Comparison of the Nidek EAS 1000 system and the Topcon SL-45 in clinical application

Determination of the density of human nuclear cataract lenses

The aim of the present study was to detect senile nuclear cataract lens density and provide a quantitative measurement of lens density for the long-term clinical observation of cataracts. An Anterior Segment Analysis System was used to detect the lens density of 422 simple senile cataract eyes and normal contralateral eyes. The density values were taken at the optical axis at various depths measured from the anterior capsule. The differences in lens density at various photographic orientations and in various nuclear grading groups were investigated. The density at 90˚ orientation was larger than that at 30 and 150˚ (P<0.05). The density was reduced beyond the 1.0-mm depth point in all the lenses. Prior to the 2.5-mm depth point, there were differences among the five grade groups (P<0.05). Beyond the 3.0-mm depth point, values in all the grade groups were low and not significantly different (P>0.05). In the grade 4 group, the decrease was more evident. The average density values at all the depth points showed a tendency to increase with increasing nuclear grade. Measuring the density of the anterior half of the lens at 90˚ orientation is therefore more reliable compared with that of the posterior half and at other orientations. Results of the present study have shown that the Anterior Segment Analysis System may be used to detect senile nuclear cataract lens density and provide a quantitative measurement of lens density for long-term clinical observation of cataracts.

Scheimpflug imaging for laser refractive surgery

PURPOSE OF REVIEW

To review the principles and clinical applications of Scheimpflug corneal and anterior segment imaging with special relevance for laser refractive surgery.

RECENT FINDINGS

Computerized Scheimpflug imaging has been used for corneal and anterior segment tomography (CASTm) in different commercially available instruments. Such approach computes the three-dimensional image of the cornea and anterior segment, enabling the characterization of elevation and curvature of the front and back surfaces of the cornea, pachymetric mapping, calculation of the total corneal refractive power and anterior segment biometry. CASTm represents a major evolution for corneal and anterior segment analysis, beyond front surface corneal topography and single point central corneal thickness measurements. This approach enhances the diagnostic abilities for screening ectasia risk as well as for planning, evaluating the results, managing complications of refractive procedures, and selecting intraocular lens power, type, and design. In addition, dynamic Scheimpflug imaging has been recently introduced for in-vivo corneal biomechanical measurements and has also been used for anterior segment imaging of femtocataract surgery.

SUMMARY

Scheimpflug imaging has an important role for laser refractive surgery with different applications, which continuously improve due to advances in technology.

Comparison of anterior segment measurements by 3 Scheimpflug tomographers and 1 Placido corneal topographer

PURPOSE

To compare the anterior segment measurements provided by 3 Scheimpflug tomographers and a Placido corneal topographer.

SETTING

Private clinical ophthalmology practice.

DESIGN

Evaluation of diagnostic test or technology.

METHODS

In a sample of 25 consecutive patients having either refractive or cataract surgery, the anterior eye segment was analyzed by means of a rotating Scheimpflug camera (Pentacam), 2 devices with a Scheimpflug camera combined with a Placido disk (Sirius and TMS-5), and a Placido disk corneal topographer (Keratron). Measurement results were compared using analysis of variance. Agreement was assessed using Bland-Altman plots.

RESULTS

The mean simulated keratometry (K) was different between the 4 instruments (P<.0001), with Keratron providing the highest value (44.43 diopters [D] ± 1.28 [SD]). The Pentacam and Sirius provided the lowest values (44.05 ± 1.21 D and 44.05 ± 1.27 D, respectively), without statistical difference (posttest). The mean posterior corneal power and minimum corneal thickness were statistically different between the 3 Scheimpflug cameras (P<.0001 and P=.0210, respectively); 95% limits of agreement, however, were narrow for posterior corneal power and large for corneal thickness. The only 2 devices measuring the distance between the corneal endothelium and the anterior lens surface showed a statistically but not clinically significant difference (2.90 ± 0.48 mm and 2.94 ± 0.47 mm, respectively). There were no statistically significant differences in anterior corneal asphericity between the 4 instruments.

CONCLUSION

Although the measurements of some parameters by different instruments were similar, caution is warranted before using them interchangeably.

