Aims and objectives
The primary aim the study was to determine the best method for detecting potentially sight-threatening MO in people with diabetes using photographic surrogate markers within the English and Scottish national screening programmes. Specifically we wished to:
(a) investigate whether or not particular distributions and combinations of lesions [microaneurysms/dot haemorrhages (M/DHs), blot haemorrhages (BHs) and exudates], assessed manually or automatically, are more specific photographic surrogate markers of MO than current practice, using OCT as the reference standard
(b) assess the costs and consequences of using alternative distributions and combinations of these lesions to screen for MO, using either automated or manual detection of lesions
(c) model the long-term cost and quality-of-life implications of using alternative distributions and combinations of surrogate markers to screen for MO.
Once the study was under way several screening programmes were found to be using OCT as part of the screening pathway to reduce false-positive referrals to the hospital eye service. Consequently, we added a further aim to assess the costs and consequences of using OCT within retinal screening programmes in addition to improving the photographic surrogate markers, as this would affect how photographic markers would be used in future.
Primary outcome measure
The primary outcome was the sensitivity and specificity of manual grading, computer-assisted manual annotation grading and fully automated annotation grading strategies, utilising photographic lesions to infer the presence of diabetic MO, compared with a reference standard based on the detection of diabetic MO using OCT.
Other outcome measures
The sensitivity and specificity estimates were used to assess the costs and consequences (i.e. the proportion of appropriate ophthalmology referrals) of using the alternative grading strategies for the detection of MO. The long-term costs and outcomes [visual loss and quality-adjusted life-years (QALYs)] of the alternative grading strategies were modelled using data from the epidemiological literature and available cost estimates. The effect of optionally including OCT within the screening pathway was also modelled.
Study design
The study was a multicentre prospective observational cohort study. The cohort consisted of subjects with features of diabetic retinopathy visible within the macular region attending one of seven diabetic retinal screening programmes. The specific diabetic retinopathy features of interest as surrogate markers for MO were M/DHs, BHs and exudates. All subjects were recruited and imaged at the participating centres in Aberdeen, Birmingham, Dundee, Dunfermline, Edinburgh, Liverpool and Oxford.
Main exclusion criteria
Any macular or pan-retinal laser treatment in the study eye(s) or any intraocular injection, since these interventions affect disease progression and characteristics.
Intraocular surgery (e.g. cataract surgery) within 1 year of enrolment. Cystoid MO following cataract surgery, also known as Irvine–Gass syndrome, is the most common cause of decreased vision following cataract surgery.
24Pregnancy. During pregnancy diabetic retinopathy can change much more rapidly.
25–27An inadequate OCT image or two inadequate retinal photographs. Photographs are considered inadequate if either (1) the clarity is insufficient as the macular vessels are not clearly visible or (2) the field of view does not include a circular region with a radius of at least two disc diameters centred on the fovea.
Contraindications to pupillary dilatation should pupil dilatation be necessary. Pharmacological pupillary dilatation is necessary where the pupil is too small to allow imaging, either retinal photography or OCT. Typically the pupil diameter must exceed 4 mm to allow imaging.
Reference standard for macular oedema
Optical coherence tomography was chosen as the reference standard for identifying the presence of diabetic MO.1 Although other technologies are available for measuring retinal thickness (see Appendix 1), OCT has the highest resolution, good repeatability, and is the only method that provides detailed anatomical cross-sections.
Rationale for the macular oedema reference
The reference standard should identify all the cases with MO that would benefit from assessment (though not necessarily treatment) by an ophthalmologist in the eye clinic. The first large study to investigate MO (in the context of determining the efficacy of laser treatment for potentially sight-threatening retinopathy) was the ETDRS.28 Stereoscopic fundus photography was used to determine whether or not retinal thickening was present and the following criteria were used to define clinically significant MO.
Thickening within 500 µm of the centre of the macula.
Exudates within 500 µm of centre of macula with adjacent thickening.
Thickening of one disc area or larger where any part is within one disc diameter of the centre of the macula.
Optical coherence tomography now provides a more sensitive and specific test for MO than was available for the ETDRS. The ETDRS defined a circular grid consisting of nine regions4 (see ) where the central region has a radius of 500 µm for the ETDRS central thickening region and the surrounding four regions have a diameter of a DD for the non-central thickening region. All current OCT scanners generate retinal thickness values for these regions of interest. Many studies have investigated the normal range of retinal thickness in these regions of interest. Subjects with abnormally thickened retinas may be selected based on a thickness threshold. For the Zeiss Stratus OCT™ (Carl Zeiss Meditec International, Jena, Germany) studies have shown that biomicroscopy is able to reliably identify central thicknesses > 300 µm, and less reliably detect thicknesses between 250 and 300 µm. Setting a thickness threshold of 250 µm should therefore ensure that the majority of biomicroscopy-positive cases with central thickening will be included in the study. A Diabetic Retinopathy Clinical Research Network study that used Zeiss Stratus OCT selected eyes with previously untreated MO that were characterised by a central subfield mean thickness of at least 250 µm or an inner paracentral subfield mean thickness of at least 300 µm:29 these thickness thresholds were used in this study. However, as different OCT scanners were used at the various centres it was necessary to correct for possible differences in thickness measurements, see Chapter 3.
