Cataract

Cataract (What is cataract?)

Overview: Physiology of Vision

Light rays entering the eye are bent (refracted) as they pass over the curved surfaces of the cornea and through the various structures of the eye which have been varying densities to focus on the retina. The cornea provides the major refractive change for light entering the eye, with the lens providing the fine focus for light transmitted posteriorly to the retina.

 The eye can adjust to seeing objects at various distances by the flattening or thickening of the lens. Accommodation is also facilitated by changing the size of the pupil. With near vision the iris constricts the pupil to force light rays to pass through the shortened but thicker lens. The pupil also constricts with bright lights to protect the retina from intense stimulation.

Light rays are absorbed by the photoreceptors on the retina, changed to electrical activity, and transmitted via the optic nerve to the visual cortex areas of the brain for processing. The fibers of the optic nerve divide at the optic chiasm; the medial portion of each nerve crosses to the opposite side, and the impulses are then transmitted to the visual cortex. In this way, visual information received by each eye is transmitted simultaneously to both sides of the brain. Bilateral vision provides depth perception.

Lens (Part of the Eye involved in Cataract)

The lens is a transparent biconvex structure located directly behind the iris and the pupil. It is attached to the ciliary body by multiple sensory ligaments called zonules. The lens, which is the fine- focusing mechanism for the eye, allows a clear image to be focused on the retina.

Cataract Definition

A cataract is clouding or opacity of the lens that leads to be gradual painless blurring of vision and eventual loss of sight. In general, cataracts are classified as senile, those associated with aging; traumatic, those associated with injury; congenital, those present at birth; or secondary, those that occur following other common eye diseases.

In the United States, cataracts are the third most likely cause of preventable blindness. The prevalence of cataracts increases with age. Epidemiologic studies indicate that for persons over 65 years of age, incidence of cataracts increases in geographic areas where there are long periods of sunlight.

Cataracts occur so often in elderly persons that the term senile cataract is used. Of persons 52 to 85 years of age, 15 percent have cataracts that reduce visual acuity to 20/30 or less, and at 80 years of age, about 85 percent of all persons have some clouding of the lens. Senile cataracts are listed as the most common cause of blindness in older persons, yet the response of the condition to surgery is often excellent.

After aging, the next most common cause of cataract is injury to the eye. The transparency of the lens may be destroyed by either a penetrating wound or a contusion. Cataracts may result from the ingestion of injurious substances such as dinitrophenol or naphthalene. Some researchers report that cataracts may result from systemic absorption of hair dyes.

Cataracts may also occur secondary to eye diseases, such as uveitis (inflammation of the uvea) or eye trauma; or with systemic diseases, such as diabetes mellitus, galactosemia, or sarcoidosis.