Repeatability of automatic measurements by a new Scheimpflug camera combined with Placido topography

PURPOSE

To assess the repeatability of anterior segment measurements performed by a Scheimpflug camera combined with Placido corneal topography (Sirius) in unoperated, post-refractive surgery, and keratoconus eyes.

SETTING

Private clinical ophthalmology practice.

DESIGN

Evaluation of diagnostic test or technology.

METHODS

Three consecutive scans were acquired for each eye. The following parameters were evaluated: simulated keratometry, posterior corneal power, mean pupil power (ie, corneal power assessed by ray tracing through the anterior and posterior corneal surfaces), corneal asphericity, thinnest and apex corneal thickness, aqueous depth, anterior chamber volume, and corneal spherical aberration. Repeatability was assessed using test-retest variability, the coefficient of variation, and the intraclass correlation coefficient (ICC).

RESULTS

Sixty-four unoperated eyes, 17 eyes that had myopic excimer laser surgery, and 13 eyes with keratoconus were analyzed. High repeatability was achieved for most parameters in the 3 groups, with an ICC higher than 0.99 for all measurements except posterior corneal power and mean pupil power in keratoconus (ICC, 0.868 and 0.976, respectively), anterior and posterior asphericity in normal eyes (ICC, 0.904 and 0.977, respectively), and spherical aberration in normal eyes (ICC, 0.806), post-refractive surgery eyes (ICC, 0.980), and keratoconus eyes (ICC, 0.981).

CONCLUSION

The anterior segment measurements provided by the new Scheimpflug camera-Placido corneal topography system were highly repeatable and can be relied on in clinical routine and for research purposes.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Central corneal thickness measurements using Orbscan II, Visante, ultrasound, and Pentacam pachymetry after laser in situ keratomileusis for myopia

PURPOSE

To compare corneal pachymetry assessment using 4 measurement methods in eyes after laser in situ keratomileusis (LASIK) for myopia.

SETTING

Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, Hong Kong SAR.

METHODS

Fifty-two consecutive patients (103 eyes) who had LASIK for the correction of myopia had Orbscan II (Bausch & Lomb), Visante (Carl Zeiss Meditec), Pentacam (Oculus, Inc.), and ultrasound (US) pachymetry (Sonomed, 200P) 6 months after surgery. Data were analyzed using the paired sample t test, Bland-Altman plots, and linear regression.

RESULTS

The mean postoperative pachymetry measured by US, Orbscan (0.89 acoustic factor), Pentacam, and Visante pachymetry were 438.2 microm+/-41.18 (SD), 435.17+/-49.63 microm, 430.66+/-40.23 microm, and 426.56+/-41.6 microm, respectively. Compared with the US measurement, Pentacam and Visante measurements significantly underestimated corneal thickness by a mean of 7.54+/-15.06 microm (P<.01) and 11.64+/-12.87 microm (P<.01), respectively. There was no statistically significant difference between US and Orbscan measurements.

CONCLUSION

Pentacam and Visante measurements of corneal thickness 6 months after LASIK were significantly less than those obtained using Orbscan and US pachymetry, although all 4 measurement methods showed a high correlation with each other.

External factors in the development of cataract

AIMS

To provide an update on the risk factors for cataract development.

METHODS

Review of the literature.

RESULTS

Age and heredity are the most important risk factors associated with the different types of cataract. While the hereditary component is self-explanatory, increasing age serves as a surrogate for a number of potential external risk factors, the effect of which is cumulative. Identification of the risk factors that have a causal effect on cataract development may provide means for cataract prevention. There are only a few risk factors that satisfy the criteria for causal effect: smoking, which results in the increased risk of nuclear cataract, excessive UV-B exposure and diabetes that increase the risk of cortical cataract, and steroidal treatment, diabetes and ionising radiation that lead to the formation of posterior subcapsular opacity. The effect of medications on cataract development requires further study, since the effect of the diseases should be distinguished from that of treatment. 'Stop Smoking' and 'UV-B protection' campaigns are gaining momentum as preventative measures, while the attempts to actively prevent cataract with antioxidants have not been successful. Cataract research has been facilitated lately by improvements of precision and standardisation in measuring lens opacities. However, measurement precision on its own cannot give us a solution to this problem.

