The eye that never learned to see

Amblyopia — commonly called lazy eye — is reduced vision in an eye that is physically healthy. Nothing is wrong with the eye itself. What has gone wrong is the connection between it and the brain, which failed to develop properly during early childhood.

It affects around two to three children in a hundred, and it is the commonest cause of monocular vision loss in people under forty. Almost all of it is preventable if it is found early.

How it happens

The visual part of the brain develops by being used, in a window that runs from birth through roughly the first seven or eight years. If one eye sends a consistently poorer signal during that period, the brain suppresses it and develops around the better eye. The suppressed pathway does not mature, and after the window closes it cannot easily be made to.

Three things commonly cause the poorer signal:

An unequal prescription. One eye much more long-sighted, short-sighted or astigmatic than the other. This is the most easily missed cause, because the child sees perfectly well — with the good eye — and behaves entirely normally.

A squint. One eye turning in or out. The brain suppresses the image from the deviating eye to avoid double vision.

An obstruction. A congenital cataract, a droopy eyelid covering the pupil, a corneal opacity. This form is the most severe and the most urgent.

Why it is so often missed

A child with one good eye functions normally. They read, play, catch a ball, and never complain — they have no way of knowing that other people see with both eyes, because they have never seen any other way.

Parents do not notice because there is nothing to notice. Teachers do not notice for the same reason. A school screening that tests both eyes together will pass the child straight through; the deficit only appears when each eye is tested separately, which is why proper screening covers one eye at a time.

Very often it is discovered by accident in adulthood — at a driving medical, or when the good eye is injured. By then nothing can be done, and the person is left with one functional eye for life.

Treatment, and the window

Treatment has two parts. First, correct the cause: spectacles for the refractive difference, surgery for a cataract or a significant droopy lid. A meaningful proportion of amblyopia improves on spectacles alone, given time.

Second, make the brain use the weaker eye. Patching the better eye for a few hours a day is the standard approach. Atropine drops that blur the better eye are an alternative that some families find easier to sustain.

Treatment works best under seven. It still works, more slowly and less completely, into the early teens. There is some evidence of benefit later than that, but the realistic position is that the earlier it starts, the better the result — and that after adolescence the loss is largely permanent.

The practical consequence

Have children's eyes examined before they start school, whether or not anything seems wrong. The absence of symptoms is not reassurance; it is the expected finding. An examination at three or four years old, when a child can respond to picture charts, is the single most useful preventive step available.

Bring a child sooner if you notice an eye turning, a white or unusual pupil reflex in photographs, a persistent head tilt, one eyelid drooping, or if there is a family history of squint or amblyopia.

On patching

It is hard. The child is being asked to cover the eye that works and stumble through the day with the one that does not, in exchange for a benefit they cannot perceive. Resistance is normal and is the commonest reason treatment fails.

What helps is patching during an activity the child enjoys and that involves looking closely at something, keeping to a consistent routine, and being honest with the child about why. The schedule is set by the practitioner and adjusted at review; more patching is not automatically better, and over-patching can weaken the good eye.