New model better at detecting patients at risk for ectasia after LASIK
May 14, 2008
For most patients, LASIK surgery is safe and effective. But in rare cases, a serious complication called ectasia can occur.
Characterized by a thinning and bulging of the cornea, post-LASIK ectasia resembles a condition unrelated to refractive surgery called keratoconus. In both conditions, vision becomes distorted and cannot be corrected with eyeglasses.
Prevention is the best treatment
A relatively new treatment called corneal collagen crosslinking shows promise to stabilize ectasia and possibly allow additional vision correction surgery to improve vision.
However, to prevent the condition from occurring in the first place, doctors are looking for better ways to identify eyes that may be at risk of corneal ectasia from laser vision correction so they can avoid performing LASIK and other corneal procedures on these eyes.
Risk factors for ectasia
To create a clinical screening tool that might better identify eyes at risk for developing ectasia following corneal refractive surgery, J. Bradley Randleman, MD and other researchers at Emory Eye Center in Atlanta evaluated past cases of post-surgical ectasia and have developed a multi-factor scoring model that appears to have achieved that goal, according to a report recently published by Ophthalmology Times.
The researchers reviewed all post-surgical cases of ectasia after laser vision correction procedures performed at Emory Eye Center from 1998 to 2005 and cases from other surgery centers published in the English language through 2005.
Their search identified a total of 171 cases. Approximately 50% of the cases of ectasia occurred within 1 year of surgery, and 80% occurred within 2 years. However, some cases did not become apparent until up to 4 years after LASIK.
When compared to 186 "control" eyes that did not develop ectasia after surgery, the eyes that developed ectasia were generally younger, more myopic, had thinner corneas before surgery, were more likely to have abnormal corneal topography prior to surgery, and had a thinner residual stromal bed (the thickness of the cornea under the flap created during LASIK) after surgery.
Further analysis indicated that the most significant risk factors for ectasia – in order of importance – were:
- abnormal or suspect corneal topography
- residual stromal bed thickness less than 300 microns
- patient age less than 30 years
- preoperative corneal thickness less than 510 microns or extremely high myopia
These findings were then used to develop a risk score model wherein points are applied in a weighted fashion to each factor to arrive at a score that indicates a patient's relative risk for ectasia after LASIK or other corneal refractive surgery.
New model is more sensitive
When the risk model developed by Dr. Randleman and his colleagues was then applied to the cases in their retrospective study, 98% of the control population (eyes that did not develop ectasia) were identified as being at low risk for ectasia, and 93% of the eyes that developed ectasia were identified as being at high risk for the condition.
The traditional clinical standard of care for identifying eyes at risk for developing ectasia after LASIK has been a calculated residual stromal thickness of less than 250 microns after surgery and/or suspect or abnormal preoperative corneal topographies that suggest an atypical or incomplete manifestation of keratoconus (also called "forme fruste" keratoconus).
When compared to this traditional model, the new ectasia risk scoring system developed by Dr. Randleman and his colleagues was found to be significantly more sensitive in detecting at-risk patients, according to the Ophthalmology Times article.
"However, even with this scale, some eyes will still develop ectasia," warns Dr. Randleman.