PURPOSE. To investigate the role of posture and other systemic factors in the diurnal variation of clinically significant diabetic macular edema (CSDME). METHODS. Ten eyes of 10 diabetic subjects with CSDME underwent four OCT foveal thickness measurements with Stratus-OCT at 9 AM and 12, 3, and 6 PM consecutively on two different days, with the subject in an upright position on one and in a recumbent position on the other. For the "recumbent- position" measurements, the patients were admitted the night before and remained in bed during the entire day of testing. Clinical laboratory results at baseline included HbA1c, urinary albumin, and serum creatinine. Refraction and Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity were also measured before each OCT measurement was taken. Variations in blood pressure, body temperature, plasma glucose, renin, aldosterone, and cortisol levels were measured and then correlated with macular thickness. RESULTS. Foveal thickening decreased in all cases over the course of the day. The decrease, however, was significantly greater for the upright-position measurements (relative mean ± SD decrease of 20.6% ± 6.5% in the upright position and 6.2% ± 4.6% in the recumbent position). Visual acuity improved by at least 1 ETDRS line in three eyes in the upright position as opposed to only one eye in the recumbent position. There seemed to be no association between any of the systemic factors studied and foveal thickening, with the exception of cortisol. CONCLUSIONS. The results support the hypothesis that posture and hydrostatic pressure play a major role in determining time-related shifts in CSDME and suggest that the forces of Starling's law can in part, account for CSDME formation.
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