PRK Astigmatism Treatment & Management
- Author: Manolette R Roque, MD, MBA, FPAO; Chief Editor: Hampton Roy, Sr, MD more...
Medical therapy is limited to broad-spectrum topical antibiotics and corticosteroids for uncomplicated cases (see Postoperative details).
To understand the astigmatic ablation technique used in PARK, briefly presenting the different ablation profiles used for myopia, hyperopia, and astigmatism is important. For the surgical procedure, see Intraoperative details.
In the refractive laser correction of myopia, the excimer laser ablates using a greater amount of energy to the central cornea compared to the peripheral cornea. This is attained by the opening or closing of a central aperture via which the laser light exits or by the use of a scanning laser that sends pulses mainly on the central cornea. The central corneal tissue is ablated more than the periphery. Different machines have different peripheral blending zone sizes. This ablation results in an oblate corneal shape.
In the refractive laser correction of hyperopia, the excimer laser ablates using a greater amount of energy to the peripheral cornea compared to the central cornea. This is attained in an opposite fashion of achieving myopic correction.
In the refractive laser correction of astigmatism, the key is the use of an elliptical pattern of ablation applied along the central part of the flat meridian that leads to flattening of the steep axis. This may be performed in the setting of both combined myopic astigmatism or combined hyperopic astigmatism.
An extensive informed consent should be prepared during the screening process. This consent usually is performed on a one-on-one basis. Providing each potential patient with complete reading materials about PRK and LASIK is worthwhile. The informed consent should be discussed only after the patient has read and understood this material.
Determining and discussing each patient's motives for refractive surgery are essential. Certain qualifications can immediately raise a red flag, including those individuals who expect 20/20 vision, desire perfect results, or believe that refractive surgery will change their lives. Patients who fit this profile should be advised against refractive surgery to avoid disappointments and potential lawsuits. The perfect candidate should be motivated by a desire to reduce dependence on glasses or contact lenses. When discussing motivations, the surgeon should ask the patient, "Will you be happy with refractive surgery if your dependence on glasses or contact lenses is decreased, while knowing for a fact that the possibility of using either correction may be required for some tasks?" If the patient provides an affirmative answer, then that patient is ideal.
The surgeon should not overlook the fact that many potential patients have active lifestyles involving contact sports. Some patients may have occupations in which spectacles and/or contact lenses are troublesome. As in all other forms of surgeries, a complete systemic history should be elicited, with attention to incidental data. This history should include a search for risks associated with autoimmune disease and dermal keloid formation, as well as a complete ocular history of contact lens status, prior eye trauma, and/or past and present ocular disease. Some medications (eg, amiodarone) may cause corneal toxicities.
A complete and thorough ocular examination should be performed for each patient. Prior to any examination, advise the patient to abstain from soft contact lens use for at least 3 days and to abstain from hard contact lens use for at least 2 weeks. Some surgeons recommend a cutoff of at least 2 weeks for both soft and hard contact lens use prior to screening. This interim period is believed to decrease the possible effects of corneal warpage. The patient's old prescription eyeglasses are metered and/or old contact lenses are requested and subsequently noted in the chart.
The patient then is subjected to several refractions, including best-corrected and cycloplegic, automated, and manifest. The cycloplegic refraction usually is performed at the end of all other procedures and is compared with the manifest refraction to avoid overcorrections (and undercorrections). If the automated refractometer does not have a built-in keratometer, stand-alone keratometry is performed.[44, 45, 46]
Having the patient place the apposed thumb and indexed finger in front of the line of sight and visualizing a letter from the 20/200 line determine eye dominance. Asking the patient to alternately close each eye while the apposed fingers are in place determines the dominant eye. Determination of eye dominance is important in the event that the patient desires monovision. The effect of monovision can be simulated using disposable contact lenses for distance correction of the dominant eye.
The scotopic pupil size is noted with an infrared camera (Colvar). This helps to weed out patients with a large pupil and to decrease postoperative glare and halos. Contrast sensitivity testing preoperatively also is recommended. A slit lamp examination is performed to screen for the presence of corneal disease or other lid abnormalities. Intraocular pressure is likewise determined with a Goldmann applanation tonometer.
