Eye Didn't Know!

The Amazing Eye

Richard Dawkins elegantly shows how the eye, a complex organ, evolved through gradual steps, each small step conferring a small additional benefit to the organism.

In this video, the ophthalmologist explains the function of the eye in the same manner as I do with patients.

Chiropractic Pseudoscience

What's the rationale behind the chiropractic field?

It's easy to trace the hierarchy and progression of scientific ideas behind most medical science (just as it's easy to show that medicine was pretty primitive 100 years ago). 

Medical students learn the biology, physiology, biochemistry, anatomy, pharmacology, pathology, and so on, of human diseases. Treatment of these diseases is mostly based on evidence, the gold standard being the double-blind, controlled clinical trial.

Most physicians and most of the public have no idea what chiropractic treatment is based on, beyond a vague notion of spinal manipulation. Evidently chiropractors themselves split themselves into a wide number of camps, depending on their belief systems regarding how much spinal manipulation can affect physical conditions, and how widely they wish to wander into other areas of alternative (non-evidence based) care, such as acupuncture, herbs, homeopathy, etc.

Here is an excellent video designed to educate the public of the philosophy behind chiropractic care. What intrigues me is that it most resembles a pre-scientific, faith-based belief system, rather than a science. 

The video was made in response to the incredible lawsuit by the British Chiropractic Association against the free-speech rights of a British scientist, Simon Singh, who publicly criticized chiropractic. 

It is important that science be able to advance without being stifled by the legal intimidation of lawsuits such as this. It is also important that the public be made aware of the lack of a rational foundation behind practices such as homeopathy and chiropractic. (I'm not sure, though, if what most people believe is really guided by reason).

Luckily, few chiropractors, that I am aware of, make claims that spinal manipulation is effective in treating disorders of the eye. This may be, in my opinion, because most eye diseases have fairly objective measurements, such as visual acuity, refractive error, or intraocular pressure, which are measurable and less susceptible to the placebo effect, which is the only likely benefit of many non-scientific treatments.

There are many ways to treat physical conditions, but not all of them are based on reason or evidence. Caveat emptor.

20/20!

Did you know?  Two patients today asked me what 20/20 vision means. I suspect that every day other patients wonder as well, but don't ask.

The easiest way to think about 20/20 is that it means that the eye can see at 20 feet what the average, healthy eye can see at 20 feet. It would follow that 20/200 (poor) vision means that the eye can see at 20 feet what an average, healthy eye can see at 200 feet, or that an eye that sees 20/10 (exceptionally well) sees at 20 feet what an average, healthy eye can see at 10 feet.

Having 20/20 vision does not necessarily mean that your eyes are healthy. The presence of glaucoma or many other eye conditions may not affect central visual acuity until late in the disease. Healthy eyes, though, usually do see 20/20 or better.

If an eye does not see 20/20, it is up to the ophthalmologist to determine the cause. It is also important to test each eye individually.

Did you also know?  The standard eye chart used by most doctors today is known as the Snellen Eye Chart, developed by Dutch ophthalmologist Herman Snellen in 1862.

image: wikimedia commons 

Why do you dilate my pupils?

Did you know? Patients frequently ask why it's necessary to dilate the pupils during a complete eye examination. It's simple, really. Imagine the difference between looking into a room through a keyhole, and looking in through a large window. If you really want to get a good view inside the room, the large window gives a much more complete view.

Similarly, dilating the pupil enables the ophthalmologist to view the entire back of the eye. What are we looking for? There are hundreds of things that can go wrong inside the eye, (many of which have no symptoms). We're checking to see if everything is normal. We look at the optic nerve, the blood vessels, and the retina, including the central retina, called the macula. If you have a specific complaint, like floaters, or a family history of glaucoma, or blurred central vision, obviously we will pay particular attention to the part of the eye which is most likely involved.

Here is a diagram which shows the advantage of a dilated pupil. The diagram is incorrect, though. With a dilated pupil, and the proper equipment and technique, the entire back of the eye can be examined (including the periphery), not just the part shown in the diagram.

 

(courtesy of the National Eye Institute) 

Did you also know? In 1823, a Dr. Purkinje was the first to ever look inside a living human eye, when he devised the optics to do so. It was not until 1850 that another ophthalmologist, Hermann von Helmholtz, invented the ophthalmoscope independently and publicized his device that other doctors finally were able to look inside the eye and more accurately diagnose its conditions. Read more about the history of the ophthalmoscope here.

How the Eye Works

Did you know?  The average person has almost no idea how the eye works! When educating patients about their eye conditions, I routinely start from square one, and give them a quick overview of how the eye works. The simplest and most accurate explanation is that the eye is like a camera.

Light enters the camera through its lens, is inverted, and lands on the film (or digital sensor), where it can be processed into a picture.

Similarly, light enter the eye through the cornea (the clear window) and passes through the lens, where it is inverted. The light then lands on the retina, which is like the film in the camera. The signals from the retina travel through the optic nerve to the brain, where they are processed into a picture.

(image courtesy of the American Academy of Ophthalmology, with permission)

Did you also know? The pupil of the eye is like the aperture of the lens. In dim light, a photographer opens the aperture wider, letting more light in. Similarly, the pupil enlarges in dim light, letting more light in. In dim light, the photographer will choose a more sensitive film (or increase the sensitivity of the sensor). Similarly, the retina gradually adapts to dim light by becoming more sensitive to light (think of walking into a dim movie theater.)

So, are your eyes Nikon or Canon?