex (see Figures 1-12, 1-14, and 1-15). Because the lens is avascular and has no innervation, it must derive nutrients from the aqueous humor. Lens metabolism is primarily anaerobic owing to the low level of oxygen dissolved in the aqueous.

The eye is able to adjust its focus from distance to near objects because of the ability of the lens to change shape, a phenomenon known as accommodation. The inherent elasticity of the lens allows it to become more or less spherical depending on the amount of tension exerted by the zonular fibers on the lens capsule. Zonular tension is controlled by the action of the ciliary muscle, which, when contracted, relaxes zonular tension. The lens then assumes a more spherical shape, resulting in increased dioptric power to bring nearer objects into focus. Ciliary muscle relaxation reverses this sequence of events, allowing the lens to flatten and thus bringing more distant objects into view. As the lens ages, its accommodative power is gradually reduced as lens elasticity decreases.

PHYSIOLOGY OF SYMPTOMS

Symptoms associated with lens disorders are primarily visual. Presbyopic symptoms are due to decreased accommodative ability with age and result in diminished ability to perform near tasks. Loss of lens transparency results in blurred vision (without pain) for both near and distance. If the lens is partially dislocated (subluxation), visual blur can be due to a change in refractive error. Complete dislocation of the lens from the visual axis results in an aphakic refractive state; severely blurred vision results from loss of over one-third of the eye's refractive power.

The lens is best examined with the pupil dilated. A magnified view of the lens can be obtained with a slitlamp or by using the direct ophthalmoscope with a high plus (+10) setting.

CATARACT

A cataract is any opacity in the lens. Aging is the most common cause of cataract, but many other factors can be involved, including trauma, toxins, systemic disease, and heredity. Age-related cataract is a common cause of visual impairment. Cross-sectional studies place the prevalence of cataracts at 50% in those age 65-74; the prevalence increases to about 70% for those over 75.

The pathogenesis of cataracts is not completely understood. However, cataractous lenses are characterized by protein aggregates that scatter light rays and reduce transparency. Other protein alterations result in yellow or brown discoloration. Additional findings may include vesicles between lens fibers or migration and aberrant enlargement of epithelial cells. Factors thought to contribute to cataract formation include oxidative damage (from free radical reactions), ultraviolet light damage, and malnutrition. No medical treatment has been found that will retard or reverse the underlying chemical changes that occur in cataract formation. However, some recent evidence suggests a protective effect of estrogen use on the lenses of postmenopausal women.

A mature cataract is one in which all of the lens protein is opaque; the immature cataract has some transparent protein. If the lens takes up water, it may become intumescent. In the hypermature cataract, cortical proteins have become liquid. This liquid may escape through the intact capsule, leaving a shrunken lens with a wrinkled capsule. A hypermature cataract in which the lens nucleus floats freely in the capsular bag is called a morgagnian cataract.

Most cataracts are not visible to the casual observer until they become dense enough to cause severe vision loss. The ocular fundus becomes increasingly more difficult to visualize as the lens opacity becomes denser, until the fundus reflection is completely absent. At this stage, the cataract is usually mature, and the pupil may be white.

The clinical degree of cataract formation, assuming that no other eye disease is present, is judged primarily by the Snellen visual acuity test. Generally speaking, the decrease in visual acuity is directly proportionate to the density of the cataract. However, some individuals who have clinically significant cataracts when examined with the ophthalmoscope or slitlamp see well enough to carry on with normal activities. Others have a decrease in visual acuity out of proportion to the degree of lens opacification. This is due to distortion of the image by the partially opaque lens. The Cataract Management Guideline Panel recommends reliance on clinical judgment combined with Snellen acuity as the best guide to the appropriateness of surgery but recognizes the need for flexibility, with due regard to a patient's particular functional and visual needs, the environment, and other risks-all of which may vary widely.

