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.

10.1036/1535-8860.ch8

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