Photorefractive Keratectomy (PRK) for Astigmatism Correction 

Updated: Apr 07, 2017
Author: Manolette R Roque, MD, MBA, FPAO; Chief Editor: Douglas R Lazzaro, MD, FAAO, FACS 

Overview

Background

Because of the numerous refractive surgery terms that are used almost exclusively in the successful understanding of refractive disorders, first presenting a glossary of terms to assist the reader is essential.

Glossary of terms

Ablation - Removal. In excimer laser surgery, a frequency of energy causes corneal molecules to detach from one another from their points of attachment.

AK - Abbreviation for astigmatic keratotomy

ALK - Abbreviation for automated lamellar keratectomy

ArF - Abbreviation for argon fluoride

ASA – Abbreviation for advanced surface ablation. A collective term sometimes used to refer to the current techniques for surface treatments.

Astigmatic keratotomy - Surgical procedure in which microscopic incisions are positioned in the peripheral cornea to create a more spherical shape

Astigmatism - A refractive condition where the surface of the cornea is not spherical. A distorted image is formed because light images focus on 2 separate points in the eye.

Automated lamellar keratoplasty - An incisional refractive surgery method used in low-to-moderate myopia. An automated microkeratome placed on the eye removes, in an oscillating shaving motion, a thin layer of cornea that is microns thick. Subsequent removal of a thinner underlying corneal stroma is performed with stitchless replacement of the initial superficial cap.

Excimer - Abbreviation for excited dimer

LASEK - Abbreviation for laser epithelium keratomileusis

Laser - Abbreviation for light amplification by the stimulated emission of radiation

Laser in situ keratomileusis - A refractive laser procedure combining the use of ALK and photorefractive keratectomy in reshaping the central cornea to treat refractive errors. An automated microkeratome, similar to that used in ALK, is used to fashion a flap with a hinge. Subsequent ablation is performed on the corneal stromal bed with stitchless replacement of the corneal flap.

Laser epithelium keratomileusis - A modification of the epithelial debridement performed in photorefractive keratectomy. In this procedure, the epithelium is detached with the use of an alcohol solution (usually 20%) to lyse DNA bonds (cleave basement membrane) and to soften the epithelium to facilitate rolling over into a flap. Ablation is performed subsequently with replacement of the epithelial flap.

LASIK - Abbreviation for laser in situ keratomileusis

Light amplification by stimulated emission of radiation - Laser light is composed of one color (wavelength) traveling in one direction, and each light wave is traveling in step with the adjacent wave, making the laser light more powerful by a factor of millions.

Oblate - Shape of the cornea after conventional laser ablation profile where it is steeper in the periphery

PARK - Abbreviation for photoastigmatic refractive keratectomy

Pachymetry - Optical or ultrasonic procedure to measure the corneal thickness

Photoastigmatic refractive keratectomy - Although similar to photorefractive keratectomy, the ablation profile is specific for astigmatism.

Photorefractive keratectomy - Surgical procedure using an excimer laser to fashion the central cornea to treat refractive errors

Prolate - Normal corneal shape; steeper in the center

PRK - Abbreviation for photorefractive keratectomy

Topography - Used to measure the low or high areas of a plane

Wavefront - Describes the surface that connects all the points on a propagating light wave that are of equal phase

Wavelength - The distance between the top of one wave and the top of the next wave. In the case of an excimer laser, this is measured in nm (eg, argon fluoride has a wavelength of 193 nm).

Astigmatism and the advent of photoastigmatic refractive keratotomy

Astigmatism, a refractive condition where the surface of the cornea is not spherical, can decrease visual acuity by forming a distorted image because light images focus on 2 separate points in the eye. Clinicians and surgeons have searched constantly for the most successful device or procedure to treat this refractive error. Nonsurgical devices include spectacles and contact lenses. To date, these devices are being improved continuously to address the complex problem of astigmatism. Initial surgical approaches include astigmatic keratotomy, compression sutures, and wedge resection. Recent surgical procedures involve the use of the excimer laser in PARK and LASIK with or without wavefront-guided technology.[1]

PRK is the application of ultraviolet high-energy photons (193-nm wavelength) of the ultraviolet range generated by an argon fluoride excimer laser to the anterior corneal stroma to change its curvature and, thus, to correct a refractive error.[2, 3] The physical process of remodeling by PRK is called photoablation. This surgical procedure reshapes the central cornea to a flatter shape for people who are nearsighted and a more curved surface for people who are farsighted. Several techniques are being used to correct for astigmatism.

