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VISYAMieux voir avec le laser

Laser refractive surgery

Remember to take the following with you:

  • A list of any medicines you are taking
  • Your current contact lenses (but do not wear them)
  • Your glasses
  • Previous glasses prescriptions for the last 2-3 years

You may find it helpful to write down your questions for the surgeon so you don’t forget to ask something important. 

As you may be given eye drops that cause blurred vision, it is best to check with the clinic in advance whether you will be able to drive home.

Refractive surgery?
Introduction
Laser refractive surgery is becoming increasingly popular as people are turning to alternatives to traditional spectacles and contact lenses for vision correction. 120 000 surgeries were performed in France in 2008. About 9 million procedures have been performed in the United States since the approval of the excimer laser for refractive surgery in late 1995. About 1.4 million procedures were performed in 2007 alone (Marketscope).

Anatomy of the cornea
The transparent cornea is about one-half millimeter thick and has five distinct layers. The epithelium is the most exterior layer providing the smooth refractive surface and barrier against infection. The function of Bowman's membrane, which lies beneath the epithelium and its basement membrane, is unclear.

The stroma, made up of intertwining lamellae of collagen fibrils, provides structure and accounts for 90 percent of the corneal thickness. The endothelium and its basement membrane (Descemet's membrane) form the innermost layers. Endothelial cells,  are responsible for the natural corneal dehydration necessary for corneal clarity. Average central corneal thickness is about 550 µm, increasing to about 700 µm in the periphery. The cornea has a diameter (from the front surface) of about 11 mm vertically and 12 mm horizontally. The air-tear interface is the first refractive surface that light encounters and accounts for about 80% of the eye's total refractive power; the average corneal curvature (K readings) in the adult cornea is approximately 44.00 diopters (D).

Three types of cells are present in the epithelium: (1) basal columnar cells attached to the epithelial basement membrane via hemidesmosomes, (2) wing cells noted for thin winglike projections, and (3) surface cells joined by connecting bridges and covered by microvilli. Mucin is attached strongly to the surface. Usually, 5-7 layers of cells are present. Unlike stratified squamous epithelium in other areas of the body, the epithelium in the eye has an exceptionally smooth and regular surface, contributing to the transparency and light transmission characteristics of the cornea.

The Bowman layer is not a membrane, but rather an acellular structure consisting of collagen and representing the most superficial layer of the stroma.

The stroma makes up about 90% of the corneal thickness and consists of regularly arrayed flattened bundles of collagen called lamellae. Approximately 200-250 lamellae are present in the human cornea. Each bundle extends the width of the cornea and is about 2 µm thick and up to 260 µm wide. The parallel arrangement of these bundles together with the uniform spacing between collagen fibrils helps explain corneal transparency. Although relatively acellular, stromal fibroblasts called keratocytes can be found scattered throughout the stroma between lamellae, and they are responsible for collagen production and wound healing.

Optics, refraction and refractive error
Refraction is the bending of light rays as they pass from one transparent medium to another medium of a different density. Refraction is measured in diopters (D). The refractive power of a lens is the reciprocal of its focal length in meters. As an example, a 1D lens has a focal point of one meter; a 2D lens has a focal length of one-half meter.

The cornea and crystalline lens refract light entering the eye. The cornea is responsible for two-thirds of the eye's total focusing power while the crystalline lens accounts for one-third. Corneal power is fixed, but the lens can change its shape to become more powerful for viewing objects up close in a process called accommodation.
Many patients with myopia or hyperopia have some degree of astigmatism. This means that your eye is slightly oval and your cornea is shaped like a football rather than a sphere. People with astigmatism experience, distortion or tilting of images due to the unequal bending of rays of light entering your eyes. High degrees of astigmatism will cause blurred vision for distant and near objects. Refractive errors occur when light rays entering the eye do not focus properly on the retina:
•    In myopia (nearsightedness), the most common type of refractive error, the cornea is too curved or the lens is too powerful for the length of the globe. 

A myopic eye is too long in relation to its focal length. The image produced from a distant object by the optics of the eye will be placed in front of the retina and the projection on the retina will be blurred. Unfortunately, myopia is not only uncomfortable due to a reduced quality of vision; it may also cause, rarely, a number of serious conditions such as retinal detachment, myopic retinopathy, and glaucoma.
The most common reason to myopia is that the eye grows too long and thus shifts the retina backwards relative to the image of the world. Even though the disposition of developing myopia is partly inheritably, the exaggerated growth is beyond doubts triggered by the environment. It is the visual impressions that may control the eye growth and thereby the progression of myopia. This has been confirmed in a number of studies in animal models. The increased growth of the eye during myopia development is thus induced by the blur of the image on the retina, but today it is not known which the causing optical errors are neither do we know which parts of the retina are most susceptible. The progression of myopia might be also due to insufficient accommodation during near work (such as reading), which means that the crystalline lens is not increasing its refractive power enough to place the image on the retina when fixating a nearby object

Distant objects cannot be seen clearly because light rays are focused in front of the retina (show figure 1). The myopic eye has a focal point close to the eye. As a result, near objects can be seen clearly.

