Index
Patients with myopia, hyperopia and astigmatism can now reduce or eliminate their dependence on glasses.
Whether it’s glasses or contact lenses, you can forget about them thanks to refractive surgery that includes radial keratotomy, photorefractive keratectomy, laser-assisted in situ keratomileusis, laser thermal keratoplasty, and intrastromal corneal rings.
Since the approval of the excimer laser in 1995, stable and predictable results have been produced with an excellent safety, efficacy, predictability and safety profile, with the additional benefits of rapid vision recovery and minimal pain.
Despite the current wide publicity and media attention about laser refractive surgery, not all patients are good candidates for this surgery.
Corneal anatomy
The transparent cornea is approximately 0.5 mm thick, with five different layers: the epithelium, Bowman’s membrane, the stroma, the endothelium, and Descemet’s membrane.
The epithelium is the outermost layer, provides a uniform refractive surface and acts as a barrier against infection.
The function of Bowman’s membrane, which is located under the epithelium and its basement membrane, is not very clear.
The stroma, made up of interlocking lamellae of collagen fibrils, provides structure to the cornea and accounts for 90 percent of the corneal thickness.
The endothelium and its basement membrane (Descemet’s membrane) form the innermost layers.
Endothelial cells, through a sodium potassium pump, are responsible for the relative corneal dehydration necessary for corneal clarity.
Optics, refraction and refractive error
Refraction is the bending of light rays as they pass from a transparent medium to another medium of a different density; it is measured in diopters.
The refractive power of a lens is the reciprocal of its focal length measured in meters (for example, a one diopter lens has a 1 m focal point, a two diopter lens has a 0.5 m focal length).
The cornea and lens refract light that enters the eye. The cornea is responsible for two-thirds of the eye’s total focusing power, while the lens represents the remaining third.
The focusing power of the cornea is fixed, while the focusing power of the lens is not.
Through a process called accommodation, the lens changes its shape to focus on objects.
In emmetropia (an eye with normal vision), the focusing powers of the cornea and lens are perfectly combined with the length of the globe.
When a person with normal visual acuity sees an object, the cornea and lens focus the parallel rays of light emitted from the object precisely on the retina, and a clear image is perceived.
The focal point of the eye is at infinity. Refractive errors occur when the cornea and lens do not properly focus light rays on the retina.
In myopia, the most common type of refractive error, the cornea is too curved or the lens too powerful for the length of the globe.
Distant objects cannot be seen clearly because light rays are focused in front of the retina; however, nearby objects appear clear.
Concave lenses with negative or divergent power correct this refractive error and refocus light rays at the correct point on the retina.
In hyperopia, the cornea is too flat or the lens too weak for the length of the globe. As a result, the cornea and lens focus light rays behind the retina.
This accommodation process can focus on a distant object, however, near vision is not clear.
Convex lenses with positive or convergent power correct this refractive error and refocus light rays to the correct point on the retina.
In astigmatism , the refractive power of the eye is different in different meridians.
The cornea and lens cannot carry light rays to the precise point on the retina to provide clear vision, therefore objects will appear blurry at any distance.
Astigmatism can occur with myopia or hyperopia .
Patient selection criteria for refractive surgery
- Over 18 years of age.
- Stable refraction lasting at least one year.
- Myopia between -0.50 and -12.00 diopters.
- Astigmatism 5.00 diopters.
- Hyperopia <+6.00 diopters.
Absence of ocular contraindications such as:
- Keratoconus.
- Herpetic keratitis.
- Progressive myopia.
- Corneal disease.
- Glaucoma.
- Waterfall.
- Any other pre-existing pathology of the cornea or anterior segment, including scars, agophthalmos, dry eye, and blepharitis.
- Uncontrolled vascular disease.
- Autoimmune disease.
- Immunosuppressed or immunocompromised patient.
- Pregnant or nursing.
- History of keloids.
- Mellitus diabetes.
