Refractive surgery includes any surgical procedure that improves vision, reducing or eliminating the need for glasses or contact lenses. Refractive surgical procedures are designed to correct optical errors and bring images into focus as much as possible. The actual results of surgery vary, depending on the procedure used and the accuracy of the examination data obtained pre-operatively.
Dr. Fuerst has been performing refractive surgery for over 20 years. Among his satisfied patients are many ophthalmologists and others in the eye care field.
Dr. Fuerst’s goal is to help the patient decide whether refractive surgery is a good option and which procedure is best to achieve the desired results.
Visual Acuity and Refractive errors
Visual acuity is usually measured with a black-on-white character chart set 20 feet away, often projected using a series of screens and mirrors. A visual acuity of 20/20 means that you see what an average person with normal, healthy eyes sees from 20 feet. A visual acuity of 20/40 means that you see, at 20 feet away from the chart, what a 20/20 person can see from 40 feet. The best theoretically possible visual acuity is thought to be 20/8 (you see at 20 feet what a 20/20 person needs to move within 8 feet of the chart to see) because of the spacing of the retinal receptors known as cones.
Refractive errors are deviations from perfect focusing on the retina. In myopia (nearsightedness), the image is focused in front of the retina. In hyperopia (farsightedness), the image is focused behind the retina. Astigmatism refers to an ovalization of the image. Presbyopia is a condition which usually becomes significant after the age of 40, where the natural lens of the eye no longer allows adequate focusing for near vision. Refractive errors are measured in a unit termed the diopter.
Refractive Surgery for Presbyopia
In monovision, the non-dominant eye is set for near and the dominant eye is set for distance. This works well for many patients and can be tested pre-operatively by fitting contact lenses to mimic the planned surgical result. The vast majority of patients can adapt to monovision over time.
Some monovision patients use glasses for some tasks, such as for nighttime driving or for prolonged reading. A contact lens may still be worn to correct the near eye for specific needs, such as sports.
LASIK and PRK
Monovision can be achieved by correcting one eye for distance and the other for near using LASIK or PRK. Sometimes only one eye requires treatment to achieve monovision.
Clear Lens Extraction
The crystalline lens of the eye can be removed and replaced with a multifocal intra-ocular lens (Array? IOL) in both eyes or intraocular lenses can be selected to create blended vision. The surgical procedure is fundamentally the same as the procedure used to perform cataract surgery. Clear lens extraction with a multifocal lens may be a good option especially for those with early cataracts and for individuals who are not good monovision candidates but desire the ability to see both at distance and near in each eye. These procedures are considered ‘off-label’ uses of intraocular lenses.
Laser Vision Correction
Laser vision correction with an excimer laser can be performed using various methods. The substance of the cornea (corneal stroma) is exposed by removing the surface epithelium (PRK or LASEK) or creating a flap (LASIK). Next, the excimer laser reshapes the stroma using a patient-specific, software-controlled ablation pattern to produce the desired correction (of myopia, hyperopia and/or astigmatism).
The choice of a no-flap (PRK) or flap (LASIK) procedure generally depends on the amount of correction needed, corneal thickness, previous eye surgical history, and other factors. No-flap procedures have a longer recovery and more discomfort than flap procedures. Flap procedures have faster recoveries and minimal discomfort, but flaps can slip or wrinkle, sometimes requiring further surgery. The visual results of all of the excimer procedures are excellent for good candidates. Complications and risks include undercorrection or overcorrection, dryness of the eyes (usually temporary, but can last 3 months or longer), night vision disturbances (glare, halos), infection, inflammation, and loss of best-corrected visual acuity.
In LASIK, a hinged corneal flap is created, followed by excimer ablation of the exposed corneal stroma and repositioning of the flap. LASIK is generally selected because of a more rapid recovery, less post-operative discomfort, and relative ease of enhancement procedures. Flap-related complications may occur. These complications are rare and can be minimized by using the Intralase laser (instead of a mechanical microkeratome) to create the flap. The disadvantage of Intralase is a longer total procedure time.
PRK and LASEK
These no-flap procedures avoid potential complications of a flap but are usually associated with more discomfort, a higher risk of haze (requiring prolonged use of steroid eye drops), and a delay of visual recovery of a few days duration. PRK or LASEK may be recommended for patients who have an increased risk of flap problems (highly curved or very flat corneas, some people with recurrent corneal erosions, and those with very thin corneas). In rare situations (in addition to the risk factors listed above, anterior scleral buckles, optic nerve disease, a risky occupation or activity, or stable keratoconus) PRK or LASEK may be the only choice. Recovery from PRK is now much faster and less painful with less risk of complication with the newer excimer lasers.
Several different excimer lasers are FDA approved in the United States. Dr. Fuerst uses the AMO Visx Star S4 and the Alcon Allegretto Wavelight lasers. Both of these lasers can be used for custom LASIK.
AMO VISX S4 ACTIVE TRAK™
The VISX S4 has a variable spot beam and a video-based tracker. Iris registration is a feature that allows the laser to track the eye by identifying the unique appearance or ‘fingerprint’ of the iris. Wavefront guided custom ablation with the VISX S4 is termed CustomVue™
Custom LASIK uses a sophisticated, objective measurement of the entire visual system, linked with software to the excimer laser, to minimize visual aberrations and maximize the visual outcome.