•    In hyperopia (farsightedness), the cornea and lens are too weak for the length of the globe. As a result, light rays reach the retina before they are focused to a single point . A distant object may be brought into focus by using accommodation, but clear near vision is difficult.
•    With an astigmatism, the refractive power of the eye is not the same in different meridians. Light rays can never be brought to a single point and objects will appear blurry at any distance. Astigmatism may occur with myopia or hyperopia. 

In emmetropia (an eye with normal vision), the focusing power of the cornea and lens is perfectly matched to the length of the eye. Parallel light rays from a distant object are brought into focus precisely on the retina and a clear image is perceived. The eye's focal point is at infinity.

Many patients with myopia or hyperopia have some degree of astigmatism. This means that your eye is slightly oval and your cornea is shaped like a football rather than a sphere. People with astigmatism experience, distortion or tilting of images due to the unequal bending of rays of light entering your eyes. High degrees of astigmatism will cause blurred vision for distant and near objects.

Technically called myopia, nearsightedness means you have trouble seeing and resolving objects at a distance. When the eye is too long in relation to the curvature of the cornea, images focus in front of the retina instead of on it and objects at distances appear blurred. Patients with myopia are able to see near objects.

Technically called hyperopia, farsightedness means you have trouble with close up vision. It occurs when the eye is functionally too short..

The rays of light are focused behind the retina producing a blurred image. Some people who are farsighted are able to use their focusing muscle (accommodation) to bring the image forward allowing them to see clearly. This ability worsens with age and reading glasses or bifocals may be needed. Farsighted people see distant objects more clearly than near objects.

The normal solution for refractive errors is to wear spectacles or contact lenses to see clearly. There is one other condition for which people normally need to wear glasses. This condition is normally associated with ageing, and is known as presbyopia.

Presbyopia is a normal ageing process. As we get older, the lens loses its ability to flex and focus at near.

The onset of presbyopia typically is between 40 and 50. When this occurs people who already wear glasses may need bifocals and those who have never worn glasses may require reading glasses. Presbyopia is an important concept to understand, if you are over 40, and if you are considering laser correction.

One advantage of mild nearsightedness (myopia) is the ability to remove your glasses after presbyopia sets in and still be able to read. If you have Laser Vision Correction for nearsightedness, you will lose this ability. That's because your vision typically becomes normal, and for the ageing eye usually will require reading glasses for small print.

The excimer laser has no effect on your focusing muscles and therefore cannot treat presbyopia. However, there are ways around this problem, and it is best to talk to one of our expert eye doctors to discuss the best solution for you.

Presbyopia is a different type of refractive error in which loss of accommodation occurs as the lens hardens with age. This results in an inability to bring a near object into focus on the retina and requires the use of reading glasses, typically in patients in their 40s. Presbyopia is not directly corrected by laser refractive surgery.

Corrective lenses — Corrective lenses are the traditional method of treating refractive errors. Myopia is treated with concave lenses with minus or divergent power to focus light rays on the retina. Convex lenses with plus or convergent power help correct vision in the hyperopic eye, and cylindrical lenses are used to neutralize astigmatism.

Spectacle prescriptions are used to correct the sphere and cylinder components of refractive errors. The first part of the prescription is the sphere. A plus number indicates hyperopia and a minus number indicates myopia. The second number of the prescription is the cylinder, otherwise known as the astigmatism, either written in a plus or minus. The third number is the axis of the astigmatism.

Wavefront testing — In a standard eye examination, the refractive surgeon will test for myopia, hyperopia, and astigmatism. However, patients may have irregular astigmatism defined as higher order aberrations eg, coma or spherical aberrations. These higher order optical aberrations can have significant impact on vision. In the past, the ophthalmologist had no way to correct a patient's irregular astigmatism. Spectacles only correct lower order aberrations such as sphere and cylinder.

Advantage of LASIK

  • LASIK makes permanent changes to correct refractive errors
  • Quick surgery and rapid recovery   
  • lasik is painless
  • Minimal temporary side effects
  • Quick return of usable vision
  • No injections, no sutures (only numbing eyedrops)
  • easy retreament

LASIK and the Improvement of Quality of Life

  • LASIK can eliminate risks associated with long term contact lens wear
  • LASIK can make you more eligible for certain professions
  • LASIK can increase convenience in everyday life
  • LASIK can allow more freedom in many activities, especially water and outdoor sports.
  • LASIK can improve your self-confidence.

Customized LASIK

  • There is a greater chance that you will achieve a twenty-twenty vision
  • There is a greater chance that you will get better vision than twenty-twenty
  • There is a lower chance of losing the optimal correction for your vision
  • There is a lower chance of losing the quality in your vision
  • There is a lower chance of losing sensitivity in terms of contrast
  • There is a lower chance of getting glares.