CONCLUSION

The major studies repeatedly measure the exposure to the traditional health hazards, while the missing parts in the equation are those risk factors that we do not know about and therefore do not measure. New approaches and new hypotheses are needed.

Agreement and Repeatability of Central Thickness Measurement in Normal Corneas Using Ultrasound Pachymetry and the OCULUS Pentacam

PURPOSE

To compare the accuracy and repeatability of the OCULUS Pentacam (a new Scheimpflug-based imaging system) with ultrasound pachymetry in the measurement of central corneal thickness (CCT).

METHODS

CCT was measured in 21 subjects (21 normal corneas) on 2 separate occasions by the same examiner, using an Allergan-Humphrey 850 ultrasonic pachymeter and an OCULUS Pentacam instrument.

RESULTS

Mean values of CCT for both visits for each instrument were 534 +/- 47 microm and 528 +/- 45 microm using the ultrasonic pachymeter and the Pentacam, respectively. Plots of differences against means displayed relatively good agreement (limits of agreement were -13.0 to +26.6 microm). The repeatability (limits of agreement) of the ultrasound pachymeter was -18.3 to +17.7 microm, while for the OCULUS Pentacam it was -24.1 to +21.1 microm.

CONCLUSIONS

Our data showed that the Pentacam instrument provided measurements that were slightly but systematically lower than the measurements provided by ultrasonic pachymetry, which is currently the clinical gold standard method. The results, coupled with a unique ability to image and analyze the anterior chamber in vivo, make the OCULUS Pentacam a promising new instrument for anterior eye evaluation.

Anterior chamber angle measurement with Anterior Eye Segment analysis system Nidek EAS-1000: improving the repeatability

Previous studies have reported good intraobserver and interobserver repeatability of anterior chamber angle (ACA) assessment using the Anterior Eye Segment analysis system, EAS-1000. This study investigated the influence of fixation pattern, corneal plotting method and camera position on ACA measurements. A Nidek EAS-1000 system was employed for image acquisition and analysis, to obtain measurements of ACA. Twenty-five young healthy subjects participated. The ACAs were different in different quadrants and using different corneal plotting methods (Repeated measures 3-way anova, p < 0.05). However, the difference was <5 degrees and therefore not clinically significant. Alignment along the visual axis or optical axis did not yield any significant difference. Repeatability was improved by asking the subjects to fixate at the flashing light-emitting diode (LED). Temporal and nasal angles also demonstrated better repeatability than vertical angles. In measuring the vertical angles, the camera can be set at either the temporal or nasal side (Paired t-test, p > 0.05). The Anterior Eye Segment analysis system has good clinical repeatability in measuring ACA. It can be further improved with the use of the flashing LED during image acquisition.

Cataract classification

Opacifications of the eye lens--generally defined as cataracts--develop in various different parts of the lens. Therefore, one has to differentiate the types of opacities. For epidemiological studies it is prerequisite to classify the cataracts according to their localization within the lens as well as to the size and intensity of the opacified area. Two approaches have been used in the past: 1) subjective methods of lens observation (based on slit lamp microscopy) and 2) objective methods with measurements of lens transparency or lens opacity respectively based on slit image documentation according to the Scheimpflug principle combined with the retroillumination technique. With ageing, the light transparency of the lens is subjected to considerable changes. Even without the formation of an opacity the transmission of the wavelengths in the UV-B/UV-A and the visible range is diminished. The single lens layers are affected by this phenomenon to different degrees. These changes which might also indicate an early stage of 'cataract formation' cannot be discerned by subjective methods. The densitometric image analysis of Scheimpflug slit images, however, allows the exact measurement of the light scatter in the single lens layers and enables the early recognition of disturbances in transparency which is of crucial importance particularly in cataract epidemiology. In view of our present knowledge the evaluation of risk factors which might be of importance in multifactorial cataract processes will hardly be possible by carrying out prevalence and/or incidence studies involving a single examination of the population. In this case follow-up studies (cohort studies) with repeated examinations are prerequisite. The 'objective methods' for classification alone are able to ensure the necessary reproducibility and the possibility to measure transparency changes in the lens before visible (and therefore subjectively recognizable) opacifications occurred. The methodical procedure with respect to an epidemiological study on the involvement of UV-B radiation in the processes of cataract formation in man requires the application of objective methods for cataract classification.