AGE-RELATED CATARACT (  Figures 8-1, 8-2 and   8-3)

The normal condensation process in the lens nucleus results in nuclear sclerosis after middle age. The earliest symptom may be improved near vision without glasses ("second sight"). This occurs from an increase in the refractive index of the central lens, creating a myopic shift in refraction. Other symptoms may include poor hue discrimination or monocular diplopia. Most nuclear cataracts are bilateral but may be asymmetric.

Cortical cataracts are opacities in the lens cortex. Changes in the hydration of lens fibers create clefts in a radial pattern around the equatorial region. They also tend to be bilateral, but are often asymmetric. Visual function is variably affected, depending on how near the opacities are to the visual axis.

Posterior subcapsular cataracts are located in the cortex near the central posterior capsule. They tend to cause visual symptoms earlier in their development owing to involvement of the visual axis. Common symptoms include glare and reduced vision under bright lighting conditions. This lens opacity can result also from trauma, corticosteroid use (topical or systemic), inflammation, or exposure to ionizing radiation.

Age-related cataract is usually slowly progressive over years, and death may occur before surgery becomes necessary. If surgery is indicated, lens extraction definitely improves visual acuity in well over 90% of cases. The remainder of patients either have preexisting retinal damage or develop serious postsurgical complications that prevent significant visual improvement, eg, glaucoma, retinal detachment, vitreous hemorrhage, infection, or epithelial downgrowth into the anterior chamber. Intraocular lenses have made adjustment following cataract operation much easier than was the rule when only thick cataract glasses were available.

CHILDHOOD CATARACT (Figures 8-4 and 8-5)

Childhood cataracts are divided into two groups: congenital (infantile) cataracts, which are present at birth or appear shortly thereafter; and acquired cataracts, which occur later and are usually related to a specific cause. Either type may be unilateral or bilateral.

About one-third of cataracts are hereditary, while another third are secondary to metabolic or infectious diseases or associated with a variety of syndromes. The final one-third result from undetermined causes. Acquired cataracts arise most commonly from trauma, either blunt or penetrating. Other causes include uveitis, acquired ocular infections, diabetes, and drugs.

Clinical Findings

A. Congenital Cataract:

Congenital lens opacities are common and often visually insignificant. A partial opacification or one out of the visual axis-or not dense enough to interfere significantly with light transmission-requires no treatment other than observation for progression. Dense central congenital cataracts require surgery.

Congenital cataracts that cause significant visual loss must be detected early-preferably in the newborn nursery by the pediatrician or family physician. Large, dense white cataracts may present as leukocoria (white pupil), noticeable by the parents, but many dense cataracts cannot be seen by the parents. Unilateral infantile cataracts that are dense, central, and larger than 2 mm in diameter will cause permanent deprivation amblyopia if not treated within the first 2 months of life and thus require surgical management on an urgent basis. Even then there must be careful attention to avoidance of amblyopia related to postoperative anisometropia. Symmetric (equally dense) bilateral cataracts may require less urgent management, though bilateral deprivation can result from unwarranted delay. When surgery is undertaken, there must be as short an interval as is reasonably possible between the surgery on the two eyes.

B. Acquired Cataract:

Acquired cataracts do not require the same urgent care (aimed at preventing amblyopia) as infantile cataracts, because the children are older and the visual system more mature. Surgical assessment is based on the location, size, and density of the cataract, but a period of observation along with subjective visual acuity testing can be part of the decision making process. Because unilateral cataracts in children will not produce any symptoms or signs parents would routinely notice, screening programs are important for case finding.

Treatment

Surgical treatment of infantile and early childhood cataracts involves lens extraction through a 3 mm limbal incision utilizing a mechanical irrigation-aspiration handpiece. Phacoemulsification is rarely required. In contrast to the procedure used for adult lens extraction, the posterior capsule and anterior vitreous are removed by many surgeons using a mechanical vitreous suction-cutting instrument. This prevents formation of secondary capsular opacification, or after-cataract (see below). Primary removal of the posterior capsule thus avoids the necessity for secondary surgery and enhances early optical correction.

