FOR THE CORRECTION OF myopia, astigmatism, hypermetropia and presbyopia
What is it?
Laser technology allows the surgical correction of the most common vision defects or refractive errors (myopia, astigmatism and hypermetropia) and, in some cases, even presbyopia. As a beam of coherent light, the laser can reshape the cornea: the transparent front part of the eye covering the iris is, in fact, a natural lens, whose focusing ability (“dioptric power”) can be improved.
How can a refractive error be corrected?
The cornea accounts for about 60% of the eye’s total refractive power (40 diopters): it can change the direction of light beams, converging them towards the retina, due to its curvature. By altering corneal thickness, laser refractive surgery aims at ensuring that images come into focus on the retina. In short, we could say that we must flatten the cornea to correct myopia, whereas we need to increase it curvature to correct hypermetropia.
WHAT are the main techniques?
There are three techniques that are currently the most widespread:
Lasik: a small “flap” is created on the cornea, by cutting horizontally through the corneal epithelium, Bowman’s layer and superficial stroma. This action can be performed with a precision scalpel called microkeratome, or with another high precision high speed laser (femtosecond laser). After folding the flap back, the cornea is made thinner with an excimer laser and then the flap is repositioned.
- Advantages: Lasik surgery is usually not painful and sight is recovered immediately after the treatment.
- Disadvantages: creating the flap is a risky procedure and its success largely depends on a surgeon’s technical ability. The flap could still be slightly raised after one year, thus never fully adhering to the stoma below; in this case, an accidental shift can occur after a trauma. Infectious contaminations can also occur under the flap itself.
PRK (PhotoRefractive Keratectomy): with this procedure, the laser alters the curvature of the anterior cornea after the removal of the corneal epithelium (the most external layer of the cornea). After the surgery, patients are required to use soft contact lenses with no refractive power, which are intended to protect their eyes while the cornea’s outermost layer regenerates (re-epithelialization takes 4-5 days).
- Advantages: PRK is the easiest procedure, from a technical point of view. The absence of a surgical flap also reduces long term complications.
- Disadvantages: post-surgery pain and increased risk of developing post-surgery corneal opacity.
Lasek: this procedure is substantially comparable to PRK, as both techniques imply the removal of corneal epithelium. However, while the PRK procedure entirely removes it, the epithelium is repositioned over the eye after the LASEK treatment. With this technique, the corneal epithelium is first lifted, then a laser beam reshapes the cornea to achieve the desired refractive results and, at the end, the epithelium flap is put back into place (protected by a contact lens). The repositioned epithelium protects the regenerating one, but its accidental shift doesn’t compromise the surgical result.
Which errors can be treated by excimer laser?
La miopia, l’ipermetropia e l’astigmatismo (che può essere associato ad altri vizi refrattivi), a cui, negli ultimi anni, si è aggiunta la presbiopia (da valutare però con grande attenzione). Astigmatismo e miopia sono difetti per la cui correzione è necessario asportare una quantità di tessuto corneale maggiore, a parità di diottrie, rispetto all’ipermetropia.
WHY CHOOSE ONE TECHNIQUE OVER ANOTHER?
The choice pertains to the ophthalmologist who performs the surgery. Generally speaking, LASIK is preferred for slight-moderate myopia, whereas PRK and LASEK are chosen in the case of high myopia.
WHEN IS THE SURGERY CONTRAINDICATED?
Refractive surgery aims to correct refractive errors by reshaping the cornea through a reduction of its thickness. One of the main contraindication is a cornea which is too thin. Therefore its thickness must always be measured before surgery (corneal pachymetry).
Another contraindication is ocular dryness. The quantity of secreted tears can be evaluated through a test of lacrimation and people with dry eyes can be excluded. Also, all pathologies that affect the cornea represent a limit to laser refractive surgery, particularly in people affected by keratoconus.
Myopia can be treated successfully and without risks up to 10-12 dioptres; beyond these values the risks for the eye become serious; the same is for hypermetropia that is greater than 5-6 dioptres, for which laser treatment is not always totally successful.
WHEN SHOULD YOU HAVE SURGERY?
The best age to undergo refractive surgery is between 25 and 40 years of age. Refractive errors can significantly worsen before 25 years of age, therefore nullifying the effects of the treatment. After 40 years of age, the onset of presbyopia increases the risks of suffering from dry eye syndrome, with a higher chance of developing a burning sensation after surgery. Also, the refractive error must be stable for at least 1-2 years prior to surgery.