It is unlikely that you will have problems seeing at night

There may be a chance to correct previous refractive surgeries and make seeing better with the custom LASIK refractive surgery

Patients who cannot cooperate with procedures under a topical anesthetic and cannot accurately fixate or lay flat without difficulty are poor candidates for refractive surgery.

 What are the advantages of using the IntraLase as compared to the traditional LASIK?

Here are the following advantages that can be derived when you use the Intra Lase over the traditional LASIK method.

  • you will heal faster and you will recover much quicker
  • the surgery will be more accurate which makes better results possible
  • more and more patients have reported to achieve perfect twenty-twenty vision
  • less and less patients are reported to have deteriorated vision
  • you will have better cornea flap cuts
  • the process is much safer because you are relying on lasers instead of blades
  • the cut will be cleaner which makes it less prone to infections and inflammations

La Clinique de la Vision in Paris  ( cliniquedelavision@gmail.com ) has 4 lasers excimers ( Allegetto 400 eyeQ, Visx4, Technolas 217 Z 100 ACE-UG 2009 ) and 2 femtosecond laser ( ABBOTT FS 150 IEK ) this is the only such center in U.E.

Surgical techniques — The majority of refractive surgical procedures can be divided into five broad categories: incisional procedures (radial keratotomy); surface altering procedures (photorefractive keratectomy [PRK] and laser epithelial keratomileusis [LASEK]); lamellar procedures (laser-assisted in situ keratomileusis [LASIK]); intracorneal procedures (intracorneal ring); and intraocular procedures (anterior or posterior chamber intraocular lens). PRK, LASIK, and LASEK are most commonly performed  )

Photorefractive keratectomy — Photorefractive keratectomy (PRK) effectively treats low to moderate myopia, myopia with astigmatism, and low to moderate hyperopia without astigmatism . PRK is performed as an outpatient procedure using topical anesthetic. First, the corneal epithelium in the ablation zone is removed or pushed to the side to allow a more accurate ablation of the corneal tissue. The laser treatment is then applied to the exposed corneal stroma  Immediately after the laser is completed, the ophthalmologist applies topical antibiotic, topical steroid, and a topical nonsteroidal antiinflammatory drug. Then a disposable bandage contact lens is placed over the operated cornea.

PRK is usually performed as an outpatient procedure. The PRK procedure is very quick. It can take less than a minute. You will be awake for the rest of the procedure and not feel anything. Some oral sedatives will be given to you in order to minimize any discomfort.

In general, people will not feel any pain when they go through the PRK procedure. Anaesthetic drops will be put on your eyes in order to minimize discomfort. You will have to lie down as the doctor puts the laser into its proper place. Eyes are operated on one at a time and a retainer is placed on the eyes in order to keep them open. With the anaesthetic drops, this will not be uncomfortable.

There will be a laser machine that makes a steady clicking sound during the procedure. This sound is from the laser pulses that are being emitted. You may smell a faintly acid odour when tissues from your eye are being removed. The surgeon will have complete control over this laser during the procedure. It can be shut down any time just in case something wrong happens. When the procedure is finished, you will be asked to rest for awhile. If both of your eyes are getting operated on the same day then there may be a little time interval between eyes. There are even some people who have the next eye operated on the next week.

In the early postoperative period, patients may have significant tearing, photophobia, blurred vision, and discomfort because of the central corneal abrasion. With the use of the bandage contact lens and topical nonsteroidal antiinflammatory medications, postoperative pain is usually mild to moderate; however, patients occasionally require systemic analgesia for more severe pain.

The contact lens remains in the eye until the epithelial defect is healed, averaging three to four days. Antibiotics are usually continued for two to three days after the defect has healed and topical corticosteroids may be continued for up to three months postoperatively. Visual acuity improves once the epithelial defect heals, usually within one week postoperatively, and typically fluctuates for several months after the surgery before stabilizing at around three months. Glare, halos, and dry eye symptoms are common in the first month following surgery, but usually diminish or disappear entirely by three to six months. Patient follow-up is variable, but a typical schedule is postoperative day one and three and then again at one week if the cornea is not reepithelialized.