Thickening seen on OCT, however, is not specific to diabetic retinopathy. When an abnormal thickness measure was found within the inner five ETDRS regions the OCT cross-sections were examined for intraretinal cysts or subretinal fluid. This is similar to the grading protocol in the Diabetic Retinopathy Clinical Research Network study29 which states: ‘Retinal morphology was assessed at baseline from OCT images for cystoid abnormalities and subretinal fluid’. Although the Diabetic Retinopathy Clinical Research Network study29 used the selection criterion of a central field thickness threshold of at least 275 µm rather than the more common 250 µm. Other studies note that non-diabetic pathology is specifically excluded from the study (e.g. the RESTORE study22 and the READ-2 study21).
In this study we used the Diabetic Retinopathy Clinical Research Network criteria for defining MO.29 MO was deemed to be present where the following two criteria were met:
The central ETDRS region thickness was > 250 µm, or any of the inner five regions were > 300 µm.
A visible intraretinal cyst or area of subretinal fluid on the OCT cross-sections.
Sample size
A previous study found a 14% prevalence of macular lesions within a screening programme,30 the subjects making up the target cohort for this study. Of those with macular lesions, 10% are expected to have MO. Thus, the majority of the study cohort will not have MO and hence the precision of the sensitivity measurement will be the determining factor in the calculation of study power and sample size.
To detect a 3% difference in sensitivity between two diagnostic tests with 80% power using the McNemar test requires a sample of 400 positive cases. Using the assumed prevalence of positive cases above of 10% this would mean recruiting 4000 subjects.
Regulatory approval
The study was approved by the North East Scotland Research Ethics Committee on 17 December 2007 (reference 07/S0801/107). Each participating centre obtained approval from their local ethics board. The study was cosponsored by the University of Aberdeen and NHS Grampian. All participants provided signed informed consent after reading the patient information sheet and following discussion with the local study representative. A Trial Steering Committee oversaw the conduct of the study. The study was registered with the United Kingdom Clinical Research Network (UKCRN reference 9063).
Recruitment and data collection
All patients were recruited at one of seven study centres in Aberdeen, Birmingham, Dundee, Dunfermline, Edinburgh, Liverpool and Oxford.
Visual acuity
Visual acuity was measured using either best corrected visual acuity or a pin hole (the method used was noted). Visual acuity was recorded using the log-MAR scale. Where subjects could not resolve characters on the vision chart they were asked if they could count fingers, see hand movement or perceive light, otherwise the eye was recorded as having no perception of light. Count fingers was assigned a log-MAR value of 2, and hand movement a log-MAR value of 3.31 Where vision is worse than hand movement the subject is unable to resolve any object and therefore the visual acuity is undefined.
Pupillary dilatation
In the English centres all patients received mydriasis, while Scottish centres only used mydriasis if the pupil size was too small for imaging. Both retinal photograph and OCT scanning require a pupil size of at least 4 mm. Studies have shown that the use of pupillary dilatation does not affect OCT thickness measurements.32,33
Retinal photograph acquisition
All photographs were acquired by ophthalmic photographers or retinal screeners and the name of the photographer was noted for each image. A single retinal photograph was acquired for each eye meeting the following criteria:
45 degrees field of view
macula centred
colour digital photograph (between 3 and 8 megapixels)
JPEG image compression (if used) set for high quality
adequate field of view (the image should show a region having a radius of at least two disc diameters around the fovea)
adequate clarity (i.e. adequate to see macular microaneurysms, if present)
the fundus camera small pupil facility was acceptable providing a region of at least two DD was still visible around the fovea.
Optical coherence tomography reference image acquisition
All OCT scans were acquired by operators who had been accredited for the study. There was a maximum time limit of 4 weeks between the retinal photograph and OCT reference scan as the disease was unlikely to progress significantly during this period of time. Eighty-nine per cent of OCT scans were acquired on the same day as the retinal photograph.
Accreditation
As in other multicentre imaging studies, to avoid intercentre variation all OCT operators were required to be accredited before submitting data for the study. Operators submitted a portfolio that included the following images, collected using the OCT scanner they would be using for the study:
Normal eye Repeat macula maps of the same normal eye as per scanner model protocol.
MO eye Repeat macula maps of an eye showing obvious MO (i.e. central thickness of at least 300 µm).
The images were uploaded using the same website as the study data. Images were checked for:
Foveal position (where visible) The foveal minimum should be within 250 µm of the centre of the thickness map.
Adequate image quality Image quality was assessed by visual inspection as well as quantitative parameters such as signal strength and standard deviation (SD) in the central measurement, where present.
Repeatability Repeatability was assessed for each region as the absolute percentage difference between the repeat scans. These should be < 10% in all regions.