Using today's sophisticated surgical techniques, operative and postoperative complications are similar to those reported with adult cataract procedures. Optical correction can consist of spectacles in older bilaterally aphakic children, but most childhood cataract operations should be followed by contact lens correction. The use of intraocular lenses in early childhood is under active investigation and, if successful, may reduce the difficulty in optical rehabilitation associated with contact lenses in children.

Prognosis

The visual prognosis for childhood cataract patients requiring surgery is not as good as that for patients with age-related cataract. The associated amblyopia and occasional anomalies of the optic nerve or retina limit the degree of useful vision that can be achieved in this group of patients. The prognosis for improvement of visual acuity is worst following surgery for unilateral congenital cataracts and best for incomplete bilateral congenital cataracts that are slowly progressive.

TRAUMATIC CATARACT

Traumatic cataract (Figures 8-6, 8-7 and 8-8) is most commonly due to a foreign body injury to the lens or blunt trauma to the eyeball. Air rifle pellets are a frequent cause; less frequent causes include arrows, rocks, contusions, overexposure to heat ("glassblower's cataract"), and ionizing radiation. Most traumatic cataracts are preventable. In industry, the best safety measure is a good pair of safety goggles.

The lens becomes white soon after the entry of a foreign body, since interruption of the lens capsule allows aqueous and sometimes vitreous to penetrate into the lens structure. The patient is often an industrial worker who gives a history of striking steel upon steel. A minute fragment of a steel hammer, for example, may pass through the cornea and lens at a tremendous rate of speed and lodge in the vitreous or retina.

CATARACT SECONDARY TO INTRAOCULAR DISEASE ("Complicated Cataract")

Cataract may develop as a direct effect of intraocular disease upon the physiology of the lens (eg, severe recurrent uveitis). The cataract usually begins in the posterior subcapsular area and eventually involves the entire lens structure. Intraocular diseases commonly associated with the development of cataracts are chronic or recurrent uveitis, glaucoma, retinitis pigmentosa, and retinal detachment. These cataracts are usually unilateral. The visual prognosis is not as good as in ordinary age-related cataract.

CATARACT ASSOCIATED WITH SYSTEMIC DISEASE

Bilateral cataracts may occur in association with the following systemic disorders: diabetes mellitus (Figure 8-9), hypoparathyroidism, myotonic dystrophy, atopic dermatitis, galactosemia, and Lowe's, Werner's, and Down's syndromes. (These entities are discussed in Chapters 15 and 18.)

DRUG-INDUCED CATARACT

Corticosteroids administered over a long period of time, either systemically or in drop form, can cause lens opacities. Other drugs associated with cataract include phenothiazines, amiodarone, and strong miotic drops such as phospholine iodide, used in the treatment of glaucoma.

AFTER-CATARACT (Secondary Membrane)

After-cataract (Figure 8-10) denotes opacification of the posterior capsule due to partially absorbed traumatic cataract or following extracapsular cataract extraction. Persistent subcapsular lens epithelium may favor regeneration of lens fibers, giving the posterior capsule a "fish egg" appearance (Elschnig's pearls). The proliferating epithelium may produce multiple layers, leading to frank opacification. These cells may also undergo myofibroblastic differentiation. Their contraction produces numerous tiny wrinkles in the posterior capsule, resulting in visual distortion. All of these factors may lead to reduced visual acuity following extracapsular cataract extraction.

After-cataract is a significant problem in almost all pediatric patients unless the posterior capsule and anterior vitreous are removed at the time of surgery. Up to one-half of all adult patients develop an opaque secondary membrane after extracapsular cataract extraction. Before the neodymium:YAG laser came into use, this condition was treated by performing a small capsulotomy with a knife or barbed 27-gauge needle, either at the time of the original operation or as a secondary procedure.

