Chapter 7 and BCSC Section 8, External Disease and Cornea). Although some early case reports have suggested that combining CXL treatments with PRK may offer some benefit to keratoconus patients, the clinical experience remains very preliminary.
Alessio G, L’Abbate M, Sborgia C, La Tegola MG. Photorefractive keratectomy followed by cross-linking versus cross-linking alone for management of progressive keratoconus: two-year follow-up. Am J Ophthalmol. 2013;155(1):54–65. Epub 2012 Sep 27.
Ambrósio R Jr, Alonso RS, Luz A, Coca Velarde LG. Corneal-thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract Refract Surg. 2006;32(11):1851–1859.
Belin MW, Asota IM, Ambrósio R, Khachikian SS. What’s in a name: keratoconus, pellucid marginal degeneration, and related thinning disorders. Am J Ophthalmol. 2011;152(2): 157–162. Epub 2011 Jun 25.
Binder PS, Lindstrom RL, Stulting RD, et al. Keratoconus and corneal ectasia after LASIK. J Cataract Refract Surg. 2005;31(11):2035–2038.
Kilic A, Colin J. Advances in the surgical treatment of keratoconus. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2012: module 2.
Randleman JB, Russell B, Ward MA, Thompson KP, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology. 2003;110(2):267–275.
Saad A, Gatinel D. Topographic and tomographic properties of forme fruste keratoconus corneas. Invest Ophthalmol Vis Sci. 2010;51(11):5546–5555. Epub 2010 Jun 16.
Post–Penetrating Keratoplasty
Refractive unpredictability after penetrating keratoplasty (PKP) is extremely common owing to the inherent imprecision of the operation. Most series document a mean postoperative astigmatism of 4.00–5.00 D. In many cases, these refractive errors are not amenable to spectacle correction, and between 10% and 30% of patients require contact lens correction to achieve good vision after PKP. However, contact lens fitting may not be successful in this patient population because of either the abnormal corneal curvature or the patient’s inability to tolerate or manipulate a contact lens.
Surgical alternatives for the correction of post-PKP astigmatism include corneal relaxing incisions, compression sutures, and wedge resections. In a series of 201 corneal transplants for keratoconus, 18% of patients required refractive surgery to correct the astigmatism. Although these procedures can significantly decrease corneal cylinder and are highly effective, they have minimal effect on spherical equivalent. In addition, they can be unpredictable and may destabilize the graft– host wound.
Patients with pseudophakia who have significant anisometropia after PKP surgery may be candidates for intraocular lens (IOL) exchange or piggyback IOL implantation; new options include toric IOLs (see Chapter 8). These alternatives require another intraocular procedure, which increases the risk of endothelial decompensation, glaucoma, and cystoid macular edema and may incite graft rejection.
Given the successful use of the excimer laser in treating myopia and astigmatism, PRK has been studied and used to treat post-PKP refractive errors. PRK has the disadvantages associated with epithelial removal in a corneal transplant and may result in corneal haze when high refractive errors are treated. With the use of prophylactic topical mitomycin C, PRK has become a more common treatment option for refractive errors after PKP. Although the refractive results are often good, PRK in post-PKP patients is generally less predictable and less effective than it is for naturally occurring astigmatism and myopia.
LASIK after PKP is subject to the same patient-selection constraints as conventional LASIK is. Without extenuating circumstances, monocular patients or patients with limited vision potential in the fellow eye are not candidates. In addition, patients with a wound-healing disorder, significant dry eye syndrome, or a collagen-vascular disease should be offered other options. Finally, patients should have realistic expectations for their rehabilitation after post-PKP LASIK. The goal of LASIK following PKP is to return the patient to spectacle-corrected binocularity or to enable the patient to wear contact lenses successfully, as the accuracy of the procedure is less predictable than that of conventional LASIK. Also, note that there are no FDA-approved procedures to treat irregular
astigmatism. Pre-operative evaluation of the post-PKP patient who is considering refractive surgery should include the original indications for the PKP. Patients with low endothelial cell counts may be at increased risk of flap dislocation after LASIK because of impairment of the endothelial cell pump function.
