Objective: To identify fundus characteristics associated with intraocular and intraindividual variations in the distribution of macular oedema in patients wit h diabetes. Methods: A review was carried out of fundus photographs and fluoresc ein angiograms from 226 diabetes patients who received photocoagulation treatmen t for macular oedema. Cases with strictly unilateral clinically significantmacul ar oedemawere identified. Results: Strictly unilateral macular oedema was identi fied in five patients with non-prolifer- ativ-ediabetic retinopathy, three of whom demonstrated angiographic leakage c onfined to a single venous drainage unit, and two of whom demonstrated leakage c onfined to two adjacent venous drainage units opposing one another on either sid e of the temporal circulatory watershed. Involved drainage units were delimited by arteriovenous crossings displaying signs of venous compression. Affected eyes had more frequent and more severe crossing signs involving macular drainage tha n fellow eyes. Although all patients had been examined regularly, no patient had had branch retinal vein occlusion and no patient developed such occlusion durin g 5 years of follow-up. Conclusion: In the present study, unilateral macular oe dema in patients with diabetes was associated with angiographic leakage from ven ous drainage units where compression signs indicated a higher than normal likeli hood of upstream congestion being present. Presumably, a modest increase in veno us pressure induced macular oedema because of an underlying abnormal vascular vu lnerability induced by diabetes.
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