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.

Thursday, January 24, 2008

7.12 I see 'little' people

(Gulliver's Travels: Lilliput hallucination?)

Occasionally, an elderly patient with counting finger vision reports seeing black cats darting across the room, zig-zag lines, little people moving about (hence the term "Lilliput hallucination"), various pulsating geometric shapes, closing and opening curtains, etc. Needless to say, the patient is very anxious ("Am I losing my mind?") and family members are equally concerned - until it is explained that these are a form of visual hallucination.

A Swiss gentleman by the name of Charles Bonnet documented in 1760 that his grandfather, nearly blinded from cataracts, complained of seeing vivid phantom images - hence the name, Charles Bonnet Syndrome (CBS). If you have never heard of it, you are not alone.

First and foremost, patients with CBS are mentally healthy. The hallucination is not the pharmacologically-induced type or from psychosis/dementia, but are, in a way, similar to the phantom-limb syndrome. In the latter, amputees can still sense itch and pain in the now removed limbs.

What causes CBS? Theories abound, e.g., sensory deprivation, dream activation, creative visual cortex, etc. The only way to make sure is to map the brain with MEG or fMRI during hallucination, at least to see which brain area is activated. This will be extremely difficult, if not impossible.

Not everyone experiences CBS, though - in only about 15% of low-vision patients according to one study. There are other studies, statistical numbers may vary but all are in the low percentiles. Some clinicians have reported CBS in patients with recent vision loss. We have encountered only patients with long-term (i.e., years of) loss. So, the onset of CBS may be patient-specific. The cause of vision loss per se, however, does not appear important.

Unlike the real pain in phantom-limb syndrome, CBS cannot be or even needs to be treated. Just know that patients with CBS do not have any psychiatric issues. Instead, their vision needs to be re-evaluated and possibly further rehabilitated.

Sunday, January 20, 2008

7.11 Flower child?

No, iris and pupil, part of the eye. Not being insulting, this is just to refresh your memory: Iris is of course the colored part of the eye and the central opening is the pupil. And we all know the latter regulates the amount of light entering the eye.

The color of the iris is determined by the proportion of two pigments, melanin (brown, controlled by chromosome 15) and lipochrome (yellowish brown, chromosome 19). A blue eye looks blue is because it has less melanin than lipochrome and less pigments in general. And the more melanin, the darker the iris. An iris without pigments would appear pale pink. A cross section of the iris is shown below:

The cornea (not shown) is to the left. The dark vertical structure in this image is the iris proper and the pink area to its right is the crystalline lens. In the posterior part of the iris, the darker band is the pigment epithelium with all the pigments. Without shielding from much melanin, blue eyes are inherently more sensitive to bright lights. Dark irises are pharmacologically more resistant to dilating eyedrops, however.

You can have one hazel eye and the other brown, too - known as heterochromia. Presumably Alexander the Great (356-323 BC) was heterochromic. This happens when one iris fails to generate enough melanin. It can be hereditary or from childhood trauma to the sympathetic nerve along the carotid artery. One can imagine someone (King Philip II?) picked up little Al by the head in a Macedonian horseplay, thereby causing the said injury. To all the dads/uncles out there: please don't do this to the little ones.

Sometimes, however, the pigments can get dislodged, as seen below in a retro-illumination photo. In which the light is reflected through the backside of the iris. And in the posterior surface of the cornea, a vertical band of pigments also appears (i.e., the Krukenberg's spindle).
This is known as pigmentation dispersion syndrome. If the pigments clog up the aqueous outflow, the intraocular pressure can rise, then we have the pigmentary glaucoma. Often filtration surgery is needed.

The pupils of all humans are round. Animals are not all so. Some animals have vertical pupils while others horizontal ones. They can be slits, ovals, or even W-shaped. And the reason why all these different shapes? You can propose your own theory and whatever it is, is probably right. It is usually something like slit pupils can shut quicker than round ones, nocturnal animals have vertical pupils to detect a prey scurrying away, and horizontal slits are better for navigating rocky terrains, etc.

Here are a few examples:

Above: Vertically oval pupil in an alligator, also seen in cats and some lizards. And below: horizontal slits in a goat (also in octopuses).
And a W-shaped pupil found only in the cuttlefish:Other than it looks like a miniature Batman symbol, a reasonable explanation is still lacking.

In humans, some are born without the iris (i.e., aniridia). Supposedly, during Renaissance, the courtesans would take Belladonna extracts (rich in scopolamine and atropine) to maximally dilate their pupils - to make the eyes more attractive. And the pupils are sometimes re-shaped by trauma from, e.g., recoiling bungee cords with a metal hook, or playing paintballs without the goggles. Or more frequently by surgery, e.g., during cataract removal (see image below):
Then we may have a case of light and glare sensitivity because the iris no longer functions normally. And sometimes a small opening in the iris is created to facilitate aqueous flow as a treatment for chronic narrow-angle glaucoma (see the small hole, 11 o'clock position, from laser iridotomy):The iris is such an interesting structure, some have gone further and developed iris-reading (as in palm-reading):
Although it is still unclear, for example, why/how/where/when the iris sphinctor muscle area should be associated with the digestive tract (and so on).