Optimal timing of refractive surgery after PKP is controversial. All sutures should be removed, and the refraction should be stable. To avoid wound dehiscence, many surgeons wait at least 1 year after PKP, and an additional 4 months after all sutures are removed, before performing the refractive surgery. An interval of at least 18–24 months after PKP provides sufficient wound healing in most cases. No matter how much time has elapsed since the PKP surgery, the graft–host wound should be carefully inspected to make sure it appears strong enough to undergo a LASIK procedure, as there is a small but significant risk of keratoplasty wound dehiscence during application of the vacuum ring used to create the LASIK flap.
Refraction and corneal topography should be stable, as documented by 2 consecutive readings on separate visits at least 1 month apart. Areas of suspected ectasia should be confirmed with pachymetry to avoid perforation. Refractive surgery should be avoided if the corneal graft shows evidence of inflammation, diffuse vascularization, ectasia, inadequate healing of the graft–host interface, or refractive instability or if there are signs of rejection or decompensation.
Because eye alignment under the laser is crucial for accurate treatment of astigmatism, some surgeons mark the vertical or horizontal axis of the cornea at the slit lamp before placing the patient under the laser. Suction time should be minimized to decrease stress on the corneal wound and to lessen the potential for the devastating complication of wound dehiscence. If the corneal curvature is very steep, cutting a thicker flap during the microkeratome pass may decrease the risk of buttonhole formation. PRK should also be considered in steep corneas to avoid flap complications.
Another potential problem specific to post-PKP LASIK is that the creation of a lamellar flap may itself cause a change in the amount and axis of the astigmatism. Therefore, some surgeons perform LASIK in 2 stages. First, the flap is cut and laid back down. Second, several weeks later, after the curvature and refraction have stabilized, the flap is lifted and laser ablation is performed. Some reports describe minimal refractive changes after flap creation, and some surgeons prefer to perform LASIK in 1 step to avoid increasing the potential for the complications associated with performing 2 separate procedures, including infection, graft rejection, and epithelial ingrowth. Flap retraction and necrosis have been reported in patients undergoing LASIK after keratoplasty.
The mean percentage reduction of astigmatism after LASIK following PKP ranges from 54.0% to 87.9%. Although most series report improvement in UDVA, up to 42.9% of patients require enhancement because of cylindrical undercorrection. In addition, up to 35% of patients lose 1 line of CDVA. Corneal graft rejection has been described after PRK; thus, higher and more prolonged dosing with topical corticosteroids should be prescribed for post-PKP refractive surgery patients to decrease this risk.
Alió JL, Javaloy J, Osman AA, Galvis B, Tello A, Haroun HE. Laser in situ keratomileusis to correct post-keratoplasty astigmatism: 1-step vs 2-step procedure. J Cataract Refract Surg. 2004;30(11):2303–2310.
Busin MB, Arffa RC, Zambianchi L, Lamberti G, Sebastiani A. Effect of hinged lamellar keratotomy on postkeratoplasty eyes. Ophthalmology. 2001;108(2):1845–1850.
Hardten DR, Chittcharus A, Lindstrom RL. Long term analysis for the correction of refractive errors after penetrating keratoplasty. Cornea. 2004;23(5):479– 489.
Huang PY, Huang PT, Astle WF, et al. Laser-assisted subepithelial keratectomy and photorefractive keratectomy for post-penetrating keratoplasty myopia and astigmatism in adults. J Cataract Refract Surg. 2011;37(2):335–340.
Kollias AN, Schaumberger MM, Kreutzer TC, Ulbig MW, Lackerbauer CA. Two-step LASIK after penetrating keratoplasty. Clin Ophthalmol. 2009;3:581– 586. Epub 2009 Nov 2.
