Reproducibility of Optical Measurements in Pseudophakic eyes Using Double-Pass Measurement System St Thomas Hospital, London Anish Dhital, David Spalton,

Reproducibility of Optical Measurements in Pseudophakic eyes Using Double-Pass

Measurement System

St Thomas Hospital, London

Anish Dhital, David Spalton, Kavita Gala

ASCRS 2010No Relevant Financial Disclosure

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

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 46:12:4468-44742005Nanavaty et al Ophthalmogica in press

Optical Quality Analysis System II (OQAS)

• Uses double-pass measurement system

• Images point spread function• Derives MTF, Strehl ratio, light

scatter (objective scatter index - OSI)

(Visionmetrics, Spain)

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

Methods • Prospective reproducibility study

Population • Normal pseudophakic eyes,• At least three months post-cataract surgery

Measures• Refraction• OQAS measurements used customised OQAS II

machine

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.

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

Results

0

0.05

0.1

0.15

0.2

0.25

0 0.05 0.1 0.15 0.2 0.25

R2 = 0.78

Average of 2nd reading

Average of 1st reading

Correlation of Strehl ratio values

Results

0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16

Correlation of OSI values

R2 = 0.95

Average of 1st reading

Average of 2nd reading

Results Bland-Altman Plot of Strehl ratio

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

-0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3

Coefficient of Repeatability = 0.033

Strehl ratio

Difference

Standard Deviation = 0.017

Results Bland-Altman Plot of OSI

Difference

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10 12 14 16 18

Ocular Scatter Index

Coefficient of Repeatability = 1.56

Standard Deviation = 0.79

Conclusion

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