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Original Paper

Ophthalmologica 2011;225:61–66 DOI: 10.1159/000316690

Use of the Double-Pass Technique to Quantify Ocular Scatter in Patients with Uveitis: A Pilot Study

Mayank A. Nanavaty Miles R. Stanford Rohit Sharma Anish Dhital David J. Spalton John Marshall

Department of Ophthalmology, St. Thomas’ Hospital, London , UK

Introduction

Breakdown of the blood-aqueous barrier leads to leak-age of protein and cells into the ocular media, which is a cause of light scatter in eyes with uveitis. Intraocular scat-tering affects image quality [1] and thereby reduces vi-sion. Quantification of these changes is important in the understanding of the disease process, response to treat-ment and its complications. Several methods have been proposed to estimate scatter, including psychophysical techniques [2] , and others based on Scheimpflug imag-ing, dynamic light scattering [3] or the analysis of Hart-mann-Shack images [4] .

The Hartmann-Shack aberrometric system consists of a microlens array conjugated with the eye’s pupil and a camera placed at its focal plane [5] . If a plane wavefront reaches the microlens array, the camera records a per-fectly regular mosaic of spots. However, if a distorted wavefront (i.e. wavefront aberration) reaches the sensor, the pattern of spots is irregular. The displacement of each spot is proportional to the derivative of the wavefront over each microlens area. Although the wavefront sen-sors are extremely useful, their main drawback is the lack of information on scattering due to limitations imposed by lens sampling.

The double-pass technique has been proposed to esti-mate retinal image quality for over half a century [6] , and has been well studied over the past few decades [7–12] .

Key Words

Uveitis � Ocular inflammation � Double-pass technique

Abstract

Purpose: To assess whether the double-pass technique can be employed to quantify the amount of light scattering in patients with uveitis. Methods: 56 eyes of 44 patients with intraocular inflammation were consecutively recruited from the uveitis clinic over 9 months. The degree of intraocular in-flammation was recorded according to the Standardization of Uveitis Nomenclature criteria and the eyes were grouped as having anterior, intermediate, posterior or pan-uveitis. Ob-jective scatter index (OSI) was assessed using a double-pass technique with the Optical Quality Analysis System II. Re-

sults: Twenty-four eyes had anterior uveitis, 9 eyes had inter-mediate uveitis, 10 eyes had posterior uveitis and 13 eyes had panuveitis. The OSI was significantly different between all 4 groups (p = 0.0005). The mean OSI was highest in eyes with anterior uveitis (2.6 8 3.1) and lowest in posterior uveitis (1.9 8 1.3). Anterior chamber cells significantly correlated with OSI (R 2 = 0.8726, p = 0.007), unlike posterior chamber cells(R 2 = 0.0189, p = 0.588) and flare (R 2 = 0.0048, p = 0.471). Con-

clusion: Patients with anterior uveitis have more ocular scat-ter, and anterior chamber cells scatter more light. This pilot study opens new avenues for research in use of the double-pass technique to assess light scattering in uveitis.

Copyright © 2010 S. Karger AG, Basel

Received: November 20, 2009 Accepted after revision: January 28, 2010 Published online: August 14, 2010

Ophthalmologica

Mayank A. Nanavaty, DO, MRCOphth, MRCS (Ed) Department of Ophthalmology St. Thomas’ Hospital London SE1 7EH (UK) E-Mail mayank_nanavaty   @   hotmail.com

© 2010 S. Karger AG, Basel0030–3755/11/2251–0061$38.00/0

Accessible online at:www.karger.com/oph

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Ophthalmologica 2011;225:61–66 62

