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 ...