Laser-assisted in situ keratomileusis LASIK — Laser-assisted in situ keratomileusis (LASIK), now the most commonly performed refractive surgery, is an effective treatment for low, moderate, and high myopia with and without astigmatism, as well as hyperopia with and without astigmatism  ]. Like PRK, LASIK is an outpatient surgery performed with topical anesthesia. A Laser femtoseconde, is used to raise a corneal flap about the size of a contact lens. This flap usually averages 100/120 microns thick and is folded back to expose the underlying stroma . Microkeratom is an old method still used with success but including an higher percentage of complications a thicker flap and a less predictability

The excimer laser is used to ablate a precise amount of corneal stroma and the flap is irrigated and replaced . The cap is stabilized without sutures by the natural corneal dehydration created by the endothelial pump. Flap stability and adherence to the corneal stroma are checked after surgery and patients are usually sent home on topical steroid, topical antibiotic,or topical nonsteroidal drops. The patient also is instructed to use an eye shield overnight with follow-up typically scheduled on postoperative day one and then at one week. The patient is usually seen again at one, three, and six months

LASIK has significant attractions for the patient. It causes little pain, authorizes a bilateral procedure, provides quick recovery of vision, and has the potential for treating higher levels of myopia [  LASIK has also been found to be safe and effective to treat both eyes on the same day, while PRK is usually done on two separate days  . LASIK enhancements are more easily performed, at least within the first 6 to 12 months, by lifting the original flap and retreating the stromal bed to correct any residual refractive error  . Unlike PRK, LASIK produces less stromal haze and does not require continuous steroid therapy. Nevertheless, despite the different surgical techniques of PRK versus LASIK, the refractive outcomes are similar

Now, surgeons use a new procedure called Ultra Lasik or Premium which associates femto second laser excimer laser (upgraded in 2008) and customized algorithm with 4 D recognition. These progress like others like Topo Link...Custom Vue...Zyoptix...) reduce sides effects and night vision difficulties. Complications are far less important with these new methodology.

 Are you a good candidate ?

  • Feel that spectacles/contact lenses are a nuisance in your job, sporting activities or
    personal life
  • Suffer from the effect of scattered light (aberrations)
  • Have been told that your pupils are too big, or your corneas too thin,for standard treatment
  • Need vision correction and have a prescription range of up to – 12.00 diopters sphere
    and – 7.00 cylinder
  • Have had stable vision for the past 2 years
  • Have healthy eyes, good general health and are not taking medication that affects the healing process  
  • Are older than 20

 Refraction — The initial patient visit includes a testing of uncorrected visual acuity (UCVA) and a measure of the current spectacle prescription with BSCVA. The patient's refractive error must remain stable for one year, defined as no more than a 0.50 D change in either the manifest cylinder or manifest spherical equivalent. The ophthalmologists will review all previous eyeglass prescriptions, prior medical records, and measure the current spectacle prescription on the initial preoperative evaluation.

Sometimes the initial visit requires a cycloplegic refraction to paralyze the eye's ability to accommodate. The patient will return on another day for a repeat manifest refraction if there is a greater than –0.50 D difference between manifest and cycloplegic refraction.

Slit lamp examination — A slit lamp examination evaluates the lids for evidence of blepharitis, meibomian gland dysfunction or infection, and the use of cosmetic makeup. The conjunctiva and sclera may reveal injection from chronic irritation or abnormal pooling of the tear film. The cornea may show superficial punctate keratopathy indicative of dry eye syndrome, epithelial irregularities such as epithelial basement, membrane dystrophies, scarring from previous trauma, or vascularization into the cornea from previous disease. The cornea may show signs of Fuchs endothelial dystrophy, which can lead to corneal decompensation and poor flap adhesion. The anterior chamber may show signs of inflammation from chronic uveitic processes. The iris may have abnormalities and the lens may have cataractous changes, which will affect the postoperative visual acuity.

Pupil size — Pupil size evaluation is determined in the initial refractive surgery screening. The pupil is measured with a device called a pupillometer that measures the pupil using infrared light in a dark room. Large pupils are associated with a higher risk of postoperative glare, halos, and night vision difficulties 

A typical myopic ablation has an optical zone of 6.0 mm; patients with a pupil size greater than 7.0 mm in dim illumination are at the highest risks for these postoperative complaints. The goal of the ablation is to treat the cornea overlying the pupil in the dark in order to prevent light from entering the eye at the edge of the treated and nontreated cornea

Dry eye evaluation — The risk of dry eye symptoms increases after laser refractive surgery. Thus, the ophthalmologist must rule out a diagnosis of dry eye before performing surgery. A The examination for dry eyes includes a basal tear secretion test , the tear breakup time (TBUT) evaluates the oil secretions of the meibomian glands.   Rose bengael  or green lyssamine stainarealso used to stain conjunctival epithelial cells that are inadequately protected by a mucin tear coating secondary to chronic inflammation or dry eye syndrome.

Intraocular pressure — The screening examination also includes intraocular pressure (IOP) measurement.  Postoperatively the patient's cornea will be thinner, creating a falsely lower IOP when performing any applanation tonometry.  The postoperative PRK patient may develop elevated IOP secondary to topical corticosteroid use. The LASIK candidate with high IOP may have undiagnosed glaucoma.

