Retinal Detachment and Congenital Glaucoma

Dear Editor: Miyamoto et al show that intravitreal triamcinolone reduces macular edema as early as 1 hour after injection. They suggest that the time pattern demands that nongenomic mechanisms must be involved in the therapeutic effect of triamcinolone on retinal edema. Vasogenic edema in all tissues, including the retina, may be understood in terms of Starling’s law, by which the balance of hydrostatic and oncotic pressure differences between vascular and tissue compartments determines the formation or disappearance of edema. Triamcinolone has generally been thought to influence capillary permeability and, thereby, the oncotic pressure gradient between vessel and tissue, and little attention has been paid to possible hemodynamic effects of the drug. However, Paques et al have recently reported that retinal vessel tortuosity decreases after intravitreal triamcinolone injection. Retinal vessel length, tortuosity, and diameter change together, and a decrease in tortuosity (length) is associated with a reduction in vessel diameter. The early nongenomic effect that Miyamoto et al predict may well be the hemodynamic effect, by which triamcinolone induces retinal vasoconstriction and thereby reduces the hydrostatic pressure in the capillaries and reduces edema according to the hydrostatic pressure arm of Starling’s law. The genomic effects of the corticosteroid, which decreases the permeability of retinal capillaries, will later affect the oncotic arm of Starling’s law and further reduce edema. Both hydrostatic and oncotic pressure changes need to be considered to understand the mechanism of edema treatment fully.