Approximately one fourth of the world's population is myopic. Radial keratotomy (RK) for myopia has developed slowly over the last century. Since it was first performed in the United States, the operation has been the subject of interest and lively debate among ophthalmologists and patients with myopia. See the image below.
 View Image | Optical zones and pupil dilation discussed by radial keratotomy surgeons. |
The use of corneal incisions to alter corneal curvature began in the late 19th century.
Choi et al[1] reported the following:
The earliest report of using an incision to alter the shape of the human cornea was in 1885, when the Norwegian ophthalmologist Schiotz used a limbal relaxing incision in a patient who underwent cataract surgery. Schiotz observed that the placement of the incision in the steep meridian of the patient's cornea resulted in flattening in the incised meridian. In 1894, Bates made the observation that traumatic peripheral corneal scars flattened the cornea in the meridian of the scar without affecting the meridian that was 90 degrees away. His observation advanced the idea that anterior corneal incisions could affect shape in a way to create more symmetry in astigmatic corneas.
Modern surgery for myopia was initiated by the original ideas and experiments of the late Professor Tsutomu Sato of Juntendo University, Tokyo. In 1939, he observed that spontaneous breaks in the Descemet membrane in keratoconus produced flattening of the cornea as the breaks healed. This observation provided hints of his idea of posterior corneal incisions. He also observed that injury to the Descemet membrane was greater than that induced by injury on the Bowman layer, suggesting that surgery on the posterior cornea would be more effective than surgery on the anterior cornea. In the late 1940s, anterior incisions were added to enhance the effect of the posterior incisions after experimental studies on radial and tangential incisions in rabbits to correct astigmatism. Myopic surgery was performed most frequently in Japan in the early 1950s.
As the contributions of Sato and his colleagues to the development of keratotomy wilted and died in Japan, they took root and blossomed in the former Soviet Union. There, ophthalmologists (first, Yanaliev, who performed 426 incisional refractive surgery cases between 1969 and 1977, and, later, Fyodorov and Durnev) found identical conclusions to Sato's over thousands of operations. Anterior radial incisions were not very effective, and posterior incisions were necessary for the desired effect. They concluded that the effect of the surgery was contingent on the length, the distance from the limbus, and the depth of the transverse incisions.
During the 1970s and 1980s, Fyodorov and his colleagues in Moscow made important contributions to modern anterior refractive keratotomy. Fyodorov started performing surgery in humans in 1974 using a freehand razor blade fragment in a blade holder, checking the depth of the incision with a depth gauge and deepening the incisions as needed.
The intense interest in RK in the United States in 1979 and 1980 triggered a stampede of ophthalmologists traveling to Moscow to learn the technique from Fyodorov. Fyodorov reported a high success rate; however, complications of the procedure were not well documented (see Complications).
The clinical and scientific development of keratotomy in the United States involved the experience, insights, and studies of dozens of ophthalmologists working rapidly in an unstructured setting. Identifying who was responsible for which innovations and advances is difficult.
The introduction of RK to the ophthalmologist's arsenal of surgical procedures sparked criticism and controversy.
RK for myopia was the most commonly performed refractive procedure in the late 1970s and 1980s. Most patients who elected to have RK did so to avoid being dependent on glasses or contact lenses. Other patients sought the surgery for occupational, athletic, or cosmetic reasons.
Hundreds of thousands of RKs were performed by ophthalmic surgeons around the world during that period, but now they are rarely, if ever, performed. New techniques with fewer complications and adverse effects have supplanted RK in the 1990s, starting with photorefractive keratoplasty (PRK) and followed by laser in situ keratomileusis (LASIK).
RK is considered an elective surgery because the surgery is not necessary to achieve functional emmetropia. The most common motivation for patients to have RK is to see well without having to depend on spectacles and contact lenses.
The refractive error is typically the first item used to screen patients. Those patients with myopia less than 1.00-1.50 diopters (D) and greater than 10.00 D are eliminated from consideration, as are patients with hypermetropia. Best results were found on individuals with myopia from 1.50-6.00 D.
In patients who have less than 1.50 D of myopia, the danger for overcorrection is usually high, and, in those patients who have more than 6.00 D of myopia, a decreased probability of an acceptable result exists.
