Introduction
The excimer laser revolutionized the world of vision correction because of its precision and unique abilities to reshape the corneal surface. This laser can remove scars and other opacities from the cornea in the procedure termed phototherapeutic keratectomy (PTK). It can also predictably reshape the anterior surface of the cornea in the procedure termed photorefractive keratectomy (PRK). The use of PRK as a mainstream refractive modality declined during the late 1990s and early 21st century due to the dramatic increase in laser in-situ keratomileusis (LASIK). PRK has remained a significant procedure because there is no risk of flap creation and less risk of ectasia. PRK has been shown to be safe and effective in long-term studies and is the only corneal refractive procedure that, with proper healing, it is not discernible that a procedure has been performed, even when observing with a slit lamp.
Early excimer laser experiments were among the first attempts to produce lasers in the visible portion of the electromagnetic spectrum, more specifically the ultraviolet (UV) range. Research in the mid-1970s resulted in excimer lasers consisting of rare gas-halogen mediums, which, upon electrical stimulation, created an unstable fusion of these two molecules (e.g., excited dimer = excimer) followed by immediate dissociation with subsequent fluorescence of UV energy, which, when properly harnessed through sophisticated optical focusing mechanisms, produced a laser beam of considerable energy. Different rare gas-halogen excimer laser combinations produced different wavelengths of UV laser light ( Table 18.1 ).
| Laser Medium | Wavelength (nm) |
|---|---|
| Argon fluoride | 193 |
| Krypton chloride | 222 |
| Krypton fluoride | 248 |
| Xenon chloride | 308 |
| Xenon fluoride | 351 |
Trokel, working with Srinivasan at the IBM Watson Research Center, was the first to suggest that the excimer laser had unique qualities for performing corneal surgery. In their 1983 paper, they suggested that this laser could be used to remove a lamellar portion of tissue to reshape the corneal curvature and to perform precisely placed incisions in the cornea. This early work stimulated considerable interest and research activity in laser corneal surgery with the excimer laser.
In 1985, Theo Seiler performed the first human corneal excimer laser treatment in the form of an astigmatic keratotomy, followed the next year by the first excimer laser PTK. Marshall’s group first described the technique of anterior surface ablation to reshape the anterior corneal curvature. Munnerlyn was the first to describe a computer-generated algorithm relating the treatment zone diameter with the depth of ablation to effect a specific dioptric change. Marguerite McDonald performed the first PRK on a sighted, myopic eye. An enormous amount of research has followed to make excimer laser PRK a procedure that has changed the face of medicine and eye care.
Excimer Laser Physics and Beam Tissue Interaction
The word excimer is a contraction of two words: “excited” and “dimer.” A simplified description of an excimer laser system involves a cavity filled with rare gas and halogen gas molecules. A high-voltage electric discharge is sent across the laser cavity, which causes an unstable bond to be formed between these different types of molecules, resulting in high-energy dimers rare/halogen gas dimers. These dimers then spontaneously dissociate and fall to a lower energy level, releasing a photo of energy at a wavelength of 193 nm that is harnessed through a series of focusing lenses and mirrors, which ultimately reshapes the corneal surface.
Before exiting the laser, the excimer laser beam can be shaped by various methods. In general, there are two categories of excimer lasers based on the beam size and method of delivery. Broad beam excimer lasers were the most common laser in the beginning of laser vision correction and the main method used to shape the excimer laser beam was an iris diaphragm that gradually opened with every pulse delivered so as to deliver more energy to the central cornea than to the periphery in a myopic correction ( Fig. 18.1 ).
Scanning lasers have become the method of choice for excimer laser deliver in a refractive correction for a multitude of reasons. First, it is much more forgiving on the optics of an excimer laser to focus a small beam or slit beam than it is to focus a large beam. Small beams can be also be computer scanned over the corneal surface in many patterns.
Two features of an excimer laser ablation that can create a negative effect if not controlled are the plume that is released during the photoablative process and the heat that is dissipated during the same process. With every laser pulse, a plume of ablated tissue is released. With a broad beam laser myopic treatment, every pulse hits the center of the ablation as the iris diaphragm opens. As a result, the plume that is released can block the delivery of the total amount of energy to the central cornea. The end result is that less tissue is removed centrally than predicted, which can cause a central steepening (island) when compared to the rest of the more peripheral ablation ( Fig. 18.2 ). With scanning laser technology, the small laser beam (typically in the 0.9- to 1.0-mm diameter range) is moving constantly. The advantage of this is that by the time a laser pulse is delivered in an area it just treated the plume from the previous pulse should be dissipated. This is why central islands have not been a problem with scanning lasers, as they were with broad beam technology. In the past, PRK procedures using scanning technology typically took longer than those using the large-area photoablation technique, but with advancements—including increased repetition rates (500–1000 Hz)—the scanning laser time has been greatly reduced. Also, it has been shown that reduced peripheral ablation can induce high-order aberrations. Scanning laser technology has the ability to address this issue by placing more pulses peripherally and lessening the induction of these unwanted aberrations.