ARE THERE PERSONS FOR WHOM LASER SURGERY IS PARTICULARLY RECOMMENDED?
People who use contact lenses inappropriately tend to contract infections, and therefore could benefit from ambulatory surgery. However, certain categories of people may experience more problems from wearing contact lenses or eyeglasses - because of their job, hobby or sport - than from undergoing laser surgery.
In some visually impaired people, there’s a considerable difference between the two eyes; this situation cannot be completely corrected with eyeglasses, because our brain doesn’t tolerate differences greater than 3 dioptres. In these cases, the possible solutions are laser refractive surgery or the correct use of contact lenses.
In conclusion, refractive surgery not only has aesthetic purposes (eliminating the use of eyeglasses) but also functional ones. However, it is generally considered a form of cosmetic surgery and is only paid for by the National Health System in some specific cases, i.e. when there is a strong difference of visus between the two eyes (severe anisometropia) or in the case of intolerance to contact lenses .
CAN EYES RECOVER FROM MYOPIA?
No, but myopia can be corrected. A myopic eye stays that way, but after surgery, a patient can see clearly (even though a second laser surgery can sometimes be necessary). This means that problems with the retina, eye pressure and other issues are not eliminated by refractive surgery. People with retinal problems, who undergo regular check-ups every year, must continue to do so even if they can see clearly after refractive surgery. Short-sighted people are used to seeing well at a close distance. Refractive surgery makes them emmetropic, that is to say without any apparent visual impairment. However, after the age of 40 presbyopia isn’t compensated by myopia and patients will see clearly at a close distance only by using eyeglasses (a likely event in any case, since refractive errors tend to physiologically worsen with age).
WHAT NEEDS TO BE DONE BEFORE SURGERY?
The use of contact lenses must be avoided for as long as possible before surgery. The length of the interruption depends on the patient’s eye; but in any case, surgery cannot be performed until two weeks from the interruption. Contact lenses can deform the cornea and cause problems. Patients should frequent environments that can favor ocular infection in the days before the surgery; in fact, even a simple conjunctivitis could undermine the result. Exposure to strong wind must be avoided at all costs (i.e. going on a motorbike without protection), as well as contact with people affected by infective conjunctivitis or keratitis.
WHAT NEEDS TO BE DONE AFTER SURGERY?
Refractive surgery lasts a few minutes and its results are immediate. After surgery, a patient gets up from the surgical table and can generally see clearly. This is why postoperative risks and the doctor’s recommendations are sometimes undervalued. On the contrary, it is extremely important to follow the doctor’s prescriptions closely and adhere to their therapy with regularity and precision.
In the period following the operation, the eye is more delicate. A possible infection could jeopardise the surgical result and cause severe consequences. Patients should avoid outdoor activities, smoky environments, using motorbikes or scooters and going to the swimming pool (as chlorine causes eye irritation). Reading, using a computer, going to the cinema and watching TV is perfectly fine. Keeping the eyes well hydrated is essential and artificial tears should frequently be applied. Eye drops dosage also depends on the environment: working spaces are often very dry, due to heating in winter and air conditioning in summer, causing the tear film to evaporate too quickly.
WHAT TYPE OF ANESTHESIA IS USED?
Refractive surgery with excimer lasers requires topical anesthesia: anesthetic eye drops are instilled on the ocular surface. Patients undergoing surgery won’t feel any pain. They will have to stare at a light (called “mira”). This kind of anesthesia doesn’t inhibit ocular movements and patients must keep their eyes as still as possible.
HOW SURE CAN PATIENTS BE OF NO LONGER NEEDING THEIR GLASSES?
It is possible, but not guaranteed, that patients will no longer need glasses after surgery. If the surgery is successful, there should be no problem, at least in the short term. However there is no assurance that the refractive defect will be completely eliminated, as many factors come into play. Furthermore the refractive error could return (i.e. myopia), even if with reduced intensity compared to the level prior to surgery. In this case a second surgical intervention could be necessary, although it is possible that it will not permanently eliminate the refractive error and patients may still need eyeglasses or contact lenses. To give an idea of the incidence of myopic regression, out of a sample of 90 eyes with about -8 diopters on average (before surgery), six months after the laser surgery was completed, a measurement of -1.5 diopters was recorded (considering only people with a myopic regression).