This technique is based on the recording of the retinal image after double-pass through the ocular media and retinal reflection. From the double-pass images, the point spread function (PSF) and the ocular modulation transfer function can be determined. The modulation transfer function provides information on the overall optical performance of the human eye, including all the optical defects involved in retinal image degradation, such as diffraction, aberrations and scattering. This property of the double-pass method renders the ap-proach extremely powerful in many conditions that spe-cifically affect scattering, e.g. eyes with uveitis. Díaz-Doutón et al. [13] indicated that in normal young eyes with very little ocular scatter, Hartmann-Shack and the double-pass technique provide similar estimates of im-age quality, whereas in eyes with severe scattering the double-pass technique accurately quantifies quality of vision. Moreover, investigators in another study involv-ing the double-pass approach have also suggested the po-tential of this technique to demonstrate ocular scatter [14] . Estimates of ocular scatter can be accurately derived from the PSF [15] . In brief, the width of the measurable PSF at the base (approximately lower 10% of measurable PSF) is directly proportional to scatter. The Optical Quality Analysis System (OQAS II; Visionmetrics, Ter-rassa, Spain) is based on the double-pass technique, in-corporating improved features adapted for routine mea-surements in clinical practice.

We designed this study to assess the use of the double-pass technique in quantifying ocular scatter in eyes with uveitis.

Material and Methods

We performed a cross-sectional observational clinical study on patients with intraocular inflammation attending the Uveitis Clinic, Department of Ophthalmology, St. Thomas’ Hospital, London between December 2007 and October 2008. Informed consent was taken from each patient, and the study followed the tenets of the Declaration of Helsinki.

Inclusion criteria were patients with active intraocular in-flammation who presented to the Uveitis Clinic with anterior or posterior chamber cells or flare. Exclusion criteria were patients with media opacification, such as corneal opacity, keratic precip-itates, lenticular opacities, aphakia, pseudophakia and spherical equivalent of ! 1.5 dpt. Patients with posterior synechiae leading to small non-dilating pupils, pupil sizes ! 4 mm, coexisting cor-neal edema and patients with recent intraocular surgery were also excluded.

The patients were assessed by 2 independent observers (M.R.S. and R.S.) in the Uveitis Clinic. The details of each patient who met the inclusion criteria were entered in individual enrolment forms.

After a detailed slit lamp examination, patients were either diag-nosed as having anterior, intermediate, posterior or panuveitis. The quantification of cells in the anterior and posterior chamber was performed by adopting the Standardization of Uveitis No-menclature (SUN) criteria for diagnosis using a slit lamp with the light beam size of 1 ! 1 mm. Flare quantification was also per-formed using the SUN criteria, published elsewhere [16] . One or both eyes of a patient were included, depending on the presence of cells and flare.

The pupils were then dilated and once dilatation was more than 4 mm, the eyes were assessed for ocular light scatter by an observer who was blinded to the details of clinical observation. Light scattering in these patients was assessed using OQAS II and the objective scatter index (OSI) was determined. OQAS II cap-tures the PSF of the eye and computes the OSI mathematically from the PSF.

Clinical and in vitro Analysis to Assess the Validity of the Double-Pass Technique In our ongoing prospective reproducibility study [17] , 38 nor-

mal pseudophakic eyes at least 3 months after successful uncom-plicated phacoemulsification and intraocular lens implantation were examined using the double-pass technique on OQAS II. Re-fraction was performed to obtain the cylindrical correction to op-timize measurements on the double-pass system. To standardize the measurement technique, the pupils were maximally dilated. The OSI and Strehl ratio were measured on the OQAS II machine which has a fixed exit pupil size of 4 mm. The averages of 3 sets of measurements were considered for analysis. The same measure-ments were repeated 1–2 weeks later to check for reproducibility.

An in vitro experiment was also performed to assess whether the machine measures the OSI accurately and whether the OSI measured by this machine increases proportionally with the den-sity of suspension in fluid. A small square clear glass tube was taken and filled with 5 ml of distill water. This tube was fixed on OQAS II with clear transparent tapes to get five OSI measure-ments. Two drops of milk were added and mixed to this clear dis-till water, and the OSI measurements were repeated 5 times. The same procedure was repeated after adding 5 drops of milk to the distilled water. The mean OSI measurements were calculated at each occasion.

Statistical Analysis All the data was entered in an Excel spreadsheet (Microsoft

Office 2007, Redmond, Wash., USA) and further analyzed using Excel and GraphPad Prism version 3 software (GraphPad Soft-ware, La Jolla, Calif., USA). Normality of the data was checked using the Kolmogorov-Smirnov test. One-way ANOVA with Bartlett’s correction was employed to analyze the difference in OSI between anterior, intermediate, posterior and panuveitis eyes. Pearson correlation was calculated to analyze the correla-tion between cells in anterior or posterior chamber or f lare ver-sus OSI. Correlation coefficient was assessed to check for the reproducibility of OSI and Strehl ratio measurements in our clinical reproducibility study. Values of p ! 0.05 were considered significant.

