visual acuity from a number of causes, but typically caused by early visual
deprivation or high refractive error. Early diagnosis and treatment before
the age of 5 years can lead to a successful outcome.
Snellen acuity that can be obtained with correction. BCVA can be reduced
following keratofractive surgeries and certain eye diseases such as keratoconus,
macula degeneration, and optic nerve disease.
of vision obtained when both eyes are working together as a team. The advantages
of binocular vision are better visual acuity and fine depth perception,
for Conductive Keratoplasty, a non-incisional technique that involves the
local heating and reshaping of corneal tissue to alter its refractive properties.
Microwaves are applied very precisely to localized areas of the cornea in
a circular pattern in order to shrink the collagen. This produces a steepening
effect in the center. CK is approved only for the temporary reduction of
farsightedness and for the temporary reduction of presbyopia in one eye
for Clear Lens Extraction, the procedure whereby the natural lens of the
eye is removed to correct high myopia. For example, if the power of the
natural lens is approximately +12 diopters, then its removal will result
in a one-to-one reduction in myopia of 12 diopters. However, when a CLE
is performed in a young patient without presbyopia, the patient will lose
the ability to see up close. The risks of a CLE are about the same as with
any cataract surgery.
of the crystalline lens due to age, trauma, certain eye diseases, drugs
|Contrast Sensitivity||The ability
to perceive different spatial frequencies under conditions of varying contrast.
An example of high contrast and high spatial frequency would be the 20/20
letter on a well-lit eye chart. An example of low contrast and low spatial
frequency would be the ability to discern facial features under dim illumination.
A loss of contrast sensitivity often occurs following keratorefractive surgeries
such as LASIK, PRK, RK as well as certain eye diseases.
A corneal dystropy is a condition in which one or more parts of the cornea lose their normal clarity due to a buildup of cloudy material. There are over 20 corneal dystrophies that affect all parts of the cornea. Examples include Salzmann's Nodular Dystrophy, Groenouw's Nodular Corneal Dystrophy and Fuchs Dystrophy.
|Depth Perception||There are
two kinds of depth perception: stereopsis and monocular. Stereopsis is the
ability to perceive objects in 3D, or three dimensions. It can only occur
when both eyes are working as a team. Other depth cues, such as shadow,
relative size, relative motion, are called monocular cues because binocular
vision is not required.
is a disease causing breakdown of the blood vessels and capillaries within
the body. This can lead to small microaneurysms and hemorrhages in the retina,
called background diabetic retinopathy. When severe oxygen deprivation occurs,
new vessels are formed on the surface of the retina. This is called neovascularization.
The new vessels are fragile and bleed. This is called proliferative diabetic
retinopathy. At some point, intervention with laser treatment can prevent
further neovascularization and hemorrhaging. Called “panretinal photocoagulation,”
the aim is to destroy healthy retinal tissue to reduce oxygen demand.
|Double Vision||When the
eyes do not line-up with the object of regard, double vision can be the
result. This is called binocular diplopia. This is caused by the images
falling on non-corresponding points of the two retinas. When a double image
is perceived by one eye only, it is called monocular diplopia. Monocular
diplopia can result from a distortion of the optical media, such as high
astigmatism or corneal warpage, or a cataract.
|Dry Eyes||The tear
film is a complex layer of lipid, aqueous, and mucus components, and many
other biochemical constituents. Those substances are secreted by a number
of different glands. The major lipid glands are the meibomian glands of
the eyelids. The major aqueous-secreting gland is the lacrimal gland, located
upper-outer quadrant of the orbit, and the various small accessory glands
throughout the conjunctiva. Dry eye can result when any of the constituents
is altered in terms of quantity or quality. Causes of dry eye are manifold,
and include certain drugs such as antihistamines, Acutane, Retin-A, autoimmune
diseases such as arthritis, or common hormonal changes. Dry eye is much
more prevalent in women.
