Saturday, January 26, 2008

7.13 Computer vision syndrome

Does this picture look familiar? Yes, a CPA who's been working on spreadsheets all day. It is the beginning of the tax season, you know. And he is already feeling the eye strain (not to mention the stress).

CPA or not, all ye who work with the computers, welcome to the world of CVS (computer vision syndrome).

No doubt most of you have searched high and low for CVS and its remedies. Here, we will try to explain what, physiologically, may lead up to CVS. Quite a few factors, actually:

First, the attention factor. The prefrontal cortex in the brain is responsible for attention (its development is delayed by ca 3 years in children with ADHD) - needed for analytical tasks such as computing. This area is also coded for eye blinking. With the attention turned on, the blinking rate changes from a normal of once every 4 sec to a whopping 20 sec. That is plenty of time for the tear fluid to evaporate, which is further aggravated if you work in a "sick" building or if you suffer from dry eyes in the first place. With the tear film (known as the tear lens) disturbed, vision also will be affected. Often if you blink a few times to refresh the tear film, then the vision improves.

You can remind yourself to blink more often; although this, much like the New Year's resolution, is usually forgotten after a few days. There is actually pop-up software to remind you to blink. Some may find it useful while others more a distraction. The more reasonable approach is to regard attention-induced non-blinking as a dry eye issue and treated as such.

Next, the "dark focus" factor. Computer work is a type of near work. This actually requires 3 activities working together: the two eyes turn inwards, pupils constrict, and the crystalline lenses focus. So we have the medial rectus muscles, the iris sphinctor muscle, and the ciliary muscle all working at the same time. Hacker-style computing is not without merits: by sitting away from the monitor (preferably in a well-padded sofa), these muscles will not have to work as hard as at close range (usually 17-18 inches) - perhaps the reason why the hackers can hack 24/7 non-stop. Until...


There is, however, the "dark focus" factor. Computer screens, be they CRTs or LCDs, are all pixelated. Presumably the eyes have a difficult time remaining focussed on the pixels. And as a result, the accommodation tends to relax to a point farther away from the monitor, i.e., at the Resting Point of Accommodation (the "dark focus"). Since the eyes must constantly re-focus, the strain on the ciliary muscle can be a major source of eye strain.

How to avoid this strain? Well, if the refraction is done using a computer monitor as the visual target, then the final prescription may be different from that based on printed Snellen reading charts. Usually more plus power is needed and the two eyes may need different powers as well. The prescription is known as, what else, the computer glasses - although not to be confused with those US$5-15 OTC readers available at your local computer stores or pharmacies.

Sustained accommodation also can cause transient pseudo-myopia. In other words, after an 8-hour day, some people become (more) myopic. Driving home may become noticeably unsafe.

Then there is the glare factor. Glare generally refers to intolerance to bright lights, e.g., oncoming car headlights at night especially for people with early cataracts. In computer use, glare from the monitor can be a source of eye irritation as it is similar to staring at a light bulb all day long. Actually it's more than that, the refresh rate of a monitor in a way is a measure of how fast the lights flicker - similar to pulsating strobe lights or single fluorescent light tubes if you will. So now we have an uncomfortable repetitive on-and-off, as opposed to sustained continuous activation of retinal photoreceptors.

Of course, manufacturers of monitors do strive to provide you with glare-free, high-resolution, high-refresh-rate monitors. Well worth the purchase price, we might add. And anti-glare screens for CRTs may still be popular. Anti-reflection (AR) coating on your spectacles works quite well also.

Finally, the sick building factor. In office buildings with poorly-ventilated poorly-filtered arid air, ozone generated from the computers can be another source of eye irritation. This is, however, beyond the scope of eye care. Talk to your landlords.

Unfortunately, this is the digital age. No one escapes CVS.

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