The second most common criterion used to screen patients is age. Individuals younger than 21 years should not have the procedure performed because their refraction may not be stable.
Individuals who cannot be corrected better than approximately 20/40 with spectacles are seldom considered for RK, including patients with amblyopia, maculopathies, myopes with previous retinal detachment, and higher myopes with macular degeneration.
The cornea protects the intraocular contents and refracts the light. It is approximately 550 µm thick centrally and 700 µm thick peripherally and has, on average, a 12-mm diameter horizontally and an 11-mm diameter vertically.
The human cornea has 5 primary layers, as follows: epithelium, Bowman layer, stroma, Descemet membrane, and endothelium.
Knowing the corneal thickness is fundamental in keratotomy surgery because it forms the basis for setting the length of the knife blade in an attempt to make uniformly deep incisions through approximately 90% of the cornea.
RK works by altering cornea anatomy to create a new shape, flatter in the center and steeper in the periphery. The incisions cut a graded amount of corneal stroma, allowing the biomechanical forces to produce a gaping of the incisions and repositioning of the uncut corneal tissues.
For more information about the relevant anatomy, see Eye Globe Anatomy.
Contraindications to RK include abnormal corneal thickness or topography, keratoconus, inflammatory corneal disease, glaucoma, herpes simplex keratitis, pathologic myopia, pregnancy, active systemic diseases, atopy, and connective tissue diseases.
Pachymetry was designed to measure corneal thickness and to enhance the understanding and management of disorders of the corneal endothelium. It has become an integral part of the clinical practice, initially with radial keratotomy (RK) and then with other types of refractive surgery, since making an accurate measurement of corneal thickness is a necessity.
An ultrasound pachymeter uses the principles of A-scan ultrasonography. It provides a convenient means of measuring corneal thickness. To determine corneal thickness, the ultrasonic beam is aligned precisely perpendicular to the corneal central surface. Ultrasonic echoes are obtained from the anterior and posterior surfaces of the cornea. The time interval between the echoes can be used to determine the corneal thickness if the ultrasonic speed of propagation in the cornea is known. The cornea thickness is the speed of sound in the cornea multiplied by the time interval between corneal echoes divided by 2. Mean corneal thickness values are 0.51-0.58 mm.[2, 3]
Manifest and cycloplegic refractions should be measured, but cycloplegic refraction should be the basis for calculating the surgical plan and for comparing preoperative and postoperative results. The manifest refraction tends to overestimate the amount of myopia because of accommodation.
Used to establish the normal corneal anatomy, a careful slit lamp microscope examination can reveal early keratoconus, corneal scars, mild lens opacities, subluxation of the lens, and syneretic cavities in the vitreous, suggesting pathological myopia or vitreous degeneration.
The major objective of measuring the intraocular pressure (IOP) before surgery is to identify individuals who have elevated pressures and to exclude them from keratotomy surgery. This screening is particularly important because myopes are more likely to develop elevated IOP and glaucoma than emmetropes, and patients with fluctuating IOPs have unstable vision. Eyes with IOPs in the reference range of approximately 10-20 mm Hg are acceptable for keratotomy surgery.
This is the measure of the central cornea curvature. Little correlation exists between preoperative central keratometric power and the effect of keratotomy. Some contend that steeper corneas achieve more change in refraction; others find more change in refraction in flatter corneas. Still others contend that it is the overall corneal topography that affects the outcome, not just the central keratometric power. Even though many formulas and nomograms include keratometric power, the preoperative keratometric power plays little role in designing the surgical plan.
Qualitative keratography has a minor role in evaluating patients with myopia for RK, but qualitative keratography does identify individuals who may have irregular astigmatism associated with keratoconus or warpage caused by contact lens wear. It plays a major role in planning surgery for patients with astigmatism, particularly for those with penetrating keratoplasty or ocular trauma, where asymmetric, irregular astigmatism may be present.
Corneal topography is considered a mandatory test in all refractive patients to rule out conditions such as keratoconus or other corneal ectatic disorders that would contraindicate incisional keratotomy.
Indirect ophthalmoscopy with visualization of the ora serrata is important because of the increased propensity of myopes, particularly intermediate and pathologic myopes, to develop lattice degeneration of the retina, retinal holes, and retinal detachment.
