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M.A. Johnstone, A. Jamil, E. Martin, R. Mills; Brimonidine–Dependent Pulsatile Aqueous Discharge to the Episcleral Veins . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5472.
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To determine whether pulsatile aqueous flow into the episcleral veins increases in the acute interval following instillation of the selective alpha agonist brimonidine and to correlate the findings with IOP reduction.
Research microscope (80–power magnification), micrometer scale, videography. Six eyes of 6 normal subjects, mean age 32 (R 24–51), F/M 6/0, race C5, O1 were examined. The following intervals in minutes were recorded: 1,2,3,4,5,10,20,30,60,90,120. Where aqueous veins join episcleral veins to form mixing veins, separate lamina of aqueous and blood are regularly present. Measurements were made of the mixing vein diameter (MVD) and the aqueous lamellae diameter (ALD). In 3 eyes (Group 1) aqueous completely filled the vein post–brimonidine precluding amplitude measurements. In the other three eyes, (Group 2) (E–1, E–2, E–3) measurements of pulse–dependent aqueous lamellae diameter in systole and diastole defined the amplitude of the aqueous pulse wave pre and post brimonidine.
Following brimonidine in each eye, an increased pulsatile aqueous flow into the aqueous veins was visible within 3 minutes and was marked at five minutes. Group 1 mean mixing vein diameter was 47.5 µ (R 34.3–54.5). Pre–brimonidine these veins completely filled with blood; Post–brimonidine the same veins completely filled with aqueous entering in pulsatile waves made visible by intermittent passage of minute groups of RBCs. Aqueous displaced the slower moving lamellae of blood in the mixing veins and increased the amplitude of the aqueous pulse wave making the increased force of the aqueous wave apparent. Veins of Group 2 pre–brimonidine had visible oscillating lamellae of blood and aqueous allowing measurement of aqueous pulse wave amplitude. Mean vein diameters in E–1, E–2 and E–3 were respectively: 63.3µ, 58.6µ, 33.1 µ. Pre–brimonidine aqueous pulse wave amplitudes in E–1, E–2 and E–3 were respectively 39.4 µ, 35.3 µ, 8.2 µ. Pre–brimonidine aqueous pulse wave amplitudes as a % of vein diameter in E–1, E–2 and E–3 were respectively: 62.2 %, 60.2 %, 24.8%. Post–brimonidine aqueous pulse wave amplitudes were 100 % of vein diameter in E–1, E–2 and E–3. Mean IOP before brimonidine (N=5) was 14.2 mm Hg (R 12–18). After brimonidine (10–20 min.), mean IOP (N=5) was 11.7 mm Hg (R 8–14). Lowest IOPs were attained in the 60–120 minute time interval in each subject. The mean of the lowest pressure readings was 8 mm Hg (R 5–10), representing a 43.7% mean IOP decrease.
Brimonidine caused a marked acute interval increase in pulsatile aqueous discharge that precedes IOP reduction.
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