Ectasia after laser in-situ keratomileusis (LASIK) 長 庚 紀 念 醫 院 眼 科 蕭 靜 熹
Post-LASIK ectasia A rare complication of LASIK Manhattan jury awarded a former investment banker a record $7.25 million for post-lasik ectasia Seven members of AAO, ISRS, ASCRS produced a consensus paper on this subject
Definition of post-lasik ectasia A progressive corneal steepening, usually inferiorly, with an increase in myopia and astigmatism, loss of uncorrected and bestcorrected visual acuity that can present days to years after LASIK.
Mechanism Remains unknown Guirao proposed a complex biomechanical model: corneal curvature, Young s modulus, Poisson ratio, corneal thickness, intraocular pressure (IOP), ablation profile JRS 2005;21:176-85.
Incidence Actual incidence: undetermined 0.04%, 0.2%, 0.66% ISRS survey in 2004, more than 50% had at least one case of ectasia 17 million procedures performed 34000 (0.2%) cases? PK due to refractive surgery in UK from 1999 to 2004: 26/300,000 (0.0087%)
Risk Factors Topographic abnormalities Low residual stromal bed (RSB) High myopia (>-8.0D) Low pre-operative corneal thickness Multiple enhancements
Risk Factors Preexisting corneal pathology: keratoconus of frome fruste keratoconus 1% candidates were excluded due to abnormal topographies Mechanical instability produced by weakening the residual stromal bed due to excessive tissue removal
Forme Fruste Keratoconus (FFKC) Subclinical keratoconus Placido-based computerized topography: Rabinowitz criteria Central keratometry > 47.2D Inferior- superior value > 1.4D Skewing of the radial axis of astigmatism > 21 o Orbscan Posterior float elevation > 40 um (50 um)
Topographic Abnormalities Asymmetric inferior corneal steepening Asymmetric bow-tie patterns with skewed steep radial axes above and below the horizontal meridian Inferior crab-claw pattern accompanied with central flattening (pellucid marginal degeneration suspect)
Asymmetric bow-tie patterns with skewed steep radial axes above and below the horizontal meridian(ab/srax) No skewing Skewed > 30 0 from the vertical meridian
Pellucid marginal degeneration
Pre-op LASIK screening alogrithm OPH 109;9:1642-6
Pre-op grading system for detection of patients who are at risks for ectasia Grade 1 Grade 2 Grade 3 Keratometry <45 45-47 >47 Oblique cylinder <0.5 0.5-1.5 >1.5 Pachymetry >520 500-520 <500 Posterior surface elevation <30 30-40 >40 I-S values <1.0 1.0-1.4 >1.4 Post BSF/ Ant BSF <1.20 1.20-1.27 >1.27
Low residual stromal bed (RSB) thickness Lower limit of RSB: 250um (Seiler) Andreassen et al: elastic modulus of keratoconic cornea is 1.6-2.5 (average:2.1) 525-550 um/ 2.1 = 250 um - Barraquer s recommendation: RSB of 250-300 um to prevent corneal ectasia (550-300=250 um) 50% minimal RSB (?)
Residual stromal bed (RSB) thickness RSB = Corneal thickness Flap thickness Ablation depth
JRS 2006;22:861-71.
Ectasia without risk factors Inadequate RSB due to unrecognized deep microkeratome cut Undetected pre-op subtle topographic abnormalities Biomechanically unstable corneas (abnormal biomechanical properties of the cornea) Each cornea s response to LASIK is unique
Management First and foremost Recognition and Prevention!
Algorithm to prevent ectais Clinical history Stability of refraction Keratoconus in fellow eye Family Hx of keratoconus Slit-lamp evaluation Corneal thinning Clinical signs of keratoconus or pellucid Curr Opinion in OPH 2006;17:421-6.
Algorithm to prevent ectais Cycloplegic retinoscopy Scissoring of red reflex Videokeratography Irregular topography AB/SRAX pattern (FFKC) Crab-claw pattern: inferior blue dot (pellucid suspect) R/o contact lens warpage Curr Opinion in OPH 2006;17:421-6.