Fundoscopic evaluation — A dilated fundus examination may uncover other eye diseases or causes of decreased visual acuity. The lens may have cataract changes, which can prevent optimal results with any laser refractive surgery. Patients with symptomatic cataracts may need a cataract surgery evaluation instead. The patient's vitreous may have opacities such as posterior vitreous detachments that cannot be corrected by refractive surgery. The optic nerve may show signs of atrophy or glaucoma; this will prevent patients from seeing better postoperatively. The refractive surgery candidates may have macular diseases such as macular degeneration or inherited retinal dystrophies. The peripheral retina may have signs of lattice degeneration, retinal tears, retinal holes or retinal breaks needing further evaluation and treatment prior to undergoing refractive surgery.

Evaluation of the cornea — The reshaping of the cornea is the fundamental principle of laser refractive surgery. The ophthalmologist must calculate certain corneal measurements. The steepness or flatness of the cornea is defined by keratometry. The shape and contour of the cornea is measured using topography or videokeratography. The keratometry and topography will rule out corneal diseases such as keratoconus or posterior corneal ectasia. The thickness is measured by a pachymeter. Pachymetry is useful for identifying unusually thin corneas and calculating the depth of the ablation. Orbscan, Pentacam, ORA tesst are the most popular cornea screens used at this step.

How is the LASIK procedure performed?
The LASIK procedure will take around an hour to complete and can be performed on an outpatient basis. You will be awake for the entire procedure and in order to reduce discomfort and numb the pain, local anaesthetic will be applied. Your vision will be blurred throughout the procedure. Here are the steps of the LASIK process

  • The doctor will ask you to sit down on the reclining chair
  • In order to numb the area and reduce discomfort, anaesthetic drops will be applied on your eyes
  • Your eyes will be cleaned with an antibacterial wash
  • In order to keep your eyes open throughout the procedure, a special instrument will be used so that the eyelids are held in place
  • A suction ring will be placed on the eye so that there will be pressure on the cornea prior to cutting it
  • You will be asked to focus on a coloured light, this may be coloured green or red
  • On the surface of the cornea, a flap will be cut with femtosecond laser and folded back so that the doctor can reach the tissue underneath it. A special blade called a microkeratome will be used
  • The tissue that has been exposed will be dried
  • The laser will be put into position
  • The laser will be used to remove corneal tissue. A computer will be used to guide the laser in reshaping the cornea. You may notice a smell that is like burning hair during this process
  • When the right amount of tissue has been removed, the flap is put back into place within seconds
  • No stitches will be placed
  • You will be allowed to leave after a few minutes but you should take care of yourself and have someone drive you home

The Epi-LASIK surgery is not meant for everyone although it may be a better option for those people who aren’t going to be fit for a LASIK surgery. People with thin corneas and people without enough tissue for a flap should consider an Epi-LASIK surgery instead of a LASIK surgery.

You may be allowed to remove the contact lenses after the third day of surgery although you will have to get permission from your doctor. Your vision will not be perfect right away although patients have been reported to have twenty forty and even twenty-twenty vision in three to six months. You should be able to drive after a week from the surgery.

The recovery times are much longer with the LASIK method but it gives people better vision on the day of the surgery up to a few weeks ahead. They will be able to drive after a week.

  • Corneal topography is used primarily as a screening tool to evaluate prospective refractive surgery candidates and a diagnostic aid in evaluating refractive surgery patients with poor outcomes.7,8 Irregular corneas are poor candidates for refractive surgery since results with current lasers can be unpredictable. Keratoconus and contact lens warpage are the most common causes of irregular corneas in the screening population. Steep (ie, red) areas isolated to the inferior cornea suggest keratoconus, and many topographers come equipped with programs to alert the clinician when a diagnosis of keratoconus is likely. Postoperative patients with poor vision should have topography; such problems as central islands, irregular ablation profiles, and decentered laser ablations can be assessed with these devices.
  • Custom laser treatments incorporate a specific algorithm to help limit the induction of spherical aberration. This algorithm is based on a patient's unique wavefront measurement of their individual eye to some extent. However, the most important aspect of treatment is a blend or tapering of the peripheral treatment zone. Some lasers have incorporated a noncustom approach to this problem and create the transition zone at the edge of the ablation based on an empirical approach that takes into account the patient's prescription glasses and corneal curvature readings instead of using unique patient wavefront data.

    A wave front test is performed in all cases to appreciate refraction, pupil, aberrations…The results can modifie indication or leads to a customized treatment decide by the surgeon. This test also helps keratoconus detection. Everyone considering any corneal-based refractive surgery, such as Lasik, LASEK, PRK, or Epi-Lasik should have a wavefront diagnostic. A wavefront diagnostic will determine if critical HOA are below normal, normal, or elevated. If the HOA are elevated, either wavefront-guided surgery is required, or no surgery is appropriate. If HOA are normal, wavefront-guided surgery may be wise. If HOA are below normal, wavefront-guided continues to be an option, but not a requirement. This is assuming, of course, that you meet all other requirements for wavefront-guided Lasik, LASEK, PRK, or Epi-Lasik laser eye surgery

 

In addition to standard visual errors, Zyoptix also measures and treats small vision errors (higher order aberrations) that scatters light in a pattern that is unique to you. Scattered light errors could, for example, cause shading seen round one edge of a perfectly round light, and certainly they are an important cause of poor quality night vision. About 70% of people could benefit from Zyoptix treatment.