A baseline Schirmer test is performed to determine the normal tear production. Tear break-up time also is recorded. All patients with dry eyes or blepharitis are treated extensively prior to the procedure. Proceeding with surgery is recommended only with adequate improvement.
A dilated fundus examination also is performed to screen for retinal or optic nerve abnormalities. Indentation ophthalmoscopy is essential to visualize the possible presence of peripheral retinal lesions (eg, lattice, tear).
After these procedures, the patient has the final 2 examinations. The first examination involves the use of the corneal topographer. This is very useful in determining the effects of corneal warpage or screening for keratoconus. The final examination entails the use of a wavefront aberrometer that gives a detailed wavefront imprint and a highly accurate automated refraction. This enables the surgeon to screen patients with highly irregular astigmatism and higher order aberrations. These patients are offered the advantage of the use of wavefront-guided aberrometry in the excimer laser correction of their refractive error. The use of an aberrometer may not be available in all centers.
The patient is advised to sleep adequately prior to the procedure and is requested not to wear makeup and perfume on the day of the surgery. Some patients may require restraint from intake of caffeine-containing drinks to decrease agitation. Other patients may warrant the intake of calming medications.
The excimer laser is placed in a stable environment with 24-hour monitoring and maintenance of temperature, humidity, and electricity. A specific setting that is recommended for a Bausch and Lomb Technolas 217 is 20°C and 40% humidity. A voltage regulator with unlimited power supply sufficient to complete laser firing for both eyes is installed. In the unlikely case that power fluctuates or loss of electricity occurs, sufficient back-up power is required to complete the procedures.
The entire surgical suite is sterilized at the beginning of each day with strict maintenance of semisterility during the entire day. No cell phones, pagers, computers, and electronic planners are allowed inside the surgical suite. In addition, no street clothes are allowed inside the surgical suite.
Perform a thorough check of the laser each day, including tests for an adequate homogenous beam profile, alignment, and power output. Fluence testing is performed for every eye.
The patient is seated under a slit lamp biomicroscope and preplaced fiducial marks are placed with either a Gentian violet marker or scored with a Sinskey hook at the limbal area (ie, 3- and 9-o'clock positions). This is essential to PARK to lessen the chances of ablating at the wrong axis due to torsion eye movements when the patient assumes a supine position. Each patient is advised about the sounds and smell of the excimer laser. Continuous communication is maintained to calm the patient. Sometimes, the patient requires an intake of oral diazepam to soothe the senses. For unilateral cases, the appropriate eye is marked clearly. Some surgeons desire miosis secondary to pilocarpine instillation for papillary centration; however, this may not be necessary, and, in fact, it may distort the natural papillary position.
Maintaining a preoperative room adjacent to the surgical suite is recommended, where all patients can be prepped surgically prior to the procedure. Both eyes are prepared in this preoperative room. The second eye is covered appropriately with a sterile eye shield. The opposite eye is patched for protection and prevention of cross-fixation. Usually, the right eye is treated first, followed by the left eye. The patient is now ready to be assisted into the laser room.
Once inside the laser room, the patient lies on the bed with the head appropriately placed under the laser apparatus. Place a lint-free blanket over the entire body to minimize clothing particles from flying into the surgical zone. Verify the patient's identity with the patient chart that is located inside the room. To minimize data inputting errors, only a single chart at a time is allowed within the room.
In the eye to be operated on, place the lid speculum. Sterile topical tetracaine anesthetic eye drops are dropped into each eye. The preoperatively determined amount of laser correction is entered into the laser. The patient is reminded to fixate on the red laser centration light, while the surgeon focuses the helium-neon (He-Ne) beam. Several reports claim that patient fixation during actual laser surgery results in better centration than manual globe immobilization by the surgeon.
