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The crystalline lens is a remarkable structure that in its normal state, functions to bring images into focus on the retina. It is positioned just posterior to the iris, and is supported by zonular fibers arising from the ciliary body. These fibers insert onto the equatorial region of the lens capsule. The lens capsule is a basement membrane that surrounds the lens substance. Epithelial cells at the lens equator continue to be produced throughout life, so that older lens fibers are compressed into a central nucleus; younger, less compact fibers around the nucleus make up the cortex (see Figures 1-12, 1-14, and 1-15). 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.

PHYSIOLOGY OF SYMPTOMS

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 results in blurred vision (without pain) for both near and distance. If the lens is partially dislocated (subluxation), 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 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.

10.1036/1535-8860.ch8

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