Refractive surgery
Radial keratotomy, photorefractive keratectomy, laser-assisted in situ keratomileusis, laser thermal keratoplasty, and intrastromal corneal rings are the most common refractive surgeries currently performed in the treatment of patients with myopia, hyperopia, and astigmatism.
The goal of these procedures is to change the refractive state of the eye by changing the shape of the cornea.
Radial keratotomy
In the past, radial keratotomy was performed to treat myopia patients.
The surgeon makes a series of microscopic corneal incisions in a radial or ray-like pattern.
This allows the outer cornea to relax so that the central cornea flattens out.
The new shape of the cornea is permanently retained as the cornea heals.
Possible serious complications include loss of best-corrected visual acuity, corneal perforation, infection, and balloon rupture.
Some of the major concerns with this procedure relate to surgery-induced significant corneal instability, including diurnal fluctuation of refractive error, overcorrection, hyperopic shift, and potential balloon rupture with blunt trauma.
The excimer laser
The excimer laser is used to perform procedures, changing the shape of the cornea, with the help of an ultraviolet beam, it breaks the intermolecular bonds of the cornea. This procedure is called photoablation.
Because adjacent tissue produces little or no thermal damage, it is often referred to as a “cold” laser beam.
A computer, programmed with the patient’s refraction and corneal topography, controls the laser beam to precisely remove corneal tissue.
With the improvement of technology, the width of the laser beam has continued to decrease to less than 100 μ.
Additionally, laser eye tracking systems are now available that allow precise corneal ablation during eye movements.
In the case of myopia, the beam of light from the laser decreases its focusing power, flattening the central cornea.
In hyperopia, the laser indirectly magnifies the central cornea by removing tissue from the periphery, increasing the focusing power of the cornea.
In astigmatism, the corneal meridian is treated with an elliptical or cylindrical beam, flattening it and making it more inclined.
Photoreactive keratectomy
Photorefactive keratectomy is a treatment for patients who present myopia, astigmatism and hyperopia at low to moderate levels without the presence of astigmatism.
This procedure is performed on an outpatient basis with the use of topical anesthesia.
The corneal epithelium in the ablation site is first removed or pushed to the side to allow more precise ablation of the corneal tissue.
Laser treatment is then applied to the exposed corneal stroma.
Immediately after laser treatment, the ophthalmologist applies topical antibiotics, steroids, and non-steroidal anti-inflammatory drugs.
And finally a disposable bandage contact lens is placed over the cornea.
During the early postoperative period, patients may experience significant tearing, photophobia, blurred vision, and discomfort due to central corneal abrasion.
With the use of contact lenses for bandages and eye drops with non-steroidal anti-inflammatory drugs, 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 heals (an average of three to four days).
Antibiotic therapy is usually continued for two to three days after the defect has healed, and topical steroid drops can be continued for up to three months after the operation.
Visual acuity improves once the epithelial defect heals, usually one week after the operation and usually fluctuates after surgery before stabilizing at approximately three months after the operation.
Glare, halos, and dry eye symptoms are common during the first month after surgery, but generally decrease or disappear completely within three to six months after surgery.
In-situ laser-assisted keratomileusis
A microkeratome is used to lift a corneal flap the size of a contact lens.
This flap, averaging 160μ thick, is folded back to expose the underlying stroma.
The excimer laser is used to remove a precise amount of corneal stroma, and the flap is irrigated and returned to its original position.
The corneal flap is stabilized without sutures by the relative corneal dehydration created by the endothelial pump.
Corneal flap stability and adherence to the corneal stroma is verified after surgery, and patients are generally sent home with topical steroids, topical antibiotics, and topical non-steroidal anti-inflammatory eye drops.
In addition, the patient is instructed to wear an eye shield overnight, with a scheduled follow-up on postoperative day 1 and again a week.
The patient is usually seen again in the first, third, and sixth months.