Algorithm to prevent ectais Ultrasonic pachymetry Pre-operative: central, inferior Intraoperative: If RSB close to lower limit and ALL enhancements Curr Opinion in OPH 2006;17:421-6.
Recognition and prevention: future directions Utilizing new technology to identify abnormal corneas Corneal interferometry Corneal hysteresis measurements (ocular reponse analyzer) Dynamic corneal imaging Utilizing alternative treatment strategies for at-risk patients Surface ablation Phakic intraocular lens implantation
Utilizing alternative treatment -- Surface ablation
Ectasia after PRK for low myopia (OPH 2006;113:742-6) KM: OD: 43.77/43.32, OS: 43.88/43.26 Pachymetry: OU: 495 um Attempted correction OD:-1.25/-0.75x105 (32um),OS:-1.25/- 0.75x70 (30um) 4 years later, bilateral corneal ectasia
Utilizing alternative treatment strategies for at-risk patients Surface ablation (PRK, Epi-LASIK, LASEK) Thin but normal cornea Corneal topographic irregularities, but not keratoconus suspect Phakic intraocular lens implantation JRS 2005;21:734-41
Options for treatment Lower intraocular pressure Rigid gas permeable lens Intracorneal ring segments (Intacs) Deep lamellar keratoplasty Penentrating keratoplasty Collagen cross-linking: New hope!
Intraocorneal ring segments
Collagen Crosslinking Treatment Using collagen crosslinking by the photosensitizer riboflavin and ultraviolet A light
Collagen Crosslinking Biomechanical measurements: an increase in corneal rigidity of 328.9% in human corneas after crosslinking Goal: to stop the progression of the keratoconus or corneal ectasia, not to improve the patient s UCVA
Consensus opinion on post- LASIK ectasia Videokeratography should be performed prior to refractive surgery; by itself it is not diagnostic of keratoconus since keratoconus is a clinical diagnosis. Computer generated diagnosis of keratoconus suspect is not necessarily a contraindication to surgery. A decision to perform LASIK should take the entire clinical picture into account, topographically based, computer-generated warnings may occur in the presence of normal corneas JCRS 2005, 31:2035-8.
Consensus opinion on post- LASIK ectasia Although some risk factors have been suggested for ectasia, none of them are absolute predictors of its occurrence. The occurrence of ectasia per se is not a deviation from the standard of the care, and does not mean the patient was a poor candidate for refractive surgery. JCRS 2005, 31:2035-8.
Consensus opinion on post- LASIK ectasia Because keratoconus can occur in the absence of LASIK, its occurrence does not necessarily mean that LASIK was a contributing factor to its development. Wang et al, JCRS 2003, 29:2015-2018 Bilateral keratectasia after unilateral LASIK JCRS 2005, 31:2035-8.
Consensus opinion on post- LASIK ectasia FFKC is a topographic diagnosis; patients who have asymmetric inferior corneal steepening or asymmetric bow-tie patterns with skewed steep radial axes above and below the horizontal meridian are at risk for ectasia. They could be offered PRK with the correct informed consent. LASIK should be avoided in the patients with an inferior crab-claw pattern accompanied with central flattening. JCRS 2005, 31:2035-8.
Consensus opinion on post- LASIK ectasia Risk factors for ectasia after LASIK do not necessarily predict ectasia after surface ablation. Intraoperative pachymetry should be considered in cases in which the calculated residual bed might be near the safe lower limits and/or the reproducibility of the microkeratome might result in a residual stromal bed less than those limits. JCRS 2005, 31:2035-8.
Is LASIK safe? 21-year outcome of Barraquer s cryolathe keratomileusis: 1606 eyes, -2.0 D to 27.0D, 45 eyes (2.8%) ectasia 300 um vs 160 um Corneal topography to screen for keratoconus Rarely treat myopia >-11.0D Technology for high-resolution biometric assessment of the individual layers of the cornea in three-dimensions is available now Reinstein and Waring. JRS 2006;22:843-5
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