insufficiency, accommodative infacility, or accommodative spasm are all
disorders of the focusing system. The most common focusing disorder is simply
due to the aging of the eye and is called presbyopia. Other forms are due
to uncorrected refractive error, such as hyperopia(farsightedness), certain
drugs such as atropine, antihistamines, and antidepressants, or genetic.
used to describe the visual phenomena resulting from keratorefractive surgeries
such as LASIK, PRK, and RK. It stands for Glare, Astigmatism, Starbursting,
and Halos. The GASH symptoms are more intense, or likely to occur under
conditions of low light when the pupils dilate. It is generally accepted
that GASH symptoms are the result of a mismatch between the ablation zone
or correction zone of the surgery and the pupil. RGP contact lenses are
generally effective in resolving this problem. Additional surgeries are
|Graft Vs. Host Disease|
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. High astigmatism is usually defined as any amount greater than 2.50 diopters.
|Higher order aberrations||
Optical imperfections of the eye are also known as aberrations. Aberrations are generally considered to be either lower order such as nearsightedness, farsightedness, and astigmatism, or higher order. In general, lower order aberrations are those that can be corrected by conventional means such as eyeglasses and soft contact lenses. In the normal eye, lower order aberrations comprise about 85% of the total. In certain conditions such as keratoconus, pellucid marginal degeneration, corneal transplants, cataracts, and following some refractive surgeries, higher order aberrations are significantly elevated and may actually comprise most of the total aberrations of the eye. Higher order aberrations cause halos, ghosting, monocular diplopia, starbursts, and other phenomena. In most cases, the source of the higher order aberrations is the surface of the cornea. Specialty GP lenses have been shown by Dr. Gemoules and other researchers to be the most effective means of reducing these higher order aberrations.
Contact Lens. Also called a phakic IOL, or phakic intraocular lens, because
it is placed in an eye without removing the natural lens. An ICL is used
to correct high refractive errors such as nearsightedness and farsightedness
that are beyond the safe range of other, non-invasive procedures such as
LASIK or PRK. Complications of ICL implantation include glaucoma, secondary
cataract, and infection.
for Implantable Corneal Ring Segments. The commercial name for ICRS is Intacs.
Available in various curves and sizes, these acrylic ring segments are implanted
within the cornea to reduce myopia by flattening the center of the cornea.
Another use of Intacs is to improve the vision of patients with keratoconus.
The effects of Intacs are somewhat limited to people with lower amounts
of myopia. Also, the technique lacks the higher precision of other techniques
such as LASIK and PRK. However, the effects are somewhat reversible, and
there is no disturbance of corneal tissue within the optical zone.
The boundary zone between the clear cornea and the white sclera is known as the limbus. A contact lens that fits within this zone is called and intra-limbal lens. Intra-limbal lenses are generally GP lenses between 10.0 mm and 12.0 mm in diameter.
that cannot be completely resolved with sphero-cylindrical lenses is called
irregular. Irregular astigmatism is sometimes induced by keratorefractive
surgeries such as LASIK, CK, RK, and corneal transplants. A rigid contact
lens will usually resolve this type of astigmatism.
A type of corneal dystrophy, keratoconus is caused by a thinning central or paracentral cornea, with subsequent bulging from the intraocular fluid pressure. The onset is generally during the second decade of life. Keratoconus is marked by increasing myopia, astigmatism, and eventual distortion of vision that conventional eyeglasses or soft contact lenses cannot correct. Progressive keratoconus may result in a corneal transplant.
for Laser Assisted Sub-Epithelial Keratomileusis, a surgical procedure designed
to reduce or eliminate refractive errors such as nearsightedness, farsightedness,
and astigmatism. The procedure involves removing the top layer of the cornea,
called the epithelium, in one piece instead of the thicker flap as is done
in LASIK. After the excimer laser is applied to remove the desired amount
of corneal tissue, the layer of epithelium is replaced. LASEK is probably
more similar to PRK, in which the epithelium is removed first and them allowed
to re-grow over a period of two or three days. Currently, there do not seem
to be any clear advantages of LASEK over conventional PRK. However, like
PRK, flap complications are avoided.
