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Refractive Error

A refractive error means that the optical system of the eye is out of focus. There are two main types, long-sighted and short-sighted.

Long-sight is where the eye has a relatively weak focusing power usually because the eye is a little smaller than average. In long-sight, the vision close up is blurred. It’s a little more complicated in children, see below.

Short sight is where the focusing power is too strong, usually because the eye is a little larger than average. Here the distance vision is blurred.

 Young children have slightly smaller eyes than adults so it is normal for them to be longsighted. In most children, this corrects itself as the child and their eye grow.

In addition there is another type of refractive error called astigmatism which can occur on its own or in combination with long or short sight.

Optometrists test for refractive errors and this test is called a refraction.

Long Sight

The official name is hypermetropia. As stated, most babies start out as long-sighted and this corrects itself as the eye grows. People who end up longsighted as adults started out with even smaller eyes as babies. In spite of the eye growing along with the rest of their body, as adults they still end up with smaller eyes than average and hence long-sighted. As children therefore, they will have started out more longsighted than normal for their age.

 

 (Illustration Courtesy Ted Montgomery)

So it can be confusing when a parent is told their child is long-sighted but does not need glasses.

If they are long-sighted how come everything is not blurred when looking close up?  This is because a child has the ability to markedly alter the focusing power of the eye by using internal eye muscles to change the shape of their lens. In this way a long-sighted child with an eye of less optical power (due to its smaller size) can overcome this by increasing the focusing power of the lens.

This process is called accommodation.

Put another way, a long-sighted adult, who is less able to alter the strength of their natural lens, would find everything close up blurred. To increase the focusing power of their eye they require a convex lens in front of the eye, glasses.

Children who are very longsighted can only overcome a certain amount by increasing their eye’s focusing power. To completely focus the eye they need additional power and like adults have to wear glasses. As they and their eye grow, they may become less long-sighted and not as reliant on their glasses. This is not always the case however.

Short Sight

The official name is myopia. Here the eye’s optical system is too strong, usually because the eye is relatively large. As a result the vision is blurred in the distance. Children can increase the power of an optically weak (longsighted) eye by changing the shape of the internal lens. In a short-sighted eye no-one can reduce the power of the lens when focused in the distance, so even children cannot compensate for being short-sighted. They need glasses in the form of weakening (concave) lenses.

If a young child is short-sighted, they can expect to become more short-sighted as they grow and their eyes inevitably enlarge.

Illustration Courtesy Ted Montgomery

Astigmatism

This term can sound like a serious disease but should be regarded as just another type of refractive error.

The front window of the eye is called the cornea.  Here the majority of focusing takes place. As such it is provides most of the optical power of the eye. Ideally it would be shaped like part of a perfect sphere, giving the same amount of focusing power over it whole area. In fact it tends to be more curved in certain directions. As such the focusing power varies in different parts. If this difference is large enough it can cause blurred vision. A simple way of looking at this is regarding the eye as being shaped very slightly like a rugby ball instead of a football. You would never be able to detect this simply by looking at someone’s eye.

AstigmatismAlmost all eyes have a degree of astigmatism. If however there is more astigmatism than normal, it
can cause blurred vision and glasses are given to correct this. It’s very common to have astigmatism as well as being long or short sighted.

In young children, if one eye has more astigmatism than the other this can cause a lazy eye (see lazy eye) and glasses would be given for this reason.

Illustration Courtesy Ted Montgomery

 

Childrens eye problems

Blepharitis
/
Chalazion
/
Conjunctivitis
/
Lazy Eye
/
Refractive Errors
/
Squint
/
Sticky Eye

 

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