The neodymium:YAG laser provides a noninvasive method for discission of the posterior capsule (see Chapter 24). Pulses of laser energy cause small "explosions" in target tissue, creating a small hole in the posterior capsule in the pupillary axis. Complications of this technique include a transient rise in intraocular pressure, damage to the intraocular lens, and rupture of the anterior hyaloid face with forward displacement of vitreous into the anterior chamber. The rise in intraocular pressure is usually detectable within 3 hours post treatment and resolves within a few days with treatment. Rarely, the pressure does not return to normal for several weeks. Small pits or cracks may occur on the intraocular lens but usually have no effect on visual acuity. In the aphakic eye, rupture of the vitreous face with anterior displacement of vitreous may predispose to development of rhegmatogenous retinal detachment or cystoid macular edema. No significant damage seems to be done to corneal endothelium with the neodymium:YAG laser.

CATARACT SURGERY

Cataract surgery has undergone dramatic change during the past 30 years with the introduction of the operating microscope and microsurgical instruments, improvements in suture materials, the development of intraocular lenses, and alterations in techniques for local anesthesia. Further refinements continue to occur, with automated instrumentation and modifications of intraocular lenses allowing surgery through small incisions.

The generally preferred method of cataract surgery in adults and older children preserves the posterior portion of the lens capsule and thus is known as extracapsular cataract extraction. Intraocular lens implantation is part of this procedure. An incision is made at the limbus or in the peripheral cornea, usually superiorly but sometimes temporally. An opening is formed in the anterior capsule, and the nucleus and cortex of the lens are removed. The intraocular lens is then placed in the empty "capsular bag," supported by the intact posterior capsule. In the standard form of extracapsular cataract extraction, the nucleus is removed intact, but this requires a relatively large incision. The cortex is removed by manual or automated aspiration. The technique of phacoemulsification utilizes a handheld ultrasonic vibrator to disintegrate the hard nucleus such that the nuclear material and cortex can be aspirated through an incision of approximately 3 mm. This same incision size is then adequate for insertion of the recently developed folding lenses. If a rigid intraocular lens is used, the wound needs to be extended to approximately 5 mm. The advantages of small-incision surgery are more controlled operating conditions, avoidance of suturing, rapid wound healing with lesser degrees of corneal distortion, and reduced postoperative intraocular inflammation-all contributing to more rapid visual rehabilitation. The phacoemulsification technique does, however, run the risk of posterior displacement of nuclear material through a posterior capsular tear, which generally necessitates complex vitreoretinal surgery. After all forms of extracapsular cataract surgery there may be secondary opacification of the posterior capsule that requires discission using the neodymium:YAG laser (see After-Cataract, above). Lens extraction through the pars plana during posterior vitrectomy is called phacofragmentation. This type of cataract removal is only performed in conjunction with the removal of an opaque or scarred vitreous.

Intracapsular cataract extraction, consisting of removal of the entire lens together with its capsule, is less frequently performed today. The incidence of postoperative retinal detachment and cystoid macular edema is significantly higher than after extracapsular surgery, but intracapsular surgery is still a useful procedure, particularly when facilities for extracapsular surgery are not available.

Intraocular Lens

There are many styles of intraocular lenses, but most prostheses consist of a central biconvex optic and two legs or haptics to maintain the optic in position. The optimal intraocular lens position is within the capsular bag following an extracapsular procedure. This is associated with the lowest incidence of postoperative complications, such as pseudophakic bullous keratopathy, glaucoma, iris damage, hyphema, and lens decentration. The newest posterior chamber lenses are made of flexible materials such as silicone and acrylic polymers. This flexibility allows the lens implant to be folded, thus decreasing the required incision size. Lens designs that incorporate multifocal optics have also been produced. The goal of this design is to provide the patient with good vision for both near and distance without glasses, which current monofocal designs are unable to do.

After intracapsular surgery-or if there is inadvertent damage to the posterior capsule during extracapsular surgery-intraocular lenses can be placed in the anterior chamber or sometimes fixated in the ciliary sulcus.

Methods of calculating the correct dioptric power of an intraocular lens are discussed in Chapter 20. If an intraocular lens cannot be safely placed or is contraindicated, postoperative refractive correction generally requires a contact lens or aphakic spectacles.

