Pachymetric evaluation prior to laser in situ keratomileusis

Pentacam pachometry: comparison with non‐contact specular microscopy on the central cornea and inter‐session repeatability on the peripheral cornea

PURPOSE

Our aim was to compare specular microscopy (SM) and a new corneal topographer with rotating Scheimpflug camera (Pentacam) for non-contact pachometry of the central cornea. The repeatability of Pentacam for topographic pachometry was also studied.

METHODS

Thirty-nine subjects were recruited and one eye was selected randomly for non-contact pachometry by SM and by Pentacam, in random order. The corneal thickness (CT) was monitored at 30-second intervals for 10 minutes. Baseline CT was defined as the average of all measurements taken over the 10 minutes. Subjects were required to return within one to two weeks for a second corneal evaluation by Pentacam. Comparisons were made on the central CT between the two devices and on the topographic CT from Pentacam between the two visits.

RESULTS

The spontaneous variation of central CT was similar for the two instruments. Central CT varied within +/-2.3 microm during the 10 minutes. The 95% limits of agreement between SM and Pentacam were within +/-15 microm for the central CT. The spontaneous variation of peripheral CT was within +/-3.1 microm. There was no significant difference (p > 0.05) between the two visits on CT at different regions. Further analysis found that with Pentacam three measurements should be taken to obtain topographic CT measurement of one per cent precision.

CONCLUSION

Non-contact specular microscopy and Pentacam have good agreement for central CT measurement. Topographic pachometry from Pentacam requires three repeated measurements for one per cent precision.

The distribution of corneal thickness in a 40- to 64-year-old population of Shahroud, Iran

PURPOSE

To determine the distribution of central and peripheral corneal thickness values and its determinants in an Iranian population.

METHODS

This population-based study was conducted between 2009 and 2010 in Shahroud, with a response rate of 82.2%. Here in, we present the data on central and peripheral corneal thickness in 3890 participants of the study, which included all 40- to 64-year-old participants except those with a history of ocular surgery. Measurements were done with the Pentacam, and correlations with age, gender, height, body mass index, and refraction were tested.

RESULTS

The mean age (± standard deviation) of the participants was 50.7 years (± 6.2 years). Mean corneal thickness in the thinnest point, the center, and 3 mm away from it in the inferior, superior, nasal, and temporal directions were 525.5 ± 35.9 μm, 528.5 ± 35.8 μm, 608.2 ± 39.8 μm, 635.5 ± 41.9 μm, 611.7 ± 43.2 μm, and 601.0 ± 40.4 μm, respectively. The thinnest point was in the inferotemporal quadrant in 87.5% of cases. The center of the cornea was thinner than 500 μm in 36.6% of myopic cases and 21.8% of hyperopic cases. The thickness and location difference between the center and thinnest point were statistically significant (P < 0.001). Central corneal thickness was directly correlated with refraction (P = 0.028) and inversely with age (P = 0.027).

CONCLUSIONS

In this population-based study, the thinnest point was usually located inferotemporally and the thickest point superiorly. The peripheral cornea was significantly thicker than the central cornea but thinned faster with age. The thickness difference between the thinnest and central points was significantly greater in men. People with hyperopia had thicker corneas compared with those with myopia.

Novel pachymetric parameters based on corneal tomography for diagnosing keratoconus

PURPOSE

To describe pachymetric progression indices (PPI) of the Pentacam HR (Oculus Optikgeräte GmbH) and the concept of relational thickness, and to test their accuracy for differentiating keratoconic and normal corneas compared with single-point thickness values.

METHODS

One hundred thirteen individual eyes randomly selected from 113 normal patients and 44 eyes of 44 patients with keratoconus were studied using the Pentacam HR by acquiring central corneal thickness (CCT), thinnest point (TP), position of the TP and PPI at minimal (PPI Min) and maximal (PPI Max) meridians, and the average (PPI Ave) of all meridians. Relational thickness parameters were calculated as the ratios of TP and CCT and PPI values. Mann-Whitney U test assessed differences in groups for each variable. Receiver operating characteristic (ROC) curves were calculated for all variables and pairwise comparisons were performed.

RESULTS

Statistically significant differences were noted between normal and keratoconic eyes for all parameters (P<.001), except for horizontal position of TP (P=.79). The best parameters, named Ambrósio's Relational Thickness (ART), were ART-Ave (TP/PPI Ave) and ART-Max (TP/PPI Max) with areas under the ROC curves of 0.987 and 0.983, respectively. The best cutoffs were 424 μm and 339 μm for ART-Ave and ART-Max, respectively. Pachymetric progression indices and ART had a greater area under the curve than TP and CCT (P<.001); TP (0.955) had a greater area under the curve than CCT (0.909; P=.002).

