The (crystalline) lens 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.
Accommodation is the eye’s ability to adjust its focus from distance to near due to changes of the shape of the lens. Its inherent elasticity allows the lens 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 that, when contracted, relaxes zonular tension. The lens then assumes a more spherical shape, resulting in increased dioptric power to bring near objects into focus. Ciliary muscle relaxation reverses this sequence of events, causing the lens to flatten and bringing distant objects into view. As the lens ages, accommodation gradually reduces as lens elasticity decreases.
Symptoms associated with lens disorders are primarily visual. Presbyopia is the reduced ability with age to perform near tasks due to decreased accommodation. Loss of lens transparency (cataract) results in blurred vision for near and distance. Surgical removal of the lens or its complete dislocation 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.
The term cataract refers to 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. The prevalence of cataracts is around 50% in individuals age 65–74, increasing to about 70% for those over 75.
The pathogenesis of cataracts is incompletely understood. They are characterized by protein aggregates that scatter light and reduce transparency and other protein alterations that result in yellow or brown discoloration. Factors that contribute to cataract formation include oxidative damage (from free radical reactions), ultraviolet light damage, and malnutrition. ...