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These are definitions that will help you understand some
of the terms used in this website.
A - E . F - L . M - P . R - V
Aberrations Imperfections in the visual
or optical system that result in distortions and out-of-focus
images.
Ablation Removal of tissue by vapourisation
(in this case by the Excimer laser).
Amblyopia Also known
as lazy eye. A condition where the structure of the eye is
normal but the visual system in the brain is poorly developed.
This occurs because of some form of visual deprivation before
the age of seven. Amblyopia results in poor vision that cannot
be corrected by spectacles, contact lenses, PRK or LASIK.
Astigmatism A condition where the assymetrical
shape (like a rugby ball) of the eye (cornea) results in two
points of focus.
Bilateral Both; in this case, both eyes.
Crystalline lens The natural human lens
used for fine and adjustable focusing in the eye. This does
not provide the main power of focus in the eye.
Cornea The transparent front of the eye,
similar to the glass surface of your watch. This provides
the majority of the power of your eye.
Corneal pachymetry
This is the measurement of your corneal thickness. The cornea
is like the glass surface of your watch. It is clear and has
a certain thickness. The average thickness of a cornea is
500 to 600 microns or 0.5 to 0.6 millimetres thick.
Corneal topography A computerised system
to measure the shape and contour of the cornea. Essential
in LASIK.
Decentration This occurs when the treatment
area of the laser is not in line with the visual axis of the
patient.
Dioptre This is the unit of measurement
of the power of the eye or spectacles. In Singapore, we use
the term degrees. 1 dioptre = 100 degrees
Emmetropia This is the term for a normal
eye with no short- or long-sightedness. An eye with zero dioptre
or degree.
Endothelium The innermost layer of cells
of the cornea.
Epithelium The outermost layer of the
cornea. It consists of five layers of cells which can be scraped
off, causing an abrasion. This is removed in PRK but not in
LASIK or LASEK.
Excimer laser A type of laser (light amplification
by stimulated emission of radiation)-excited dimer made from
argon-fluorine gas. A dimer is a combination molecule of two
gases; in this case, argon and fluorine. It produces an invisible
far ultraviolet light that vapourises the cornea with almost
no damage or scarring.
A - E . F - L . M - P . R - V
Flap-and-zap Nickname
for LASIK. The flap is cut from the surface of the cornea
and the Excimer laser zaps the cornea, changing its shape
and power.
Haze This is a complication of PRK Excimer
laser surgery. The cornea develops scarring and cloudiness
after laser treatment. It does not occur with LASIK.
Hypermaetropia Also
called long-sightedness, this is a condition of the eye where
the cornea and the lens focus light behind the retina. The
retina is the layer at the back of the eye which is sensitive
to light.
Iris The coloured tissue behind the cornea.
It is usually brown in Asians and blue in Caucasians. It controls
the amount of light entering the eye. The pupil is the opening
in the iris. By regulating the size of the pupil, the iris
controls the amount of light entering the eye.
Irregular astigmatism This is a condition
where the irregular surface of the eye causes multiple images
and scattering of light.
Kerato Greek for cornea. Many medical
terms have their origins in the Greek language.
Keratoconus A disease of the cornea in
which the cornea is conical shaped, thin and bulging forward.
Most patients who have keratoconus have very high short-sightedness
and astigmatism. This disease of the eye is not correctable
by LASIK.
Keratomileusis One of the original surgical
procedures that was finally refined to LASIK. In this procedure,
a thin layer of the cornea is removed, reshaped and replaced
with stitches. This procedure was extremely difficult to master
with any consistency. Limited success was achieved in South
America, but it has now been abandoned because of the advent
of LASIK.
Laser Stands for light amplification
by stimulated emission of radiation. This light is of pure
wavelength and is of usually high energy.
Laser-assisted in-situ keratomileusis (LASIK)
A surgical procedure where a flap is made in the superficial
cornea and the Excimer laser is used to vapourise the remaining
cornea bed to change its shape. The flap is then replaced
to complete the procedure. The nickname for LASIK is flap-and-zap.
Lazy eye See Amblyopia.
Lens A light-focusing device.
Long-sightedness See Hypermaetropia.
A - E . F - L . M - P . R - V
Microkeratome The instrument
used in LASIK surgery to create a thin flap on the cornea.
Micron (΅m) One-millionth of a metre.
Monovision The use of one eye for near
focus while the other eye is for distance focus. Used in LASIK
surgery and sometimes recommended to contact lens users as
an alternative to bifocal or multifocal spectacle wear.
Myopia Also known as short-sightedness.
