Medicine For People!

June 2018

Human Eye

Table of Contents

  • Cataracts
    • When Surgery Shouldn't be Delayed
    • What is a Cataract, Anyway?
    • Options in Lens Replacement
  • The Visual Cortex is a Wonderful Piece of Wetware
    • Dynamic Range
  • Macular Degeneration
  • Building a Landscape
  • Take care of your eyes!

Appendix

  • Cataracts in the Days of the Pyramids

Cataract

When Surgery Shouldn't be Delayed

Mary had trouble seeing. Her optometrist told her that she had cataracts but her scores on the standard vision test were not bad enough for Medicare to approve the surgery. Nonetheless, Mary didn't feel safe driving at dusk anymore. Sometimes she didn't see children playing near the street until she was almost next to them. She complained to me about this.

A quick test in the office showed that Mary had diminished contrast sensitivity, a problem her optometrist hadn't tested for.

Functional Acuity Contrast test
Photo by D Rienstra

While she could easily distinguish black letters on a white background, she had trouble when there was less contrast. Her eyes were not sensitive to grey objects against a slightly greyer background. At dusk, when colors start to fade, she couldn't pick out the children against the bushes.

I suspected that Medicare guidelines did cover cataract removal when contrast sensitivity was as poor as Mary's. She took the test result to my ophthalmologist colleague. He agreed, did the surgery, and now Mary drives safely.

What is a Cataract, Anyway?

Schematic Diagram of the Human Eye

The part of your eye that is blue or brown we call the iris. The black opening we call the pupil. The iris closes the pupil when light is bright in order not to damage the retina—the "film" at the back of the eyeball. The iris relaxes when it is dark, allowing more light to enter the eye.

The lens of the eye occupies the dark circle of the pupil and focuses the incoming light on the retina. The lens is perfectly transparent, in the same way an uncooked egg white is transparent. Put enough heat on the egg white, however, and it loses its transparency. In the same way, decades of UV light passing through the lens makes it opaque as well. We call that a cataract.

Intemescent Cataract

In the U.S., we usually operate before the lens becomes this cloudy. Surgery consists of removing the cloudy lens and replacing it with a plastic implant.

After surgery, the world appears clear and brightly colored again. The plastic lens can incorporate whatever eyeglass correction you may have needed for perfect vision.

A physician acquaintance of mine asked his surgeon to use a replacement lens that would focus on objects near at hand, as the physician's work life didn't require the distance vision of, for example, an aircraft pilot. My friend was more concerned about reading the ingredient labels on the supplements his patients were taking, and the small print in the reference materials needed in his work. His eye doc complied: a close-up lens was inserted into the left eye, and my friend was spared the hassle of putting on and taking off his readers all day.

A couple of years later my friend needed to have a cataract removed in his other eye. This time, he asked his eye doc for a lens appropriate for distance vision in the right eye. After surgery my friend easily passed the visual test at the driver's license bureau and safely retired his driving glasses.

The Visual Cortex is a Wonderful Piece of Wetware

You are familiar with computer hardware and software. Computer geeks have another word—wetware—for the processing power between your ears.

What my friend discovered with his two new implanted lenses was this: He could see close-up without readers. He could see far away without his regular glasses. He was not aware that in each case only one eye was doing the job. Only when he was reading and blocked his left near-vision eye with his coffee cup did he notice the magic going on behind the scenes. Sometimes to prove the point he'd look at something far away. It was clear and distinct until he put a hand in front of his right eye, when it became as blurry as it had been before the surgery. Closing one eye at a time gave the same effect.

Otherwise, what he saw, or thought he saw, was the print in the book clearly focused in both eyes, and the lighthouse in the distance clearly focused in both eyes.

Thinking of all the surgical procedures of modern medicine, cataract surgery is in the top rank of benefit to cost.

Dynamic Range

Ever see one of your outdoor photos turn out like the image on the left?

Dynamic Range
Image courtesy of Dave Dugdale [https://www.flickr.com/photos/davedugdale/14561000184] and used under a Creative Commons license.

The light intensity varies greatly from the top to the bottom of the photo. No camera can capture the bright and dark areas of a scene as well as our human eye, as (still imperfectly) illustrated on the right panel.

