Sight Defects

  
Vision 
Braille 

 
CNIB 
Canadian organization representing the blind. Extensive vision information area. 
LensCrafters 
Retailer of eyecare supplies such as glasses, contact lenses, as well as eye examinations.

Hyperopia, mypoia, and astigmatism are refractive errors. In normal eyes, the lenses adjust as necessary to focus light into the retina properly. If the eye’s shape or length is abnormal, the lens won’t adjust correctly, and the light won’t reach the retina at the proper time, resulting in a blurred image. These conditions are often inherited from parents. 

Hyperopia, also known as far-sightedness is the inability to clearly view objects upclose, though distance vision is normal. This can be corrected with prescribed convex corrective lenses. 

Myopia, the opposite of hyperopia, is known as near-sightedness. It can be defined as the inability to clearly view distant objects, though close objects can be seen normally. This can be corrected with prescribed biconcave corrective lenses. Though uncommonly recommended by opthalmologists , refractive surgery to treat the condition is available. 

Normally, the cornea of the eye is symmetrically curved. If this curve becomes distorted, objects will appear blurry at any distance. This is known as astigmatism. The condition is very common, and often requires no treatment. Astigmatism is mostly hereditary, though anyone may have slight alterations in the dimensions of their cornea at anytime in life. If the problem does require treatment, the cornea will need to be reshaped. This can be done by using specially fitted contact lenses, or replacing the cornea with donated human corneal tissue. 

Most people’s vision declines with age. After the age of forty, eyes flexibility is often reduced, causing difficulty in focusing for close viewing, such as reading. This aging of the eyes is called presbyopia, and is normal. If the person already wears corrective lenses, this problem can be corrected by increasing the strength of the prescription. 

Eye Diseases 
The eye is by far one of the most complicated organs in our bodies, and they are many different disorders which affect them. They can be classified according to what part of the eye they affect. 

For the eyelids, hordeolum, or sty, an infection of the lashes’ follicles, is very common. There are other problems though, such as congenital defects like cleft eye and ptosis (drooping upper lid). Injuries can cause the eyelid to adhere to the eyeball’s surface, which is caused by burns. Chronic muscular contractions can force the lid inward or outward, and they are commonly subject to skin diseases such as eczema and acne. 

Disease or injury may damage the cornea, resulting in impairment, as will cataracts, which cloud the crystalline lens. The middle coating of the eyeball, the choroid, can contract infections or cancer, as can the vital light sensitive retina. Finally, the optic nerve can become inflamed. When this occurs within the eye it is optic neuritis or papillitis; behind the eye, retrobulbar neuritis; or when skull pressure is elevated, often from tumours, edema or swelling of the optic disc where the nerve enters the eyeball, it is referred to as papilledema, or choked disc. 

Colour Blindness 
Another common eye defect is colour blindness. This problem involved the inability to distinguish colours, and usually occurs in males. The problem is caused by a defect in the retina or nerve (usually one of the three colour cells of the retina), and severe cases mean all colours are perceived as shades of grey, specifically known as achromatopsia or monochromatism. More commonly, people are affected with dichromatism, or partial colour blindness, which affects 7% of all men and under 1% of women. This problem causes the inability to differentiate either reds from greens or yellows from blues. 

For those affected, all other aspects of vision are normal. Many people with the defect unconsciously learn to associate certain sensations of brightness with colours. To discover if they are colour blind most people will need to be diagnosed by using a special test.