Currently, several different techniques are available for preparing an eye for PRK. These techniques include removal by a sharp blade or a blunt spatula, removal with a rotating corneal epithelial brush, debridement after diluted alcohol solution application, alcohol solution application with replacement of the epithelial flap, and transepithelial laser ablation.
The most widely used technique for epithelial debridement is the use of a sharp blade or a blunt spatula. In this procedure, the surgeon uses an optical zone marker to define the outer borders of the area to be deepithelialized. Beginning from the periphery, the surgeon then proceeds with the debridement, attempting to scrape as evenly as possible. Near the end of the debridement, the surgeon switches to a moistened surgical cellulose sponge to even out the surface and to remove leftover epithelium. This entire procedure should be performed efficiently to avoid hydration changes in the stroma that could affect the ablation results.
When using the Amoils epithelial scrubber, the following tips are recommended.
Apply a round eye shield soaked in proparacaine on the corneal surface for around 90 seconds. This will help loosen the corneal epithelium from the Bowman membrane. Irrigate the cornea with a chilled balanced salt solution. If the excimer unit has a ring light, turn it on to assist in seeing the corneal epithelium during scrubbing. Use a circular motion while applying the brush to the eye. This will prevent a central area of residual epithelial tissue and can be used to control or enlarge the zone of epithelial removal. Always wet the cornea and brush tip with a balanced salt solution prior to use. Invert the brush and place a few drops of the balanced salt solution on the concave tip.
Turn the brush on before placing it on the eye. It is often useful to tell the patient that this is the sound of the brush as it creates a smooth surface for the laser to work. Turn the magnification in the microscope as low as possible while using this device. Be sure to turn off the fixation occluder before proceeding with laser ablation. Flood the cornea with a chilled balanced salt solution (5-7°C) immediately after PRK to equilibrate the corneal temperature, approximately 7-10 seconds.
In using the LASEK technique in the removal of the epithelium, a standard radial keratotomy marker is centered on the cornea and lightly indented on the cornea to create a groove on the epithelial side. A special modified corneal ring with a diluted alcohol solution dispenser is then placed over the preplaced groove, and a measured amount of 20% alcohol solution is allowed to bathe the underlying epithelium. Care is maintained not to allow the alcohol to overflow or spill onto the rest of the cornea.
After a variable amount of time (30 s to 2 min), depending on the surgeon, the alcohol is removed with a surgical sponge, while maintaining both the ring and the dispenser in place. Then, the ring is removed. A Sinskey hook is used to score the groove 300° around, leaving an area attached. A surgical sponge is used to push the epithelium to the hinge.
This technique is now much easier because of the special LASEK instruments that have been developed. These products are recommended because of the ease with which the procedure is performed.
The head of the patient should be leveled properly, ensuring that the eye is in a neutral position. The laser-aiming beam is centered and focused. Remember that each machine has a different recommendation for centration and focusing. If the machine has an eye tracker, then the amber-enhancing lights should be positioned properly to provide the best illumination. Once proper position is achieved, switch on the eye tracker. A viewing monitor displays whether or not the tracker is in place and is working.
Remind the patient to maintain the head position and to keep the other eye open at all times. Constant reminders to continue looking at the red light during ablation are essential for proper centration. If the patient exhibits a doll's eye phenomenon or begins to lose fixation, then the surgeon should stop firing the excimer laser until the patient exhibits appropriate fixation.
A never-ending debate is ongoing regarding which system is better with respect to eye tracking. All the current systems are fast enough to deal with microsaccades. The argument of patients with nystagmus benefiting from faster trackers is defeated by the fact that, in this subset of patients, screening and refraction are difficult immediately from the outset. Achieving an accurate topography, much less an aberrometry, is a sizable task. For these patients, first exploring other avenues of refractive care is advisable.
During ablation, the surgeon can opt either to complete the entire process without wiping off ablated tissue or to intermittently wipe the cornea with a surgical sponge. When visible ablated debris is on the cornea, a few strokes with a surgical sponge to remove the debris may help the excimer laser to deliver the appropriate energy to the cornea.
