Iatrogenic Keratectasia after LASIK
(Progressive Corneal Ectasia)
by Gail Keziah
1. What is "iatrogenic keratectasia" or "progressive corneal ectasia" after LASIK?
Iatrogenic keratectasia is a complication after LASIK that is induced inadvertently by the surgeon performing the procedure. The cutting of the flap and the subsequent laser ablation weakens the cornea and causes a bulging in the weakest area. The cornea continues to bulge because it has been destabilized. This bulging results from the cornea being thinned to the point that the internal pressure of the eye affects the structurally weakened cornea. The bulging usually is not even, so the effect is about the same as keratoconus -- a form of irregular astigmatism that is extremely difficult to treat. The process is irreversible once it begins.
2. What causes progressive corneal ectasia?
Patients with high myopia ( -8.00 D or higher), thin corneas ( less than 500 microns ), and keratoconus suspects are at highest risk. A family history of keratoconus is a warning sign.
There are two known causes of iatrogenic keratectasia after LASIK: 1) the removal of excessive amounts of tissue from the posterior stromal layers, and 2) previous keratoconus (forme fruste keratoconus). It is believed that the progression of ectasia is likely to be hastened by the removal of central corneal tissue. Forme fruste keratoconus is a definite contraindication to LASIK.1
According to Amoils1 , LASIK has certain intrinsic problems, and the combination of incisional surgery and laser ablation has a potential for serious short- and long-term problems.
Keratectasia after LASIK has been reported most frequently when 250 microns of corneal bed are not left remaining after the flap cut and laser ablation. The anterior part of the cornea consists of alternating collagen fibrils and a more complicated interwoven structure. This layer of tissue is about 100-120 microns thick and appears to be the strongest layer of tissue of the cornea. The epithelium (outer skin) is on top of this layer. It is generally about 50 microns thick. This anterior part of the cornea is believed to be the major stress-bearing layer. This layer which is approximately 150 to 170 microns is also the layer used in making the flap. After LASIK, the corneal flap, though apposed to the bed, does not weld to it for years. According to Seiler2 , the anterior flap after LASIK does not contribute to the mechanical strength of the cornea. The stress is supported by the residual thickness of the bed alone.
The strongest part of the cornea becomes a chronically "fractured" cornea. The remaining tensile strength is only that of the corneal bed. The fibrils in the corneal bed fit a very regular pattern as compared to the more structurally complicated anterior corneal layer. The deeper stroma appears to have less bio-mechanical strength as compared to the anterior corneal layer according to Park et al3.
The amount of residual stromal bed thickness that is necessary to avoid progressive corneal ectasia after LASIK is unknown. The consensus among most LASIK surgeons is 250 microns. Barraquer4 recommended a minimal thickness of 300 microns of stress-bearing stroma based on years of experience with lamellar refractive surgery. It appears the opinions of doctors vary from 250 to a more conservative figure of 325 microns.
The causes of iatrogenic keratectasia are not completely known. It is known that intraocular pressure (IOP) can cause forward bowing and thinning of a structurally compromised cornea. What is not known is the amount of posterior stromal thickness needed to provide stability. Reducing the central thickness of the cornea increases the surface-parallel stress in the center. This increase in stress is a long-term condition. Individual corneas could react differently. The structural rigidity of the cornea could vary from patient to patient along with the variations in IOP. Patients with a history of eye rubbing may also be at risk.
There are possible contributing factors that have not yet been proven. Could the treatment zone size be a factor? It makes sense that a larger treatment zone would yield a larger area of thinned cornea thus increasing the risk of ectasia. There is a lot that is not understood about wound healing after LASIK and the biochemical changes that occur during corneal thinning.
Calculating the residual corneal bed to protect a patient from ectasia can be challenging. It is not known at the time of surgery the actual thickness of the corneal flap. The flap thickness can vary greatly from one eye to the other even using the same microkeratome. The flap thickness can vary as much as 40 microns. The precise laser ablation depth within the corneal stroma can also vary. It is not known with certainty how much residual cornea is needed to protect a patient from progressive corneal ectasia.
