Chapter 8. Lens

The crystalline lens is a remarkable structure that contributes to focusing of images on the retina. It is positioned just posterior to the iris and is supported by zonular fibers arising from the ciliary body and inserting onto the equatorial region of the lens capsule (see Figure 1–12). The lens capsule is a basement membrane that surrounds the lens substance. Epithelial cells near the lens equator divide throughout life and continually differentiate into new lens fibers, so that older lens fibers are compressed into a central nucleus; younger, less-compact fibers around the nucleus make up the cortex. 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.

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 (cataract) results in blurred vision (without pain) for both near and distance. If the lens is partially dislocated (subluxation) due to congenital, developmental, or acquired causes, 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 majority still being provided by the curvature of the cornea.

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.

A cataract is any opacity in the lens. Aging is the most common cause, 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 substance is opaque; the immature cataract has some transparent regions. 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 (Figure 8–1).

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 patient's symptoms and the visual acuity. 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 visual 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.

See Figures 8–1 and 8–2.

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 refractive power of the central lens, creating a myopic (nearsighted) 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 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 adjacent to the 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 also result 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 improves visual acuity in over 90% of cases. The remainder of patients either has preexisting retinal damage or, in rare cases, develop serious postsurgical complications that prevent significant visual improvement, for example, glaucoma, retinal detachment, intraocular hemorrhage, or infection.

See Figures 8–3 and 8–4.

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.

About one-third of childhood cataracts are hereditary, while another third are secondary to metabolic or infectious diseases or associated with a variety of syndromes. The final one-third results 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

Congenital Cataract

Congenital lens opacities are common and often visually insignificant (see also Chapter 17). 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 (see also Chapter 17) 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 surgeries on the two eyes.

Acquired Cataract

Acquired cataracts often do not require the same urgent care (aimed at preventing amblyopia) as infantile cataracts because the children are usually 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 cataract in children will not produce any symptoms or signs parents would routinely notice, screening programs are important for case finding.

Traumatic cataract (Figures 8–5, Figures 8–6 and 8–7) 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.

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 metal upon metal. 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 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.

Bilateral cataracts occur in many systemic disorders including diabetes mellitus (Figure 8–8), hypocalcemia (of any cause), myotonic dystrophy, atopic dermatitis, galactosemia, and Down, Lowe (oculo-cerebro-renal), and Werner syndromes (see Chapters 15 and 18).

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.

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. An incision is made at the limbus or in the peripheral cornea, either superiorly or temporally. An opening is formed in the anterior capsule (anterior capsulorhexis), and the nucleus and cortex of the lens are removed. An intraocular lens can then be placed in the empty “capsular bag,” supported by the intact posterior capsule.

The technique of phacoemulsification is now the most common form of extracapsular cataract extraction in developed countries. It utilizes a handheld ultrasonic vibrator to disintegrate the hard nucleus such that the nuclear material and cortex can be aspirated through a small 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. In developing countries, particularly rural areas, the instruments for phacoemulsification are not available. Manual sutureless small incision cataract surgery (MSICS) is based on the traditional nuclear expression form of extracapsular cataract extraction, in which the nucleus is removed intact, but utilizing a small incision. The cortex is removed by manual aspiration. MSICS may be indicated for dense cataracts unsuitable for phacoemulsification.

The advantages of small-incision surgery, either phacoemulsification or MSICS, 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 main intraoperative complication of extracapsular surgery is posterior capsular tear, for which the main predisposing factors include previous trauma, dense cataract, unstable lens, and small pupil, possibly leading to displacement of nuclear material into the vitreous (“dropped nucleus”) that generally necessitates complex vitreoretinal surgery. Postoperatively there may be secondary opacification of the posterior capsule that requires discission using the neodymium:YAG laser (see Posterior Capsule Opacification later in the chapter).

