Andrew K. C. Lam

The repeatability and accuracy of axial length and anterior chamber depth measurements from the IOLMaster

Background: Axial length and anterior chamber depth have been measured clinically using conventional ultrasound biometry. Recently, a non‐contact device has become available to measure these parameters. This study evaluated the repeatability and accuracy of this device.

Intrasession and intersession repeatability of the Pentacam system on posterior corneal assessment in the normal human eye

PURPOSE: To study the intrasession and intersession repeatability of the Pentacam system (Oculus Inc.) in measuring the posterior corneal shape. SETTING: School of Optometry, Hong Kong Polytechnic University, Hong Kong, China. METHODS: The posterior corneal shape in 1 eye (randomly selected) was measured with the Pentacam system. Three consecutive readings were taken for intrasession repeatability analysis. Measurements were repeated 1 to 2 weeks later for intersession repeatability analysis. The anterior best‐fit sphere (BFS) and posterior BFS at the 5.0 mm and 8.0 mm zones, as well as the elevation of the posterior cornea at these 2 zones, were compared. RESULTS: The anterior and posterior BFS demonstrated good intrasession repeatability (Cronbach α ≥0.997; intraclass correlation coefficients ≥0.990). No significant difference was found between the 2 visits. The 95% limits of agreement of posterior BFS were 52.3 to −41.7 μm at the 5.0 mm zone and 66.1 to −60.9 μm at the 8.0 mm zone. The elevation was similar between the 2 visits. The 95% limits of agreement were 4.27 to −5.29 μm at the 5.0 mm zone and 5.33 to −5.77 μm at the 8.0 mm zone. CONCLUSIONS: Pentacam provided good performance in assessing the posterior cornea. To improve the intersession repeatability, it is suggested that 3 consecutive readings, rather than 1 image, be taken to generate an average BFS for analysis. When comparing the posterior corneal elevation between sessions, an average BFS generated from the first visit should be used for elevation calculation.

Corneal deformation measurement using Scheimpflug noncontact tonometry.

Purpose To determine the intraexaminer repeatability and intersession reproducibility of corneal deformation measurement using Scheimpflug noncontact tonometry (Corvis ST) on normal subjects. Methods Thirty-seven adults aged 20 to 48 years were invited to have their corneal deformation and curvature measurements taken using Corvis ST and Pentacam, respectively. Three consecutive measurements were taken for each instrument between 9:00 and 11:00 AM for intraexaminer repeatability analysis. Participants returned between 3:00 and 5:00 PM the same day for intersession reproducibility analysis. Results The most repeatable corneal parameter measured by Corvis ST was central corneal thickness ([CCT] ICC, 0.96; precision, 10.85 &mgr;m; repeatability, 15.34 &mgr;m; CV, 1.01%), followed by deformation amplitude ([DA] ICC, 0.80; precision, 0.08 mm; repeatability, 0.13 mm; CV, 4.33%), first applanation time ([1st A-time] ICC, 0.77; precision, 0.22 milliseconds; repeatability, 0.31 milliseconds; CV, 1.42), and intraocular pressure ([IOP] ICC, 0.75; precision, 1.39 mm Hg; repeatability, 1.97 mm Hg; CV, 4.98). Other parameters showed poor repeatability. The DA and 1st A-time showed good intersession reproducibility. The 95% limits of agreement were +0.13 to −0.13 mm for DA and +0.27 to −0.33 milliseconds for 1st A-time. The DA was negatively correlated with central corneal thickness (r = −0.53, p < 0.001) but not with corneal curvatures (flattest curvature, r = 0.13, p = 0.46; steepest curvature, r = 0.05, p = 0.75). Conclusions Corneal deformation parameters DA and 1st A-time were repeatable and reproducible. A thinner cornea was associated with a higher corneal deformation. Measurement of DA serves as an indicator of corneal biomechanical properties.

The performance of four different corneal topographers on normal human corneas and its impact on orthokeratology lens fitting.