Until recently, refractive errors were measured using only sphere (nearsightedness or farsightedness), cylinder (astigmatism), and axis (direction of astigmatism). These are considered lower order aberrations. Although machines called autorefractors give close approximations, refractive errors are usually determined subjectively (“Which is better, number 1 or number 2?”) by the doctor and the patient. The accuracy of this subjective process is not ideal. The sphere, cylinder, and axis information is programmed into the laser.
Higher order aberrations, those that cannot be determined by refraction, exist to a varying degree in different individuals. Higher order aberrations, which are dependent on pupil size, may contribute to glare, halos, ghosting, and other causes of a decreased quality of vision. These higher order aberrations are not corrected with glasses, contact lens, or conventional LASIK.
Higher order aberrations can be measured by a method known as wavefront analysis. This technology, which has been used for decades in astronomy, has been adapted for use with the excimer laser. A computerized instrument performs the wavefront analysis to determine both low order (sphere, cylinder, axis) and high order (coma, trefoil, tetrafoil, etc.) aberrations of the patient’s visual system. Light waves generated by the wavefront-measuring instrument pass through the eye, which then measures the aberrations in the wavefront of the reflected light. After analyzing the data for reproducibility and consistency, the data is transferred to the excimer laser to determine the treatment needed to correct the visual aberrations and optimize the result.
Wavefront analysis can be obtained for most people, but factors such as small pupil size and previous eye surgery may prevent useable wavefront information. When Custom LASIK is possible, it is the preferred method. Dr. Fuerst uses AMO VISX Star S4 with CustomVue™. He uses the intralase femtosecond laser for LASIK.
Refractive Surgery Complications
Serious, vision threatening complications with LASIK or PRK are rare.
Flap complications are the most common of these uncommon problems. Corneal abrasions may occur when a mechanical microkeratome is used. Abrasions are more common in patients with certain pre-existing corneal disorders (corneal basement membrane dystrophy) and in diabetics. Free caps may occur when the flap hinge is too thin; free caps are realigned into position and will adhere without stitches. The visual result is generally unaffected. Incomplete flaps, poorly centered flaps, and damaged flaps may rarely occur. In these cases, the laser procedure is not done and the flap is put back into position and allowed to heal for a few months. In most cases, a new flap can be created and LASIK performed once healing is complete. Flap striae (wrinkles) or flap slippage which effect vision may require repositioning of the flap and ironing out the wrinkles. Bleeding from peripheral corneal vessels can be controlled using eye drops and stops when the flap is replaced. Blood under the flap may cause a delayed visual recovery.
Corneal ectasia (bulging) may occur if the stromal bed, the part remaining after lifting the flap and treating with the excimer laser, is too thin. Corneal ectasia may be treated with glasses or rigid gas permeable contact lenses. Severe ectasia may necessitate corneal transplant surgery. Corneal perforation or penetration is a serious complication but has been practically eliminated with the use of modern microkeratomes.
Laser complications include decentered treatments and central islands (central areas that are incompletely treated). Both of these complications are rare with current excimer lasers.
Infection, epithelial ingrowth, and diffuse lamellar keratitis (DLK) may occur in the early postoperative period. All of these conditions are serious, but fortunately rare. Infection is largely avoided by using the antibiotic drops provided and avoiding exposure to contaminants. Strict avoidance of swimming, diving, or other exposure to water is critical for the first week after surgery. Infection symptoms include blurred vision, pain, worsening redness, or a hazy or white spot on the cornea. Infection must be treated urgently to prevent or lessen permanent vision loss. DLK usually occurs one to three days after surgery. This non-infectious corneal inflammation may cause blurred vision and light sensitivity. If treatment with steroid drops is started right away, visual loss can be prevented. Significant epithelial ingrowth, requiring surgical treatment to remove the epithelium, is detected during one of the early post-operative visits.
Dry eye syndrome is extremely common after LASIK and may last 3 months or longer. Treatment with preservative-free artificial tears and/or placement of punctual plugs can control the symptoms of blurred vision, scratchiness, foreign body sensation, and burning.
Glare, halos, and difficulty with nighttime vision may occur for several weeks post-operatively. They usually resolve over time, but may be permanent. These symptoms are more common in patients with large pupils and large refractive errors. Persistent symptoms may be treated with eye drops that prevent pupil dilation or retreatment with one of the custom LASIK programs.
Overcorrection and undercorrection of the desired refractive result may occur, depending on many factors. These factors include variable environmental conditions during surgery (temperature, humidity), different healing response in different individuals, and regression of effect. In most cases, an enhancement (retreatment) can be performed when the measurements have stabilized, generally after 3 months.
Loss of best-corrected visual acuity is possible. In this case, even glasses or contact lenses may not improve the visual acuity as well as before surgery. Enhancement is an option in some cases.
After any type of excimer laser surgery, your central cornea will be thinner and the intraocular pressure measured with standard office instruments may measure falsely low. It is important to let any doctor checking your eye pressure to know that you have had refractive surgery.