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Glossary

A | B | C | D | E | F | H | I | K | L | M | O | P | R | S | T | U | V

A
  • 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.
B
  • Bilateral
    Both; in this case, both eyes.
C
  • 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.
D
  • 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
E
  • 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.
F
  • Flap-&-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.
H
  • 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.
I
  • 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.
K
  • 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 ab&oned because of the advent of LASIK.
L
  • 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-&-zap”.
  • Lazy Eye
    See "Amblyopia".
  • Lens
    A light-focusing device.

    Long-sightedness
    See "Hypermaetropia".
M
  • Microkeratome
    The instrument used in LASIK surgery to create a thin flap on the cornea.
  • Micron (Ám)
    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.
O
  • 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.
P
  • 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 eye?s 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).
R
  • 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 &, 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 underst& 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.
S
  • 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.
T
  • 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.
U
  • Under-correction
    By far, the most common complication of any refractive surgery: too little tissue is removed and insufficient correction of refractive error results.
V
  • 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.
JERRY TAN EYE SURGERY
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