Overnight thickness variation in diabetic macular edema.

PURPOSE: To study the evening-to-morning variation in retinal thickness in patients with fovea-involving diabetic macular edema.
METHODS: Twelve eyes in 12 patients aged 39 to 78 years (mean, 57) with fovea-involving diabetic macular edema and 14 eyes in 7 healthy volunteers aged 30 to 70 years (mean, 57) were examined by optical coherence tomography, in the evening and in the morning after > or =6 hours of sleep in the recumbent position in darkness followed by 0.5 hour wakefulness in the same position in room light with both eyes open.
RESULTS: In patients with diabetic macular edema, macular thickness increased overnight, from 316 +/- 72 microm in the evening to 336 +/- 81 microm in the morning (P = 0.003). Visual acuity decreased from a mean of 41 ETDRS letters (Early Treatment of Diabetic Retinopathy Study; range, 4-61) in the evening, to a mean of 36 letters (range, 2-60) in the morning (P = 0.03). No overall change was found in mean arterial blood pressure (MABP; P = 0.48), blood glucose (P = 0.25), or corneal thickness (P = 0.26). The overnight change in macular thickness correlated directly with the change in MABP (r = 0.65, P = 0.03) but not with baseline MABP or blood glucose. The overnight increase in retinal thickness remained significant after statistical adjustment for the effect of arterial blood pressure (P = 0.002). Healthy subjects demonstrated no significant change in any parameter.
CONCLUSIONS: In fovea-involving diabetic macular edema, a reduction in visual acuity accompanies overnight retinal thickening, the magnitude being related to the nocturnal change in blood pressure. The results indicate that deficient regulation of retinal capillary filling pressure promotes edema, but the bulk of the overnight increase in macular edema is caused by other mechanisms, of which postural variation in venous blood pressure and increased retinal metabolism in the dark merit further study.