Common vision defects

Short-sightedness (Myopia) 

Shortsightedness is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it. This means a shortsighted person will see close up objects clearly, but objects in the distance will appear blurred. Squeezing the eyelids together (‘squinting’) will tend to make distant objects seem clearer.

It is most commonly corrected through the use of corrective lenses, such as glasses or contact lenses. These will be concave which means that they will be thicker at the edge than in the middle. For people who require strong lenses to correct high levels of shortsightedness, newer materials mean this thickness can be avoided.
There are 2 main types of myopia:
  • Axial myopia – this is when the physical length of the eye is greater than the optical length and is the more type
  • Refractive myopia – this is when the front of the eye is too steeply curved resulting in the light entering being focused too much
Shortsightedness affects males and females equally and people who have a family history of shortsightedness are more likely to develop it. Most eyes with shortsightedness are healthy, but a small number of people with severe myopia develop a form of retinal degeneration.

Long-sightedness (Hypermetropia) 


Longsightedness (hypermetropia) is a defect of vision causing difficulty focusing on near objects, and even causing a sufferer to be unable to focus on objects at any distance if the weakness is moderate or severe.

This can be caused by:
  • The eyeball is too short and so the rays of light are not properly focused at the back of the eye (retina)
  • The cornea is not curved enough (it’s too flat)
  • The lens inside the eye is unable to become round enough – often in older age groups (see Presbyopia)
People with hypermetropia can experience:
  • Blurred vision
  • Asthenopia (eyestrain)
  • Accommodative dysfunction
  • Binocular dysfunction
  • Amblyopia (lazy eye)
  • Strabismus (turning eye or squint)
Longsightedness is most commonly corrected through the use of corrective lenses, such as glasses or contact lenses. These will be convex which means that they will be thicker in the middle than at the edge. For people who require strong lenses to correct high levels of longsightedness, newer materials mean this thickness can be avoided.

Astigmatism  


Astigmatism is a common and usually minor condition of the eye that causes blurred or distorted vision.

It occurs when the cornea or lens is not a perfectly curved shape (spherical, like a cricket ball), but rather is elliptical (like a rugby ball).
Most people who wear glasses also have astigmatism.
If left untreated, astigmatism can cause:
  • Headaches
  • Blurred vision
  • Eye strain and fatigue (tiredness) – particularly after doing tasks that involve focusing on something for a long period of time, such as reading or using a computer
Astigmatism is usually corrected with spectacles or contact lenses together with the underlying short- or long-sightedness. For contact lens use, often, specially designed lenses (Toric contact lenses) will be necessary to ensure clear and comfortable vision.

Presbyopia 


Presbyopia is similar to long-sightedness (hypermetropia) but is specifically caused by age.

In order to see close-up objects, our eyes have to accommodate (focus). This means that the lens has to change its thickness. It can do this because of special muscles that are attached at either end and as these muscles tighten, the lens becomes more thickened and curved allowing the light rays from close objects to be brought into sharp focus on the retina.

As we get older the lens of the eye becomes stiffer and less elastic which makes it more difficult for the lens to change shape, resulting in increased difficulties in seeing close objects clearly.
The first signs of presbyopia are:
  • Eyestrain (asthenopia)
  • Difficulty seeing in dim light
  • Problems focusing on small objects and/or fine print
  • Having to extend one’s arms to read print
These are usually first noticed between the ages of 40 and 50 and this ability to focus on near objects generally declines further throughout life.
Presbyopia is corrected using spectacles or contact lenses. These can be especially for close work only (‘reading glasses’) or bifocals or varifocals.

Squint (turning eye)


A squint (strabismus) is a condition where your eyes look in different directions. One eye turns inwards, outwards, upwards or downwards while the other eye looks forwards. The affected eye may turn all the time (constant strabismus) or some of the time (intermittent strabismus).

In some people the eye will only turn when under binocular stress, but will remain straight under normal viewing conditions. This is known as a latent squint or heterophoria.
How does a squint affect vision?
  • Blurred vision
  • Double vision (diplopia)
  • Lazy eye (amblyopia)
If your child has a squint, their eyes will no longer work together and they may see two images (double vision) instead of one. To avoid double vision, your child’s brain will ignore the signals from the eye with the squint and only recognize the images from the normal eye. As the squinting eye is not being used, it eventually becomes ‘lazy’.
The cause, severity and direction of a squint will vary from person to person. It is present in about 4% of children and treatment should be started as soon as possible to ensure the best possible visual outcome.

Lazy eye (amblyopia)


A lazy eye (amblyopia) is a childhood condition that occurs when the vision in one eye does not develop properly. This means that the child can see less clearly out of one eye and relies more on the ‘good’ eye.

A lazy eye is a relatively uncommon condition, affecting about that 1 in 50 children.
How do I know if my child has a lazy eye?
Younger children are often unaware that there is anything wrong with their vision and if they are, they are usually unable to explain what is wrong. Older children may complain that they can’t see as well through one eye.
Most cases of lazy eye are diagnosed during routine eye tests before parents even realise that there is a problem.
The NHS recommend that children should have an eye test before they start school and then further tests every year.
The common causes of lazy eye are:
  • Squint
  • Refractive errors – this could be due to short- or long-sightedness or anisometropia. This is where one eye is short-sighted and the other is long-sighted
  • Congenital cataract – clouding of the lens of the eye that is present from birth
  • Droopy eyelid (ptosis)
  • Glaucoma– a group of eye conditions that affect vision
  • Corneal scarring
Treatment Options:
  • The younger the child is when a lazy eye is diagnosed, the more successful treatment is likely to be. If treatment is started after the age of eight, it is unlikely to be as successful.
  • Treating, or correcting, any underlying eye problems
  • Encouraging the use of the affected eye so that vision can develop properly
Treating underlying eye problems
  • Glasses – Vision problems, such as short, or long- sightedness, can be corrected using glasses or contact lenses. These usually need to be worn constantly and are regularly checked. Glasses may also help to straighten a squint and, in some cases, can fix the lazy eye without the need for further treatment. Your child may say that they can see better without their glasses. This is because their eyes have become used to working hard to focus, and they now find it hard to let the glasses focus for them. They will need plenty of encouragement to wear their glasses continuously.
  • Surgery – Cataracts can be removed to treat blurred and distorted vision.
  • Surgery – To improve the appearance of a squint. The operation will either strengthen, or weaken, the eye muscles of the lazy eye to change its position. This means that the lazy eye will appear to be better aligned with the good eye. The child’s vision will not improve, but their eyes will appear straighter and it will help the eyes work better together.
  • Surgery – A droopy eyelid can also be corrected using surgery.
Encouraging the use of the eye
A number of different treatments options can be used to encourage your child to use the affected eye. These are outlined below:
  • Patching – Using a patch is known as occlusion. It involves placing a patch with a sticky rim over the ‘good’ eye so that the lazy eye is forced to work. It can be very effective in improving the sight in the lazy eye. The treatment using patching can be done in community optometry practices or in hospital eye clinics. The length of time the child will need to wear the patch will depend on how old they are, how serious the problem is and how much they co-operate with wearing the patch. Regular follow up checks and constant encouragement to the child are important. Patches are most effective before a child reaches seven or eight years of age and most children will need to wear the patch for a few hours a day for several weeks. While the patch is on, the child should do close-up activities, such as colouring, reading or schoolwork. Patches can be worn with glasses.
  • Eye drops – Atropine eye drops can be used to blur the vision in the good eye which will encourage the use of the lazy eye. As side effects from atropine use are common, this can only be done under the supervision of an ophthalmologist (eye surgeon).