Postoperative Care

If a small-incision technique is used, the postoperative recovery period is usually shortened. The patient may be ambulatory on the day of surgery but is advised to move cautiously and avoid straining or heavy lifting for about a month. The eye can be bandaged for a few days, but if the eye is comfortable, the bandage can be removed on the first postoperative day and the eye protected by spectacles or by a shield during the day. Protection at night by a metal shield is required for several weeks. Temporary glasses can be used a few days after surgery, but in most cases the patient sees well enough through the intraocular lens to wait for permanent glasses (usually provided 6-8 weeks after surgery).

DISLOCATED LENS (Ectopia Lentis)

Partial or complete lens dislocation (Figure 8-11) may be hereditary or may result from trauma.

Hereditary Lens Dislocation

Hereditary lens dislocation is usually bilateral and is commonly associated with homocystinuria and Marfan's syndrome (Chapter 15). The vision is blurred, particularly if the lens is dislocated out of the line of vision. If dislocation is partial, the edge of the lens and the zonular fibers holding it in place can be seen in the pupil. If the lens is completely dislocated into the vitreous, it can be seen with the ophthalmoscope.

A partially dislocated lens is often complicated by cataract formation. If that is the case, the cataract may have to be removed, but this procedure should be delayed as long as possible because vitreous loss, predisposing to subsequent retinal detachment, is likely during surgery. If the lens is free in the vitreous, it may lead in later life to the development of glaucoma of a type that responds poorly to treatment. If dislocation is partial and the lens is clear, the visual prognosis is good.

Traumatic Lens Dislocation

Partial or complete traumatic lens dislocation may occur following a contusion injury such as a blow to the eye with a fist. If the dislocation is partial, there may be no symptoms; but if the lens is floating in the vitreous, the patient has blurred vision and usually a red eye. Iridodonesis, a quivering of the iris when the patient moves the eye, is a common sign of lens dislocation and is due to the lack of lens support. This is present both in partially and in completely dislocated lenses but is more marked in the latter.

Uveitis and glaucoma are common complications of dislocated lens, particularly if dislocation is complete. If there are no complications, dislocated lenses are best left untreated. If uveitis or uncontrollable glaucoma occurs, lens extraction must be done despite the poor results possible from this operation. The technique of choice is limbal or pars plana lensectomy using a motor-driven lens and vitreous cutter.