These small errors are detected with wavefront technology and treated by the laser with a combination of smaller and larger beams, allowing the surgeon to deliver a more precise, personalised treatment.

RISKS — Complications may arise from errors in the planned ablation, intraoperative mechanical factors, postoperative medications, and wound healing .  The most common subjective complaints in one LASIK study were night driving difficulty and glare 

Under and overcorrections — Undercorrections and overcorrections can occur if the excimer laser removes an incorrect amount of corneal stroma. Undercorrections occur more frequently with increasing amounts of myopia. An undercorrection for a myopic patient would leave residual stroma, thereby leaving the patient myopic postoperatively. Enhancements are usually performed within the first year after the original procedure to remove additional corneal stroma and correct for postoperative refractive error . In a prospective study, LASIK retreatment was required in 10.7 percent of eyes

Overcorrections occur when too much corneal stroma is removed and are harder to treat.

Astigmatism — Induced astigmatism occurs with decentered ablations or excessive patient movement during the ablation . Retreatment of postoperative astigmatism can be corrected with glasses, contact lenses, or repeat laser treatments with newer corneal custom ablation profiles

Irregular astigmatism (central islands) — In some cases, laser treatment or an abnormal healing process may result in astigmatism that is "irregular." This includes "central islands," which are irregular elevated areas in the treated cornea. Such irregularities may result in decreased visual acuity. These "central islands" may not be correctable with laser, and the decrease in visual acuity may not be correctable with glasses or contact lenses 

Regression — Regression of the postoperative refraction toward the preoperative refractive error can occur over time  . Regression may occur due to discontinuation of topical corticosteroids, abnormal wound healing, and pregnancy or other hormonal imbalances.

Glare, halos, diplopia — Glare, halos, and monocular diplopia can occur postoperatively   night halos can remain a persistent problem for many years after PRK   The etiology is thought to originate at the interface of the treated and nontreated corneal stroma. Patients with large pupils in dim illumination may develop symptoms as light rays are refracted differently at this junction, thereby causing glare symptoms at night, halos around bright lights, and the perception of double images out of one eye.

The newer generation excimer lasers have ablation profiles with larger blend zones to minimize the transition of treated and nontreated cornea in an attempt to reduce these symptoms [45].

Dry eyes — Dry eye syndrome is very common following laser refractive surgery   During creation of the LASIK flap, the superficial corneal nerves are cut with the microkeratome. The corneal nerves will reinnervate the corneal stroma over a six-month period  . The few months post-LASIK may require a patient to use nonpreserved artificial tears. In moderate to severe dry eye, the ophthalmologist may place temporary or permanent punctual plugs into the eyelid punctae to prevent tears from being pumped into the lacrimal system . Patients may experience a foreign body sensation, fluctuating vision, or decreased vision.

Loss of contrast — Some postoperative patients develop a loss of contrast sensitivity following laser refractive surgery  . A patient may see Snellen acuity equal to 20/20, but the sharpness and clarity are decreased   Further research to include wavefront testing is ongoing in order to understand the causes of decreased contrast sensitivity.

Epithelial defect — PRK leaves a large epithelial defect postoperatively that causes postoperative pain with delayed corneal epithelial healing. Since the epithelial barrier to infection is removed, patients also are at higher risk of corneal infections. The incidence of infection is rare, but it can cause corneal scarring, corneal melting, and irregular astigmatism.

Stromal haze — PRK patients also tend to have a higher incidence of stromal haze and scarring, especially when treating myopic patients over –6.0 D  The haze peaks between 6 to 12 weeks and then declines.

Flap complications — Creation of the LASIK flap also presents unique risks. During creation of the flap the microkeratome may create a thin flap, incomplete flap, buttonhole flap, or free flap   These complications may require the surgeon to stop the procedure and allow the cornea to heal before another attempt at refractive surgery, an average of nine months postoperatively  . Postoperatively, the corneal epithelium can grow underneath the flap requiring another procedure to lift the flap and remove the underlying epithelium   The corneal flap may also dislodge spontaneously or with trauma. These complications are not more observed when femtosecond laser is used

Diffuse lamellar keratitis — A sterile inflammatory response without an unknown etiology called diffuse lamellar keratitis (Sand's of Sahara Syndrome) may occur at the stromal interface beneath the flap. Diffuse lamellar keratitis is treated with topical corticosteroids. Severe cases require the ophthalmologist to lift and irrigate the flap postoperatively 

Other keratopathy — Cases have been described of noninflammatory central corneal opacification occurring within two weeks of surgery and regressing two to eighteen months postprocedure . Lamellar keratitis preceded the corneal opacification in 18 of 19 patients; opacification did not respond to systemic or topical corticosteroids.
Infectious keratitis — Infectious keratitis can occur after LASIK and can be vision-threatening. The estimated rate is between one in 1000 and one in 5000 procedures, and can involve bacterial, mycobacterial, or fungal organisms

Visual loss — There is a 0 to 3.6 percent loss of two or more lines of BSCVA reported with laser refractive surgery . As an example, a patient who sees 20/20 with glasses will have a small chance of not seeing any better than 20/40 with glasses following refractive surgery. There are rare case reports of visual loss and corneal perforations secondary to laser ablation through the corneal stroma or from microkeratome complications .

