Hi, I have a question!
If prolonged near work and wearing minus lenses makes one myopic, then will prolonged far work and wearing plus lenses basically make one hyperopic? Isn't that kind of like trading one condition for the other?
Because if that is the case then it would be a problem. I want to be able to see clear at any distance near or far.
Hi Reuben,
Someone at the AHS conference asked me essentially the same question. The answer is: it depends on the flexibility of your crystalline lens.
As background, keep in mind that your range of focus is determined by two factors: the ability of your crystalline lens to change shape (via contraction and relaxation of the ciliary muscles) and the overall axial length of your eye. Think of the the axial length as the setting the baseline focal length for distance, and the lens as providing the fine focus control that allows you to see close up periodically.
To understand how myopia develops, we should realize that our primate eyes evolved to be used most of the time for distance viewing, and only periodically for close up work. It is the evolutionary mismatch caused by a radical shift towards close work that has led to "spasm" of the ciliary muscles, causing blur in distance viewing. This is known as pseudo myopia. When followed by the prescription of minus lenses as a short term "fix" to restore distance focus, there is a secondary defocus for near viewing that induces axial lengthening and a more sustained type of myopia known as axial myopia. From that point there is vicious cycle of stronger and stronger minus lenses prescribed that temporarily fixes the distance viewing problem, but induces a new round of near work defocus and further axial lengthening.
Viewing objects right beyond the far point blur (between what I call D1 and D2) -- with or without plus lenses -- will induce a defocus that induces axial shortening, reversing axial myopia. If you keep pushing this, you will arrive at emmetropia, or normal eye length. If you keep pushing further, you can induce axial hyperopia, or far-sightedness, making it difficult to view print or objects up close.
However that pertains only to the axial element of focusing ability. So long as your lens remains flexible and able to be contracted by the ciliary muscles, you'll be able to achieve close up focus, even with a certain degree of axial hyperopia. This is why it is important to maintain flexibility of the lens. As we age, the lens tends to harden, and the ciliary muscles atrophy. The resulting condition of presbyopia refers to a restricted focal range -- both near and far. This is why older folks often need correction that allows for both close up reading and distance, such as bifocals.
There are basically two strategies for avoiding this situation. The first is to maintain good vision hygiene by alternating between close and distance viewing throughout the day using your natural eye. Use print pushing, with plus lenses as needed, when reading or at the computer. And build in plenty of breaks with distance viewing and active focus to resolve distant details, throughout the day. David DeAngelis' book, The Secret of Perfect Vision, has good suggestions for eye exercises that maintain the flexibility of the lens and reduce the risk of ciliary spasm.
The second strategy is to eat an eye-healthy diet. There is evidence that elevated insulin levels and blood glucose leads to weakening of scleral tissue in the eye and induces glycation that can impair the eye and stiffen the lens. Keep insulin and glucose levels low by moderating intake of carbohydrates and protein. Eating a phytonutrient rich diet will also stimulate your endogenous antioxidants, preventing oxidative stress and glycation in the eye.
Todd
