Why is my child’s squint worse since wearing glasses? Accommodative esotropias and the mechanism behind them.
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This question and the question, ‘Will glasses fix the turn in my child’s eye?’ are two of the most common questions I get in practice. The hospital does a fantastic job, but often we can fill in a few gaps regarding young children’s care.
Before we can understand why it goes wrong, often it is helpful to get an idea of how it works. I liken in the human eye to a projector and a screen. There are two lenses in the eye, the cornea, and the crystalline lens inside the eye. The combination of these two lenses and a variable aperture in the form of the pupil, forms a projector, projecting an image of the real world onto the lining of the eye. The lining of the eye (the retina) samples the image changing it from light information into electrical information. The image is then sent out of the eye via the optic nerve to the brain; actually to the primary visual cortex at the very back of the head, where we perceive the visual information. Bizarrely none of what we see happens in the eye, it all happened in the brain.
Often people think that we can see at more than one distance at one time, but if we focus on something up close, objects in the distance are blurry and if we focus on something in the distance, objects up close are blurry. It is actually a combination of a muscle and the crystalline lens inside the eye, which enables us to adjust focus to different distances – I call this our autofocus mechanism. In the human eye the autofocus mechanism, the convergence mechanism (which pulls the eyes in when we look at things up close), and parts of the pupil mechanism are all inherently linked.
Young patients who are longsighted need to use more autofocus when not wearing glasses, meaning that they sometimes coincidentally converge too much, causing their eye to turn in. By wearing glasses, we reduce the need for the eye to autofocus, allowing the eyes to stay straight. For this reason, often children who have an eye that turns in, a pair of glasses to correct their long sightedness can keep their eyes straight. Frequently, when they take their glasses off, the eye will turn in.
As we go through life the amount of autofocus we have reduces. Very young children have loads of autofocus, and people at the age of 55 have none. As the amount of autofocus reduces, the effort to autofocus through uncorrected longsightedness is greater. This means the eyes are more likely to turn in without glasses.
Understandably, parents are keen to see a reduction in the tendency for an eye to turn in when glasses have been prescribed – as you can see, sometimes the opposite is true. Unfortunately if we were to operate on the eye to reduce the chances of the eye turning in without glasses, it would increase the chance of the eye turning out with glasses. We also need to remember that the child will have an increased need for the glasses simply to correct their eyesight, as the amount of autofocus they have reduces. This means that they will need the glasses to be able to see, consequently rendering the need for the eye to stay straight without glasses irrelevant.
In conclusion, my rather blunt response to the question in the title is, having explained all this to understandably anxious parents, that it is the wrong question to ask. The best question to ask, is if their little one’s eyes are absolutely straight with glasses – that there is not a tiny, cosmetically invisible turn in one eye, even with glasses on. What we are really aiming for here with these patient’s care, is that both eyes are perfectly aligned with glasses on, which will give the small person the best chance of both eyes developing the very best level of vision.
Additionally, it will give them the best chance of becoming ‘binocular’ where their brain can properly combine and interpret the information coming from both eyes, in order to very accurately determine relative distance of objects and speed of travel. Often we would refer to this is ‘depth perception’ but if you think about it, everyone has depth perception – I don’t have any patients who live in a 2D world. Binocularity gives the gold standard in depth perception, enabling improved performance in sport, better enjoyment of 3D media, and opening up more career opportunities.