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Background
Diabetic retinopathy is a highly specific vascular complication of both type 1 and type 2 diabetes. The prevalence of retinopathy is strongly related to the duration of diabetes. After 20 years of diabetes, nearly all patients with type 1 diabetes and more than 60 % of patients with type 2 diabetes have some degree of retinopathy. Diabetic retinopathy poses a serious threat to vision. Overall, diabetic retinopathy is estimated to be the most frequent cause of new cases of blindness among adults aged 20 to 74 years.
Vision loss due to diabetic retinopathy results from several mechanisms. First, macular edema or capillary non-perfusion may impair central vision. Second, the new blood vessels of proliferative diabetic retinopathy and contraction of the accompanying fibrous tissue can distort the retina and lead to tractional retinal detachment, producing severe and often irreversible vision loss. Third, the new blood vessels may bleed, adding the further complication of pre-retinal or vitreous hemorrhage.
One of the main motivations for screening for diabetic retinopathy is the established efficacy of laser photocoagulation surgery in preventing visual loss. Two large National Institutes of Health sponsored trials, the Diabetic Retinopathy Study and the Early Treatment Diabetic Retinopathy Study, provide the strongest support for the therapeutic benefit of photocoagulation surgery. Timely laser photocoagulation therapy can prevent loss of vision in a large proportion of patients with severe diabetic retinopathy and/or macular edema. Since some patients with vision-threatening pathologies may not have symptoms, ongoing evaluation for retinopathy is a valuable and required strategy.
The American Diabetes Association recommends retinopathy screening with yearly retinal examinations beginning at the time of diagnosis of diabetes for all patients age 30 years and older. For patients under age 30 years, annual retinal examinations are recommended beginning within 3 to 5 years after diagnosis of diabetes once the patient is 10 years old or older.
Diabetic retinopathy telescreening systems involve taking digital pictures of the retina of diabetic patients in the primary care physician's office, and electronically transmitting these pictures to a reading center for evaluation for diabetic retinopathy and macular edema by trained non-physician technicians. Because diabetic retinopathy telescreening can be performed in conjunction with a primary care physician office visit without referral to an ophthalmologist or optometrist, these systems have the potential to improve compliance with retinopathy screening. A cost-effectiveness analysis performed by the British National Health Service Centre for Reviews and Dissemination concluded that screening using a digital camera may be more accurate than screening by the general practitioner, and offers an opportunity to reduce costs of diabetic screening, especially as the costs of digital cameras come down. The UK NHS National Coordinating Centre for Health Technology Assessment (NCCHTA) has initiated a primary research project on the value of digital imaging in diabetic retinopathy.
The Inoveon System of retinopathy screening (iScore, Inoveon Corp., Oklahoma City, OK) involves 7-standard field stereoscopic 30° digital fundus photographs through dilated pupils obtained by a trained photographer located in or near the primary care physician's office, electronic transmission of these digital photographs, examination and grading of these images by non-physician technicians, and rereading of a selected sample of images for assessment of inter- and intra-rater reliability. In addition, any image sets with questionable pathology or non-typical findings are referred to Inoveon's ophthalmologist medical director for secondary evaluations following initial technician reader evaluation. If images are not adequate to allow the technician readers to make an assessment of the patient's diabetic retinopathy or macular edema status, Inoveon recommends to the primary care physician that the patient be referred for further evaluation by an ophthalmologist or optometrist. According to Inoveon Corp., these quality assurance protocols meet or exceed HEDIS specifications for reading centers providing diabetic retinopathy evaluation services.
In a study comparing high-resolution digital stereoscopic fundus photographs (Inoveon System) to plain film stereoscopic fundus photographs (the gold standard), Fransen et al (2002) reported that the digital photographs provided highly accurate diabetic retinopathy referral decisions. Seven standard field stereoscopic retinal photographs were obtained in 290 adult patients with diabetes by a trained photographer using both a 35-mm plain film camera and a digital camera. In this double-masked study, each image was independently graded by trained technicians for retinopathy severity (ETDRS severity scale) and for macular edema. A third technician was used to adjudicate any discrepancies between independent readings. The primary endpoint was the detection of threshold events requiring referral, which was defined as an ETDRS retinopathy severity level greater than 52, questionable or definite clinically significant macular edema, or ungradable images. The prevalence of threshold events in the study population was 19.3 %. The investigators found that the sensitivity of the digital photography system in detecting threshold events, compared to plain film photography, was 98.2 % (confidence interval [CI]: 90.5 % to 100%) and the specificity was 89.7 % (CI: 85.1 % to 93.3 %). The positive-predictive value was 69.5 % and the negative-predictive value was 99.5 % for this sample.