Double-Pass Technique in Quantifying Ocular Scatter

Ophthalmologica 2011;225:61–66 63

Results

In total, 56 eyes of 44 patients were included. The mean age of the patients was 42 8 17 years (range 14–81 years).

Twenty-four eyes were diagnosed with anterior uveitis, 9 eyes with intermediate uveitis, 10 eyes with posterior uveitis and 13 eyes with panuveitis. The OSI was signifi-cantly different between the groups (p = 0.0005; fig. 1 ). The mean OSI was largest in eyes with anterior uveitis and smallest in eyes with posterior uveitis ( fig. 1 ).

Anterior chamber cells significantly correlated with OSI (p = 0.007; fig. 2 ), whereas posterior chamber cells and flare did not show any correlation ( fig. 3 , 4 ).

In our clinical reproducibility study on normal pseu-dophakic eyes, the correlation coefficient (r) was 0.96 for OSI and 0.84 for the Strehl ratio, with values of p ! 0.0001 for both groups of data. The in vitro experiment to ascer-tain the validity of our measurement technique on OQAS II showed a proportional increase in OSI with increases in particulate suspension. The mean OSI with clear dis-tilled water and after adding 2 and 5 drops of milk were 1, 1.1 and 1.3, respectively.

0

2

4

6

8

10

OS

I

Anterior Intermediate Posterior Panuveitis

p = 0.0005

–2

Fig. 1. Comparison of OSI in the 4 groups. Data presented as means 8 SD; p value calculated by 1-way ANOVA with Bartlett’s correction.

0

2

4

6

8

10

OS

I

Trace +0.5 +1 +2 +3

y = 0.9065x + 0.0494

R = 0.8726, p = 0.0072

Anterior chamber cells

–2

Fig. 2. Correlation of anterior chamber cells and OSI. Data pre-sented as means 8 SD.

0

1

2

3

4

5

6

7

8

9

10

OS

I

Trace +0.5 +1 +2 +3

y = 0.0618x + 1.8019

R = 0.0189, p = 0.5882

Posterior chamber cells

Fig. 3. Correlation of posterior chamber cells and OSI. Data pre-sented as means 8 SD.

0

1

2

3

4

5

6

7

8

9

10O

SI

Trace +0.5 +1 +2 +3

y = –0.0767x + 2.9733

R = 0.0048, p = 0.4712

Flare

Fig. 4. Correlation of flare and OSI. Data presented as means 8 SD.

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Ophthalmologica 2011;225:61–66 64

Discussion

We undertook this prospective cross-sectional obser-vational pilot study to assess the usefulness of the double-pass technique (OQAS II) for quantification of ocular scatter in eyes with uveitis. We found significantly more ocular scatter in eyes with anterior uveitis. There was a strong correlation between ocular scatter and anterior chamber cells as compared to posterior chamber cells or flare.

Quantification of changes in the blood-aqueous bar-rier by quantifying cells and flare during ocular inflam-mation play an important role in the understanding of the inflammatory processes, the assessment of anti-in-flammatory drugs and the management of uveitis. In the past this has been done by a number of methods, includ-ing fluorophotometry which assesses blood aqueous bar-rier breakdown [18, 19] . The need for fluorescein injec-tion and the duration of the test means that this remains as a research tool. Other non-invasive methods described before were not commercially available and they had lim-ited clinical use [20–22] . The Kowa Laser Flare Cell Me-ter, which uses a scanning laser, has been extensively studied as it allows quantification of aqueous flare and cells with the great advantages of rapidity, non-invasive-ness and simplicity of operation [18, 23, 24] . However, none of these methods gives a robust assessment of ocular light scattering. Light scatter in the eye will depend on the scattering angle, which is the angle between the initial and final paths traveled by a scattered particle or photon and the size of the scattering particle. The size of the cells

in uveitic eyes is approximately 15 � m, and hence Scheimpflug imaging cannot detect minor variations in densitometric indices in anterior chamber. With the Hartmannn Shack system, there is lack of information on scattering, due to the limitation imposed by the lenslets in the system [14] .

