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PE Miller, E Bentley, MA B Croft, AL Whiteman, PL Kaufman; The Effect of 0.005% Latanoprost on the Canine Iridocorneal Angle: An Ultrasound Biomicroscopic Study . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4073.
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Purpose: Primary angle closure glaucoma (PACG) occurs spontaneously in dogs and humans. We and others have noted that topical 0.005% latanoprost can reduce IOP from over 60 mm Hg to less than 20 mm Hg within 2 hours in dogs with acute PACG, but not in dogs with similarly marked increases in IOP which occur immediately following phacoemulsification lens extraction (IOVS 2001, 42:S840). The mechanism of this dramatic and rapid IOP reduction is not clear, but it is not consistent with a remodeling of the uveoscleral outflow pathway. In this study we evaluated the effects of 0.005% latanoprost on the ultrasound biomicroscopic (UBM) appearance of the normal canine iridocorneal angle (ICA). Methods: 50 MHz UBM (Model #840, Humphrey Instruments) images of the ICA were captured prior to, and 1 and 2 hours after the topical application of 30 µl of 0.005% latanoprost to one eye of 6 anesthetized normal dogs. Measurements included pupil diameter; IOP; maximum length, width and area of the ciliary cleft (meshwork); peripheral iris thickness; the angle of the iris to cornea; and angle opening distance (the perpendicular distance between the posterior cornea and the anterior iris) at the termination of Descemet’s membrane and at 250 and 500 microns anterior to Descemet’s termination. Results: Pupil diameter was significantly reduced by 41% and 48% (P < 0.01), and the area of the ciliary cleft significantly increased by 18% and 28% (P < 0.05) at 1 and 2 hrs post-treatment respectively. IOP decreased over time (16.5 4.3 mm Hg at baseline to 13.3 4.8 mm Hg at 2 hrs) but this did not achieve statistical significance. There were no statistically significant changes in the other parameters measured. Conclusion: Latanoprost is not only a potent miotic in dogs but it also increases the surface area of the ciliary cleft (meshwork), thereby potentially improving outflow via the conventional route. Whether this increase in surface area is the result of contraction or relaxation of the ciliary musculature, or miosis, remains to be elucidated. Nevertheless, the ocular hypotensive effect of latanoprost in dogs appears to be more complex than simply improving uveoscleral outflow, which may also occur. The ability of 0.005% latanoprost to reduce IOP from ≷ 60 mm Hg in acute PACG, but not from similar levels after lens extraction, may be the result of the drug’s ability to induce miosis in the face of high IOP, thereby relieving pupil block, rather than solely via its action on the ciliary body/cleft or improving uveoscleral outflow.
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