Excimer lasers have several desirable characteristics based on their photon energy. The energy per photon in an excimer laser beam is very high, 6.4 electron volts (ev), which easily overcomes intermolecular bond energies (carbon–carbon bonds equal 3.4 ev and peptide bonds equal 3.0 ev) at the corneal surface. This allows for accurate layer-by-layer tissue removal on a molecular level. The penetration depth of each laser pulse is also minimal so that adjacent tissue damage is minimized. These unique features provide accuracy on the micron level and control to lamellar cornea surgery never seen previously because a single pulse of the laser removes approximately 0.122 to 0.25 µm of tissue, depending on the laser parameters used. This ability to treat in an accurate lamellar fashion allows this laser to be useful in removing scars and opacities from the cornea in addition to its unique ability to reshape the cornea in a refractive procedure.
All patients being considered for PRK need to have a complete examination of their anterior and posterior segments. Excimer laser PRK details, alternatives to the procedure, and informed consent should be discussed.
Contact lens status and history, general health history, eye health history, medications review, and allergy history are also documented. Patients with collagen-vascular diseases, such as systemic lupus erythematosus (SLE), are relatively contraindicated from undergoing PRK because of potential problems with delayed epithelial healing and potential corneal melting.
As with any refractive procedure, ocular dominance, manifest refraction, cycloplegic refraction, and measurement of pupil size in dim light should be performed. Patients who cannot be refracted to 20/20 or better need close evaluation. If the cornea, lenses, maculae, and optic nerves appear to be fine, particular attention to computed topographic analysis, which all refractive evaluations should include, is important. Because patients with permanent irregular astigmatism, such as clinical keratoconus or pellucid marginal degeneration, often yield poor results after refractive surgery, they are considered contraindicated for refractive surgery ( Figs. 18.3 and 18.4 ). Others have found encouraging results in using PRK to treat preclinical or even clinical keratoconus in an attempt to improve the clinical picture in a topographically guided fashion. Caution should be taken in these types of situations, obtaining appropriate informed consent and providing patient education. A different mindset and patient education must be applied to cases such as these, going from a refractive surgery to combination refractive/therapeutic surgery.
A thorough slit lamp examination should be performed on all patients undergoing laser vision correction. Blepharitis should be ruled out and, if present, treated aggressively before scheduling PRK. An evaluation for dry eyes is performed, looking for a healthy tear strip and no evidence of any punctate staining of the epithelium with fluorescein. Tear film osmolarity, anesthetized Schirmer test, and analysis of tear film breakup time can all be useful in the preoperative evaluation of the PRK patient. Patients with documented dry eye should have aggressive treatment of this condition prior to undergoing PRK.
The cornea is evaluated for evidence of keratoconus, such as a Fleischer ring or Vogt lines. Stromal scars are evaluated closely and old herpetic disease is considered. If herpetic disease is felt to be a possibility, refractive surgery is not recommended owing to the risk of recurrence and resultant potential stromal scarring and damage to vision. The iris is examined for any evidence of iris transillumination defects since myopes especially are at increased risk for pigment dispersion syndrome. A quiet anterior chamber is expected, and the lens is thoroughly evaluated for any cataractous change. The vitreous and retina are evaluated thoroughly for any evidence of retinal pathology, such as macular disease or peripheral retinal pathology. Intraocular pressure (IOP) is evaluated because myopes are at increased risk for developing glaucoma and because steroids may be used postoperatively. In the event of a steroid-induced pressure rise, documentation of a normal IOP preoperatively is important.
Other tests we find useful include measurement of the optical scatter index (OSI) with the HD Analyzer. This device measures forward scatter and is a valuable clinical tool to measure objectively what the patient’s image quality is like. Objectively measuring the effects of ocular scatter on total vision can help with early cataract diagnosis, dry-eye diagnosis, early keratoconus detection, and can improve your outcomes with refractive treatment selection. If the OSI is low, we can be confident that the tear film is functioning optically well, the cornea does not have a visually significant irregularity, and the crystalline lens is optically clear ( Figs. 18.5A and 18.5B ). We also find wavefront analysis very helpful in measuring the aberration state of the patient’s eye. We perform conventional (treating just the sphere and cylinder), wavefront-optimized, wavefront-guided, and topographically guided PRK procedures. All of these procedures have their intricacies and preoperative testing that are important to completely understand.
