Before Lasik
Cycloplegic Refraction
One of the most important factors in obtaining an excellent result from laser vision correction is the accurate measurement of your prescription (the “refraction”) before laser treatment. Although this can be somewhat time-consuming, there can be no substitute for meticulous attention here.
Because the lens within the eye has the ability to change its shape and therefore focus light to a different extent from moment to moment, it is not always possible to accurately measure the eye-glass prescription required by a patient unless this focusing mechanism is somehow inactivated. This is readily accomplished by the administration of a special “cycloplegic” eye drop that simultaneously dilates the pupil and paralyzes the focusing muscle of the lens. With the lens unable to change its focus, an accurate refraction can be achieved. This is most important for individuals under the age of 35, because younger people have a much greater ability to focus their eye and therefore it is more difficult to determine their most accurate prescription.
Advice: To ensure the accuracy of LASIK, your eyeglass prescription should be measured during a cycloplegic refraction if you are 35 years old or younger.
Pupil Diameter
In order to understand the importance of pupil diameter, we must first discuss the dimensions of both the cornea and the LASIK treatment. The diameter (width) of the human cornea is approximately 12 mm (1/2 inch); the diameter of the area treated by the laser during the LASIK procedure is referred to as the “treatment zone”, and it is usually about 6 mm (1/4 inch) for nearsighted individuals. For farsighted patients, the treatment zone is approximately 9 mm (3/8ths of an inch). The cornea is made steeper by laser removal of the peripheral cornea, and pupil diameter is of much less concern.
The figure below represents a side view of the human cornea in which the front surface (f) of the cornea is indicated. Three possible laser treatments a, b and c are shown, all of which would cause the same amount of vision correction because they would flatten the cornea equally. In other words, the curvature of the cornea would be the same after each treatment. However, the figure demonstrates that the amount of cornea removed becomes greater as the treatment zone becomes wider. This is easier to understand if you notice that lines a, b and c are parallel to each other, and therefore the amount of corneal flattening achieved is the same for each treatment. However, it is obvious that the widest treatment zone (c) removes more of the cornea than the intermediate width b, and b removes more of the cornea than a. Because there is a well-known limit to the amount of cornea that can be safely removed, the diameter of most laser treatments for nearsightedness does not exceed 6.5 mm.
Three examples of laser treatment (a, b and c) which would cause the same amount of vision correction (see text).
In this figure we can also see that the junction (j) of the treated and non-treated areas of the cornea is similar to a step; this step becomes more abrupt as the amount of nearsighted treatment increases. This region of corneal curvature change causes distortion of light rays that pass through it. Happily, this is harmless when the pupil is smaller than the size of the treated cornea because the distorted light rays cannot enter the pupil and therefore are not seen by the patient. However, if the pupil becomes wider than the treated area, these distorted rays can pass through the pupil and causes the patient to experience glare. This undesirable situation occurs most frequently at night, because the pupil automatically enlarges (dilates) in order to permit more light to enter the eye so that we can see as well as possible in this low-light situation. Therefore, the risk of glare after LASIK is greatest in patients who have both a large pupil and a large amount of nearsightedness.
It is obviously important to precisely determine the patient’s pupil size in the dark before LASIK treatment. However, this is not the simplest measurement to make because, if the room is darkened, the examiner cannot see the pupil! In order to overcome these obstacles, infrared cameras that can photograph the pupil in the dark have been developed. However, these cameras must be placed very close to the eye and this can result in an involuntary reflex by the eye to focus on the camera. This can cause an incorrect pupil measurement, because the pupil automatically becomes smaller whenever the eye tries to focus on a close object. An alternative method that I have found to be extremely accurate uses a telescopic lens system, requires almost no light and does not have to be placed near the eye. A greatly enlarged image of the eye is displayed on a monitor, and the diameter of the pupil can be accurately measured to within 0.25 mm (1/100th of an inch). Armed with this information, we can accurately predict who is most at risk to develop glare after LASIK and thereby make the best decision as to whether or not the procedure should be performed as well as the diameter (width) of the laser treatment that is best suited for the patient.
Measurement of pupil diameter
Advice: You should discuss both the size of your laser treatment zone and your pupil size in dim light with your LASIK surgeon. This is extremely important if you are very nearsighted (prescription of 4 diopters or more), or if your pupil enlarges to more than 6 mm in dim light.
Corneal Curvature
A good way to think of the cornea is that it acts like a funnel for light rays, gathering them into a narrow bundle that then passes through the pupil. This is much the same as a funnel that gathers a large steam of water and condenses it into a narrower stream. This is an important concept because excessive flattening of the cornea will reduce its light gathering capacity, and this can result in poor night vision. Also, fibers within the cornea find it difficult to remain stable if the cornea is too flat, and the cornea will tend to return toward its original, more curved shape. This can cause a return of some of the nearsightedness (“regression”).
The cornea gathers light rays, which pass through the pupil.
With regard to farsighted individuals, the opposite is true. Laser vision correction improves the vision of farsighted people by steepening the cornea, but excessive steepening is also not well tolerated, and can cause blurred vision and glare because the pattern of the cornea fibers become irregular. Just as there are limits to the amount of flattening of the cornea that can be safely accomplished for myopia, there are limits to the amount of steepening that can be safely induced to correct farsightedness. As with eyeglass prescriptions, the amount of curvature of the cornea is measured in diopters. The greater the number of diopters of corneal curvature, the more steeply curved is the cornea. Dr. Mackool has found that the best results with LASIK are most likely to be obtained when the corneal curvature is maintained between 37 and 48 diopters. Although good results can occur when this rule is not adhered to strictly, the chances of a suboptimal result are increased if the cornea is flattened to less that 37 diopters or steepened to more than 48 diopters.Advice: Verify with your LASIK surgeon that your cornea can be expected to remain between 37 and 48 diopters after LASIK. If this is not the case, an alternative method of vision correction should probably be considered.
Corneal Thickness
The thickness of the central region of the cornea must be determined before LASIK for nearsightedness, and an ultrasonic instrument is used for this purpose. With this information, the LASIK surgeon is able to calculate the thickness of the cornea that will remain after LASIK in order to be certain that it will be adequate. If too much corneal tissue is removed, the excessively thin cornea may become weak and structurally unstable; this can cause it to bow forward, causing a serious condition called keratoconus that may require corneal transplantation.
As a general rule, patients whose nearsighted prescription is 6 diopters or less will usually have adequate corneal thickness for LASIK. If this is not the case, PRK can nearly always be safely performed.
Advice: Ask your LASIK surgeon if he/she is certain that your cornea is thick enough to permit LASIK to be safely performed.
