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How Does Light Scatter
Through Eyes After LASIK?
Editor: The otherwise excellent brief review article, “The Optical
Aberrations of LASIK” (Refractive Surgery Update, February) is marred by
one misstatement of optical theory. The authors’ claim that “the longer
the wavelength, the more scatter.”
For small-particle coherent scatter and even (to a lesser extent) in
large-particle incoherent scatter, the scattering is proportional to the
reciprocal of the wavelength raised to the fourth power. Smaller wavelengths,
not larger ones, scatter more. This is why the sky appears blue.
In a medium like the cornea, coherent scatter occurs and is mediated
by the highly organized very small collagen fibrils. For other, quite complicated
optical reasons, the scatter is less wavelength-dependent.
What occurs is a phenomenon known as “forward scattering” which greatly
reduces contrast sensitivity almost irrespective of the incident light
color. Scuba divers are all too familiar with this degradation in underwater
vision conditions. The degradative effect of forward scattering probably
also accounts for the loss of contrast in post-LASIK patients. A full explanation
of this complicated optical aberration can be found in Michael P. Keating’s
book,
Geometric, Physical, and Visual Optics (1988, Butterworth-Heinemann).—Irwin
M. Siegal O.D., Ph.D., professor, research ophthalmology, New York University
School of Medicine, New York.
Paul M. Dowd, O.D., co-author of the article in question, replies:
The sky is not always blue. I’ve seen red sunsets and sunrises, and
I’ve seen rust-brown smog. These are cases where long wavelengths scatter
more than short ones.
I’ve also seen with my own two eyes long wavelengths scatter more than
short ones when they pass through my interface post LASIK.
I’m sure this phenomenon, which seems contrary to the way light scatters
off water vapor in air, is not the same thing. It has to do with light
waves of varying lengths and how they pass through the interface and are
diffused by the stromal lamellar fibers, which are no longer spaced evenly
to allow transmission without scatter.
Perhaps the new alignment is more disruptive to longer wavelengths than
shorter ones. For certain, as they reorganize spacing with the healing
process, the scatter becomes less noticeable.
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OSHA and Computer Vision: How We Got To This Point
Editor: Your News Review article, “OSHA’s Ergonomics Proposal OK with O.D.s—for
Now,” in the January issue addresses an issue that we at PRIO Corporation
have taken very seriously since 1998. We believe that the ergonomics standard
proposed by the Occupational Safety and Health Administration (OSHA) will
affect more of vision care than just the treatment of computer vision syndrome
(CVS). Your readers deserve to know the history of how we got here.
Obviously, the PRIO Vision Tester generates a unique Rx for use in computer-specific
eyeglasses. PRIO got involved in legislative action because our future
is wed to optometry’s. In fact, PRIO employs its own government relations
director who tracks any legislation or regulation that impacts how vision
care and optometry may participate in any health and safety program.
So, OSHA asked PRIO in February 1998 to testify at the first stakeholde
rmeeting in Washington, D.C. We testified that the federal ergonomic standard
should include CVS as a repetitive-stress injury, and also recognize optometrists
as licensed physicians able diagnose and treat CVS. Despite our efforts
to garner support from other ophthalmic industry groups, no other companies,
organizations or doctors testified at that meeting.
OSHA’s rule-making team then asked PRIO to return to Washington in April
1998 to provide more specific information on the relationship between CVs
and musculoskeletal disorders (MSDs). At our request, Kent Daum, O.D.,
from the University of Alabama-Birmingham School of Optometry, also attended
and represented the AOA’s position.
At that meeting, Dr. David Cochrane, leader of the OSHA rulemaking team,
assured us that the standard would not exclude vision or optometrists.
However, he cautioned us not to push to include specific language because
it would raise objections from businesses and conservative members of Congress.
We heeded his caution because we understood that this broad ergonomic standard,
when passed, would be a tremendous gain for optometry.
Many of the 1,200 doctors in the PRIO Provider Network then sent letters
of support, which OSHA later told us were very important to the agency’s
continued push for a broad standard.
In late 1999 we learned through our doctor network that some influential
optometrists had become concerned when OSHA released the proposed draft
of the ergonomic standard, and its broadly worded language did not include
any reference to vision or eye correction. Dr. Cochrane assured us thatnothing
had changed with regard to the issues with which optometry has been concerned—that
the draft standard did not exclude any body part or health practitioner.
With this information, we were able to meet with and convince the concerned
doctors that any effort to pressure OSHA to include more specific language
in the standard would be destructive for optometry. For now, the best course
is to build support for this version of the standard.
PRIO has lobbied hard to ensure that OSHA’s ergonomic standard contains
the best possible wording, given the political realities. We continue to
encourage everyone in the ophthalmic industry to support it.—Jon Torrey,
president/CEO PRIO Corporation, Beaverton, Ore.
Editor’s note: You may contact Cynthia Katz, PRIO’s government relations
director at (831) 462-4496; or you can e-mail her at cindyk@prio.com.
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Here’s One Idea for Channeling Dr. Vickers’ Talents
Editor: I took the January 2000 issue of your magazine with me to
catch up on my journal reading while waiting with my wife in a physician’s
crowded reception room.
It was just my luck to begin reading Dr. Montgomery Vickers’ column
(Chairside, “The O.D.’s Curse: Don’t Leave Home Without it”). I burst into
uncontrolled laughter that lasted 4 1/2 minutes. The doctor’s receptionist
must have been tempted to call the nearest psychiatrist. All eyes were
on me, and my wife gave me a look that would have withered the Statue of
Liberty.
But frankly, I didn’t give a damn. That guy, Vickers, makes me laugh.
Chain him to his typewriter and never let him go.—Elmer Friedman, O.D.,Wyncote,
Pa.
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LASIK for Each Rx Change? Ouch!
Editor: Regarding the article “Tomorrow’s Contact Lens Practice”
in the February issue: Yes, surgical corrections have made an inroad into
the contact lens picture. But, consider the practical problems for the
baby boomers, today’s early presbyopes. Vision and Rx changes are coming
soon for them. What will they do? Multiple “re-touches” after surgery?
That is really crazy. Conventional disposable contacts allow routine and
regular changes.
In my 38 years of wearing monovision, I’ve needed six changes in Rx
for my reduced myopia. Think of it: Cut my eyes six times to continue superacuity?
That would have been truly crazy. We must alert patients about future developments
logically.—George Elmstrom, O.D., Carmel, Calif.
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