for Laser Assisted In-Situ Keratomileusis, a surgical procedure designed
to reduce or eliminate refractive errors such as nearsightedness, farsightedness,
and astigmatism. The procedure is performed by first making a thin flap
of corneal tissue, folding it out of the way, then altering the shape of
the cornea by subtracting tissue using an excimer laser. The flap is then
replaced over the treated area. This method results in more rapid visual
recovery and less discomfort than other refractive surgery methods.
|Over/under Correction||A result
of refractive surgery in which the target correction is missed. Theoretically,
an enhancement can be performed if the magnitude of the error is sufficiently
large. On the average, 10 percent of laser refractive surgeries will result
in an enhancement. Not all enhancements will result in total resolution.
for Photorefractive Keratectomy, a surgical method of correcting refractive
errors such as nearsightedness, farsightedness, and astigmatism by removing
corneal tissue with an excimer laser. PRK was approved in the U.S. in 1997,
and precedes the development of LASIK. PRK is still very much in use, although
the visual recovery time is longer, and there is more post-operative discomfort.
However, surgical complications involving the LASIK flap are avoided.
PKP is the medical term for a full thickness corneal transplant.
A type of corneal dystrophy, pellucid marginal degeneration is a rare corneal thinning disorder in which affects the peripheral cornea. In that sense it is similar to keratoconus, which affects the central or paracentral cornea. The symptoms are the same as with keratoconus: increasing astigmatism and visual distortion not correctable with conventional eyeglasses or soft contact lenses.
Each eye contains a tiny opening in each eyelid, called the punctum. This is the entrance to the nasolacrimal canal, and functions to remove excessive tears from the eye. Placing a small silicone plug in one or more of the puncta can improve the symptoms of certain dry eye conditions. The procedure for temporarily occluding the punctum in this manner is known as punctal occlusion. It is reversible.
for Radial Keratotomy, an older surgical procedure for correcting nearsightedness
and astigmatism by making various incisions in the cornea mostly in a radial
pie-shaped arrangement. The technique has largely been abandoned in recent
years in favor of the safer and more precise excimer laser procedures such
as PRK and LASIK.
|Recurrent Corneal Erosion – RCE||The chronic
condition in which an area of the outermost layer of the cornea suffers
from repeated spontaneous and painful erosions, typically on first awakening
when the eye is first opened. RCE can occur in an area of previous trauma
or abrasion in which the epithelial layer heals imperfectly, forming a relatively
loose attachment to its basement membrane. RCE can also occur in epithelial
basement membrane dystrophy, in which there is a genetic or acquired flaw
in the basement membrane in the absence of trauma. Treatment is generally
prophylactic, using light petrolatum ointments, sometimes with sodium chloride
added, at night to prevent adhesion of the epithelium to the inside of the
|Refractive Conditions – Refractive Errors||Also known
as myopia, hyperopia, and astigmatism, or nearsightedness, farsightedness,
and astigmatism respectively. In the new terminology, they are also called
“lower order aberrations.” Myopia results when the image plane
of the eye is located in front of or anterior to the retina. Hyperopia results
when the image plane is located behind or posterior to the retina. Astigmatism,
which may be hyperopic, myopic, or a combination, results when the image
does not fall in a single plane, but is spread out, typically in two major
planes 90 degrees apart. Myopic eyes are typically longer than average,
and hyperopic eyes are shorter than average.
Sjögren's syndrome is a chronic autoimmune disease in which people's white blood cells attack their moisture-producing glands. Although the hallmark symptoms are dry eyes and dry mouth, Sjögren's may also cause dysfunction of other organs such as the kidneys, gastrointestinal system, blood vessels, lungs, liver, pancreas, and the central nervous system. Patients may also experience extreme fatigue and joint pain.
Copyright 2008 by Greg Gemoules, O.D.