Myopia, hyperopia and astigmatism: what each one means

These three words come up constantly at appointments and on prescriptions. All three are versions of the same underlying problem: the optics at the front of the eye cannot land the image exactly on the retina.

The cornea and the lens work like a camera objective. If the focal point sits in front of or behind the retina, the picture blurs. The direction and the shape of that miss is what gives the error its name.

In myopia the image focuses in front of the retina. The result is familiar: a child who cannot make out the board but reads a book comfortably.

Myopia usually shows up during the school years and tends to progress through growth, which is why a childhood prescription is remeasured at intervals.

As far as amblyopia goes, myopia carries less risk than the other two. The reason is simple: near vision stays sharp, so the brain still receives a crisp image regularly during the developmental years. The risk rises when the myopia is very high or when the two eyes differ markedly.

In hyperopia the image focuses behind the retina. Here a feature specific to young eyes comes into play: the focusing muscle can contract and cancel out part of the error.

This is why a hyperopic child can still read the chart. It is not free, though. Sustained effort brings headaches, early fatigue during near work and, in some children, an inward turn of the eye.

Hyperopia is the refractive error most often linked to amblyopia. When it is high, or when it differs between the eyes, the brain receives a persistently weaker image from one side.

Mild hyperopia is expected in children and decreases as the eye grows. What draws attention is a value well outside that natural correction.

In astigmatism the cornea is not equally curved in every direction. Picture a rugby ball rather than a football: steeper along one axis, flatter along the other.

Light therefore does not converge at a single point but spreads along a line. Unlike myopia and hyperopia, both distance and near are affected, so a child may not get a fully sharp image at any distance.

A significant uncorrected astigmatism, especially when it differs between the eyes, can cause amblyopia. Its orientation matters too, which is why a prescription carries an axis alongside the amount.

The first is size. Small errors usually affect neither acuity nor development.

The second is the difference between the two eyes, and it is often the most decisive. A child with an equal error on both sides sends the brain a similar quality of image from each; where there is a gap, one side stays permanently behind.

The third is timing. Left uncorrected during the years the visual system is developing, the same error can turn into a lasting loss of acuity. The identical error in an adult is simply a need for glasses.

Hyperopia is common in childhood and usually decreases as the eye grows, so a value at one age does not predict the value a few years later.

Myopia runs the other way: it tends to progress through the school years and usually persists. Astigmatism is comparatively stable.

All three are corrected with glasses. The glasses do not remove the error; they put the image back where it belongs on the retina. Where amblyopia is involved, that correction is the first step of treatment, not a decoration on it.