If you enjoy looking at optical illusions you are aware that our brain can fool us. It's not trying to. It's trying to give us the best picture it can of what is around us. And so it kindly erases our blind spot for us. For my physician friend, it builds the best picture it can. For all of us, it builds an image far better than any camera can reproduce.

It does one other wonderful thing, but first let me tell you a little bit about macular degeneration.

Macular Degeneration

The lens focuses the incoming scene on a point on the retina we call the macula, a slightly reddish area where the light-sensitive cells concentrate. These cells contain molecules that change form when light hits them and send a signal to the visual cortex. These light-sensitive molecules are then quickly regenerated to be ready for the next scene. Let your friend touch off a camera flash that makes most of the cells discharge at once and the momentary blindness will illustrate the point to you.

We call these light-sensitive molecules visual pigments because these molecules are colored. Refreshing them to be ready for the next image requires nutrients. People with damage to the macula—which we call macular degeneration—can somewhat re-invigorate these visual pigments from the eye vitamins recommended by the eye doc. You will find some of these nutrients, called zeaxanthin and lutein, in spinach and tomatoes. Regular use of these help delay or prevent macular degeneration.

Our retina covers the back and sides of the eyeball. Only the macula can distinguish one letter from another or let you see the details of your world. The rest of the retina provides less detail.

And herein lies the problem of macular degeneration. Damage that little bit of retina and you may become legally blind. Sure you can make out the general contours of your surroundings but you most likely will not read nor see the details of the kind glances of your loved ones.

Building a Landscape

And this gives you a window into another miracle of the visual cortex. Go out into the country sometimes and admire the landscape. You can see wall-to-wall details from one side of the valley to the other.

That is an illusion. You can pick up details only in the direction your eyes point. As you glance about, your visual cortex builds up an image area by area, in the same way a great landscape artist would, a piece at a time. As you shift your gaze to the left, your cortex remembers what it saw on the right and shows you those now-imagined details as if you could still perceive them. Your cortex images a world you can see only a part at a time https://xkcd.com/1080/, and gives it to you whole.

Take care of your eyes!

  • Wear eye protection while using tools. Even a hammer can cause a small piece of metal to spall off from a workpiece and imbed itself into your cornea.
  • Sunglasses must block 99% or more of UV light. Polarized sunglasses are best. $20 (2018 dollars) usually does the job.
  • Wear eye protection while engaging in contact sports.
  • Eat fresh fruit and vegetables.
  • Don't look at the sun.
  • Seek medical help promptly if you suspect a problem.

Appendix for the historically minded

Cataracts in the Days of the Pyramids

Doctors in ancient Egypt and India introduced the treatment of cataracts with a surgery called couching.  This spread to Europe and later the entire developed world.  Except in backward areas of Africa, modern techniques have largely replaced it.

Those ancient doctors used tools to hold the eye perfectly still, then inserted a sterile instrument from the side of the eye, entering the eye where the iris abuts the white part of the eye. Advancing the needle, they pushed the lens out of its normal location, "couching" it on the other side of the iris out of the way of the light.

Traditional Eye Practices
http://www.uniteforsight.org/traditional-eye-practices/module3

Remove the needle, pray for no infection, and voilà, cataract gone!

One problem; no lens and the patient can't focus light on the retina. Blurry vision. They probably solved that by using the principle of the pinhole camera. Or, rather, eyeglasses where the glass was replaced with an opaque sheet pierced by many pinholes. Try it with aluminum foil pierced over much of its surface with a needle. Except in low light, this replaces eyeglasses.

Rasterbrille
https://commons.wikimedia.org/wiki/File:Rasterbrille.jpg

Endnotes

[1] Wu J, Cho E, Willett WC, Sastry SM, Schaumberg DA. Intakes of Lutein, Zeaxanthin, and Other Carotenoids and Age-Related Macular Degeneration During 2 Decades of Prospective Follow-up. JAMA Ophthalmol.2015;133(12):1415–1424. doi:10.1001/jamaophthalmol.2015.3590

Newsletter edited by Jill Buhler with valuable assistance from Michael Rienstra.

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Medicine for People! is published by Douwe Rienstra, MD at Port Townsend, Washington.