Topical drops (eg, antibiotic, steroid, preservative-free lubricant) are administered. A soft bandage contact lens (BCL) is applied prior to lid retractor removal. The surgeon has the option of applying a single drop of nonsteroidal anti-inflammatory drug (NSAID) after BCL application.
If the LASEK step was performed for deepithelialization, then additional steps are warranted. After the excimer laser ablation is completed, the cornea is wiped carefully with a surgical sponge to remove ablated corneal debris. Once the ablated area is clear from debris, the rolled epithelium is teased slowly back into place. This is tricky and sometimes leads to a so-called jigsaw puzzle experience. Some surgeons believe that prefiducial marks with Gentian violet all around the groove of proposed deepithelialization makes the epithelial replacement easier, whereas other surgeons see no difference. Once the epithelium is replaced, topical drops (eg, antibiotic, steroid, NSAID) are administered. Then, a soft BCL (preferably nonionic, low-water content) is placed prior to lid retractor removal.
After placing the shaded protective polycarbonate eyeglasses, the patient is escorted to the recovery room. Here, while receiving postsurgery orders, the patient is advised to rest for the next few minutes. Post-LASEK patients are observed at the slit lamp 30 minutes after the surgery to check for epithelial and soft BCL positioning.
Each patient is advised to go home directly and to sleep for the next few hours. Prior to sleeping, the patient is advised to take oral NSAIDs or painkillers. The polycarbonate glasses are kept on for at least the first 24 hours for protection.
Patients are monitored closely until full reepithelialization is complete, which usually occurs after 72 hours. Patients then receive follow-up care on postoperative week 1 and postoperative months 1, 3, 6, and 12. The surgeon should observe patients for postoperative complications (see Complications).
PARK is used more often in combination with a LASIK flap than as a solitary procedure. For purposes of simplification, LASIK flap complications associated with cases involving PARK are not discussed herein. These LASIK-PARK complications are discussed in Astigmatism, LASIK. Understanding that the complications in performing PARK are similar to those encountered with PRK is important. These complications include corneal haze, optical aberrations, halos, decentered ablation, regression, overcorrection, undercorrection, and endothelial effects. In LASIK-PARK, also consider epithelial defects and infiltrates and elevation of intraocular pressure. These complications are discussed further in Myopia, PRK.
When a high degree of cylinder is corrected, a smaller optical zone in the steep meridian is fashioned. This leads to glare and halos, especially with night vision. The Melbourne Excimer Laser Group reported an observable higher retreatment rate after performing astigmatic corrections compared with spherical corrections.
Exclusively seen complications related to PARK usually involve the axis and the magnitude of astigmatism. No consensus exists on whether to use cycloplegic or manifest refraction when a discrepancy is presented between each refraction. Occasionally, the keratometric and topographic cylindrical axis differs from the patient's subjective refraction. Some surgeons choose to follow the patient's subjective refraction because of the possibility of lenticular astigmatism; other surgeons believe otherwise.
Often, the primary source of error in proper axis alignment stems from the initial placement of corneal markings prior to the procedure. Placement of fiducial markings while the patient is seated at the slit lamp biomicroscope is advocated. This simple step of corneal marking may eliminate the cyclotorsion that occurs when a patient assumes a supine position. The alignment of the patient's eye under the excimer laser is achieved easily because of these markings, thereby allowing the steeper meridian to receive more laser energy. Continuously asking the patient to look at the blinking fixation light during the course of the ablation minimizes treatment drift off-axis.
Delayed epithelialization and corneal haze has been reported in cases in which NSAIDs are applied directly on the cornea post-PRK prior to BCL application. The author no longer applies NSAIDs directly on the cornea post-PRK. It is applied after BCL application and is given only 4 times daily for 2 days, then discontinued. This regimen has eliminated delayed epithelialization and corneal haze.
