Identifying normal values

March 1, 2014

Corneal anterior segment imaging has emerged as a screening tool for refractive surgery, cataract and glaucoma. In this article, the authors discuss their recent study that identified a set of normative data to correct for sources of bias in pre-surgery screening.

There are a number of published normative values to date, which are currently used for a number of screening displays (e.g., Belin/Ambrosio Enhanced Ectasia Display). All of the currently published normal values suffer from some investigative and selection bias. Three major sources of bias were identified; selection of patients, use of pooled refractive data and lack of homogeneity.

Prior studies used refractive surgical patients, who were pre-selected by the surgeon. Patients with 'suspicious' maps were therefore excluded when they may in fact just be a normal variant. The current values are also based on pooled data from a number of databases, which include both hyperopes and myopes. Kim et al.1 have subsequently shown that hyperopes have significantly different values especially for posterior elevation than myopic individuals. Finally Feng et al.2 have added that there is a geographic/racial variation, albeit small and often not clinically significant. We recently published a set of normative data that sought to correct for these sources of bias.3

Data was obtained from 682 paired eyes of 341 adult patients from a single refractive practice in North America. One eye was randomly selected to be included in the analysis.

All study patients were screened by two fellowship trained refractive surgeons and patients were included whether or not they proceeded to refractive surgery. Only simple and compound myopic patients were included. All patients had a minimum post study observation period of at least three years after initial examination as post LASIK ectasia and keratoconic progression should become evident during that interval thereby decreasing the likelihood of false negatives.

We reported on 21 parameters we felt most clinically applicable for pre-surgical refractive evaluation. Results were reported as percentiles rather than standard deviations as not all data was normally distributed. Five and 95 percentiles and 2.5 and 97.5 percentiles were chosen to identify outliers

The following values were reported for each selected parameter:

Results

Conclusions

Normal values are used for screening purposes throughout medicine. They allow for the identification of outliers, prompting the clinician to pursue further investigation or proceed with caution.

This study sought to provide a set of normative values commonly used in refractive surgery with elimination of potential sources of bias in previously published data.

References

1. J.T. Kim et al., J. Clinic. Experiment. Ophthalmol., 2011;2:130

2. M.T. Feng et al., J. Cataract Refract. Surg., 2011;37(10):1817–1821.

3. F. Gilani, M.W. Belin, et al., J. Cataract Refract. Surg., 2013;39(11):1707–1712.