Optical coherence tomography measurement of central retinal thickness to diagnose diabetic macular oedema

Background
Diabetic macular oedema (DMO) is a thickening of the central part of the retina, the macula, that may affect people with diabetic retinopathy (DR). Diabetic retinopathy is a complication of diabetes in which the retina (a layer of tissue at the back of the eye) becomes progressively damaged. Diabetic macular oedema is detected by means of visual examination by an ophthalmologist. The most severe form of DMO - clinically significant macular oedema (CSMO) - is associated with sight loss in the long-term. This condition is treatable. Laser photocoagulation (where a laser is used to burn off blood vessels) has been used for many years to reduce the risk of visual loss. More recently, antiangiogenic therapy (which prevents fluid leakage from retinal vessels) has been approved to try to improve vision.

Review question
Optical coherence tomography (OCT) is based on how light is reflected. It can be used to measure retinal thickness. We originally aimed to assess the accuracy of OCT for diagnosing diabetic macular oedema (DMO), as well as to investigate differences in diagnostic performance. However, the role of OCT is expanding so in the update of this review we also aimed to assess whether OCT might be considered the new standard for diagnosing DMO.

Search date
This review is updated as of June 2013.

Study characteristics
Our review included 10 studies (830 participants, 1387 eyes) published between 1998 and 2012. Nine of these studies investigated the ability of OCT to diagnose CSMO.

Study funding sources
There were no overt declarations of potential conflicts of interest in terms of the manufacturer of the OCT device being involved in funding the research.

Key results
We found that OCT retinal thickness measurement is not sufficiently accurate to detect CSMO, involving the centre of the macula, using clinical fundus examination as the reference standard. Of 10 patients with diabetic retinopathy, 5 of whom have CSMO, 1 of 5 with no CSMO would be wrongly diagnosed as having CSMO, and about 1 of 5 with CSMO would be missed.

However, researchers have found that disagreements between OCT and clinical examination occur because OCT can detect early, subclinical retinal thickening in people without CSMO and more advanced retinopathy. They suggested that such cases of subclinical macular oedema are followed more closely, since they are at increased risk of progression to CSMO. Furthermore, OCT is an essential tool to manage antiangiogenic therapy in patients with DMO and is believed by many to be a new reference standard for its diagnosis.

Quality of the evidence
Study quality was often unclear because of incomplete reporting or because it was at risk of bias. Specifically, this concerned how patients were selected in the study, who referred them and how, and exclusion of those for whom poor quality images were obtained. Furthermore, many studies included both patient's eyes, which is a problem in data analyses.

Authors' conclusions: 

Using retinal thickness thresholds lower than 300 µm and ophthalmologist's fundus assessment as reference standard, central retinal thickness measured with OCT was not sufficiently accurate to diagnose the central type of CSMO in patients with DR referred to retina clinics. However, at least OCT false positives are generally cases of subclinical DMO that cannot be detected clinically but still suffer from increased risk of disease progression. Therefore, the increasing availability of OCT devices, together with their precision and the ability to inform on retinal layer structure, now make OCT widely recognised as the new reference standard for assessment of DMO, even in some screening settings. Thus, this review will not be updated further.

Read the full abstract...
Background: 

Diabetic macular oedema (DMO) is a thickening of the central retina, or the macula, and is associated with long-term visual loss in people with diabetic retinopathy (DR). Clinically significant macular oedema (CSMO) is the most severe form of DMO. Almost 30 years ago, the Early Treatment Diabetic Retinopathy Study (ETDRS) found that CSMO, diagnosed by means of stereoscopic fundus photography, leads to moderate visual loss in one of four people within three years. It also showed that grid or focal laser photocoagulation to the macula halves this risk. Recently, intravitreal injection of antiangiogenic drugs has also been used to try to improve vision in people with macular oedema due to DR.

Optical coherence tomography (OCT) is based on optical reflectivity and is able to image retinal thickness and structure producing cross-sectional and three-dimensional images of the central retina. It is widely used because it provides objective and quantitative assessment of macular oedema, unlike the subjectivity of fundus biomicroscopic assessment which is routinely used by ophthalmologists instead of photography. Optical coherence tomography is also used for quantitative follow-up of the effects of treatment of CSMO.

Objectives: 

To determine the diagnostic accuracy of OCT for detecting DMO and CSMO, defined according to ETDRS in 1985, in patients referred to ophthalmologists after DR is detected. In the update of this review we also aimed to assess whether OCT might be considered the new reference standard for detecting DMO.

Search strategy: 

We searched the Cochrane Database of Systematic Reviews (CDSR), the Database of Abstracts of Reviews of Effects (DARE), the Health Technology Assessment Database (HTA) and the NHS Economic Evaluation Database (NHSEED) (The Cochrane Library 2013, Issue 5), Ovid MEDLINE, Ovid MEDLINE In-Process and Other Non-Indexed Citations, Ovid MEDLINE Daily, Ovid OLDMEDLINE (January 1946 to June 2013), EMBASE (January 1950 to June 2013), Web of Science Conference Proceedings Citation Index - Science (CPCI-S) (January 1990 to June 2013), BIOSIS Previews (January 1969 to June 2013), MEDION and the Aggressive Research Intelligence Facility database (ARIF). We did not use any date or language restrictions in the electronic searches for trials. We last searched the electronic databases on 25 June 2013. We checked bibliographies of relevant studies for additional references.

Selection criteria: 

We selected studies that assessed the diagnostic accuracy of any OCT model for detecting DMO or CSMO in patients with DR who were referred to eye clinics. Diabetic macular oedema and CSMO were diagnosed by means of fundus biomicroscopy by ophthalmologists or stereophotography by ophthalmologists or other trained personnel.

Data collection and analysis: 

Three authors independently extracted data on study characteristics and measures of accuracy. We assessed data using random-effects hierarchical sROC meta-analysis models.

Main results: 

We included 10 studies (830 participants, 1387 eyes), published between 1998 and 2012. Prevalence of CSMO was 19% to 65% (median 50%) in nine studies with CSMO as the target condition. Study quality was often unclear or at high risk of bias for QUADAS 2 items, specifically regarding study population selection and the exclusion of participants with poor quality images. Applicablity was unclear in all studies since professionals referring patients and results of prior testing were not reported. There was a specific 'unit of analysis' issue because both eyes of the majority of participants were included in the analyses as if they were independent.

In nine studies providing data on CSMO (759 participants, 1303 eyes), pooled sensitivity was 0.78 (95% confidence interval (CI) 0.72 to 0.83) and specificity was 0.86 (95% CI 0.76 to 0.93). The median central retinal thickness cut-off we selected for data extraction was 250 µm (range 230 µm to 300 µm). Central CSMO was the target condition in all but two studies and thus our results cannot be applied to non-central CSMO.

Data from three studies reporting accuracy for detection of DMO (180 participants, 343 eyes) were not pooled. Sensitivities and specificities were about 0.80 in two studies and were both 1.00 in the third study.

Since this review was conceived, the role of OCT has changed and has become a key ingredient of decision-making at all levels of ophthalmic care in this field. Moreover, disagreements between OCT and fundus examination are informative, especially false positives which are referred to as subclinical DMO and are at higher risk of developing clinical CSMO.

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