CONCLUSIONS

Tomographic-derived pachymetric parameters were better able to differentiate normal and keratoconic corneas than single-point pachymetric measurements. Further studies are needed to evaluate the role of tomography in identifying early forms of ectasia as well as ectasia risk among LASIK candidates.

Corneal-thickness spatial profile and corneal-volume distribution: Tomographic indices to detect keratoconus

PURPOSE

To evaluate whether the corneal-thickness spatial profile and corneal-volume distribution differentiate keratoconic corneas from normal corneas using new tomography parameters.

SETTING

Subspecialty cornea and refractive practice, Fluminense Federal University, Rio de Janeiro, Brazil.

METHODS

Forty-six eyes diagnosed with mild to moderate keratoconus and 364 normal eyes were studied by the Pentacam Comprehensive Eye Scanner. Corneal thickness at the thinnest point and the averages of the points on 22 imaginary circles centered on the thinnest point with increased diameters at 0.4 mm steps were calculated to create a corneal-thickness spatial profile. Corneal volume was calculated within diameters from 1.0 to 7.0 mm with 0.5 mm steps centered on the thinnest point to create the corneal-volume distribution. The percentage increase in thickness and the percentage increase in volume were calculated for each position of the corneal-thickness spatial profile and corneal-volume distribution from their first value. Statistical analysis was done using the Wilcoxon 2-independent-sample test to compare mean levels using S-Plus-4.0 software (MathSoft) and a normal linear model under a Bayesian frame for estimating the mean variation in thickness and volume using the BUGS 0.6 package.

RESULTS

Statistically significant differences were observed between the groups (P<.05) in all positions of corneal-thickness spatial profile and corneal-volume distribution and in the percentage increase in thickness and percentage increase in volume between 3.5 mm and 7.0 mm diameters.

CONCLUSIONS

Corneal-thickness spatial profile, corneal-volume distribution, percentage increase in thickness, and percentage increase in volume were different between keratoconic corneas and normal corneas and could serve as indices to diagnose keratoconus and screen refractive candidates. Further studies are necessary to evaluate whether these tomographic indices are more sensitive and specific than the classic Placido-based topography.

Ectasia after laser in situ keratomileusis

PURPOSE OF REVIEW

The potential for litigation has resulted in increased interest in ectasia after laser in situ keratomileusis. This article summarizes papers written during this past year on this subject.

RECENT FINDINGS

A panel of refractive surgery experts have summarized the current state of knowledge on this subject and provided guidelines to minimize risk. Because ectasia may occur in the absence of risk factors, there may be corneas with biomechanical factors not detectable with present technology which are at risk. Several articles point out the limitations of the Orbscan (Bausch & Lomb, Rochester, New York, USA), in particular its accuracy and reproducibility with regard to data generated from the posterior surface of the cornea. A biomechanical model of the cornea is proposed which suggests that each cornea's response to laser in situ keratomileusis is unique. Several case reports are also reviewed which provide insight into pathogenesis and potential new treatment options; these include reports on histopathology, topographic pseudokeratectasia, ectasia from transient raised intraocular pressure, poor response to laser in situ keratomileusis after incisional surgery, intracorneal rings, collagen cross-linking and conductive keratoplasty to stabilize and sphericize the cornea with ectasia.

SUMMARY

Our knowledge about ectasia is still in evolution. Insights into the biomechanics and genetics of the cornea may allow us to further reduce its occurrence.

Orbscan Global Pachymetry: Analysis of Repeated Measures

PURPOSE

The Orbscan II is a hybrid slit-scanning and Placido disc corneal topographer capable mapping global pachymetry over a 10-mm corneal diameter. In this study, the repeatability of the Orbscan global pachymetry was determined.

METHODS

Five consecutive Orbscan examinations were performed on one eye of 20 healthy volunteers by one examiner in one session. Mean, standard deviation, range of readings, and coefficient of variance were calculated for the central, thinnest point and global pachymetry.

RESULTS

Orbscan repeatability for the central pachymetry was 3.62 microm (95% confidence interval [CI], 0.57-6.66) with a coefficient of variance of 0.67% (95% CI, 0.09-1.25) and intraclass correlation coefficient (ICC) of 0.978 (95% CI, 0.959-0.990). Thinnest point pachymetry had a repeatability of 4.25 microm (95% CI, 1.19-7.31) with a coefficient of variance of 0.80% (95% CI, 0.19-1.42) and ICC of 0.973 (95% CI, 0.950-0.988). Global pachymetry had a coefficient of variance <1.5% in the central 4 mm and <<2.5% across the entire cornea. Global pachymetry showed an area of greatest repeatability (<< +/- 5 microm) within a central 3.0-mm horizontal and 4.0-mm vertical diameter.

DISCUSSION

Orbscan is capable of giving repeatable pachymetry readings that are comparable to the ultrasound pachymeter within the central and thinnest point of the cornea. There is a gradual loss of repeatability toward the peripheral cornea possibly as a result of a lack of overlapping data points.