Purpose. To evaluate the performances of Humphrey Atlas 991, Orbscan II, Dicon CT200, Medmont E300 on young Chinese adults. Methods. Three sets of corneal topography measurements were obtained from each topographer from 22 subjects—two sets by the same examiner and one set by another examiner on the same day. Results. There were no significant within-examiner and between-examiner differences for any of the parameters tested for each topographer. However, only the repeatability and reproducibility (of apical radius[Ro], eccentricity, and elevation) of the Humphrey and Medmont were good. There was no statistically significant between-topographer difference in Ro, but significant differences in eccentricity and elevation values were found. The number of repeated readings that should be taken for a precision of 2 &mgr;m (elevation) were 12 for the Humphrey and 2 for the Medmont. Conclusions. The performance of both the Humphrey and the Medmont was very good. Roand eccentricity values of different topographers cannot be used interchangeably, but the agreement in elevation values was good for these topographers. The number of repeated readings required for maximum precision varies with the topographer used, and they are not interchangeable.

The effect of an artificially elevated intraocular pressure on the central corneal curvature

Twenty-three optometry students with normal corneal condition were recruited. The intraocular pressure and central corneal curvature of the right eye were measured in a sitting and a 30 degrees head-down posture. The mean (standard deviation) IOPs before and during posture change were 15.6 (2.4) mmHg and 22.1 (2.3) mmHg respectively. This 6.5 mmHg mean rise in IOP was found to be statistically significant which is similar to the results from previous studies. The mean changes in radius of corneal curvature and the orientation of the axis of the vertical principal meridian were 0.02 mm (SD 0.025 mm) and 2.4 degrees (SD 10.4 degrees) respectively. No significant variation was demonstrated on these keratometric results due to the 30 degrees head-down posture. The maximum change in radius of curvature was only 0.055 mm for one subject. Perhaps this amount of pressure rise was not sufficient enough to distort the corneal surface centrally. Another possibility could be an even distribution of the elevated pressure around the cornea, or the distribution of pressure is not even but could not be revealed by a conventional keratometer.

Comparison of IOP measurements between ORA and GAT in normal Chinese.

Purpose. To compare intraocular pressure (IOP) obtained from the ocular response analyzer (ORA) and Goldmann applanation tonometer (GAT) on a group of normal Chinese. Methods. One hundred twenty-five normal subjects were recruited, with one eye randomly selected for this study. Each eye was measured first with the noncontact tonometer ORA, followed by the GAT and ultrasound pachometry, in a randomized order. Four readings were obtained from the ORA, and three measurements were taken with the GAT. The mean was used for analysis. The ORA provided a Goldmann-correlated IOP (IOPg) and a corneal-compensated IOP (IOPcc). Three central corneal thickness (CCT) values were measured using an ultrasound pachometer, and the mean was used for analysis. Results. IOP obtained from the ORA was similar to that from the GAT (IOPg minus GAT: mean difference = 0.33 mm Hg, 95% limits of agreement = 4.55 to −4.44 mm Hg; IOPcc minus GAT: mean difference = 0.24 mm Hg, 95% limits of agreement = 4.83 to −5.07 mm Hg). CCT was positively associated with corneal hysteresis (CH) (r2 = 0.30, p < 0.01), corneal resistance factor (r2 = 0.38, p < 0.01), GAT (r2 = 0.09, p < 0.01) and IOPg (r2 = 0.16, p < 0.01). IOPcc was not associated with CCT (r2 = 0.01, p = 0.33). Conclusions. Both IOPg and IOPcc have good agreement with GAT on normal subjects. The influence of CCT on IOPcc was insignificant.

Reliability and repeatability of the Pentacam on corneal curvatures

Background:  The purpose of this study was to assess the intra‐observer reliability and inter‐session repeatability of the Pentacam system (Oculus, Inc.) on the simulated keratometry readings (Sim K) and peripheral corneal curvatures (PCC) at both the anterior and posterior corneal surfaces.

Does the change of anterior chamber depth or/and episcleral venous pressure cause intraocular pressure change in postural variation?