WHAT ARE THE POSSIBLE RISKS?
According to the Food and Drug Administration (FDA), the highest US government body for the protection and promotion of public health, the following side effects may occur (or even real eye damage in the worst cases):
● ocular dryness (dry eye syndrome), “which can be severe”. In fact, after surgery, it may be necessary to frequently instill artificial tears and use humectants (eye gel), even though there was no need of them before;
● the use of eyeglasses or contact lenses may still be necessary after laser surgery (even though with a lower prescription). In fact it is not always possible to eliminate the refractive error. According to a study published on the American Journal of Ophthalmology “up to 28% of patients who undergo refractive surgery continue to experience a worsening of their sight” (in the case of lasik surgery for myopia);
● the risk of experiencing halos, glare, starburst (star-shaped light vision) and double vision, all visual problems “that could be debilitating”;
● According to the FDA, the “loss of sight” can occur in extreme cases. Although very rare, this is anyway a possibility.
WHAT IS WRITTEN IN THE INFORMED CONSENT THAT YOU SUBSCRIBE TO IN ITALY?
In brief, the informed consent approved by the Italian Ophthalmology Society (SOI), reads as follows:
1. refractive errors (myopia, hypermetropia and astigmatism) “can be corrected with a wide margin of safety and precision by excimer lasers”;
2. “refractive surgery aims to only correct refractive errors and does not alter other pathologies that may be associated to such visual defects”;
3. refractive surgery will not improve eyesight any better than what can be achieved through the use of eyeglasses or contact lenses
4. the intervention cannot guarantee the best vision possible without eyeglasses. In some cases a small “retouch” could be necessary to optimize the result;
5. “in the case of myopic patients aged above 45 years, the complete elimination of myopia will result in the immediate need for an optical correction for farsightedness” (to correct presbyopia, previously compensated by myopia);
6. Even if the surgery is correctly performed, unpredictable individual factors, which are “unrelated to a surgeon’s skill and laser precision, can affect the recovery and therefore the results. As a consequence, it is not possible to guarantee with absolute certainty the planned result”;
7. not all individuals and not all eyes are suitable to laser surgery on the cornea. In fact, people affected by certain general or systemic diseases (immunosuppression, autoimmune diseases, infectious diseases, diabetes, epilepsy, etc…) or in the presence of certain general conditions (pacemaker, professional exposure to UV or blue light, pregnancy, breast feeding) and people taking a series of drugs (hypotensive, contraceptives, hormones, amiodarone, chloroquine, drugs against migraine, anti-acne) that can affect postoperative recovery and make the surgery result unpredictable;. therefore the advisability to undergo refractive surgery should be carefully assessed on a case-by-case basis.
There are various eye diseases and conditions (high degenerative myopia, shallow anterior chamber, glaucoma, cataract, recurrent anterior and posterior ocular inflammations, burns consequences, ocular surface disease such as dry eye and all palpebral anomalies), and in particular affecting the cornea (keratitis, corneal ectasia, keratoconus, keratoglobus, endothelial dystrophy) that can condition or even compromise the progression of the refractive error after surgery, making the result unpredictable. For this reason, the possibility of surgery must be carefully assessed in person;
8. “the cornea is the structure that will be thinned by the surgery, therefore it must have a thickness which is appropriate to the extent of the error to be corrected and to the diameter of the optical area to be treated, which is necessary to ensure complete coverage of the pupil even in poor lighting conditions”;
9. after surgery (PRK, lasik, lasek) “the patient is required to apply with extreme care the prescribed eye drop medications, and according to the indicated modalities.” In fact “negligence in following the postoperative therapy and in undergoing specialist check-ups can affect the final refractive result and cause complications”.
WHAT ARE THE MAIN COMPLICATIONS OF EXCIMER LASER SURGERY?
According to the Italian Ophthalmology Society (as written in the informed consent approved in 2011) the possible complications/problems are:
● infection: it is an extremely rare complication. In the case of an infection that is resistant to antibiotics and where there is reduced immunity”, the situation may be a serious one and lead to the loss of sight or even of the eye. This eventuality is so exceptional that it is impossible to evaluate its frequency”;
● Decentered treatment (the cornea is not reshaped in the right place): extremely rare with contemporary lasers, which are equipped with a centering control system;
● Incomplete refractive results: over or under correction are possible, in particular after the correction of high refractive errors. A second procedure can be performed, if necessary.