We used a commercially available instrument, OQAS II, which employs an asymmetric double-pass technique based on recording of the retinal image after double-pass through the ocular media and retinal reflection. This method uses an infrared wavelength of 780 nm, which is more comfortable for the subject and in addition has been shown to provide adequate estimates of the retinal image quality compared with those obtained with visible light [25] . The peak of the PSF obtained on OQAS II is affected by both aberration and scatter and it is difficult to isolate these to see their individual effect on PSF [15] . However, in the peripheral zone of this PSF, there is negligible in-fluence of aberrations if the patient’s lower order aberra-tions are corrected (i.e. spectacle correction) and so all the light in this region is only due to scattering. In addition to the evidence of good repeatability of OSI measure-ments on OQAS II from a recently published study [26] , our study also showed good reproducibility of the mea-surement technique [17] . Our in vitro experiment mea-suring the OSI of clear distilled water, and repeating the measurements by adding milk drops to the solution, showed a linear increase in OSI with increases in par-ticulate suspension of the distilled water. This clearly shows good accuracy of the double-pass technique with OQAS II.

2-mm entry laserbeam

Cornea Lens

Cells in anteriorchamber

Retina

a

2-mm entry laserbeam

Cornea Lens

Retina

Cells in posteriorchamberb

Fig. 5. a First pass in eyes with anterior uveitis showing wider scatter during the first pass as the rays travel lon-ger in eyes with anterior chamber cells. b First pass in eyes with posterior uveitis showing narrower scatter dur-ing the first pass as the rays travel longer in eyes with anterior chamber cells.

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Ophthalmologica 2011;225:61–66 65

Interestingly, we found more ocular scatter in eyes with anterior uveitis and the OSI correlated significantly with anterior chamber cells. This does not correlate with the clinical observation that eyes with posterior uveitis tend to have increased visual loss compared to those with anterior segment inflammation only. This may be be-cause diseases in the posterior segment also affect retinal function. One possible explanation is that in eyes with anterior uveitis during the first pass of the double-pass system, the scattered light after hitting the anterior cham-ber cells will have to travel a longer distance to reach the retina as compared to that in eyes with posterior chamber cells. Therefore, the anterior chamber cells will produce wider scatter during the first pass compared to the pos-terior chamber cells. Hence, in the second pass of the light, the eyes with anterior chamber cells will have wide-ly scattered rays reflecting and hence they will give more scatter compared to the eyes with posterior chamber cells ( fig. 5 ). This is a possible theory, which needs further val-idation with in vitro experiments.

The potential limitation of this technique is that the pupil size should be larger than 4 mm in order to get an accurate measurement. This may not always be possible in patients with uveitis due to the presence of synechiae. Also, the natural pupil size (if ! 4 mm) acts as an addi-tional aperture to the system and may influence the light scattering. This technique computes OSI from the PSF,

and hence it is not measured directly. The scatter derived by this technique may be a combination of forward and backward scatter from the retina and reflections from various ocular structures. Although we excluded patients with a spherical equivalent of 1 1.5 dpt, aphakic and pseu-dophakic eyes, it is possible that some residual aberra-tions in the eye may interfere with the PSF measurements [27] . Nevertheless, this technique can potentially be use-ful for monitoring the severity and progression of the dis-ease and quantifying the response of various treatment modalities in patients with uveitis. Further large-scale in vitro and clinical studies are required to explore this as-sessment technique in eyes with inflammation.

Several objective techniques (like laser flare measure-ments, fluorophotometry, etc.) that can be employed to quantify the clinical signs in patients with uveitis have been described in past, but these techniques are neither simple, quick nor safe. The double-pass technique (OQAS II) may be useful in objective monitoring of the therapeu-tic effects of various anti-inflammatory treatment mo-dalities in inflammatory eye diseases as it is simple, quick and safe. In summary, we have described a novel objective approach to quantify ocular scatter in patients with uve-itis using double-pass technique (OQAS II). This study found more ocular scatter in patients with pure anterior uveitis.

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