Purpose. Previous studies have found that the intraocular pressure (IOP) variation from postural change is due to the obstruction of aqueous outflow by an increase in episcleral venous pressure. This study investigated if any shift of anterior lens position from postural variation would be another contributing factor. Methods. Thirty-three Chinese subjects were recruited with their IOP and anterior chamber depth (ACD) measured in the sitting, supine, and prone postures. The IOP was measured using a Pulsair 2000 noncontact tonometer and ACD with a Nidek US-2000 EchoScan unit. Results. The highest IOP was obtained in the prone position and this value was significantly different from the IOP obtained in other postures, whereas there was no significant difference in ACD. Conclusions. Because no significant variation in ACD was demonstrated, the prone and supine IOP variation could be due to something other than the change in lens position. However, a higher IOP in the prone position rather than in the supine position also suggests that it is not merely the episcleral venous pressure causing the IOP change. Investigation of the entire iris profile at different postures would be more informative in future studies.

Retinal nerve fiber layer thickness in normal Hong Kong chinese children measured with optical coherence tomography.

PurposeTo obtain the retinal nerve fiber layer (RNFL) thickness normative values of Hong Kong children by using optical coherence tomography (OCT) and to investigate the effect of age and axial length (AL) on RNFL thickness. Patients and MethodsOne hundred and four normal children aged between 6 and 17 years were recruited for this study. The RNFL thickness was measured by OCT (Stratus OCT). AL was measured by the IOLMaster. ResultsThe median age of our subjects was 9.75 years (range: 6.08 to 17.58 years). The mean full circle peripapillary RNFL (RNFL-FC) was 113.5 μm (SD 9.8) and 113.1 μm (SD 10.8) in the right and left eyes, respectively. Multiple linear regression analysis showed a significant negative correlation between AL and RNFL-FC in both the right (r=−0.23, P<0.05) and left eyes (r=−0.25, P<0.05). The effect was the strongest in the superior quadrant. Every 1 mm increase in AL would result in a decrease in RNFL-FC by 2.7 to 2.9 μm. Age did not show significant effect on RNFL-FC. ConclusionsThis study provides normative RNFL values for Hong Kong Chinese children. It can be used as a reference for the screening of glaucoma in children. AL demonstrates a significant negative correlation with RNFL thickness.

The effect of age on ocular blood supply determined by pulsatile ocular blood flow and color Doppler ultrasonography

Purpose. Pulsatile ocular blood flow (POBF) assessment measures the choroidal circulation and provides diagnostic value to certain ocular diseases such as glaucoma. This technique assumes a constant ocular rigidity and is influenced by axial length, diurnal variation, and age. This study investigated the effect of age on POBF, with consideration of the above factors. Ocular blood supply in the ophthalmic artery was also determined using color Doppler ultrasonography. Methods. A total of 118 healthy subjects aged 19 to 75 years were recruited. They were divided into five groups (below age 30, 30 to 39, 40 to 49, 50 to 59, and 60 or above) of at least 20 subjects each. Only one eye of each subject, with axial length <24.5 mm, was considered. The subject’s supine POBF was determined followed by a measurement of the blood flow velocity in the ophthalmic artery using color Doppler ultrasonography. All the measurements were at around the same time of day to eliminate any effect from diurnal variation. The scleral rigidity was measured using a Schiotz tonometer with 5.5- and 7.5-g weights. Results. Linear regression analysis demonstrated a significant increase of scleral rigidity with age (Pearson correlation coefficient r = 0.26, p < 0.01) and a significant decrease of POBF with age (r = −0.35, p < 0.01). The reduction in peak systolic velocity in the ophthalmic artery with age was significant (r = −0.28, p < 0.01). Both the systolic and diastolic brachial pressure showed significant increase with age (r = 0.55, p < 0.01; r = 0.40, p < 0.01, respectively). Using multiple regression analysis, POBF showed a significant correlation with age (partial correlation r = −0.36, p < 0.01), but not with scleral rigidity or systolic or diastolic brachial pressure. The peak systolic velocity in the ophthalmic artery also showed significant correlation with age (partial correlation r = −0.29, p < 0.01). Conclusions. The reduction in POBF with age was significant. Although aging affects scleral rigidity and systemic blood pressure, multiple regression analysis indicates that the most influential factor affecting POBF is aging. The peak systolic velocity in the ophthalmic artery also decreased with age, indicating reduced ocular blood supply.