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Understanding
Refractive Disorders
------[ Your Prescription in Diopters
]
The History
of Vision Correction Procedures
LASIK
Laser
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PRK
- Photorefractive Keratectomy
Your
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Results
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Potential
Risks

Understanding
Refractive Disorders
Your Prescription
in Diopters

Refractive disorders for myopia
(nearsightedness), hyperopia (farsightedness) and astigmatism
are measured in units called diopters. Diopters represent the amount of
correction needed to provide 20/20, or normal vision. The greater the
degree of nearsighted or farsighted vision, the higher your prescription
in diopters. Measurements in negative diopters refers to nearsighted vision
while measurements in positive diopters refers to farsighted vision.
Also see Presbyopia
Diopter measurements are often referred to with the letter "D".
An example, -5.00 D means the degree of nearsighted correction is 5.00
diopters of correction to obtain 20/20 vision.



MYOPIA:
(Nearsightedness)
Nearly 70 million people (25% of the population) in North America are
myopic. With myopia, the cornea or optical schematic is too steep and
light rays are focused in front of the retina instead of directly on the
retina, creating blurred vision. People who are nearsighted can see images
clearly with near vision, but distance vision is blurred.
Nearly 90% of all myopic patients have corrections of less than -6.00
D, which is in the mild to moderate range. The severity of myopia is demonstrated
below:
Mild
Moderate
Severe
Extreme |
< -6.00 D
-6.00 D to -10.00 D
-10.00 D to -15.00 D
> 15.00 D |
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to Understanding Refractive Disorders

HYPEROPIA:
(Farsightedness)
Another 62 million people (22% of the
population) in North America are farsighted. With hyperopia, the cornea/optical schematic
is too short and light rays entering the eye focus behind the retina. Farsighted
people can see better at a distance, but have increased blurred vision at close range.
Mild
Moderate
Severe
Extreme |
+1.00 D to +2.00 D
+2.00 D to +4.00 D
+4.00 D to +5.00 D
> +5.00 D |
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to Understanding Refractive Disorders

ASTIGMATISM
Regular astigmatism occurs when light
is focused at two focal points, both in front and behind the retina. This happens
because the cornea is shaped more like a football than a basketball. People with
astigmatism have difficulty seeing clearly with both near and far vision.
Astigmatism is also measured in diopters. Over 50% of myopic patients have mild astigmatism
as well.
Mild
Moderate
Severe
Extreme |
<1.00 D
1.00 to 2.00 D
2.00 to 3.00 D
> 3.00 D |
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to Understanding Refractive Disorders
PRESBYOPIA
Presbyopia is a normal process of aging. This happens when the fibers
attached to the lens lose power and then lose elasticity and flexibility
to change from distance vision to near vision. Most people begin experiencing
presbyopia after they turn 40 years of age. This problem is treated with
reading glasses or bifocals. Presbyopia cannot be treated with
laser or any other form of vision correction surgery.
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to Understanding Refractive Disorders


