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What is axial length in myopia?

Updated: Apr 20, 2023

Our eyes may either be windows to the soul or onto the world, depending on which quotation you are looking at, but how much do we know about the workings of these vital organs? Look at someone’s face and what we see is only the outer aspect of a complex mechanism that bends and translates light into signals that can be interpreted by our brains.

Because they are so complex, every aspect of our eyes plays a vital part in how we see and interpret the world. There is an intricate relationship between the curvature of the optical surfaces of the eyes and the length of the eyes from front to back.

When balanced in proportion, those differences help the eye to work at an optimum level. But vision changes can occur if the eye grows outside of those limits. The most common of these is myopia. However, most people are born long-sighted (hyperopia). As we babies grow, the eyes try to change size to make vision perfect (emmetropisation).

In general, when we are born, the distance from the front to the back of the eye, known as the axial length, is around 16mm. By the time we reach adulthood, axial length has grown to some 24mm. Most of the growth occurs in the early childhood years, being most rapid in the first ten months of life.[1] Generally, the changes of the eye surface curvatures and eye length stay in ratio to keep vision clear for distance. Sometimes, though, the eyes grow longer than they should, or the surfaces grow steeper than they should.

Within normal boundaries, the axial length lets light focus on the retina, enabling us to see clearly. If the axial length goes beyond 24mm, the point at which light is focused within the eye may move in front of the retina. As a result, it becomes harder to see objects clearly at a distance. This is known as myopia or short-sightedness, specifically axial myopia.

Because the eye grows fastest in childhood, in most cases, myopia is most likely to develop.

Interestingly, in 2022 a Chinese study identified a correlation between growth spurts and increases in axial length, recommending that when children grow quickly, axial length elongation should be monitored.[2]

Understanding this and other potential trigger points could help parents to identify whether their child might be starting to develop myopia. Parents can then go to optometrists to take actions that could help slow down myopia's progression. If you think your child may have myopia, use our find a specialist tool to find a practice that offers myopia management near you.

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