A broad overview of ultra widefield imaging in the field of diabetic retinopathy

Since the advent of human flash fundus photography in the 19^th century, substantial advances have been made in the area of fundus imaging. A traditional fundus camera is unable to capture images from the posterior pole, covering a field of view between 20-50 degrees. This part of the fundus is the place for the maximum ocular diseases such as glaucoma, optic neuropathy and macular degeneration. In addition to this, the images obtained from the macula and optic nerve are frequently used for diagnosing, managing and following up diabetic retinopathy and retinal vascular occlusions. There are three methods to obtain widefield images– creating montage images, using a wide angled camera specially designed for the purpose or using a special lens on a traditional fundus camera. Montage images visualise approximately 75 degrees of the retina with the help of 30-degree images. Early Treatment Diabetic Retinopathy Study group introduced stereoscopic colour fundus photography in 7 standard fields for detecting and classifying diabetic retinopathy as the gold standard. Although 7 field photography is reliable in assessing diabetic retinopathy, it requires skilled photographers and pharmacological pupil dilation and is quite time-consuming.

Optos pioneered retinal imaging with non-contact laser technology to take ultra-wide images. The 200° field of view provided by Optos images covers approximately 82% of the retinal surface. A recent study found that Optos images cover a much larger retinal surface area and they also show a wider, temporal view of the retina and nasal cavity. Despite the wide field of view possible to achieve with Optos images, a small but critical part of the peripheral retina may be completely obscured in the axial images, especially in the vertical meridian. This could possibly be due to the inherent limitations in imaging the peripheral retina, an obscuration by either the pupil or eyelid or a combination of both. Thus, a special software was developed by Optus to merge axial images with those obtained by steering to a different gaze. This image then covers the entire field view of the retina. This has proved vital in obtaining normative data that serves as a potential reference for future studies. Significant differences in the optic disk to periphery distance based on the quadrant and age were found. It thereby made it possible to calculate lesion areas in anatomically correct physical units.

Visualising the peripheral retina with ultra widefield imaging has led to a revolution in the assessment of diabetic retinopathy. Multiple studies conducted have revealed that ultra widefield imaging could demonstrate diabetic retinopathy (including neovascularisation and non-perfusion) not visible in a 7 field standard view in around 10% of the eyes surveyed. They also showed that additional peripheral lesions that were identified by ultra widefield images led to a more severe assessment of DR in 10% of the eyes than was identified by the ETDRS. Recent studies have probed the role of ultra widefield FA imaging in diabetic macular oedema cases. There seems to be a significant correlation between peripheral retinal ischemia and diabetic macular oedema in ultra widefield FA images. The association of peripheral lesions in ultra widefield FA images of diabetic retinopathy and the severity was also observed. Predominantly peripheral lesions were linked to an increased non-perfusion index and non-perfusion area.

Future research might highlight the link of peripheral diabetic lesions with the macular oedema and diabetic retinopathy stage. The relationship between the extent peripheral non-perfusion and presence with various kinds of diabetic macular oedema could also be a topic of potential further studies. This is very important for patients suffering from chronic persistent macular oedema. Various results have been found after laser photocoagulation of the ischemic area of eyes with diabetic retinopathy and retinal vein occlusion. Additional studies are recommended to prove the role of ultra widefield guided peripheral laser photocoagulation in the treatment of diabetic macular oedema. The automatic detection of diabetic retinopathy with standard fundus photography has been studied before with specificity for posterior lesions and high lesions. Future studies could focus on automated detection of peripheral lesions on ultra widefield images. Ultra wildfield imaging dramatically improves the peripheral diabetic retinopathy lesions to be detected. Diabetic Retinopathy Research Network protocol AA hopes to evaluate the link of FA images with the progression of the diabetic retinopathy and peripheral findings on ultra widefield colour. The results of this study are anticipated in the year 2020.