Population factors improve screening for refractive surgery

Indianapolis-Findings from an international, cross-sectional study investigating normal variation in intrasubject asymmetry of central corneal pachymetry in patients presenting for refractive surgery are consistent with those previously reported in a refractive surgery population from North America. The data also support undertaking further research with the ideal goal of establishing tomography-derived pachymetric parameters for refractive surgery screening that are population-specific, said Matthew T. Feng, MD.

The research presented by Dr. Feng was conducted in collaboration with Renato Ambrosio Jr., MD, Michael Belin, MD, and colleagues, and analyzed bilateral pachymetry data acquired with a rotating Scheimpflug camera system (Pentacam Eye Scanner, Oculus Inc.) in 555 adults being evaluated for refractive surgery. The study population represented eight countries and six continents.

Paired right and left eye pachymetry measurements were compared at the corneal thinnest point, pupil center, and apex. Statistical analyses determined normative thresholds for right-left eye asymmetry, and the data were compared with findings from a previous study involving a North American cohort.

The data for mean intrasubject pachymetric differences and normative threshold were similar to those identified in the prior study. Overall, persons having more than 23 micrometers of central pachymetry asymmetry were statistical outliers. With the patients divided by country, however, the normative thresholds were slightly lower for China, Japan, and Egypt (19 or 20 micrometers) and slightly higher for New Zealand, the United States, and India (26.5 or 27 micrometers).

“We believe asymmetry in these pachymetric parameters beyond the normal range warrants suspicion and correlation with other refractive surgery risk factors,” said Dr. Feng, who is a fellow with Corneal Consultants of Indiana, Price Vision Group, Indianapolis. He was involved in the study while he was an ophthalmology resident at the University of Arizona, Tucson.

“In addition, we believe it is preferable to establish geography- and race-specific normative values so that clinicians can use the most applicable normative threshold for their locale and patient population. To this end, recruitment is ongoing to increase the number of populations sampled and the sample size for each,” he said.

Explaining the background for the study, Dr. Feng observed that tomographic evaluation has emerged as a useful tool in refractive surgery screening, but researchers who have developed screening criteria are continuing their work to fine-tune the identification and weighting of parameters and thresholds in order to improve screening performance. Because most of the work to date, as well as an existing screening model, compare individual eye data against collective normative data derived from patient populations mainly in North and South America, there was concern about generalizing the findings to patients in other geographic regions representing other racial-ethnic groups.

“Our previous research conducted in an international cohort validated thresholds for anterior and posterior corneal elevation, anterior chamber depth, and central pachymetry across a wide geographic base. The present research represents the next logical step of looking at other established thresholds, including intrasubject variation in central pachymetry that is of particular interest for refractive surgery screening based on the knowledge that keratoconus often presents asymmetrically,” Dr. Feng explained.

The eight countries represented in the study included the United States, New Zealand, Japan, China, Brazil, India, Egypt, and Germany. Eligible patients were aged 25 to 65 years with normal ocular health and were emmetropic or had simple myopia or myopic astigmatism. Patients were excluded if they were foreign-born, had mixed astigmatism, hyperopia, prior eye surgery, or a personal or family history of corneal ectatic disease.

Dr. Feng reported that data from measurements of corneal pachymetry at the thinnest point, pupil center, and apex were normally distributed. Data for the intrasubject differences were skewed, however, so that analyses for determining thresholds were done using nonparametric methods.

For all three parameters, median right-left eye differences were 6 or 7 micrometers. Taking into account two interquartile ranges (8 or 8.5 micrometers), the upper limit of the normal difference thresholds was 22 micrometers for the apex and 23 micrometers for the thinnest point and pupil center. Superimposing the 23-micrometer threshold on the summary histogram plotting data for intrasubject differences showed that the threshold encompassed 97% of patients, said Dr. Feng.

As a caveat, Dr. Feng said that even though the patients included in the study were selected to represent a normal population, the values for intrasubject differences ranged up to 62 or 67 micrometers.

“Therefore, variation between eyes in and of itself is not the only factor that needs to be considered,” he said.

Dr. Feng has no financial interest in the subject matter he presented. Drs. Belin and Ambrosio are consultants to Oculus.

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