Pars plana lensectomy or phacofragmentation, in which the lens is removed via the pars plana in conjunction with posterior vitrectomy using automated lens and vitreous cutters, may be performed to facilitate vitreo-retinal surgery, although conventional phacoemulsification surgery is now more commonly undertaken under such circumstances, or to remove a completely dislocated lens or a partially dislocated lens that is not amenable to phacoemulsification (see later in the chapter). Whether phacofragmentation is required depends upon the severity of cataract.

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 when facilities for extracapsular surgery are not available and occasionally for treatment of a dislocated lens.

Intraocular Lenses

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 or partially restore accommodation, have also been produced. The goal of these designs is to provide the patient with good vision for both near and distance without glasses, which current monofocal designs are less likely 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 21. 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 is usually ambulatory on the day of surgery but is advised to move cautiously and avoid straining or heavy lifting for about a month. The eye may be patched on the day of surgery. Protection at night by a metal shield is often suggested for several days after surgery. 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 4–8 weeks after surgery).

Complications of Adult Cataract Surgery

Cataract surgery in adults has a very low rate (2%–5%) of complications that result in permanent impairment of vision. The rarest, but also most serious complications include intraocular infection (endophthalmitis, 0.1%) and intraocular hemorrhage (less than 0.5%), either of which can result in severe visual loss. Suspicion of endophthalmitis requires vitreous tap for microscopy and culture, and intravitreal injection of antibiotics (see Table 22–1). Vitrectomy is sometimes indicated (see Chapter 9). Other complications include retinal detachment, cystoid macular edema, glaucoma, corneal edema, and ptosis. The most common complication is posterior capsule opacification but this is amenable to treatment.

Posterior Capsule Opacification

In the past, up to 50% of eyes developed opacification of the posterior capsule (“after-cataract”) (Figure 8–9) after uncomplicated adult extracapsular cataract extraction. Improved surgical techniques and new intraocular lens designs, particularly sharp posterior edges on the optics, have significantly reduced the incidence. About 10% of eyes require treatment for posterior capsule opacification following uncomplicated phacoemulsification surgery but the recorded incidence depends upon duration of follow-up.

Persistent subcapsular lens epithelium favors 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.

The neodymium:YAG laser provides a noninvasive method for discission of the posterior capsule (see Chapter 23). Pulses of laser energy cause small “explosions” in target tissue, creating an opening 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, potentially leading to rhegmatogenous retinal detachment or cystoid macular edema. The rise in intraocular pressure is usually detectable within 3 hours after 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. No significant damage seems to be done to corneal endothelium with the neodymium:YAG laser.

Childhood Cataract Surgery

Cataract surgery in young children is hindered by more difficult anterior capsulorhexis, as well as the need to make an opening in the posterior capsule (posterior capsulorhexis) and to remove part of the vitreous (anterior vitrectomy) to reduce the incidence of posterior capsule opacification, which is much higher than after adult cataract surgery. The cataracts are less dense than in adults and can usually be removed by an irrigation–aspiration technique, without the need for phacoemulsification.

Optical correction can consist of spectacles in older bilaterally aphakic children, but most childhood cataract operations are followed by contact lens correction, with adjustment of power as the refractive status of the eye changes with growth. Intraocular lenses are being used. They avoid the difficulties associated with contact lens wear, but there are difficulties calculating the appropriate power.

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. Glaucoma is a common long-term complication.

Partial or complete lens dislocation (subluxation) (Figure 8–10) may be hereditary or may result from trauma.

Hereditary Lens Dislocation

Hereditary lens dislocation is usually bilateral and may be an isolated familial anomaly or due to inherited connective tissue disorder such as homocystinuria, Marfan's syndrome or Weill-Marcehsani 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 there is a significant risk of vitreous loss, predisposing to retinal detachment. 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 if left untreated. If uveitis or uncontrollable glaucoma occurs, lens extraction must be done despite the poor results possible from this operation. For completely dislocated lenses, the technique of choice is pars plana lensectomy or phacofragmentation, depending upon the density of cataract. Some partially dislocated (subluxed) lenses are amenable to phacoemulsification with various adaptations, such as capsular tension rings or support hooks.