3. What are the symptoms?
· Progressive myopia
· Irregular astigmatism
· Ghosting
· Fluctuating vision
· Problems with scotopic vision (vision in darkness or dim lighting)
· Progression of ectasia leads to severe loss of -corrected visual acuity
· Eyes at high risk of corneal ectasia following LASIK may have unstable refractions and variable posterior surface bowing prior to developing frank ectasia.
4. How is it diagnosed?
It is identified topographically as a central or paracentral progressive steepening after LASIK. This is associated with a posterior corneal bulging greater than 60 microns as seen on Orbscan topography. An eccentric posterior bulge below the centre of the laser ablated area is most ominous.9
Keratectasia after myopic LASIK is clinically observed as a central steep island accompanied by regression of the refractive effect. This is not to be confused with conventional central steep islands seen immediately after LASIK that are caused by a corneal ablation irregularity. Ectasia generally manifests itself weeks to months to possibly years later.
5. How common is ectasia?
The true incidence of the occurence of ectasia isn't known. Theo Seiler, MD, PhD, has suggested that the number of cases may be significantly underestimated.8 As of 2006, there have been about 200 cases reported in the medical literature, however, most cases have not been written up. This should change as doctors submit more cases for publication to help better understand this complication.
6. How long after LASIK can the onset of ectasia occur?
The onset of ectasia after surgery is highly variable, ranging from one week to 27 months reported in the literature. There is concern that other ectasia cases may develop more slowly.
In patients who have undergone RK (another incisional surgery), many years elapsed before the onset of progressive hyperopia. In the Prospective Evaluation of Radial Keratotomy (PERK) Study of eyes treated by radial keratotomy, it took over 5 years to recognize that the late keratectasia of the paracentral corneal could lead to progressive hyperopia. Is it possible that after LASIK, alterations in the biomechanics of the cornea will cause instability and progressive myopia? There have already been cases reported.
Damiano5 , in a correspondence in the Journal of Refractive Surgery stated that in all his cases of myopic keratomileusis done 15 years ago with more than 8 diopters of myopia, the cases experienced a slow return of the myopia after 5 years. He attributes this ectasia to a long-term weakening of the cornea. Years of pressure on the cornea result from factors including, but not limited to blinking, sleeping in a prone position, normal wear and tear, and inadvertent eye rubbing.
The minimal thickness of the cornea needed to preserve corneal shape for the lifetime of a patient is not known. Another consideration: "The cornea gradually becomes flatter, thinner, and slightly less transparent (with age)."6 If the cornea thins with age, could it weaken further as it tries to withstand IOP? If it does, this possible weakening could trigger the onset of ectasia which could cause the bulging and subsequent myopia and astigmatism. It seems logical that some post-LASIK patients with borderline beds could be pushed into kerataectasia as they get older. Only time will tell.
7. What Steps Can Be Taken to Prevent Iatrogenic Keratectasia? 7, 9
Until this problem is better understood, one can only suggest some general guidelines to minimize its occurrence:
1. Calculate the estimated corneal thickness for each LASIK case.
2. Determine a threshold for posterior corneal thickness that will serve as a cutoff; if calculations suggest that the procedure will remove too much stromal tissue, options include canceling the procedure, reducing the size of the correction zone, or switching to PRK.
3. Consider measuring flap thickness and posterior stromal thickness (before and after ablation) in your own patients so future calculations will be based on known values rather than unproven assumptions.
4. Be especially wary of performing LASIK in eyes with abnormal topography and have a low threshold for rejecting these patients for LASIK.
5. Evaluate topography and pachymetry before every LASIK procedure and avoid additional procedures in eyes with steep topographic zones that develop following LASIK. 7 Re-operations are especially dangerous with virgin residual beds under the flap of less than 300 microns and more so if they are repeated more than once.9
"Dandelion keratectasia," is when the myopia increases after every re-operation. It is a feared complication.9
6. There is danger in doing LASIK in corneas with a curvature greater than 48.5 dioptres.9
7. Report cases of keratectasia so the mechanism can be better understood and, as Geggel and Talley have done, submit excised tissue for histological examination, preferably to a center that specializes in corneal histology.
"Until the riddle of iatrogenic keratectasia is solved, we as LASIK surgeons must exert appropriate caution with our patients and provide any new data that might enhance our understanding of this problem." Douglas D. Koch, MD
8. What are the possible treatments for ectasia?
Non-surgical:
1) Hard contact lenses are worn to hopefully delay the progression of ectasia.