Many surgeons prefer to operate on a patient's nondominant eye first; if complications occur, the presumably more valuable dominant eye can be left unoperated.
Endothelial morphology does not play a role in patient selection for RK. Specular microscopy of the endothelium is limited to studies in which careful preoperative and postoperative examinations are performed in the same locations in the central cornea and in the areas of incisions.
The steps in keratotomy surgery are outlined below.
1. Topical anesthesia: In addition to the application of topical drops, flushing the fornices with anesthetic through a blunt cannula and applying anesthetic on a sponge at the limbus enhance topical anesthesia.
2. Marking the center of the pupil: A blunt instrument, such as an intraocular lens (IOL) hook, marks the cornea by indenting it over the center of the pupil.
3. Marking the central clear zone: A clear zone marker of appropriate diameter indents the cornea. The clear zone diameter is selected, referencing the surgeon’s choice of nomogram. The smaller the clear zone, the greater the effect of the surgical correction and the more likely stray light will become a problem, especially at night.
4. Marking the location of the incisions: A marker with ridges imprints the location of the incisions in the epithelium as it is depressed on the cornea. The meridian for astigmatic surgery may be marked using a circular protractor and linear marker.
5. Measuring corneal thickness with an ultrasonic pachymeter: The fluid-filled probe tip is held perpendicular to the corneal surface, and the instruments obtain paracentral measurements just outside the circular clear zone mark.
6. Setting the length of the knife blade with a micrometer: A micrometer setting is completed away from the surface of the patient's eye, and the micrometer is turned forward until the desired setting is achieved. The length of the blade determines the depth of the cut and is based on a percentage of the thinnest pachymetry reading obtained.
7. Verifying the length of the knife blade: The micrometer handle is placed in a gauge block cradle, and, under high magnification, the tip of the blade is aligned with the edge of the gauge block at an appropriate setting.
8. Calibrating the length of the knife blade: A compound microscope with micrometer stage is used to inspect the knife blade and to measure the accuracy of its extension.
9. Removing the excess surface fluid: The ocular surface, the fornices, and the tip of the knife are dried to prevent misinterpretation of surface fluid as aqueous from corneal perforation.
10. Centrifugal incision with an oblique knife blade: Double-pronged forceps fixate the globe at the limbus, and the knife is held perpendicular to the corneal surface at the edge of the clear zone, commencing a centrifugal incision.
11. Centripetal incision with a vertical knife blade: The globe is fixated at the limbus with a forceps, and the knife blade is inserted into the cornea adjacent to the limbus.
12. Transverse incision with a vertical knife blade: A knife with a vertical blade cuts a transverse incision for astigmatism, while a double-pronged forceps fixates the globe adjacent to a transverse incision.
13. Cleaning the diamond knife and irrigating the wounds: Final cleansing of the knife is completed with hydrogen peroxide and distilled water, using a fully extended knife blade to cut through the cellulose sponge. Blood and foreign material are irrigated from wounds using a fine blunt cannula, irrigating parallel to the cornea surface.
Preoperative explanation, reassurance, and preparation: The surgeon and the operating room staff should inform the patient of what to expect during the surgical procedure to reduce the patient's fear and anxiety. If needed, appropriate drops and sedative/hypnotic drugs should be administered. Surgical procedure should be planned. Surgical instruments should be selected in advance.
Management of the pupil and preoperative medication: The pupil's natural diameter does not need to be altered. The bright light of the microscope usually constricts the pupil enough and reduces the patient's light sensitivity. A dilated pupil makes the patient more sensitive to light during the procedure.
Preoperative topical antibiotics, given 15 minutes apart for 4 applications immediately before surgery, can decrease the normal conjunctival bacterial flora, but no evidence exists that they decrease the rate of infection after keratotomy.
Posting the surgical plan: A drawing of the exact surgical procedure to be performed for each patient is completed for the surgeon. Identifying information includes the patient's name, age, eye to be operated on, refraction, diameter of the clear central zone, number and location of incisions, and the direction of the incisions. The optical zone and number of incisions to be used is usually determined by referring to a nomogram, which takes into account the patient's age and refractive error.
Anesthetizing the eye: Topical anesthesia is sufficient for refractive keratotomy. A few drops of topical anesthetic easily anesthetize the cornea, which remains anesthetized throughout the operation. Irrigation of an anesthetic into the conjunctival cul-de-sac through a blunt cannula increases patient comfort.