Device description

Two different methods of energy delivery are available by the excimer laser device, a large circular beam and a scanning slit or spot.

The earlier devices initially used large area ablation. To date, some manufacturers still use large area ablation in their modern devices. The circular laser beam passes through a diaphragm that slowly enlarges to deliver more cumulative energy in the center and less in the periphery.[4, 5] Some laser-induced irregularities (central islands) have been reported in these large area systems. This method results in a shorter operating time to deliver the necessary laser pulses versus a system that uses a scanning slit system. The following manufacturers use circular beam lasers: Schwind (Coherent Medical Inc, Palo Alto, Calif), Summit (Waltham, Mass),[6, 7, 8] and VISX (Santa Clara, Calif).

The scanning slit or spot is an alternative method of energy delivery by the excimer laser. A smaller beam passes through a beam-shaping aperture, delivering a pattern of more pulses centrally than peripherally and resulting in greater corneal tissue ablation centrally. Less total energy is delivered at the corneal surface; therefore, a less powerful laser device may be used. In principle, this system is more effective in providing different ablation patterns in the treatment of astigmatism, irregular astigmatism, and hyperopia.

The use of scanning laser technology with its small moving beam has resulted in reduced thermal heating. This is visualized in a study that showed the different areas of plume production after each area of ablation following movement of the scanning beam. Central islands have not been reported in these systems. The smaller ablation size of the scanning laser consequently results in a longer operating time. Maintaining fixation has always been a problem for these scanning lasers, especially with the longer operating time, which results from more ablations by the smaller beams. Moreover, precise pulse-to-pulse registration of the scan is necessary to achieve a smooth and accurate final pattern. Automatic tracking devices are provided standard in these devices.

Manufacturers of scanning slit systems include the following: Autonomous (Orlando, Fla), LaserSite (Orlando, Fla), Meditec (Aesculap-Meditec, Heroldsberg, Germany)[9, 10] , Nidek (Fremont, Calif)[11, 12, 13, 14, 15, 16] , Novatec (Carlsbad, Calif), and Technolas (Chiron Vision Corp, Irvine, Calif). Novatec's claim to fame is its use of solid-state laser crystals that obviates the need for argon fluoride gas to create its shorter ultraviolet beam. Presently, the Food and Drug Administration (FDA) has approved only the Summit and the VISX laser systems for commercial use within the United States. All the other systems currently are being used in other countries.[17]

PRK variations

The current techniques for surface treatments (eg, PRK, PARK, LASEK, epithelial scrubber assisted PRK, epi-LASIK) are sometimes collectively termed advanced surface ablation (ASA).

LASEK

Surface ablation resurgence was primarily assisted by the introduction of LASEK. The authors believe that LASEK is an improvement of the existing PARK technique. LASEK uses a sterile diluted alcohol solution to separate a viable sheet of corneal epithelium, fashioning a flap that can be rolled back after excimer laser ablation. Several advantages have been identified upon performing this procedure, as follows:

  • No blade technique for refractive surgery

  • Corneal tissue sparing

  • Less postoperative pain as compared to PARK

  • Less postoperative corneal haze as compared to PARK[18]

Epithelial scrubber assisted photorefractive keratotomy

The use of an Amoils epithelial scrubber allows fast and accurate epithelial removal, leaving a smoother anterior stromal surface.[19] The quick and efficient epithelial removal in less time decreases the potential for corneal dehydration to occur. It produces an easily seen bull’s eye that aids in centration of the laser application. Less epithelium is removed, resulting in faster healing time.