Outcome and Prognosis
PARK in the context of application under a LASIK flap and as a solitary procedure under an epithelial LASEK flap is gaining popularity among keratorefractive surgeons. Numerous studies have been performed to determine the efficacy and safety of PARK.[56, 57, 58] Results show that the reduction in total cylinder is 15-95%. Uncorrected visual acuity of greater than or equal to 20/40 is 55-90%. Two or more lines of best-corrected visual acuity were lost in 0-28% of eyes. Postoperative results and uncorrected visual acuity of PARK versus PRK are similar.
Kremer observed 28 eyes (mild astigmatism) for 12 months and noted that the preoperative cylinder of –0.84 +/-0.22 D decreased to a postoperative cylinder of –0.40 +/-0.33 D. Uncorrected visual acuity of greater than 20/40 occurred in 89% of eyes. In 44 eyes (moderate astigmatism), the preoperative cylinder of –1.77 +/-0.42 D decreased to a postoperative cylinder of –0.54 +/-0.48 D. Visual acuity of greater than 20/40 was evident in 82% of eyes. In 20 eyes (high astigmatism), the preoperative cylinder of –3.54 +/-0.64 D decreased to a postoperative cylinder of –0.69 +/-0.32 D. Uncorrected visual acuity of greater than 20/40 occurred in 90% of eyes.
Using a whole-field technique, they found PARK to be less effective in reducing the preoperative astigmatism in individuals with low levels (< 1 D) of cylinder (48%) in comparison with individuals with moderate levels (1.25 to 2.50 D) of cylinder (68%) and high levels (2.75 to 5 D) of cylinder (81%). They postulated that the effect of treating lower levels of astigmatism might be lost in the overall healing process of the spherical part of the ablation. The efficacy of PARK could vary according to the amount of preoperative astigmatism, and, as such, the dictum of "greater the amount of preoperative astigmatism, the greater the percentage of correction" was coined.
Kremer and associates also found that the residual refractive cylindrical axis after PARK did not change significantly from the cylindrical axis preoperatively, with a range of 5-15°.
Alio observed 46 eyes for 12 months with a preoperative cylinder of –2.50 +/-0.70 D and a postoperative cylinder of –0.50 +/-0.20 D. No patient lost 2 lines of best-corrected visual acuity.
For 6 months, Kim observed 168 eyes with a preoperative cylinder of 1.51 +/-0.81 D and a postoperative cylinder of 0.67 +/-0.60 D. Uncorrected visual acuity of greater than 20/40 was evident in 91% of eyes.
Lazzaro conducted a 12-month follow-up study of 7 eyes with a preoperative cylinder of 5.32 D and a postoperative cylinder of 2.79 D. Despite results of decreasing the cylinder to nearly half, 2 lines of best-corrected visual acuity were lost in 28% of eyes. They discovered that when PARK was used to correct residual astigmatism present after penetrating keratoplasty using a whole-field technique, an average reduction in the refractive astigmatism of 38% and 48% was achieved. Differences in corneal wound healing in a grafted eye compared with a nongrafted eye were believed to have resulted in the reduced efficacy of PARK in these cases.
Gallinaro presented a 6-month follow-up study of 72 eyes with a preoperative cylinder of –2.14 +/-1.99 D and a postoperative cylinder of –1.75 +/-1.32 D. Uncorrected visual acuity of greater than 20/40 occurred in 65% of eyes. Two lines of best-corrected visual acuity were lost in 12.5% of patients.
Taylor also presented a 6-month follow-up study of 65 eyes, with 72% of them achieving an uncorrected visual acuity of greater than 20/40 and 12.5% of them losing 2 lines of best-corrected visual acuity.[64, 9]
Hamberg-Nystrom and coworkers presented a 12-month follow-up study of 113 eyes. They used a whole-field technique and found a smaller reduction in the preoperative astigmatism in individuals with low levels (< 2 D) of astigmatism (44%) in comparison with individuals with higher levels (>2 D) of astigmatism (72%). Similar to Kremer's results, the efficacy of PARK could vary according to the amount of preoperative astigmatism.
Gomez de Liano also presented a 12-month follow-up study of 53 eyes, with a preoperative cylinder of –2.28 +/-1.25 D and a postoperative cylinder of –1.40 +/-0.78 D; of these, 58% of eyes achieved visual acuity of greater than 20/40.