● Inadequate optical area: “when the diameter of the pupil in low lighting conditions is wider than the diameter of the flap, night glare may occur, which can make driving at night very difficult. This situation may occur also when using topical medications (i.e. vasoconstrictor eye drops) or general medications (antimotion sickness substances [= such as antiemetics for travel sickness)] that can dilate the pupil”;
● Dry eye: this syndrome may occur for some months after surgery, requiring the application of artificial tears several times per day. This is the most frequent complication of all the laser techniques, and in particular for lasik; it usually disappears or significantly decreases within one year from surgery (but it is not guaranteed).
Other extremely rare complications listed in the document include
● Formation of corneal ulcers (lesions of the ocular surface);
● Non-specific diffuse interstitial keratitis;
● Corneal colliquation (disintegration of the cornea);
● Other unknown complications may occur: the outcome of long term studies
may reveal additional risks that are unknown at present.
PRK and Lasek specific complication:
● Re-epithelialization (which is the reconstitution of superficial corneal tissue) may be delayed due to the nature and conformation of the patient’s epithelium;
● After the intervention, a loss of clarity of the cornea may occur at various degrees (corneal haze) and it could be associated to an irregular corneal surface in the most serious cases. “Such opacity - the SOI writes - is generally reversible in a variable time (even several months) and compromises correct vision. Sometimes a second laser surgery could be necessary to smooth the corneal surface (PTK)”.
Lasik specific complications:
● Incomplete, damaged and decentralized corneal flap: in this case, the flap will be repositioned and the surgeon may decide to postpone the surgery for several months. However, this eventuality “ is extremely rare today”, the SOI assures;
● Diffuse lamellar keratitis (also known as Sands of the Sahara) of varying severity: it heals without any inconvenience if promptly and properly treated. Careful evaluation after surgery is necessary to prevent it.
WHAT SHOULD YOU KNOW, IN BRIEF?
1. According to the Italian OphthalmologY Society, your ophthalmologist must explain, amongst other things, that: excimer laser surgery is used to reduce the dependence on eyeglasses and contact lenses;
2. Excimer laser treatment does not eliminate the need of eyeglasses or contact lenses in all cases;
3. Excimer laser treatment does not cure other eye diseases;
4. Excimer laser treatment does not stop the (physiological) progression of myopia;
5. Complications are possible, especially if the prescribed therapies are not followed and follow-up examinations are not performed;
6. After the correction of short-sightedness, a pair of glasses for farsightedness may be immediately necessary.
 At the moment of writing, the criteria for laser surgery to be considered a medical-surgical treatment are:
1) anisometropia greater than 4 diopters of spherical equivalent (difference between the two eyes), which should not have been caused by previous refractive surgery, and only after after verifying the presence of single binocular vision (when intolerance to contact lenses has been certified);
2) astigmatism of at least 4 diopters;
3) strong differences between the visus of the two eyes due to previous interventions (limited to the operated eye and not caused by refractive surgery), in order to "level" the two eyes;
4) photo-therapeutic keratectomy (PTK) performed for corneal opacities, corneal tumors, scars, irregular astigmatism, corneal dystrophies and unfortunate outcomes of refractive surgery;
5) trauma consequences or anatomical malformations such as to prevent the application of glasses (only in the cases where intolerance to corneal contact lenses has been certified).
Shojaei A, Eslani M, Vali Y, Mansouri M, Dadman N, Yaseri M., “Effect of timolol on refractive outcomes in eyes with myopic regression after laser in situ keratomileusis: a prospective randomized clinical trial“, Am J Ophthalmol. 2012 Nov;154(5):790-798.e1. doi: 10.1016/j.ajo.2012.05.013. Epub 2012 Aug 28.
Please note that the certification to contact lenses intolerance - when required - must be issued by a public institution other than the one in which the intervention is performed. Amongst other things, the documentation must be accompanied by photographs.
Information edited by the International Agency for Prevention of Blindness - IAPB Italy
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Published on 2 October 2018. Updated on: 2 October 2018.
Last scientific review: 13 marzo 2018.