(FIGURE 2) Preoperative central corneal thickness – thinnest of pentacam and three ultrasonic pachymetry readings.
(FIGURE 3) Estimated residual stromal bed.
(FIGURE 3) Topography reading (Figures courtesy of Steven C. Schallhorn, MD).
Even patients with no risk factors can develop ectasia. There is no current way to eliminate the risk entirely, according to Steven C. Schallhorn, MD.
Reviewed by Steven C. Schallhorn, MD
Researchers are considering a number of factors to determine if they could be used to assess what eyes may be at a higher at risk of developing ectasia.
Steven C. Schallhorn, MD, and colleagues performed a study of a large number of consecutive primary LASIK treatments (over 300,000 procedures performed between 2007 and 2011). Of that cohort, a number of patients experienced ectasia (213 eyes in 155 patients).
The researchers first looked at age and found that LASIK patients who developed ectasia tended to be younger than the overall population of those who underwent LASIK. But 38% of ectasia patients were over the age of 30 at the time of surgery.
So, while an older patient may be less likely to develop the condition, it certainly can still occur in older patients
The next factor examined was central corneal thickness. Patients that developed ectasia tended to have slightly thinner corneas than the rest of the cohort.
However, over 80% of ectasia patients had a central corneal thickness greater than 510 Î¼m. So again, while a patient with a thick central cornea may appear to be less likely to develop ectasia, ectasia was observed with a corneal thickness of 600 Î¼m.
The group next considered the thickness of the residual stromal bed.
In general, the ectasia group had a lower residual stromal bed than the rest of the cohort. But almost all patients in the ectasia group had a residual stromal bed of 300 Î¼m or more. So again while the thickness of the stromal bed may appear to have a relationship to developing ectasia, eyes with a stromal bed of 450 Î¼m developed ectasia.
The researchers reviewed the preoperative topography of all ectasia cases as well as a subset of the large cohort treated in 2007 who did not develop ectasia. This control group was comprised of 3,700 eyes.
The review was based on established pattern recognition of the sagittal topographic maps, such as asymmetric bow-tie, inferior steepening and skewed radial axis. In the ectasia cases, 30% of eyes were classified as normal topographic shape (central round, oval, or symmetric bowtie patterns).
A much higher percent of ectasia cases were noted to have inferior steepening, skewed radial axis and especially forme-fruste keratoconus compared to controls.
However, there were control eyes that had all of these patterns.
A simple univariate analysis showed that a number of factors were significantly associated with ectasia. However, many of these factors dropped out with more in-depth analysis.
Making the presumption that the corneal shape of the control group subset who did not develop ectasia (3,700 procedures) was reflective of the large cohort (>300,000 procedures), multivariate logistic regression analysis demonstrated that the strongest factors associated with ectasia were the age of the patient and the shape of the cornea.
Odds ratios for developing ectasia were then calculated. This analysis confirmed the much higher risk of developing ectasia in the presence of preoperative forme-fruste keratoconus (odds ratio 20.7 with 95% CI 11.7 to 36.4).
Younger age and the topographic presence of inferior steepening or skewed radial axis independently imparts a higher risk.
But all of these factors do not address the significant variance seen in the development of ectasia.
There were patients with no “risk factors” who developed ectasia and those with presumed risk factors who did not.
In this large analysis, the most important factors associated with ectasia after LASIK are younger age and an abnormal corneal topography. However, patients with no apparent risk factors can develop ectasia, so we cannot eliminate the chance it will occur.
Steven C. Schallhorn, MDE: firstname.lastname@example.org
Dr. Schallhorn is chief medical officer for Carl Zeiss Meditec and medical advisor to Optical Express.