The most commonly used topical anesthetic for refractive keratotomy is tetracaine because it is packaged in sterile unit dose vials. Tetracaine causes stinging for approximately 30 seconds after application.
Preparation of the skin and insertion of eyelid speculum: The eyelids should be cleansed and prepared in the same manner as for intraocular surgery. Povidone iodine solution is a good antimicrobial agent that does not require scrubbing. The skin of the lids, nose, and face is swabbed with this solution.
Sterile drapes are applied; the amount of draping varies among surgeons. A fenestrated plastic drape commonly is used. A nonfenestrated, adhesive plastic drape can be wrapped around the eyelid margin.
A light wire eyelid speculum, such as a Barraquer speculum, holds the drape in place and retracts the lid without excessive pressure.
Fixation of the globe: The globe has to be stabilized, and the position must be controlled so that all incisions are perpendicular and straight.
Manipulation of the microscope: The goals are to maintain adequate focus of the microscope throughout the procedure and to keep light intensity low so that the patient is comfortable.
Marking the center of the optical clear zone: The goal is to mark the center of the uncut clear optical zone, minimizing glare and irregular astigmatism over the entrance pupil. (See the image below.)
View Image | Optical zones and pupil dilation discussed by radial keratotomy surgeons. |
Alignment of both the surgeon's and the patient's lines of sight: Optimal centering of the clear zone requires the patient to fixate a target that is coaxial with the examiner's sighting eye. Osher fixation device satisfies this requirement, as long as the surgeon marks the center of the pupil and not the corneal reflex.
Marking the central clear zone and location of incisions on the epithelial surface: The goals are to outline the central circular clear zone, to mark the correct location of the radial incisions, to retain the marks in the epithelium throughout the procedure, and to avoid cutting the Bowman layer.
Intraoperative ultrasonic pachymetry: The surgeon focuses the operating microscope on the corneal surface, dries the clear zone mark to make it visible, and brings the ultrasound probe tip into the surgical field. Fixation of the globe is not necessary. The probe tip covers an area of 3-4 mm in diameter; the surgeon must estimate the location of the center of the probe because that is where the ultrasonic thickness measurement is taken.
Final topical anesthesia: A microsponge soaked in anesthetic is applied to the limbus to ensure that the patient still cannot feel the fixation forceps. Applying additional anesthesia to the cornea is not necessary.
Number of incisions: The number of incisions is determined by the surgeon's experience and technique and by the nomogram used for the surgical plan. The nomogram takes into account the patient's age and refractive error. Most commonly, 4 incisions are used for lower amounts of myopia, and 8 incisions are used for moderate amounts of myopia.
Alignment and spacing of incisions: To decrease the chance of induced astigmatism on spherical myopia, the radial incisions should be spaced equidistantly around the cornea. Four incisions should be 90° apart, and 8 incisions should be 45° apart. The incisions are placed either directly opposite from each other or perpendicular to each other. Spaced incisions are more likely to increase astigmatism.
The incisions should be cleaned to reduce the amount of blood, epithelial cells, and foreign bodies from the wound and to minimize corneal edema. Some surgeons believe that irrigation of the wounds increases a patient's pain after surgery. Therefore, these effects must be balanced, using the least amount of fluid necessary to cleanse the wound, while minimizing stromal edema. The surgeon uses a small syringe or a squeeze bottle with a blunt 26- to 30-gauge cannula with salt solution, directed from central to peripheral to avoid forcing fluid into the stroma of the clear zone and to wash any material away from the center.
A broad-spectrum antibiotic should be flooded over the eye at the end of the operation. If a corneal perforation has occurred, a loading dose of antibiotic can be administered by floating the surface of the eye continuously for 2-3 minutes, giving a rapid concentration in the corneal stroma and anterior chamber.
A set of written postoperative instructions, including a description of what to expect after surgery, medication, time and location of follow-up appointments, and emergency procedures, should be given to the patient.
Pain after RK can be severe for 24-48 hours and should be the subject of careful discussion and planning between the surgeon and the patient.
Patients must be seen on the first day, first week, first month, and 3 months after surgery; depending on any complications, patients may be seen more frequently.