Enhancements
One of the great advantages of LASIK over other refractive procedures is the ease and safety of performing enhancement surgery. Enhancements should be postponed until the refractive error is stable, usually about 3 months postoperatively. It is common to wait longer, up to 6 months, for patients who experience an overcorrection because this will often regress. The corneal flap can usually be lifted easily within the first several years after surgery; beyond this time period, consideration should be given to cutting a new flap 

Enhancement surgery is performed by first positioning the patient at the slit lamp. A special shaped spatula is used to gently lift the edge of the flap and to find the corneal plane of the original cut. Then, the patient is positioned under the laser. The cornea is marked as usual. There is no risk of a free flap, but these marks help in realigning the flap. A blunt spatula is passed under the flap and swept gently back and forth, almost to the edge of the flap but avoiding breaking through the edge to the surface. The flap is grasped firmly with non-tooth forceps and peeled back, creating a clean epithelial edge. The laser treatment proceeds as usual, replacing the flap after the procedure is complete. Some practitioners prefer to use a contact bandage lens to protect the flap and for patient comfort after surgery since the epithelial edge tends to be more irregular than if the flap were cut with the microkeratome. Occasionally, a patient will not have enough residual corneal stroma in the flap to allow for an enhancement. In these patients, the correction can be applied to the surface on top of the flap by performing PRK. In general, this requires the use of mitomycin C to prevent corneal scarring and haze after treatment.

In patients whose records may not be available, the use of a device, such as the Visante OCT, may aid in determining the thickness of the original flap.

Presbyopia — Patients who are approaching the age at which presbyopia occurs or who are already presbyopic should be educated that correction of a refractive error for clear distance vision may result in an inability to focus and read near objects, which they were able to do before surgery by removing their spectacles. Some patients may be able to avoid this problem by undergoing "monovision" correction, whereby one eye is surgically undercorrected for distance, so that the patient is left with one eye for distance and the other eye for near vision. However, not all patients can tolerate monovision. Patients can undergo a trial of monovision with contact lenses prior to refractive surgery.

Long-term risks and reducing risks — Although the current laser procedures (PRK and LASIK) were required to meet FDA safety and efficacy standards for pre-market approval in the United States, they did not (and obviously could not) evaluate risks 30 or more years out. Nevertheless, we now have over a decade of follow-up in the United States, longer worldwide, and have learned several important lessons from that experience to make current procedures safer and more effective. Examples include:
•    Early treatments with smaller optical treatment zones were associated with significant difficulties with night vision and contrast acuity. Larger optical zones and blend zones have gone a long way toward alleviating these difficulties.
•    Deeper surface ablations were found to be at higher risk for postoperative haze and regression. Strategies were developed to address this shortfall in the form of LASEK, mitomycin (either prophylactically or in retreatment), or in lamellar surgery (ie, LASIK).
•    Early LASIK was associated with potentially serious flap complications such as intraocular penetration, buttonhole flaps, and free flaps. As a result, instrument manufacturers modified microkeratome design for improved safety, and surgeons learned to identify high-risk patients to avoid some of these complications.
•    Cases of iatrogenic corneal ectasia occurred when, after LASIK, the cornea was too thin to maintain structural integrity. As a result, surgeons now tailor the procedure to keep the residual corneal stromal bed thickness greater than 250 microns and avoid this complication.

Implantable lenses
Phakic IOL are a newer surgical option for correcting high myopia and hugh hypermetropia, particularly in more extreme cases that may be unsuitable for LASIK or other vision correction surgery. They can mix spheric and astigmatic correction. Somes IOL are soft.

Phakic IOLs work like contact lenses, except they are surgically placed within the eye and typically are permanent, which means no maintenance is needed. Phakic IOLs do not replace the eye's natural lens, which is left intact
•    Intraocular lens (IOL) power calculation following corneal refractive surgery can pose problems. Newer IOL calculation formulas and empiric experience with IOLs after refractive surgery are continuing to improve predictability.