2)Topical ocular antihypertensives (to lower a patient's intraocular pressure) may be effective in reducing myopic regression and may have a role in preventing the development of corneal ectasia in high risks eyes if commenced early. Longer-term studies are needed.
Surgical (experimental)
Intacs ( Intrastromal Corneal Ring Segments ) - Intacs can be inserted in the thinned cornea to serve as a "corneal crutch." The hope is to stabilize or reduce the posterior corneal steepening. It is currently unclear whether the progression of ectasia will be slowed down or eliminated. The chances of success are affected by the extent of the progression. A longer period of follow-up time is needed.
UVA cross-linking (experimental)
This is done by applying riboflavin to the cornea, and activating the riboflavin with ultraviolet (UV) light. The activated riboflavin induces cross-linking of the collagen fibers, which strengthens the cornea.
The end result is that the ectasia-weakened cornea will hopefully become stronger and flatter.
Surgical
1) Deep Lamellar Keratoplasty (partial corneal transplant) - The recovery time is faster and visual recovery quicker when compared with penetrating keratoplasty (corneal transplant).
This technique offers no guarantees of the reversal of ectasia, and has yet to be proven. The success rate could be affected by the extent of the progression of the ectasia. The problem with this approach could be that by simply adding tissue, it may not strengthen a cornea that has already reached the point that its intrinsic strength is compromised. Hopefully, further studies will prove this to be a good alternative for the long-term. Many surgeons doubt its ability to halt the progress of the ectasia.
2) Penetrating keratoplasty (corneal transplant) - Last surgical option with a long recovery and risk of rejection of the donor cornea.
Possible Hope for Post-LASIK Ectasia
http://www.revophth.com/index.asp?page=1_634.htm
Various ectasia treatments beneficial depending on patient
http://www.eyeworld.org/article.php?sid=3125
Intracorneal ring segments hold promise for treatment of post-refractive
surgey complications
http://www.escrs.org/eurotimes/December2003/Intracorneal_ring.asp
Helpful ways to prevent and treat ectasia
http://www.eyeworld.org/article.php?sid=2360
Options to treat keratoconus expand (also can help ectasia)
http://www.eyeworld.org/article.php?sid=2968
Corralling Keratoconus With Intacs
http://www.revophth.com/index.asp?page=1_792.htm
9. Quality of life impact
The ectasia patients I have come to know did not have a clue pre-op that this complication was a possibility. It took time to grasp the seriousness of their diagnosis. Shock, denial, and depression have been their common feelings. The amount of progression of the ectasia and whether or not they can be comfortably fitted with contact lenses to restore their vision has a lot to do with their ability to emotionally handle their condition. Hard contact lenses can sometimes delay progressive corneal ectasia. When they do not, the visual decline can be rapid with the need for frequent fittings of new lenses. Dreading the long recovery of a corneal transplant as a solution weighs on their minds.
Difficulty driving at night is a common complaint. Vision can be a factor in job performance, making work a struggle. Frequent doctor appointments equals time lost from work. Feeling dependent on others, or the added worry of additional medical costs, adds to their stress.
10. Private Support Group for those with LASIK induced ectasia
Many thanks to the following doctors for their topography interpretations of the ectasia support group members. Ophthalmologists: Dr. Percy Amoils, Dr. Steve Dingeldein, Dr. Bill Trattler, Dr. Ricardo Trigo, Dr. Brian Boxer Wachler, Dr. Gregg Feinerman.........Optometrists: Dr. Gregg Russell, Dr. Greg Gemoules, Dr. David Hartzok and Dr. Jason Jedlicka.
