Vaughan & Asbury's General Ophthalmology 17th Edition
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 (such as diabetes), smoking, and heredity. Age-related cataract is a common cause of visual impairment. Cross-sectional studies place the prevalence of cataracts at 50% in individuals aged 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 from dietary carotenoids (lutein), but studies evaluating the protective effect of multivitamins have yielded conflicting results.
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.
(Figures 1, and 2)
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 focus power of the central lens, creating a myopic (near-sighted) shift in refraction. Other symptoms may include poor hue discrimination or monocular diplopia. Most nuclear cataracts are bilateral but may be asymmetric.
Figure 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 2. 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).
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 they 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 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, intraocular hemorrhage, or infection. Intraocular lenses have made adjustment following cataract operation much easier than when only thick cataract glasses or aphakic contact lenses were available.
(Figures 3 and 4)
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.
Figure 3. Congenital cataract.
Figure 4. Congenital cataract, zonular type. One zone of lens involved. The cortex is relatively clear.
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.
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, although bilateral deprivation amblyopia can result from unwarranted delay. When surgery is undertaken, there must be as short an interval as is reasonably possible between surgery on the two eyes.
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.
Surgical treatment of infantile and early childhood cataracts involves lens extraction through a small 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 are followed by contact lens correction. The use of intraocular lenses in early childhood is becoming increasingly frequent. It may lessen the difficulty of optical rehabilitation associated with contact lenses in children, but there are difficulties calculating the appropriate power of intraocular lens, which may need to be changed as the eye develops.
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 (Figures 5) is most commonly due to a foreign body injury to the lens or blunt trauma to the eyeball. Air rifle pellets and fireworks 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.
Figure 5. A. Traumatic "star-shaped" cataract in the posterior lens. This is usually due to ocular contusion and is only detectable through a well-dilated pupil. B. Traumatic cataract with wrinkled anterior capsule. C. Imprint of iris pigment on anterior surface of lens.
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.
CATACARACT 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 6), hypocalcemia (of any cause), myotonic dystrophy, atopic dermatitis, galactosemia, and Lowe's, Werner's, and Down's syndromes.
Figure 6. Punctate dot cataract. This type of cataract is sometimes seen as an ocular complication of diabetes mellitus. It may also be congenital.
AFTER-CATARACT (SECONDARY MEMBRANE)
After-cataract (Figure 7) denotes opacification of the posterior capsule 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.
Figure 7. After-cataract.
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. In the past, up to one-half of all adult patients developed an opacified posterior capsule after extracapsular cataract extraction. However, improved surgical techniques and new intraocular lens materials have significantly reduced the incidence of posterior capsule opacity.
Cataract Surgery
Cataract surgery has undergone dramatic change during the past 30 years with the introduction of the operating microscope and microsurgical instruments, 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, either superiorly or 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 nuclear expression 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 is now the most common form of extracapsular cataract extraction. It 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 foldable intraocular 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, entail a higher 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 pars plana lensectomy or phacofragmentation. This type of cataract removal is usually 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 rarely 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.
There are many styles of intraocular lenses, but most designs 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 other chapter. If an intraocular lens cannot be safely placed or is contraindicated, postoperative refractive correction generally requires a contact lens or aphakic spectacles.