Defects of the Eye

There are many different issues that can arise in the eye, such as long-sightedness and short-sightedness. Each of these defects has slightly different causes and somewhat different fixes.

Eye Defects Definition

Eye defects are conditions that can affect the function of the eye. The purpose of the eye includes its role as a sensory organ and its function in the Immune System. If either of these functions is affected, we can say that an eye defect is present.

Common Eye Defects

Eye defects may refer to conditions like myopia or hyperopia. These are short-sightedness and long-sightedness, respectively. These eye defects impact our vision and can be corrected through corrective eye surgeries, contact lenses or replacement lenses. Eye defects can be natural and caused by ageing. On the other hand, they can be caused by infection, trauma or as a result of other conditions such as Diabetes or stroke.

Myopia

One of the most common eye defects is myopia, commonly referred to as short-sightedness. It is thought that by 2030, over half the world's population will have been diagnosed with myopia.

There are two leading causes of short-sightedness;

• The eyeball seems to have been stretched, increasing the distance between the lens and the retina.

• The lens is thickened or curved, focusing the light in front of the retina rather than on the retina itself.

Look at Figure 1 to see how short-sightedness (myopia) affects the eye.

Correction of Myopia

We can place a corrective concave lens in front of the eye to correct short-sightedness, which causes the light to refract slightly before entering the eye. This refraction allows the light to be focused on the retina.

Old Age Eye Defect

Presbyopia is used to describe the eye defect caused by old age. This is caused by the thickening of the lens, which affects the ability of the lens to refract light onto the retina. Presbyopia is a type of hyperopia or long-sightedness. Hyperopia itself is not entirely related to old age, whereas presbyopia is wholly linked to ageing. There are a few different causes of hyperopia.

There are two leading causes of long-sightedness;

• The eyeball is too small, causing the distance between the lens and retina to be too small.

• The lens cannot become thick enough to focus due to a lack of elasticity (more common in the elderly).

Correction of Hyperopia

To correct long-sightedness, we need to use a convex lens to focus light on the retina rather than behind it. This convex lens is different from the concave lens that we use to correct short-sightedness, but both act to refract light and redirect it to the lens.

Causes of Eye Defects

In more detail, we discussed the causes of myopia (short-sightedness) and hyperopia (long-sightedness) in our sections 'common eye defects' and 'old age eye defects'. Below is a brief summary of the cause and effects of long-sightedness and short-sightedness.

Old age is one of the factors that can lead to eye defects, but there are some other causes too. The most common causes of eye defects are infections, trauma, and secondary causes.

The eye can also become infected, leading to conditions like conjunctivitis - an infection of the conjunctiva. These conditions can lead to temporary or permanent eye defects. A temporary eye defect can be reversed or may even heal on its own. However, a permanent eye defect may lead to permanent blindness or impaired vision that remains for the rest of your life.

Diagram of Eye Defects

It's essential we can visualise how different defects affect the eye. Each fault has a different effect on vision and a different way that we then use spectacle lenses to correct the defect. Have a look at the diagram below.

This is an example of a ray diagram which shows how visible light rays hit the back of the eye at different locations.

Correction of Eye Defects

Contact lenses, laser eye surgery and replacement lenses are all potential corrections of eye defects. The different treatments and corrections of eye defects each have their advantages and disadvantages, as well as varying times when they can be used.

Contact Lenses

Contact lenses are clear lenses that sit on top of the eye. These can be split up into hard and soft contact lenses.

• An advantage of contact lenses is that they enable people with eye defects to participate in sports.

• A disadvantage of soft lenses is that they carry more risk of infection although they are more comfortable than rigid lenses.

Laser-Eye Surgery

We can use laser eye surgery to correct eye defects. This surgery aims to change the shape of the cornea. Doing so can change how light is refracted onto the retina.

• An advantage of laser eye surgery is that it replaces the need to wear glasses and contact lenses, which is often desirable.

• A disadvantage of laser eye surgery is that there is a risk of further, potentially permanent damage to the eye due to the surgery.

Replacement Lens Surgery

The final correction that we will discuss is lens replacement surgery. Lens replacement surgery completely replaces the lens of the eye with a plastic artificial lens.

• An advantage of this surgery is that it often corrects eye defects quickly and painlessly and means the patient no longer has to wear glasses or contact lenses. This procedure tends to be more expensive than going for laser-eye surgery or wearing contact lenses due to the high quality of lenses used.

• A disadvantage of this surgery is that it is more invasive than laser surgery (which only changes the shape of the lens). Therefore, it carries a risk of damage occurring to the retina leading to sight loss, similar to the potential damages that can come from laser eye surgery.