Prophylactic topical antibiotics, such as neomycin-bacitracin-polymyxin or gentamicin, should be used 2-4 times daily, commencing on the day of surgery, for 5-7 days. Some surgeons recommend topical hypertonic drops, such as 5% sodium chloride, to decrease edema and transient overcorrection.
Because pain can be severe for 24-48 hours after surgery, medications to avoid pain should be prescribed every 4 hours. A sedative, such as flurazepam (30 mg), helps the patient to sleep better the first and second night after surgery. Patients can decrease photophobia by wearing a dark pair of sunglasses.
For excellent patient education resources, visit eMedicineHealth's Eye and Vision Center. Also, see eMedicineHealth's patient education article Vision Correction Surgery.
Corneal perforations can be subdivided into microperforations and macroperforations. Microperforations allow continuation of the operation. The incidence of small punctures in the Descemet membrane is 2-10%. Microperforations usually are in the inferior and temporal cornea, but they may appear in any location.
Factors that increase the chance of corneal perforations include an unfamiliar knife blade, centripetal incisions, elevations of IOP during incisions, recutting incisions to make them deeper, prolonged dehydration and thinning of the cornea intraoperatively, inaccurate corneal thickness measurement, and sudden movements of the patient.
The management of microperforations includes stopping the incision, drying the surface and knife, and completing the incision with a slightly retracted knife blade or slightly less pressure on the knife. As long as the anterior chamber retains normal depth, the surgery can continue. Antibiotics should be given postoperatively for prophylaxis.
The complications of a small or large perforation of the cornea include damage to the endothelium with the production of a scar in the Descemet membrane, iridocorneal adhesions if the anterior chamber remains flat, laceration of the lens, endophthalmitis, or epithelial ingrowth.
The frequency of a large perforation varies from 0-0.45%. The most important factor in prevention is the early recognition of a small perforation. The cessation of the surgery because of an aqueous leak prevents the extension of the laceration of the Descemet membrane. At the end of the surgery, the eye must be patched if the anterior chamber is well formed. If the aqueous leak persists on the first day postoperatively, patching must be continued. If it persists for more days, sutures should be placed.
The smaller the clear zone, the greater the effect of decentration, with increased glare and astigmatism.
These incisions disrupt vision by producing scarring, irregular astigmatism, and glare. Predisposing factors include the patient's movements, a sudden Bell reflex, an unfamiliar knife and forceps, and a dry cornea surface.
Placing transverse keratotomy incisions for astigmatism on the improper axis can cause significant refractive problems. This complication can occur if the surgeon does not truly understand the surgery or because of momentary disorientation.
To manage this problem, the surgeon should explain to the patient what happened, suturing the wound during the operation or later after opening and cleaning and then treating residual astigmatism.
Aching pain that persists 24 hours after surgery usually is controllable by oral analgesics, such as acetaminophen or codeine. Some patients experience severe ocular pain that lasts from 12 hours to 2 days and requires oral narcotics, such as hydromorphone, 2-4 mg every 4 hours, and intramuscular analgesics.
Photophobia probably results from epithelial incisions and abrasions, mild stroma edema, inflammation, and minimal iridocyclitis. It usually occurs for several weeks after surgery.
Epithelial abrasions may occur during surgery and usually heal in approximately 48 hours without difficulty. Artificial tears should be prescribed to help the healing process and to decrease the foreign body sensation.
Fluctuating vision and corneal edema most commonly occur in the first few weeks after surgery. Stromal edema around the incisions occurs in all patients.
The conversion of a myopic refractive error to a hyperopic error is one of the most significant complications of refractive keratotomy. Myopic persons adapt better if they remain undercorrected than if they become overcorrected. When a myopic person is suddenly overcorrected, an abrupt demand for increased accommodation occurs, with accompanying accommodative-convergence of the near reflex. This sudden stress produces more symptoms than the sudden demand for more accommodation; the greater the refractive overcorrection, the greater the symptoms.
Overcorrection can occur in the immediate postoperative period when edema and the effects of the incisions themselves cause excessive wound gaping and greater flattening of the central cornea. Although it disappears spontaneously in a few weeks, it can be minimized by reducing the amount of wound irrigation at the end of the surgery and by using some hypertonic topical solutions postoperatively. Incorrect incisions for a patient also can increase overcorrection. The other circumstance in which overcorrection occurs is the unexplained overresponder who had surgery performed correctly but in whom a large overcorrection developed. The management is to provide appropriate spectacles, contact lenses, or suture the wounds closed.