For 3 months, Cherry observed 34 eyes with a preoperative cylinder of 2.35 D and a postoperative cylinder of 1.22 D.[67, 68]
Also, for 3 months, Hersh observed 10 eyes with a preoperative cylinder of 1.48 D and a postoperative cylinder of 0.86 D, with 74% of them having uncorrected visual acuity of greater than 20/40 and 10% of them losing 2 lines of best-corrected visual acuity. Hersh used an erodible mask and found undercorrection present in the spherical correction after PARK, with no eyes achieving an overcorrection using a mask. Using the erodible mask, the postoperative axis did not rotate more than 10°.[69, 70]
Brancato presented a 6-month follow-up study of 21 eyes with a preoperative cylinder of -2.46 D and a postoperative cylinder of –1.56 D; 60% of eyes achieved an uncorrected visual acuity of greater than 20/40.
Niles presented a 6-month follow-up study of 25 eyes with a preoperative cylinder of 2.31 D and a postoperative cylinder of 0.69 D, with 76% of eyes achieving uncorrected visual acuity of greater than 20/40 and 8% of them losing 2 lines of best-corrected vision. Niles and associates used an erodible mask and found a greater overcorrection of the spherical component when performing PARK in comparison with PRK. The observed overcorrection presumably arose from increased corneal dehydration as a consequence of longer surgical times for PARK. More difficulty was noted in maintaining patient eye fixation while using the erodible mask.
Kaskaloglu conducted a 6-month follow-up study of 28 eyes with a preoperative cylinder of –2.53 +/-1.49 D and a postoperative cylinder of –0.16 +/-0.99 D, with 55% of eyes achieving uncorrected visual acuity of greater than 20/40 and 7.1% of them losing 2 lines of best-corrected visual acuity.
Dausch presented an 18-month follow-up study of 17 eyes with a preoperative cylinder of –2.53 +/-1.32 D and a postoperative cylinder of –0.44 +/-0.67 D; 93% of eyes achieved uncorrected visual acuity of greater than 20/40. The high astigmatism group had a preoperative cylinder of –4.75 +/-1.17 D and a postoperative cylinder of –0.89 +/-0.60 D, with 82% of them achieving uncorrected visual acuity of greater than 20/40. They investigated the use of PARK on patients with mixed and irregular astigmatism by designing a custom asymmetric mask, which was based on computerized video keratography, to perform the asymmetric ablation. Of the 3 patients who were treated, the preoperative cylinders of 5.50 D, 1.25 D, and 7 D were reduced postoperatively to 0 D, 0.50 D, and 1.50 D, respectively.[74, 75]
Future and Controversies
Refractive laser surgery is becoming extremely popular. More and more procedures are performed by keratorefractive specialists and by PRK/LASIK-certified general ophthalmologists. The response of the general population to this procedure is overwhelming yet expected. The relative freedom from the use of glasses and contact lenses is a tempting and irresistible offer.
As a subspecialty in ophthalmology, keratorefractive surgery is one of the more exciting and fastest-growing disciplines in recent years. Being relatively, if not totally, dependent on the precision offered by the technology associated with it, continuous advancements will occur. This marriage of surgical application and technology-driven hardware brings a new frontier in patient care. Aggressive keratorefractive surgeons and newer upgrades of excimer lasers will continue to push the envelope of refractive treatment possibilities and applications.
This advent of cutting-edge research in newer machines and newer technology promises a bright future. Smoother and more precise ablations are to be expected. Real-time topography and wavefront-guided lasers will allow surgeons to perform customized ablations. Many reports are already available on the early generations of wavefront-guided excimer lasers.
In the laboratory and pharmaceutical world, studies are underway to develop a means to control the wound-healing properties of the cornea. Hopefully, the complications that are being observed today will decrease. Newer forms of complications with the latest "tech-savvy" machines are possible. Until the time that a perfect refractive laser surgery procedure is performed consistently, innovations in medical and surgical refractive treatments will continue.
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