Sharma N, Sinha R, Vajpayee RB. Corneal lamellar flap retraction after LASIK following penetrating keratoplasty. Cornea. 2006;25(4):496.
Ocular Hypertension and Glaucoma
An estimated 9%–28% of patients with myopia have primary open-angle glaucoma (POAG). Consequently, it is likely that some patients with glaucoma will request refractive surgery.
Of particular concern in patients with ocular hypertension or POAG is the effect of the acute rise in intraocular pressure (IOP) to more than 65 mm Hg when suction is applied while the stromal flap is cut for LASIK or the epithelial flap for epipolis LASIK (epi-LASIK). Although healthy optic nerves seem to tolerate this degree of IOP elevation, ophthalmologists do not yet entirely understand the effect on compromised optic nerves. There have been reports of new visual field defects arising immediately after LASIK that are attributed to mechanical compression or ischemia of the optic nerve head from the temporary increase in IOP.
Evaluation of a patient with ocular hypertension or POAG includes a complete history and ocular examination with peripheral visual field testing and corneal pachymetry. A history of poor IOP control, nonadherence to treatment, maximal medical therapy, or prior surgical interventions may suggest progressive disease, which may contraindicate refractive surgery. The surgeon should also note the status of the angle, the presence and amount of optic nerve cupping, and the degree of visual field loss.
Several reports have confirmed that central corneal thickness affects the Goldmann applanation tonometry (GAT) measurement of IOP (see the section Glaucoma After Refractive Surgery in Chapter 11). The principle of applanation tonometry assumes a corneal thickness of 520 µm. Studies have demonstrated that thinner-than-normal corneas give falsely low IOP readings, whereas thicker corneas give falsely high readings. For example, IOP is underestimated by approximately 5.2 mm Hg in a cornea with a central thickness of 450 µm. Although all reports agree that central corneal thickness affects GAT IOP measurement, there is no consensus on a specific formula to compensate for this effect in clinical practice.
In the treatment of myopia, LASIK and surface ablation procedures remove tissue to reduce the steepness of the cornea; this sculpting process creates a thinner central cornea, which leads to artifactually low IOP measurements postoperatively. Such inaccurately low central applanation tonometry measurements hinder the diagnosis of corticosteroid-induced glaucoma after keratorefractive procedures, resulting in optic nerve cupping, visual field loss, and decreased visual acuity (Fig 10-4).
Figure 10-4 Glaucomatous optic nerve atrophy in a patient with “normal” intraocular pressure (IOP) after laser in situ keratomileusis (LASIK). A, Fundus photograph demonstrating increased cup–disc ratio in a patient who received a diagnosis of glaucoma 1 year after LASIK. The patient had decreased vision, with corrected distance visual acuity of 20/40 and IOP of 21 mm Hg. B, Humphrey 24-2 visual field with extensive inferior arcuate visual field loss corresponding to thinning of the superior optic nerve rim. C, Optical coherence tomography image demonstrates marked optic nerve cupping. (Parts A and B
courtesy of Jayne S. Weiss, MD; part C courtesy of Steven I. Rosenfeld, MD.)
Because of the difficulty that PRK and LASIK cause in the accurate measurement of IOP, these refractive procedures should not be considered for a patient whose IOP is poorly controlled. Furthermore, patients should be advised of the effect of refractive surgery on their IOP measurements and urged to inform future ophthalmologists about their surgery. Patients should be referred to a glaucoma specialist when indicated.
Patients with ocular hypertension can often safely undergo refractive surgery. Such patients must be counseled preoperatively that refractive surgery treats only the refractive error and not the natural history of the ocular hypertension, which can sometimes progress to glaucoma, accompanied by optic nerve cupping and visual field loss. The ophthalmologist should pay particular attention to the risk factors for progression to glaucoma, including older age, reduced corneal thickness, increased cup–disc ratio, and elevated IOP. Each patient needs to understand that after excimer laser ablation, it is more difficult to accurately assess IOP.