Epi-LASIK

The use of an epikeratome (plastic blade) mechanically sliding over the surface of the cornea, just underneath the epithelial layer of cells, while suction is applied, has resulted in another technique, which is basically automated LASEK without alcohol.[20, 21, 22]

History of the Procedure

The history of surgical treatment for astigmatism dates back to the late 1800s. Certain milestones in the development of this procedure can be attributed to several individuals, and a number of parallel procedures were in development at certain time periods.

In 1885, Schiotz performed limbal incision in the steep meridian to reduce iatrogenic astigmatism. Faber performed anterior transverse incisions to reduce idiopathic astigmatism. Lucciola also performed nonperforating corneal incisions to correct astigmatism. In 1894, Bates postulated that corneal incisions made at right angles to steeper meridians might correct astigmatism. Later, Lans showed that flattening in the meridian perpendicular to a transverse incision was associated with steepening in the orthogonal meridian and that a greater effect may be achieved with deeper and longer incisions.

In the 1940s, Sato began an extensive investigation of radial and astigmatic keratotomy.[23] Fyodorov is responsible for presenting several nonperforating anterior keratotomy patterns.

Modern techniques for astigmatic keratotomy are attributed to the works of Nordan, Thornton, and Nichamin. Nordan advocated a simple method of straight transverse keratotomy, with a target correction of 1-4 diopters (D). Thornton proposed a technique that included up to 3 pairs of arcuate incisions in varying optical zone sizes and with consideration of age and timing after surgery, respectively.[24] Nichamin developed an extensive nomogram for astigmatic keratotomy at the time of cataract surgery, although this technique has been modified by the use of a limbal relaxing incision during cataract surgery. Consequently, Troutman, who fancied wedge resection for reduction of postcorneal transplant astigmatism, developed another technique of astigmatism reduction.

Increased interest in using lasers to ablate tissue occurred in the late 1980s. The excimer laser initially was developed to etch out inscriptions on microchips. The postulated application of controlled ablation on corneal tissue led to its use in refractive correction.

In the late 1990s, wavefront aberrometry promised both physicians and patients the potential to achieve the so-called supervision. Initially used by astronomers, this wavefront technology reduced unwanted wavefront distortions in the creation of land-based telescopes.

Problem

Refractive errors (ie, myopia, hyperopia, astigmatism) can decrease visual acuity.[25] Astigmatism is a more challenging entity because it is determined by regularity, amount, and orientation. It also is more difficult to treat than myopia or hyperopia.

The quest to treat astigmatism began with the use of nonsurgical devices, including spectacles and contact lenses. These nonsurgical devices were followed by surgical techniques involving astigmatic keratotomy, compression sutures, and wedge resection. Newer surgical procedures include the use of intracorneal ring segments, PARK, LASIK, and LASEK, with or without wavefront-guided technology.

Epidemiology

Frequency

The frequency of astigmatism has a wide range of values as presented in modern literature. Naturally occurring (idiopathic) astigmatism is common. Surgically induced (iatrogenic) astigmatism is less common yet more problematic.[26, 27]

Clinically detectable refractive astigmatism reportedly is present in as many as 9 out of 10 eyes. However, refractive astigmatism in most of these eyes would not be clinically significant. The incidence of clinically significant astigmatism has been reported to be 7.5-75%, a wide range that primarily depends on the specific study and an author's definitions. Studies have estimated that approximately 44% of the population has more than 0.50 D of astigmatism, 10% of the population has more than 1.00 D, and 8% of the population has 1.50 D or more.

Etiology

Aside from the previously mentioned idiopathic astigmatism that is present, iatrogenic astigmatism may result after surgery. Visually significant refractive astigmatism is fairly common after different kinds of ophthalmic surgery, including cataract extraction, lamellar or penetrating keratoplasty, other corneal and anterior segment surgeries, and trabeculectomy.[28, 29, 30, 31, 32, 33]

Reportedly, astigmatism of at least 1.00 D often results after extracapsular cataract extraction, and astigmatism of at least 3.00 D is present in as many as 20% of cases with 10-mm incisions. Even phacoemulsification procedures, using the clear cornea technique, reportedly cause postoperative astigmatism, thereby guiding the cataract surgeon as to the proper placement of the corneal approach. High astigmatism can occur after penetrating keratoplasty.