More Information
The riddle of iatrogenic keratectasia http://www.ascrs.org/publications/jcrs/editapr9.html
Iatrogenic keratectasia: academic anxiety or serious risk? http://www.ascrs.org/publications/jcrs/gueditoct9.htm
Timing crucial in LASIK and other retreatment decisions http://www.eyeworld.org/article.php?sid=1297&query=Timing%20crucial%20in%20LASIK%20and%20other%20retreatment%20decisions
Wound healing in LASIK
http://www.eyeworld.org/article.php?sid=1295
Need for intraoperative measurement of corneal thickness during LASIK
http://www.ascrs.org/publications/jcrs/editdec00.html
How to Enhance a LASIK and Avoid Ectasia
http://www.revophth.com/2001/july/feature2.htm
Understanding ectasia.....Use of Ocular antihypertensives ......And the point of no return http://www.eyeworld.org/article.php?sid=613
Beware of Post-LASIK Ectasia
http://www.escrs.org/eurotimes/January2002/bewareof.asp
Post-LASIK ectasia can be unpredictable
http://www.ophthalmologytimes.com/ophthalmologytimes/article/articleDetail.jsp?id=306516&sk=&date=&&pageID=1
Ectasia Without Risk Factors
http://www.crstoday.com/PDF%20Articles/0705/CRST0705_F4_Klein.html
Ectasia After LASIK
http://www.crstoday.com/PDF%20Articles/0705/CRST0705_F3_Moshifar.html
Known Risk Factors for Ectasia
http://www.crstoday.com/PDF%20Articles/1005/CRST1005_F3_Trattler.pdf
Refractive Surgery: Insights on Ectasia
http://www.revophth.com/index.asp?page=1_695.htm
A new rub on keratoconus
http://www.eyeworld.org/article.php?sid=2051&query=Eye%20rubbing%20of%20patients%20with%20keratoconus%20should%20be%20avoided
Forme fruste keratoconus a risk factor for post-
LASIK ectasia
http://www.escrs.org/eurotimes/04May/pdf/Preoperative%20forme%20fruste%20keratoconus.pdf
REFERENCES
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2. Seiler T., Koufala K., Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg 1998; 14:312-317.
3. Park D, Perez E, Miller D. Corneal lamellar strength as determined by thickness, position, and fibril orientation. J Invest Ophthalmol Vis Sci. 1995;36(suppl):39
4. Barraquer JI. Keratomileusis for myopia and aphakia. Ophthalmology. 1981;88:701-708.
5. Damiano.RE. Letters to editor. J Refractive Surg 1999;15: 160.
6. Page 338, External Disease and Cornea, Basic and Clinical Science Course, 1998-1999, American Academy of Ophthalmology. The footnote for this quote cites this reference: "Changes with age in normal cornea," Arch Ophthalmol, 1982;100:1942.
7. Koch DD. The riddle of iatrogenic keratectasia [editorial; comment] J Cataract Refract Surg 1999;25:453-4
8. Seiler T. Iatrogenic keratectasia: academic anxiety or serious risk? J Cataract Refract Surg; 1999; 25:1307-1308.
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McLeod SD, Kisla TA, Caro NC, McMahon TT. Iatrogenic keratoconus: corneal ectasia following laser in situ keratomileusis for myopia. Arch Ophthalmol 2000 Feb;118(2):282-4
Barraquer JI. Querato Mileusis y Queratofaquia. Bogota, Columbia: Instituto Barraquer de America; 1980:342
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George O. Waring III, M.D., FRCOphth, "The Limits of Keratomileusis," Special Article, Archives of Ophthalmology, Vol 117(8);Aug99(from the Dept. of Ophthalmology, Emory University School of Medicine, Atlanta, Ga.)
Alyson G. Yashar, Risk for keratectasia seen in eyes with thin stromal bed, Ophthalmology Times, March 15, 2000
Vinciguerra P, Camesasca FI. Prevention of corneal ectasia in laser in situ keratomileusis. J Refract Surg 2001 Mar-Apr;17(2 Suppl):S187-9
Behrens A., Langenbucher A., Kus, M.M. Experimental evaluation of two current-generation automated microkeratomes the Hansatome and the Supratome. Am J Ophthalmol 2000; 129:59-67.
Genth U, Mrochen M, Salahedine MM, et al. Flap thickness during LASIK and its implication for the safety of LASIK. In press, Ophthalmology
Thomas Kohnen, MD, From the editor. J Refractive Surg; December 2000; Volume 26 Number 12
Waring, III G.O., Lynn M.J. and McDonnell P.J., Results of the Prospective Evaluation of Radial Keratotomy (PERK) Study 10 years after surgery, Arch Ophthalmol. 1994; 112:1298-1308.