Long-term instability of the refractive error with a shift toward hyperopia has been reported. This refractive shift can become manifest years following the procedure and is a particularly troublesome potential complication. The exact incidence is unknown.
Undercorrection occurs more frequently in higher myopes. After refractive keratotomy, most patients show regression of the initial effect, but whether it produces an undercorrection depends on the amount of regression. Undercorrection can occur because of a miscalculation in which insufficient surgery is completed. Some patients can remain undercorrected in spite of apparently proper surgery, and other patients can regress over a long period of time. Management of patients with undercorrected vision includes medical and surgical means. Surgical management may include repeated operations.
Residual astigmatism is defined as preexisting astigmatism not corrected by surgical procedure, and induced astigmatism is an increase after surgery. Irregular astigmatism occurs most commonly in eyes that have had repeated surgeries.
A recent report looks at the 20-year history and results of a patient with residual postoperative astigmatism after RK and generally discusses rigid gas permeable contact lens treatment of patients who have received RK.[4]
Individuals with minimal refractive errors lose their ability for close vision without correction as their accommodative reserve drops with increasing age. Those who are overcorrected become symptomatically presbyopic prematurely because of their hyperopia.
Reduced night vision in myopic individuals is very common and results from a change in the focal point of light at night when the pupil is dilated. This can be corrected by adding 0.5 D in spectacle lenses.
Cases of postoperative bacterial keratitis have been reported. The location of the infiltrate was usually within a refractive keratotomy incision, and the causative organisms included Pseudomonas species, Staphylococcus aureus, and Staphylococcus epidermis. The frequency of microbial keratitis after surgery may be reduced with the use of prophylactic antibiotics until the incisions are reepithelialized. The management of these patients should include corneal scrapings, Gram stains, and cultures, followed by fortified antibiotics with broad-spectrum coverage initially and later adjustment based on culture and sensitivity results.
Delayed bacterial or fungal keratitis occurs so rarely that long-term prophylactic antibiotics would be inappropriate. However, patients should be warned about this potential complication and instructed to consult their ophthalmologist at the earliest sign of blurred vision or redness of the eye.
Recurrent herpes simplex keratitis can be stimulated by numerous exogenous insults to the eye, including sunlight, fever, minor accidental trauma, contact lenses, menstruation, and trauma of keratotomy surgery. It can stimulate the recurrence of a viral infection and decrease wound healing. It is a contraindication for refractive keratotomy.
Surgical errors and disorders in wound healing after surgery can produce clinical problems. Intersecting incisions can lead to wound gaping, severe scarring, and corneal flaps. Irregular astigmatism can result from multiple scars from repeat operations and hypertrophic scars.
Other incisional complications include posterior plaques at perforation sites, limbal scarring, irregular incisions, vascularization of scars (especially in contact lens wearers), epithelial inclusions, and debris and deposits.
Endophthalmitis is very rare after refractive keratotomy. Only 3 published cases have been reported. All 3 patients developed a small hypopyon, and the cultured organism was S epidermis. All patients had excellent visual outcome several months later. Endophthalmitis presumably begins with the introduction of the infectious agent, probably coming from the lids, lashes, conjunctiva, or lacrimal apparatus, into the eye through a corneal perforation during or shortly after surgery.
Most of the lens opacities occurring after refractive keratotomy have resulted from direct laceration of the lens at the time of a corneal perforation. Prevention includes accurate corneal incisions without perforation and minimal use of postoperative topical corticosteroids.
After refractive keratotomy, the cornea is permanently weaker than its normal state, which increases its risk of rupture from direct trauma. Patients must be warned about this complication.
The corneal epithelium grows into the anterior chamber through a microperforation. Prevention includes minimizing corneal perforations and avoiding injection of an irrigating stream through a perforation. Management depends on the extent of the ingrowth.
After surgery, most patients develop mild anterior chamber cell and flare, which usually disappears over 1-3 weeks.
Elevated IOP produces further flattening of the central cornea, with further decrease in minus power of the refractive correction. Therefore, an undercorrected patient probably will have better visual acuity when the pressure rises, and an emmetropic or overcorrected patient will have worse acuity, depending on the accommodative ability.
