A Randomized Clinical Trial of Intravitreal dexamethasOne Versus Bevacizumab in Aboriginal and Torres Strait Islander patientS With Diabetic Macular Oedema (The OASIS Study)

The prevalence of self-reported DM in Aboriginal Australians is reported to be as high as 38%. Despite gradual improvements in underlying social determinants of health, the high morbidity and mortality attributed to DM in Aboriginal populations indicates significant ongoing issues with adherence to screening and treatment regimens. The greater prevalence of DM in the Aboriginal Australian population would be expected to account (at least in part) for the observed complication rates, including DR.

DMO is characterised by swelling of the central retina. The hypoxic retinal conditions in diabetic individuals result in structural changes in the vessel walls and a functional impairment of the blood-retinal barrier. The resultant increase in vascular permeability causes retinal oedema, and loss of central vision ensues when oedema involves the macula. Treatment is aimed at reducing visual loss by targeting factors involved in the activated hypoxia pathway, or with laser targeting dysfunctional blood vessels to limit leakage. Laser was the first treatment shown to effectively reduce DMO and improve vision; however, it cannot be applied to the very centre of the macula. More recently, DMO has been shown to respond to intraocular injections with anti-VEGF agents (bevacizumab, ranibizumab, and aflibercept), reducing reliance on laser treatments.

Corticosteroids are anti-inflammatory agents with anti-VEGF and anti-proliferative effects. Unfortunately, the increased rates of cataract and elevated IOP are the main adverse effects of the IVT corticosteroid treatments, including triamcinolone, making this a less-appealing option than anti-VEGF agents. However, their efficacy has been demonstrated in a subgroup of pseudophakic patients with DMO, where triamcinolone plus laser treatment was shown to be superior to laser treatment alone, and equivalent to ranibizumab (alone or with laser treatment). First-line treatment with triamcinolone is also the most cost-effective option for pseudophakic patients. Thus, IVT triamcinolone is considered one of the effective adjunct modalities for the treatment of DMO and has emerged as an alternative therapy to anti-VEGF agents for persistent or refractory DMO.

Ozurdex (Allergan, Irvine, CA, United States) is a unique biodegradable dexamethasone IVT implant. This slow-release preparation of dexamethasone (a highly potent steroid with a short half-life) has greater long-term efficacy than conventional forms of IVT triamcinolone, with the IVT concentration peaking within 3 months and sustained for up to 6 months post injection. This translates clinically to less frequent injections than conventional treatment with monthly IVT triamcinolone. The geography and population being studied in this trial create some unique challenges, which demand a more flexible study protocol. Longer-acting IVT agents such as Ozurdex have the potential to significantly improve DMO-associated visual morbidity with greater feasibility when used for Aboriginal patients with or at risk of DMO.