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Reproducibility of Optical Measurements in Pseudophakic eyes Using Double-Pass Measurement System

Reproducibility of Optical Measurements in Pseudophakic eyes Using Double-Pass Measurement System. St Thomas Hospital, London. Anish Dhital, David Spalton, Kavita Gala ASCRS 2010 No Relevant Financial Disclosure. Purpose.

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Reproducibility of Optical Measurements in Pseudophakic eyes Using Double-Pass Measurement System

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  1. Reproducibility of Optical Measurements in Pseudophakic eyes Using Double-Pass Measurement System St Thomas Hospital, London Anish Dhital, David Spalton, Kavita Gala ASCRS 2010 No Relevant Financial Disclosure

  2. Purpose • To measure the reproducibility of optical measurements in pseudophakic patients using a double pass measurement system.  Light scatter • Increasing interest in measurements of light scatter and impact on visual function • Methods of measuring scatter • Double-pass • Psychophysical • Hartmann-Shack

  3. Variables that affect light scatter • Tear film • Cornea • Anterior Chamber • Intra-ocular lens • Vitreous clarity • Measurement error inherent in the system may also affect the repeatability of measurements Montes -Mico et al IOVS 46:12:4468-44742005 Nanavaty et al Ophthalmogica in press

  4. Optical Quality Analysis System II (OQAS) (Visionmetrics, Spain) • Uses double-pass measurement system • Images point spread function • Derives MTF, Strehl ratio, light scatter (objective scatter index - OSI) MTF - the ratio between contrast in the retinal image and contrast of the original object. OSI - objective parameter calculated by evaluating the amount of light on the outside of the double-pass image in relation to the amount of light on the centre. Higher OSI value relates to more intraocular scattering. Strehl ratio - measurement of an optical system (ratio of the MTF to that of a diffraction limited system). Thus, Strehl ratio will be a figure between 0 and 1, the higher its value, the better

  5. Methods • Prospective reproducibility study Population • Normal pseudophakic eyes, • At least three months post-cataract surgery Measures • Refraction • OQAS measurements used customised OQAS II machine

  6. Methods • Refraction - to obtain the cylindrical correction to optimise measurement. • OQAS II - Pupils were dilated and measurements made for a 4mm exit pupil diameter. • 3 sets of measurements of OSI and Strehl ratio from a single eye at one visit were taken and the average calculated. This was repeated one to two weeks later. • Correlation plots and Bland-Altman analyses were used to assess and compare the 2 sets of measurements.

  7. Results • 51 eyes from 29 patients were assessed, with a heterogenous group of intraocular lenses: • 20 eyes with an Alcon SA60AT • 5 eyes with an Akreos MI60, • 6 eyes with an Acrismart 36a IOL, • 6 eyes with an Alcon SA60WF • 6 eyes with the Morcher 89A ‘Bag in the Lens’ • 2 eyes with Crystalens HD intra-ocular lens. • 6 eyes with unknown IOL type

  8. Results Correlation of Strehl ratio values R2 = 0.78 Average of 1st reading Average of 2nd reading

  9. Results Correlation of OSI values R2 = 0.95 Average of 1st reading Average of 2nd reading

  10. Results Bland-Altman Plot of Strehl ratio Difference Strehl ratio Standard Deviation = 0.017 Coefficient of Repeatability = 0.033

  11. Results Bland-Altman Plot of OSI Difference Ocular Scatter Index Standard Deviation = 0.79 Coefficient of Repeatability = 1.56

  12. Conclusion • Optical measurements using a double-pass system are reproducible despites concerns of changes in the anterior segment between visits.

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