•	Al-Ghoul KJ et al: Distribution and type of morphologic damage in human nuclear age-related cataracts. Exp Eye Res 1996;62:237.  [ PMID 8690033 ]
•	Andley U: Photooxidative stress. In: Principles and Practice of Ophthalmology: Clinical Practice. Vol 1. Albert DM, Jakobiec FA (editors). Saunders, 1994.
•	Apple DJ et al: Posterior capsule opacification. Surv Ophthalmol 1992;37:73.  [ PMID 1455302 ]
•	Apple DJ, Sims J: Harold Ridley and the invention of the intraocular lens. Surv Ophthalmol 1996;40:279.  [ PMID 8658339 ]
•	Benitez del Castillo JM et al: Effects of estrogen use on lens transmittance in postmenopausal women. Ophthalmology 1997;104:970.  [ PMID 9186438 ]
•	Birch EE, Stager DR: The critical period for surgical treatment of dense congenital unilateral cataract. Invest Ophthalmol Vis Sci 1996;37:1532.  [ PMID 8675395 ]
•	Bourne WM, Nelson LR, Hodge DO: Continued endothelial cell loss ten years after lens implantation. Ophthalmology 1994;101:1014.  [ PMID 8008341 ]
•	Brady KM et al: Cataract surgery and intraocular lens implantation in children. Am J Ophthalmol 1995;120:1.  [ PMID 7611311 ]
•	Cataract Management Guideline Panel: Cataract in Adults: Management of functional impairment. Clinical Practice Guideline, Number 4. Rockville, MD. U.S. Department of Health and Human Services, Public Health Service, Agency for Health Care Policy and Research. AHCPR Pub. No. 93-0542.
•	Desai P et al: Gains from cataract surgery: Visual function and quality of life. Br J Ophthalmol 1996;80:868.  [ PMID 8976696 ]
•	Graziosi P et al: Location and severity of cortical opacities in different regions of the lens in age-related cataract. Invest Ophthalmol Vis Sci 1996;37:1698  [ PMID 8675414 ]
•	Halpert M, BenEzra D: Surgery of the hereditary subluxated lens in children. Ophthalmology 1996;103:681.  [ PMID 8618772 ]
•	Hiller R et al: Cigarette smoking and the risk of development of lens opacities: The Framingham studies. Arch Ophthalmol 1997;115;1113.
•	Javitt JC et al: National Outcomes of Cataract Extraction: Retinal detachment and endophthalmitis after outpatient cataract surgery. Ophthalmology 1994;101:100.  [ PMID 8302540 ]
•	Klein BE et al: Incidence of cataract surgery in the Wisconsin Epidemiologic Study of Diabetic Retinopathy. Am J Ophthalmol 1995;119:295.  [ PMID 7872389 ]
•	Klein BE et al: Incident cataract surgery: The Beaver Dam eye study. Ophthalmology 1997;104:573.  [ PMID 9111248 ]
•	Lambert SR, Drack AV: Infantile cataracts. Surv Ophthalmol 1996;40:427.  [ PMID 8724637 ]
•	Mares-Perlman JA et al: Relation between lens opacities and vitamin and mineral supplement use. Ophthalmology 1994;101:315.  [ PMID 8115152 ]
•	Norregaard JC: International variation in ophthalmologic management of patients with cataracts. Results from the International Cataract Surgery Outcomes Study. Arch Ophthalmol 1997;115:399.  [ PMID 9076214 ]
•	Powe NR et al: Synthesis of the literature on visual acuity and complications following cataract extraction with intraocular lens implantation. Arch Ophthalmol 1994;112:239.  [ PMID 8037792 ]
•	Schein OD et al: Variation in cataract surgery practice and clinical outcomes. Ophthalmology 1994;102:1142.
•	West SK, Valmadrid CT: Epidemiology of risk factors for age-related cataract. Surv Ophthalmol 1995;39:323  [ PMID 7725232 ]

List of Figures

	Figure 8-1: Age-related cataract. A and B: "Coronary" type cortical cataract (frontal and cross-sectional views): club-shaped peripheral opacities with clear central lens; slowly progressive. C: "Cuneiform" type cortical cataract: peripheral spicules and central clear lens; slowly progressive. D: Nuclear sclerotic cataract: diffuse opacity principally affecting nucleus; slowly progressive. E: Posterior subcapsular cataract: plaque of granular opacity on posterior capsule; may be rapidly progressive. F: "Morgagnian" type (hypermature lens): the entire lens is opaque, and the lens nucleus has fallen inferiorly.
	Figure 8-2: Mature age-related cataract viewed through a dilated pupil. (Courtesy of A Rosenberg.)
	Figure 8-3: Age-related cataract. In the photo at right the scene shown at left is reproduced as if seen by a person with a moderately advanced senile cataract (opacity denser centrally). (Courtesy of E Goodner.)
	Figure 8-4: Congenital cataract.
	Figure 8-5: Congenital cataract, zonular type. One zone of lens involved. The cortex is relatively clear.
	Figure 8-6: Traumatic "star-shaped" cataract in the posterior lens. This is usually due to ocular contusion and is only detectable through a well-dilated pupil.
	Figure 8-7: Traumatic cataract with wrinkled anterior capsule.
	Figure 8-8: Imprint of iris pigment on anterior surface of lens.
	Figure 8-9: Punctate dot cataract. This type of cataract is sometimes seen as an ocular complication of diabetes mellitus. It may also be congenital.
	Figure 8-10: After-cataract.
	Figure 8-11: Dislocated lens.

10.1036/1535-8860.ch8

Customer Privacy Policy Copyright ©2002-2003 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Notice. Additional credits and copyright information. For further information about this site contact tech_support@accessmedicine.com. Last modified: October 15, 2002 .