Rudnisky and colleagues (2002) found high-resolution stereoscopic digital photography comparable to contact lens biomicroscopy in diagnosing clinically significant macular edema. A total of 120 patients with diabetes underwent clinical examination with contact lens biomicroscopy by a retinal specialist (the gold standard), and on the same day received digital photographs of the macula. The stereoscopic digital images were evaluated by a single masked grader for the presence or absence of macular edema. Agreement between digital photographs and contact lens biomicroscopy was 83.6 % for clinically significant macular edema (CSME), 83.6 % for CSME type 1, 96.1 % for CSME type 2, 88.5 % for CSME type 3, 75 % for macular edema, 77.9 % for microaneurysms, 83.7 % for intraretinal hemorrhage, and 73.1 % for hard exudates. Sensitivity for CSME overall was 90.6 %. Specificity ranged from 90.0 % for macular edema to 99.0 % for CSME type 2.
The DigiScope Diabetic Retinal Evaluation Service (EyeTel Imaging Corp., Centreville, VA) employs a DigiScope, a specialized digital camera, to obtain high-resolution, wide-field stereoscopic digital images of the retina through dilated pupils. Trained office personnel use the DigiScope to obtain retinal images. The DigiScope automatically centers on the pupil, illuminates, focuses, and estimates visual acuity. The DigiScope images 15 slightly overlapping fields providing a 55 to 60 degree overall view that centered on the macula. The images are transmitted over phone lines to a central reading center, where the images are evaluated for diabetic retinopathy and macular edema by trained technicians. The findings are transmitted to the physician and patient.
An image validation study has demonstrated high correlations between the DigiScope and 7-field stereo color fundus photography as a gold standard (Schiffman et al, 2005). In a masked prospective study, 111 patients with diabetes (222 eyes) were imaged with the DigiScope and with 7-field stereo color fundus photography. There was close agreement between the DigiScope and 7-field stereo color fundus photography between “no diabetic retinopathy” and “any diabetic retinopathy” (Kappa statistic 0.97 for the right eye (OD) and 0.94 for the left eye (OS)). This was reflected in very high sensitivities (0.99 OD, 1.00 OS) and specificities (1.00 OD, 0.92 OS). As referral on the basis of any retinopathy, no matter how mild, may result in an unnecessarily high number of referrals, the study evaluated a second threshold level of very mild non-proliferative diabetic retinopathy to reduce the number of unnecessary referrals. Using this threshold, there was substantial agreement based on “microaneurysms or less retinopathy” (which includes no diabetic abnormalities and microaneurysms only) versus retinal hemorrhages or worse retinopathy" (Kappa stastistic 0.78 OD, 0.88 OS), with corresponding sensitivities (0.95 OD, 0.98 OS) and specificities (0.81 OD, 0.87 OS). The investigators concluded that this image validation study showed that the DigiScope has excellent agreement, sensitivity, and specificity compared with the “gold-standard” 7-field color stereo photography for identifying patients with any or low levels of diabetic retinopathy who should be under the care of an ophthalmologist. The authors noted, however, that the DigiScope is not designed as a diabetic retinopathy disease management tool or to replace a comprehensive eye examination.
Recent techniques permit the acquisition of high-quality photographs through undilated pupils and the acquisition of images in digital format. Although this may eventually permit undilated photographic retinopathy screening, no rigorous studies to date validate the equivalence of these photographs with 7-standard field stereoscopic 30° fundus photography for assessing diabetic retinopathy. The use of the non-mydriatic camera for follow-up of patients with diabetes in the physician's office might be considered only in situations where dilated eye examinations can not be obtained.
Salcone et al (2010) stated that retinopathy of prematurity (ROP) is a vision-threatening vaso-proliferative condition of premature infants worldwide. As survival rates of younger and smaller infants improve, more babies are at risk for the development of ROP and blindness. Meanwhile, fewer ophthalmologists are available for bedside indirect ophthalmoscopy screening examinations. Remote digital imaging is a promising method with which to identify those infants with treatment-requiring or referral-warranted ROP quickly and accurately, and may help circumvent issues regarding the limited availability of ROP screening providers. The Retcam imaging system is the most common system for fundus photography, with which high-quality photographs can be obtained by trained non-physician personnel and evaluated by a remote expert. It has been shown to have high reliability and accuracy in detecting referral-warranted ROP, particularly at later post-menstrual ages. Additionally, the method is generally well-received by parents and is highly cost-effective.
An UpToDate review on "Retinopathy of prematurity" (Paysse, 2012) does not mention the use of digital imaging or fundus photography. It states that "screening evaluation consists of a comprehensive eye examination performed by an ophthalmologist with expertise in neonatal disorders".
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