The procedure has significant benefits for patients. It causes little pain, provides rapid vision recovery, and has the potential to treat higher levels of myopia.
Laser-assisted in situ keratomileusis has also been found to be safe and effective for treating both eyes on the same day, while photorefractive keratectomy is generally performed on two separate days.
Laser-assisted in situ keratomileusis enhancements are most easily performed (at least within the first six to 12 months after the initial surgery) by lifting the original corneal flap and re-treating the stromal bed to correct any refractive error. residual.
Unlike photorefractive keratectomy, laser-assisted in situ keratomileusis does not produce stromal haze, and patients who have undergone this procedure generally do not require topical steroid drops beyond the first postoperative week.
One year after the operation, patients who have undergone laser-assisted in situ keratomileusis procedure are more satisfied than patients who have undergone photorefractive keratectomy procedure (90 versus 52 percent, respectively).
Despite the different surgical techniques of photorefractive keratectomy and laser-assisted in situ keratomileusis, the refractive results are similar.
Thermal laser keratoplasty
Thermal laser keratoplasty is performed using the non-contact Holmium: YAG laser.
The ophthalmologist uses the laser to symmetrically place radial points that are off the visual axis.
This heats up the cornea, resulting in the contraction of the stromal collagen, which modifies the anterior corneal curvature.
This procedure is performed under topical anesthesia with a variable follow-up schedule.
Thermal laser keratoplasty is used to treat patients with a hyperopic refractive error of up to +4.00 diopters who have become presbyopic and do not have ocular pathology.
Refractive results vary with the amount of laser treatment.
The most common complications include induced and irregular astigmatism and hyperopic regression with the need for retreatment using thermal keratoplasty of the laser or the excimer laser.
Thermal laser keratoplasty is safe, effective, and provides satisfactory correction of low hyperopia with minimal complications.
Anillo corneal intrastromal
Preliminary study results indicate that the Intrastromal Corneal Ring is a promising option for correcting myopic refractive errors of less than -3.00 diopters.
During this procedure, the ophthalmologist places a polymethylmethacrylate ring on the periphery of the cornea at approximately two-thirds of its depth.
The ring causes the cornea to flatten, thus correcting the refractive error.
The benefits of the Intrastromal Corneal Ring include rapid recovery of vision after placement (because surgical manipulation does not occur on the central cornea / visual axis).
An added benefit is that the ophthalmologist can remove the device at any time. Risks include infection, abnormal wound healing, and irregular astigmatism.
Postoperative complications
Complications from refractive surgery can arise intraoperatively or during the postoperative period.
With the laser-assisted in situ keratomileusis procedure, intraoperative flap complications that occur with the use of the microkeratome include incomplete flaps, irregular flaps, small flaps, flap decentration, buttonhole flaps, or free cap flaps.
Early postoperative complications include corneal stromal flap dislodgment, flap striae, interface debris, epithelial growth on the flap, corneal stromal interface, or a sterile inflammatory response called diffuse lamellar keratitis or Saharan sands syndrome.
Complications of intraoperative photorefractive keratectomy most commonly include decentered laser ablation and central islands of higher refractive power.
Postoperative complications include pain secondary to an epithelial defect or delayed epithelial healing, which can increase the risk of infection, as well as delayed haze formation and corneal scarring.
With any procedure using the excimer laser, the refractive result may not always result in uncorrected visual acuity or better vision acuity of 20/20 or better.
Some patients may have impaired visual acuity and clarity secondary to scarring, glare, halos, monocular diplopia, and reduced contrast sensitivity.
Patients may have postoperative overcorrection, undercorrection, and astigmatism that may need enhancement to correct residual refractive error.
In addition, after excimer laser refractive surgery, most patients will complain of dry eye symptoms secondary to impaired corneal nerve innervation.
These patients are most effectively treated with non-preserved artificial tears or point plugs, as needed; dry eye symptoms generally resolve within three months.