The
History of Vision Correction Procedures
There has been interest
in vision correction procedures dating back as early as the sixteenth
century when Leonardo da Vinci contemplated the possible source of visual
disturbances. A little later, in 1619, Scheiner measured the anterior
surface of the cornea. His discoveries are still used by ophthalmologists
today who recognize that refractive surgery often depends on changing
the cornea's anterior contour. Even lens removal as a means of correcting
high degrees of myopia was discussed by Boerhaave in 1746.
Johannes E. Purkinje observed in 1823 that images form on optical surfaces
when they reflect external light. His observations led to the development
of the Purkinje principles and the four Purkinje images. From these developments
our modern-day understanding of keratometry (measuring the curvature of
the cornea) and theories of visual accommodation began to grow.
Several decades later came the advent of topical anesthesia, which led
to cataract surgery after the civil war. In 1867, with the development
of the keratometer (instrument to measure curvature of the cornea), surgeons
could measure astigmatism following cataract surgery. In 1869, Snellen
(after whom vision charts of today are named) proposed using incision
across the steep meridian of the cornea to flatten it and treat astigmatism.
However, it wasn't until 1890 before Galezowski would actually attempt
to flatten the corneal contour.
In 1895, Faber performed a full thickness corneal incision to decrease
naturally occurring astigmatism in a 19 year-old patient, enabling him
to pass his vision test for entrance into the Royal Military Academy.
But all these efforts were focused on astigmatism, not myopia or hyperopia.
At this time, a Dutch physician, Leendert Jan Lans began to systematically
study and define the principles of keratotomy (the making of incisions
in the cornea) from 1895 through 1939. By varying the number, direction
and shape of incisions on the cornea, Lans could manipulate the effects
and tailor the visual correction. This period was considered the first
trial and error for refractive surgery.
In Japan in 1936, Dr. Sato observed the flattening of the cornea in patients
who had sustained traumatic injury to the eyes. His work led assistants
to establish the value of radial keratotomy, built upon the principles
outlined by Lans. Sato brought anterior and posterior keratotomy to clinical
practices in hundreds of patients and reported the results in the 1940's.
Around 1949, Jose Barraquer in Columbia developed the idea of lamellar
(pancake and flap based) corneal surgery to alter the shape of the cornea.
He discovered that lamellar keratoplasty could flatten the cone of a keratoconus
patient, significantly reducing myopia. He changed the cornea's shape
by removing the anterior cornea (similar to today's corneal flap used
in LASIK) with an instrument called a microkeratome. In 1987, a protege
of Barraquer, Luis Ruiz of Columbia, modified the principles of microkeratome
corneal resection by using an automated form of the instrument to perform
the operation directly on the eye. This procedure, called automated lamellar
keratoplasty (ALK) was used to correct high levels of myopia and was the
first real breakthrough in the treatment of hyperopia.
By the mid-1970's, Russia's Dr. Fyodorov had determined that most of the
radial keratotomy flattening effect could be obtained with sixteen or
fewer incisions placed on the anterior cornea. He developed a system,
that by varying the number of incisions and the amount of uncut clear
central zones, permitted him to carefully control the degree of vision
correction. It was he who convinced the world that RK surgery could indeed
reduce or eliminate myopia.
RK surgery was introduced into the United States in 1978 by Leo Bores,
one of Dr. Beitman's mentors. Since its introduction, over 2 million people
have had RK surgery in the U.S. alone.
Limitations of RK prompted further research and development, leading to
today's modern laser techniques. In the early 1980's, Bechman and Peyman
and their associates used a carbon dioxide laser to create thermal shrinkage
of the cornea in order to change corneal contour. In 1981, John Taboada
reported that the argon-fluoride Excimer laser had the ability to indent
eye tissue. Work was then proceeded on ablation (removal) of corneal tissue
to flatten the cornea. Further evaluation was performed by Steve Trokel.
The first use of the Excimer laser on blind human eyes took place in 1985
by Seiler in Germany, followed by L'Esperance of the United States. The
procedure was called photorefractive keratectomy or PRK. In 1991, Michelson
and a group of elite ophthalmologists became the first five clinical investigators
of the Excimer laser in the United States, utilizing the laser manufactured
by Summit Technology.
By 1991, Pallikaris and Buratto and their associates had combined lamellar
splitting (using the blade of a microkeratome to make a corneal flap)
with Excimer laser ablation. It was Pallikaris who coined the term LASIK
(laser insitu keratomileusis). The initial clinical trials of LASIK in
the United States began in 1996. These clinical investigations culminated
in the approval by the FDA of the LASIK procedure in 1999.
When compared to PRK, the results of LASIK appear to be better and healing
is significantly faster, with fewer complications. As a result, LASIK
has become the procedure of choice by vision correction surgeons throughout
the world.

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