All patients in my practice are asked if they are eye rubbers. We know that eye rubbing increases the risk of ectasia. I prefer not to perform PRK, let alone LASIK, on eye rubbers. If they say they can stop, I then consider PRK.
After the anterior and posterior segment examination and review of the corneal topography and wavefront data, the patients are counseled thoroughly, including, if presbyopic, a discussion on monovision. Pain after the anesthesia wears off is discussed, as are the techniques for treating it, such as nonsteroidal antiinflammatory drugs (NSAIDs), bandage contact lenses, and oral analgesics. Visual return is discussed, including the reduced vision (compared to LASIK) that occurs early on, followed by gradual improvement after reepithelialization (typically within 72 hours), the appearance of functional vision typically 4 to 5 days postoperatively, and the establishment of best vision 1 to 3 months postoperatively. Patients often have a misperception of the slowness of vision return with PRK. It is common to have 20/40 vision throughout the PRK healing process and, with a properly fit bandage lens, the comfort level can be quite tolerable.
Risks of infection with the potential for ulceration, scarring, and loss of best corrected vision are reviewed. The rare chance of needing a corneal transplant with a visually significant scar formation is discussed. The normal stromal and subepithelial healing response is reviewed and haze is described. Reducing the risk of haze with mitomycin C use is reviewed. I do not use mitomycin C on all PRK patients. There is an art to using this drug to reduce haze risk but its affect is variable and debated. I do use it for all patients with previous corneal surgery history and for all PRKs that will be removing more than 40 µm of tissue. The risks of undercorrection and overcorrection are discussed, as is the potential for glare and halos, especially at night. The risks of topical steroids—including cataracts and IOP elevation, which is rarely permanent—are also discussed.
The patients’ questions are then answered and if their goals, expectations, and understanding of the process seem in order, the PRK procedure is scheduled. In general, I prefer scheduling these patients on a Thursday so I can see them day 1 postoperatively on Friday and day 4 postoperatively on Monday. The vast majority of patients are reepithelialized by Monday and the bandage contact lens can be removed.
I prefer to limit PRK to low to moderate myopia (< 6.0 diopters [D]), low hyperopia (< 3.0 D), and/or low to moderate astigmatism (< 6.0 d) if possible because of the risk of haze or variabilities in healing for higher, especially hyperopic, corrections. In addition, I have had good results with phakic intraocular lens implants in the high-myopic patients who are not good LASIK candidates because of level of correction or corneal thickness. Certain indications, such as thin corneas that are still safe for laser vision correction and preexisting anterior membrane dystrophy with or without recurrent erosion, are natural situations in which PRK can be ideal. Epithelial adherence in these patients can be improved with PRK just as in PTK.
After laser calibration and ensuring that accurate laser parameters are entered, the patient is brought into the room and positioned under the microscope, reclining on the laser bed. The nonoperative eye is patched to maximize patient fixation with the operative eye. I feel that it is important to ask the patient to close the eye not being treated that is under the patch. When topical anesthetic is in the untreated eye, the cornea can dehydrate and thin rapidly. When that eye is then treated, it can be overcorrected because the laser is removing more microns per pulse. A patch can also be placed alongside the operative eye to catch excess fluids, such as tears or topical anesthetic .
Topical anesthetic and topical antibiotics are provided preoperatively. I do not use pupil constricting drops because of pupil center shift concerns. The light of the surgical microscope works well to constrict the pupil enough for centration of the PRK procedure. Topical anesthesia may be helpful in the nonoperative eye to relax any reflex tearing or relieve discomfort of the patient. A lid speculum is then placed in the operative eye and topical anesthesia is reapplied. Studies have shown improved PRK centration when the procedure is focused on the center of the pupil rather than on the corneal light reflex. For hyperopia treatments, some surgeons advocate centering the procedure on the corneal light reflex, but there is some debate in this arena. During this whole beginning process, the physician should explain to the patient in detail what is occurring and also what will be occurring during laser energy delivery. It is important to emphasize to the patient that the eye must be fixated on the fixation light in the laser at all times ( Fig. 18.6 ). Patients should be told that the fixation light may blur during the ablation but that they should be able to see it at all times. Patients should also be told that the laser will produce a certain noise and smell, which they will be introduced to during the preliminary testing before treatment.