Pathophysiology

The means of ablation of the excimer laser seem to be photochemical in type. This removal of tissue is called photochemical ablation or ablative photodecomposition. Photochemical ablation is an extremely confined tissue interaction centered on the fact that every photon created by the ArF excimer laser has 6.4 eV of energy, which is sufficient to split covalent bonds.

The intramolecular bonds of uncovered organic macromolecules are split when a sufficient number of high-energy, 193-nm photons are absorbed in a brief period. The resulting fragments rapidly expand and are ejected from the exposed surface at supersonic velocities observed under high-resolution magnification as the plume effect. This is the reason why only the irradiated organic materials are affected and the adjacent areas are not affected.

Presentation

A patient with astigmatism may complain of shadowing, bending, loss of contrast, and distortion. Astigmatism is believed to be the most common cause of ametropia. In mild cases, it may cause blurring of vision and ghosting. In more advanced cases of untreated astigmatism, amblyopia may be noted.[34] Astigmatism may occur naturally (idiopathic) or secondary to surgical procedures (iatrogenic), such as cataract extraction and penetrating keratoplasty. Several clinical procedures may be performed to detect astigmatism. These procedures include automated and/or manifest refraction, keratometry, Placido ring reflections, corneal topography, and wavefront aberrometry.

Indications

According to a November 2000 press release, the FDA approved the VISX Star Excimer Laser System for photorefractive keratectomy (PRK) for reduction or elimination of the following:

  • Myopia between 0 and -12.0 D with up to -4.0 D of astigmatism

  • Hyperopia between +1.0 D and +6.0 D with no more than 1.0 D refractive astigmatism

  • Hyperopia between +0.5 and +5.0 D of sphere at the spectacle plane with refractive astigmatism from +0.5 to +4.0 D with a maximum manifest refraction spherical equivalent of +6.0 D

PRK is an elective procedure with alternatives, including eyeglasses, contact lenses, and other refractive surgeries.[35, 36, 37]

Despite the clear-cut indications released by the FDA, several authorities and high-volume surgeons in and out of the United States are continuing to challenge the guidelines. An increasing number of presentations are being made at conventions and in journals stating the success achieved in cases performed beyond that which is listed in the FDA guidelines.

The images below illustrate current FDA-approved lasers for PRK and other refractive surgeries.

Food and Drug Administration (FDA)-approved lasers Food and Drug Administration (FDA)-approved lasers for photorefractive keratectomy (PRK). Adapted from US Food and Drug Administration - Center for Devices and Radiological Health - LASIK Eye Surgery.
Food and Drug Administration (FDA)-approved lasers Food and Drug Administration (FDA)-approved lasers for photorefractive keratectomy (PRK). Adapted from US Food and Drug Administration - Center for Devices and Radiological Health - LASIK Eye Surgery.

The images below outline current FDA-approved lasers for LASIK with or without astigmatism.

Food and Drug Administration (FDA)-approved lasers Food and Drug Administration (FDA)-approved lasers for laser in situ keratomileusis (LASIK) with or without astigmatism. Adapted from US Food and Drug Administration - Center for Devices and Radiological Health - LASIK Eye Surgery.
Food and Drug Administration (FDA)-approved lasers Food and Drug Administration (FDA)-approved lasers for laser in situ keratomileusis (LASIK) with or without astigmatism. Adapted from US Food and Drug Administration - Center for Devices and Radiological Health - LASIK Eye Surgery.
Food and Drug Administration (FDA)-approved lasers Food and Drug Administration (FDA)-approved lasers for laser in situ keratomileusis (LASIK) with or without astigmatism. Adapted from US Food and Drug Administration - Center for Devices and Radiological Health - LASIK Eye Surgery.
Food and Drug Administration (FDA)-approved lasers Food and Drug Administration (FDA)-approved lasers for laser in situ keratomileusis (LASIK) with or without astigmatism. Adapted from US Food and Drug Administration - Center for Devices and Radiological Health - LASIK Eye Surgery.