OUTCOMES — Patients can reasonably expect to have a 90 to 99 percent chance of achieving 20/40 or better UCVA   57 to 79 percent achieve 20/20 or better UCVA   The results of every study are based upon each patient's refractive error and the amount of astigmatism. Approximately 85 percent of patients see 20/25 or better UCVA, which enables them to perform the majority of activities without corrective lenses. Patient satisfaction rates after LASIK are generally high

The factors that affect refractive outcome include low versus high myopia, myopia versus hyperopia, and astigmatism. Patients with low myopia without astigmatism have the best results, whereas high hyperopes with astigmatism have the least predictable results [

A number of additional factors also may affect the surgical outcome:
•    The type of procedure such as PRK, LASIK, or LASEK
•    The surgeon skill level and experience
•    The laser center's equipment, quality control, and maintenance are crucial for accurate corneal ablations

The laser characteristics also determine refractive outcomes. The newer generation small flying spot laser beams (less than 100 microns) with eye tracking systems theoretically offer better predictability than the older generation wide beam (4 to 5 mm) without eye tracking systems.

The surgical nomogram is also critical for a predictable result. The surgical nomogram is a calculation of the amount of corneal tissue to ablate for a desired refractive correction. This nomogram is developed by clinical trials and by numerous treatments performed at a specific laser center. The surgical nomogram analyzes such factors as laser settings, refraction, and surgical technique.

Outcome in a second eye treated with LASIK may be correlated with the results in the first eye  As an example, one study found a relatively high correlation between the outcomes in the two eyes of patients who had same-session bilateral LASIK and, after controlling for baseline visual acuity, calculated an approximately 20-fold increase in the risk of uncorrected visual acuity of 20/40 or worse in an eye treated with LASIK if such a result occurred in the other eye.

Day of surgery
Amount of time required to stay out of contact lenses:
           - Soft lenses: At least 4 days before the surgery
           - Rigid Gas Permeable (RGP) lenses or semi-hard lenses: At lease 21 days before the surgery 

2 On the day of the surgery, you should wash your hair and body ( see: déroulement ) and refrain from wearing make-up. We recommend that you wear a button-down shirt for convenience since you will be wearing eye shields. Please do not bring any valuable belongings.
3 Do not take any NEW medications prior to the surgery but don't stop all the others. Follow the surgeon prescription. Dont forget to buy before surgery post-op medications.

4. Please do not consume any alcoholic drinks or cafe, do not wear perfume, use deodorant or scents of any kind before the surgery, as the vapor and odor may affect accuracy of the laser.
5. On the day of surgery, do not drive yourself. After surgery your eyes will be covered with eye-shield(s) on the operated eye(s). Although you can see through the eye-shield, you will have decreased peripheral vision and some patients may experience irritation, a stinging sensation, watering, hazy vision, and may have difficulties opening the eye which could be obstacles for safe driving.

Postoperative Details
The patient usually is seen within the first 48 hours following surgery to check visual acuity, to inspect flap position, and to ensure that no signs of infection or inflammation in the cornea are present.

Right after the LASIK surgery, your eyes may feel itchy, watery, and blurry. In order to alleviate any discomfort, the doctor will prescribe certain medications or eye drops. In order to aid in the healing process, you will be required to wear eye patches while you sleep at night.

A regimen of postoperative antibiotics, given 4 times a day for 1 week, is recommended. Fourth-generation fluoroquinolones are a good choice because of excellent corneal penetration and broad-spectrum coverage. Currently, there are 2 commercially available preparations: moxifloxacin 0.5% and gatifloxacin 0.3%. There is much debate in the ophthalmic community as to which of these topical antibiotics is best for this setting. Similarly, consensus on the use of topical steroids does not exist. However, most surgeons prescribe their use for the first week after surgery, discontinuing or tapering rapidly thereafter. A potent and penetrating steroid, such as prednisolone acetate 1%, commonly is used. This helps prevent inflammation under the flap. The role of topical steroids in influencing postoperative healing and regression has not been determined.

Patients who are undercorrected or who appear to be regressing rapidly (increasing myopia), as determined by serial refractions, may benefit from more prolonged treatment with topical steroids and a slower tapering off of these drops. Overcorrected patients may benefit from discontinuing steroids early in the postoperative period and by the use of topical nonsteroidal drops.

Translation: D.A. Lebuisson M.D.
Human vision provides some unique challenges for application of wavefront technology. The human eye is not a telescope. The eye is dynamic, with ever changing focus, pupil size, and other normal biological fluctuations. A telescope is static and once adjusted for a certain set of aberrations, never changes. A deformable mirror can be adjusted to a very precise amount. Ablating corneal tissue is not nearly so precise. Wavefront may be an excellent mapping system, but that does not necessarily mean that you can get to where you want to go.

HOA are divided into separate terms and some HOA are much more important to good quality vision than others. Spherical aberration, coma, and trefoil are examples of HOA that are very important to keep low. An HOA measurement often used is Root Mean Squared (RMS). RMS is more or less an average of all HOA. Having a low RMS is good, but if you have an elevated HOA that is one of those that is known to cause vision quality problems, a low average may not be enough. As an example, if you have low HOA RMS, but a high spherical aberration, wavefront-guided surgery may be a requirement.

 

 

 

Dernière modification le 28/05/2013 - 14:40