Novel metric may determine ectasia risk

June 9, 2015

Using a new metric seems to be an easy way to determine a patient’s risk of developing ectasia after a refractive procedure, according to Marcony R. Santhiago, MD, PhD.

Sao Paulo-Using a new metric seems to be an easy way to determine a patient’s risk of developing ectasia after a refractive procedure, according to Marcony R. Santhiago, MD, PhD.

The high percentage of tissue altered (PTA) was identified as the major risk factor for development of ectasia after LASIK in eyes with normal corneal topography preoperatively.

“With retrospective analysis, most patients who develop ectasia after LASIK have lens fiber risk factors that place them at a higher risk for this complication, especially an irregular topographic pattern,” said Dr. Santhiago, associate professor of ophthalmology, University of Sao Paulo, Brazil.” However, postoperative ectasia that develops in patients who underwent LASIK and had normal topography remains a mystery.

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“The pathology most likely represents a reduction in the biomechanical integrity below a safe threshold that is required to maintain the corneal shape and curvature,” he continued.

This can theoretically occur, Dr. Santhiago explained, when a relatively normal cornea already destined for ectasia undergoes surgery, when a preoperatively weak but clinically stable cornea undergoes surgery, or when a completely normal cornea is weakened below a safe threshold.

The corneal tensile strength, he said, is not uniform throughout the central cornea, with progressive weakening of the posterior two-thirds, and the relative extent of the biomechanical alterations plays a role in the postoperative weakening after LASIK.

The thickness of the LASIK flap contributes to the alterations because the lamellar flap does not contribute to the corneal tensile strength.

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“Based on the structural differences, it seems reasonable that a ratio or percentage should be more representative of the change that occurs after refractive surgery than specific cut-off values related to the residual stromal bed or corneal thickness,” Dr. Santhiago said.

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Dr. Santhiago and his colleagues proposed the new metric: PTA = (FT + AD)/CCT, where FT is the flap thickness, AD is the ablation depth, and CCT is the central corneal thickness.

They conducted a study to investigate the association between the PTA and the occurrence of ectasia after LASIK in eyes with normal preoperative Placido disk-based corneal topography and to compare the metric with other recognized risk factors.

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The study included a control group of normal eyes that had undergone uneventful LASIK with a stable eye for at least 3 years postoperatively and an ectasia group with bilateral normal topography (<0.5 D of asymmetry).

Dr. Santhiago reported that the mean values found in the study were significantly higher in the ectasia group compared with the control group (45.1 ± 3.9 versus 31.9 ± 5.8, respectively) with respective PTAs of 43.6% to 46.4% compared with 31.1% to 32.8%, a difference that reached significance (p < 0.0001).

The most important finding as revealed by the receiver operating curve was that the cut-off value was 40%, i.e., the maximum combination of sensitivity and specificity (97% and 89%, respectively). Dr. Santhiago and colleagues then used that percentage to evaluate where the new metric being investigated would stand compared with other recognized risk factors.

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The comparison, he said, showed that a PTA over 40% was “by far the most prevalent risk factor compared with, for example, corneal thickness below 510 µm, myopia of 8 D and higher, and a residual stromal bed of 300 µm or less. Almost 100% of patients with normal topography presented with a PTA over 40%.”

In addition, the odds ratio of the PTA (223.3) was significantly higher than the other risk factors of the individual risk factors. The next closest odds ratio was 74.5 for a residual stromal bed of 300 µm or less.

“If only the PTA was used as a screening factor, which is not our proposal, the sensitivity is 97%, specificity of 89%, and a negative predictive value of 99.4%,” Dr. Santhiago said. “The logistic stepwise regression showed that the PTA was the single most important parameter among the ectasia risk factors.

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 “A high PTA was the most prevalent identifiable risk factor in eyes with ectasia with normal preoperative topography and should be considered when evaluating any patient for a refractive procedure and a PTA of 40% or greater was a more robust indicator of risk than all other variables in this patient population,” he added. “We are proposing a change in paradigm using an equation that is easy to understand.”