When incisions are made across the limbus, possibly damaging the Schlemm canal, development of glaucoma is theoretically possible, but no cases have been reported.
Individuals with high myopia have an increased risk of retinal detachment. Several cases were reported in the literature. No evidence indicates that surgical trauma of refractive keratotomy predisposes a patient to retinal detachment. All patients should have a careful fundus examination performed with appropriate management of any retinal pathology found.
Blepharoptosis has been reported after refractive keratotomy, although the pathogenesis remains unknown. It has been proposed that ptosis results from a dehiscence of the levator aponeurosis. Surgical repair should be delayed until both eyes have had refractive keratotomy and an adequate period has elapsed to allow for spontaneous recovery.
Both corneal hypoxic expansion in the area of RK incisions, which may lead to central corneal flattening and barometric pressure directly altering corneal shape, is believed to contribute to the hyperopic shift induced by altitude. This shift has been documented as much as +3.00 D at 5000 m. Ophthalmologists performing RK at high altitude should redesign their RK nomograms with this in mind.
It seems that now and in the future, more and more private nongovernmental activities in space will take place. This will make it necessary to take a closer look at ophthalmic issues in respect to vision and ophthalmic surgery.[5] Given that RK has dramatic shifts of Rx at high altitudes, people who have had RK should probably avoid space trips. Recently, PRK in an astronaut was studied and showed good refractive stability, and it is suspected that a LASIK patient would also be fine in space.[6]
The outcomes of this surgery probably are documented best by the Prospective Evaluation of Radial Keratotomy (PERK) study and its follow-up papers over the years.
The 10-year follow-up PERK study results confirmed that RK reduced myopia but that the effectiveness of the outcome varied among patients. Of the 427 patients (793 eyes) that underwent RK, 374 patients (88%; 693 eyes) returned for examination a decade after surgery. Of 675 eyes with refractive data, 38% had a refractive error within 0.5 D of emmetropia and 60% were within 1.00 D. Uncorrected visual acuity was 20/20 or better in 53% of 681 eyes and 20/40 or better in 85%. Among 310 patients with bilateral RK, 61% reported not wearing spectacles or contact lenses for distance vision at 10 years after surgery.
These 10-year examinations indicated that the refractive error had not been stable in these eyes during the postoperative interval. For 310 first-operated eyes, the mean refractive error was -0.36 D at 6 months after surgery, and this mean refractive error had changed to +0.51 D at 10 years, for a mean change in a hyperopic direction of +0.87 D between 6 months and 10 years after surgery. The average rate of change was 0.21 D per year between 6 months and 2 years, and +0.06 D per year between 2 and 10 years after surgery. From 6 months to 10 years, the refractive error of 43% of eyes changed in the hyperopic direction by 1.00 D or more. The hyperopic shift was statistically associated with incision length, with smaller clear zone diameters and larger overall cornea diameters being associated with a greater change in refraction.
Long-term follow-up study revealed no blinding complications. Loss of spectacle-corrected visual acuity of 2 lines or more on a Snellen chart occurred in 3% of all 793 eyes that underwent surgery.
The results were acceptable in many patients (especially those between -1.50 D to -4.00 D); however, because of the advent of newer (and better) techniques, the surgery is largely not performed anymore.
The main influential factors were as follows: the unacceptable decrease in the structural stability of the eye, with the resultant risks of perforation with trauma; diurnal variations of refraction with IOP and with altitude; and progressive hyperopic shift in some patients. Other main reasons for abandoning the surgery included the risks for perforation and infection, as well as more glare and starburst effects than with the newer laser refractive techniques.
No future likely exists for this particular procedure, although it does for its variant astigmatic keratotomy (see Astigmatism, Astigmatic Keratotomy); but, historically, RK is an important milestone in the quest for emmetropia through surgery. RK has been supplanted by PRK and LASIK. However, even these procedures could fall by the wayside as new procedures for safe intraocular surgery with preservation of accommodation are developed.
Optical zones and pupil dilation discussed by radial keratotomy surgeons.
Optical zones and pupil dilation discussed by radial keratotomy surgeons.
Optical zones and pupil dilation discussed by radial keratotomy surgeons.
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