Relevant Anatomy

The cornea is a transparent, avascular tissue that is continuous with the opaque sclera and semitransparent conjunctiva. It is covered by tear film on its anterior surface and bathed by aqueous humor on its posterior surface. In adults, the cornea measures 11-12 mm horizontally and 9-11 mm vertically. The average corneal thickness is 0.5 mm (500 µm) centrally and 0.7 mm (700 µm) peripherally.

Contraindications

Contraindications to PRK are similar to that of LASIK and other refractive surgery procedures. PRK is contraindicated in patients with collagen vascular, autoimmune, or immunodeficiency diseases; those with signs of keratoconus; patients taking isotretinoin or amiodarone hydrochloride; or those who are pregnant or breastfeeding.

PRK is not recommended in patients with a history of ophthalmic herpes simplex or herpes zoster.

Caution should be undertaken in deciding to perform PRK or other refractive surgery procedures on patients with systemic diseases (eg, connective tissue disease, diabetes, severe atopic disease, immunocompromised status) that are likely to affect wound healing. Safety and efficacy have not been established for these patient populations.

Despite these contraindications, non-recommendations, and cautions, several high-volume surgeons continue to "push the envelope" and perform these procedures beyond the FDA guidelines. Several reports exist of procedures performed on patients with forme fruste keratoconus and amiodarone toxicity.

 

Workup

Imaging Studies

Corneal topography

Corneal topography provides surgeons with effortlessly understood color-coded maps of corneal curvature in addition to quantitative indexes of irregular astigmatism that correlate with potential visual acuity.[38, 39]

Modern instrumentation produces a video keratograph, which generally is in the form of a color-coded contour map.

Different manufacturers use different methods (eg, Placido, 40 scanned slits, combination Placido and 40 scanned slits, phase modified laser holography, raster stereography).

The use of video keratography in preoperative and postoperative evaluations of all refractive surgery patients is valuable.

Dual Scheimpflug analyzers

Scheimpflug imaging differs from conventional techniques in that the object plane, lens plane, and image plane all intersect in a straight line.

This type of imaging allows assessment of anterior and posterior corneal topography, anterior chamber depth, as well as anterior and posterior topography of the lens.[40]

Pachymetry

The pachymeter (optical, ultrasonic) is used to measure corneal thickness.

Accurate determination of corneal thickness preoperatively allows the surgeon to set the depth of incision to two thirds of the measured result.

Orbscan II by Orbtek uses both 40 scan slits and Placido methods to provide anterior and posterior corneal curvature in addition to data on corneal thickness.

Other Tests

See the list below:

  • Wavefront aberrometry

  • Keratometry

  • Placido ring reflections

  • Refraction (manifest, automated, cycloplegic)

Histologic Findings

Several authors have performed and reported histologic analysis of treated tissue. The excimer laser ablation of the cornea results in a very smooth ablated surface, no damage to adjacent tissue, and an abrupt transition to the untreated tissue.[35] An estimated 0.25 µm of corneal tissue is removed with each pulse of the laser. Furthermore, with electron microscopy, an extremely narrow zone of damage may be demonstrated next to the treated area.

 

Treatment

Medical Therapy

Medical therapy is limited to broad-spectrum topical antibiotics and corticosteroids for uncomplicated cases (see Postoperative details).

Surgical Therapy

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.

Myopia

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.[41] 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.

Hyperopia

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.[42]

Astigmatism

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.[43]

Preoperative Details

Informed consent

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 photorefractive keratectomy (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.

Patient history

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.

Ocular examination

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.[47]

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.

Patient advice

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.

Intraoperative Details

Working environment

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.

Excimer laser

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.

Patient preparation

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.

Epithelium

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.

Ablation

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.[48] 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.

Postoperative Details

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.

Follow-up

Patients are monitored closely until full reepithelialization is complete, which usually occurs after 72 hours.[49] 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).

For excellent patient education resources, visit eMedicineHealth's Eye and Vision Center. Also, see eMedicineHealth's patient education article Vision Correction Surgery.

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.[50] 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.[51]

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.[52]

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.[53]

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.[54]

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.[55] 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.[59]

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.[60]

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.[45]

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.[61]

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.[62] 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.[63]

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%).[65] 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.[66]

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.[71]

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.[72]

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.[73]

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.