Optical coherence tomography data description
Although the precise OCT acquisition protocol depended on the model of scanner used, every scanner was set up to provide the following data.
A nine-region (ETDRS style) map showing average regional thickness measured in microns.
A horizontal cross-sectional view through the centre of the macula.
(Optional.) If the cross-sectional view through the centre of the macula did not contain the region of greatest thickening then a second cross-section that included the region of greatest thickening was taken.
Other recorded patient information
No patient-identifiable information left the recruiting centre. Patient identifiers were removed from images and data such as subject age were recorded with insufficient granularity to be of help in identification. In addition to the retinal photograph, OCT scan and visual acuity assessment the following subject information was collected:
Amblyopia is an uncorrectable decrease in vision in one eye with no apparent structural abnormality seen to explain it. It is a diagnosis of exclusion, meaning that when a decrease in vision is detected, other causes must be ruled out. There is wide variation in reported estimates for its prevalence. A review of UK amblyopia studies in children aged ≤ 5 years assumed a prevalence of 4.8%.34 A cohort study at an English retinal screening programme recorded a 10% amblyopia prevalence.35 Since visual acuity is used as an indicator of MO in some screening programmes, amblyopia represents a confounding factor in the detection of MO. Other non-diabetic factors affecting visual acuity, such as lenticular opacities and macular degeneration, were not recorded separately as, unlike amblyopia, they directly affect the appearance or quality of the retinal photograph:
type of diabetes (the type of diabetes was recorded as type 1, type 2, secondary or other).
ethnicity (the ethnicity categories were Asian, Black, Caucasian, Chinese, mixed, other, unknown)
first half of postcode
glitazone use within the previous 6 months.
Glitazones, or thiazolidinediones, are a group of drugs that are prescribed to increase sensitivity to insulin in people with type 2 diabetes. The most common forms are rosiglitazone and pioglitazone. Rosiglitazone had its marketing authorisation suspended during the study in September 2010 by the European Medicines Agency because of concerns regarding an increased cardiovascular risk. Oedema is a known risk factor when using glitazones and studies have suggested they are associated with an increased risk of MO,34–36 although another study found no association.37 The recording of the use of glitazones was added to the study protocol in June 2009 because of the uncertainty surrounding their role in the development of MO.
Web submission and data validation
A website was developed to enable the transfer of both subject information and image data from each recruiting centre to the database in Aberdeen. The website also allocated the unique study identifier for each subject and printed out a reference sheet for the local study folder.
The website was designed to reduce data entry errors. Tick boxes, menus and calendar entry tools were used in preference to plain text entry (). Checks were performed on the entered data, for instance that the subject was aged ≥ 18 years and therefore eligible for the study. Checks were also performed on the image data that were uploaded to ensure that the images were the expected size and format for the OCT scanner used at that centre.
Web form used to upload study data.
Check sums were calculated for all images when they were uploaded. This served two purposes: first, it allowed the continuing integrity of the image data to be verified and, second, it enabled attempted duplicate image uploads to be rejected.
Image data analysis and grading
Retinal photographs
All the retinal photographs were graded and annotated by the same research nurse, who had 3 years' experience working as a retinal grader. A screenshot of the software used for grading is shown in .
Screenshot of the software used to grade images.
Image grading
All of the images were graded for image quality and severity of retinopathy and maculopathy following the Scottish Diabetic Retinopathy Grading Scheme.38
Image annotation
Software was developed to enable the research nurse to annotate the retinal images, indicating the size and position of the optic disc, the position of the fovea, and the location, shape and size of individual lesions within two DD of the fovea (). All the lesions associated with maculopathy (M/DHs, BHs and exudates) were annotated, as well as non-diabetic features with similar appearance which could confound the analysis [flame haemorrhages, drusen and cotton wool spots (CWS)].
Screenshot of software used to annotate features on the retinal photographs.
Optical coherence tomography reference standard
Optical coherence tomography scans
The OCT scan was the study reference standard for determining whether or not oedema was present. The reference was based on the nine region thickness map and visual inspection of the cross-sectional images.
Thickness map
Each OCT scanner in the study displays the thickness map on a graphical report. Optical character recognition software was used to automatically store the thickness values in the database.
The retinal thickness was considered abnormal if the centre region thickness was > 250 µm or > 300 µm in any of the surrounding four regions (the outer four regions outside a 3 mm radius were not used). These thickness thresholds were taken from studies that used the Zeiss Stratus OCT, and were adjusted to account for the scanners used in the study (see Chapter 3). Images with abnormal thickening were then visually inspected for the presence of oedema.
Cross-sectional images
The cross-sectional images were visually assessed for the presence of oedema because several pathologies, besides diabetic MO, could affect retinal thickness. Confounding features include vitreo-macular traction, epiretinal membranes, macular holes, drusen, subretinal haemorrhage, pigment epithelial detachment and choroidal neovascular membrane. Example OCT thickness map and cross-sections are shown in .
Example of OCT cross-sections and thickness map (report from Zeiss Cirrus OCT).