A focusing error in which the eye is too long or the cornea
is too curved. The cornea and lens focus light in the middle
of the eye in front of the retina. Defocused light reaches
the retina, resulting in blurred vision.
Optic nerve The nerve at the back of the
eye transmitting images from the eye to the brain.
Optical zone The area of treatment in
the central cornea where the laser changes the shape of the
cornea. The size of the optical zone should be as large as
possible to prevent night glare and haloes, especially in
patients with large pupils.
Over-correction A problem where too much
tissue has been removed and the result is not what was intended.
This is a complication of refractive surgery.
Photorefractive keratectomy (PRK) In this
procedure, the Excimer laser is used to remove the surface
of the cornea to change its shape. A raw surface is left to
heal. Most useful in low degrees of myopia only. Poor results
in high myopia, astigmatism and long-sightedness.
Phototherapeutic keratectomy (PTK) In
this procedure, the Excimer laser is used to remove scars
and irregularities on the surface of the cornea.
Presbyopia The loss of the eyes ability
to focus on nearby objects. This is due to the hardening of
the human lens and weakening of the focusing muscles due to
ageing. This commences at about 40 years of age and progresses
until 60 years of age. Also called lau hwa yan (old age
eyes).
Pupil The centre of the coloured part
of the eye (iris). It is black. By narrowing and widening,
it controls the amount of light entering the eye. At night,
the pupil is widely opened, allowing more light to enter the
eye. In the bright sun, the pupil constricts and prevents
the dazzle of the light from affecting us.
Pupillometry Measurement of the pupil
size, usually in the dark (scotopic conditions).
A - E . F - L . M - P . R - V
Radial keratotomy (RK)
Outdated surgical procedure popularised in Russia, which corrects
short-sightedness by weakening the periphery of the cornea
and causing central flattening. Major problems are fluctuating
vision, weakening of the eye, inability to treat severe myopia
and night glare.
Refraction Measurement
of the focus of the eye. In Singapore, the power of refraction
of an eye is termed in degrees. This includes short-sightedness,
far-sightedness and astigmatism. Your spectacle refraction
is usually written as:
-4.00/-1.00 x 180°.
The first figure, if negative, is the power of the short-sightedness
and, in this case, is 400 degrees short-sighted. The second
figure is the astigmatism (in this case 100 degrees). The
third figure is the axis or direction of the astigmatism
180° is horizontal while 90° is vertical.
Refractive surgery This is the surgical
correction of short-sightedness, hypermaetropia and astigmatism
but not presbyopia. Examples are RK, PRK, LASEK and LASIK.
Regression Loss of effect of laser therapy
with the recovery of the eye. This results in the reoccurrence
of initial refractive error, i.e. short-sightedness, hypermaetropia
and astigmatism.
Retina The light-sensitive layer at the
back of the eye. Light has to be focused on the retina to
obtain a clear image.
Retinal examination Most patients requesting
for LASIK are short-sighted or myopic. Unfortunately, myopes
are more prone to certain disorders. These are: retinal tears
and detachment; cataracts; and myopic degeneration.
All these decrease vision and can lead to blindness in some
unfortunate circumstances.
It is important to understand that LASIK will reduce or eliminate
your myopia and astigmatism. However, it will not make your
eye less prone to any of the above disorders. LASIK physically
reforms the eye by flattening the cornea. The myopic eye is
still long and the retina still thinner in these eyes.
Sclera The outer white coat of the eye.
It encircles the cornea and is covered by a transparent thin
tissue called the conjunctiva.
Short-sightedness See Myopia.
Slit-lamp examination A detailed examination
of your cornea, eyelids, conjunctiva (the transparent lining
of the white of your eye), iris and lens is done with a special
instrument called the slit lamp.
The health of your cornea, its shape and evidence of corneal
disease will all be carefully examined. Scarring and irregularities
of the central cornea will exclude the possibility of LASIK
surgery.
Stroma The thick middle substance of the
cornea. It is made mostly of collagen.
Tonometry Included in the slit-lamp examination
will be the measurement of your intra-ocular pressure. This
will exclude glaucoma, a disease of the eye which causes decreased
vision and loss of visual field. LASIK will not help these
patients.
Under-correction By far, the most common
complication of any refractive surgery: too little tissue
is removed and insufficient correction of refractive error
results.
Visual acuity Measurement of the visual
ability and accuracy of the eye. Perfect vision is stated
as 6/6 (in metres) or 20/20 (in feet).
Vitreous The gel-like substance that fills
the eyeball between the lens and retina.
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