Tuesday, August 13, 2013

Presbyopia: A sign you’re not a spring chicken anymore…..

Imagine you had a camera and it stopped focusing. It might take clear pictures in the distance, but up close everything becomes a hazy mess. Well, back goes the camera to the service centre or perhaps it's time to get the next iteration of your favourite DSC or DSLR (or autofocus lens...)

Spoilt camera lens

If only we could do the same with our eyes, for this is exactly what happens when the big 4-0 arrives. The lens in our eyes becomes stiffer, and this prevents it from focusing well for both far and near. In short, the eye becomes a fixed focus camera.

In actual fact, the process is a gradual one, and the lens loses its elasticity from as young as the twenties. However, it is in the early 40s when the loss of elasticity reaches such a level that focusing at reading distances becomes a pain. By the time one reaches the mid-fifties to sixties, there is virtually no focusing ability left.

The proper term for this is ‘Presbyopia’, but people may call it ‘old sight’, ‘longsightedness’, Lao Hua in Chinese, or Rabun Tua in Malay.

How it affects different people
How the condition affects you depends on what your existing spectacle power is, and what you normally use to obtain clear vision.

Let’s talk about the simplest situation, that of people who see well in the distance and do not need to wear glasses. (This includes people who have had refractive surgery like LASIK). These people continue to see well in the distance, but find that they have to hold reading material further and further away to keep it in focus. Eventually, it just becomes too tiring to read at arms length and also having to squint forcefully causes significant brow ache. That’s when reading glasses become one’s indispensable companion.

The second situation is of people who are shortsighted, ie they need glasses to see clearly in the distance. Without glasses, these people’s eyes are like cameras with extension tubes fitted. They lose infinity focus but on the other hand can focus at macro distances. Therefore, if they take off their glasses, they can see remarkably well for near even when they reach the presbyopic age group. If they wear glasses or contact lenses for distance, though, they are no different from the people in the ‘normal’ group, ie their eyes have had their extension tubes removed and with the glasses everything nearby becomes blurry too. So for looking at near things, shortsighted people either remove their glasses, or they may get glasses with a reduced distance correction.

The final situation is of people who have real longsightedness. To give a camera analogy, these people’s eyes are like cameras where the lens has been mounted too close to the film or sensor plane. To make an object at any distance clear, some degree of focusing effort needs to be put in. Even for an object at infinity, the lens needs to be focused for closer than infinity for clarity. Because these people have to put in more effort at focusing, they will notice the problem with reading earlier than others. At some point, when they are unable to focus even for distant objects, everything becomes blurry. Distant objects are blurred, and near objects even more so. Many of these people end up wearing bifocal or progressive glasses, because they always need something to clear their vision, whether for far or near objects.

Common myths
I’m shortsighted, so ‘longsightedness’ after the age of 40 will cancel out my shortsightedness
Since shortsightedness is a focusing error related to the length of the eyeball, while ‘old sight’ is a problem related to the focusing ability of the lens, they cannot cancel each other out. A shortsighted person will remain shortsighted no matter how old he or she gets. However, they have the option once presbyopic, of seeing things close up by taking off their glasses/contact lenses.

Once I start to wear reading glasses, my presbyopia/old sight/lao hua/rabun tua will get much worse very quickly. So I’m not going to wear them!
Without wearing reading glasses, many older people manage to get by, by squinting, by reading in very bright light, and by holding things as far away as physically possible. It takes a lot of effort. Once they have the reading glasses, everything gets clearer without having to put in all this effort. They may forget that they used to have to strain very hard to see, so that now, without the extra effort and without the reading glasses everything appears blurry. They may blame it on the glasses, when it was actually due to their overcompensating in the past! In actual fact, the condition slowly progresses, with or without reading glasses. Therefore, I advise my patients to wear the reading glasses if they have to and if they feel easily tired otherwise.

(PS: If the camera analogies don't quite ring a bell, feel free to clarify with me. Unfortunately I'm going through a 'gearhead' phase...)

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