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P.E. Miller, E. Bentley, K.A. Diehl, R.T. Carter; High Resolution Ultrasound Imaging of the Anterior Segment of Dogs with Spontaneous Primary Angle-closure Glaucoma Prior to, and Following the Topical Application of 0.005% Latanoprost . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4408.
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Purpose: Although spontaneous primary angle-closure glaucoma (PACG) is common in dogs, and has many similarities to PACG in humans, the mechanism by which it develops has not yet been described. We and others have noted that 0.005% latanoprost can rapidly alleviate an attack of acute PACG in dogs, and return IOP from as high as 80 mm Hg to within normal limits in 1-2 hours, but how this is accomplished remains unclear. Therefore, we sought to characterize the mechanism(s) by which PACG occurs in dogs, and to elucidate how 0.005% latanoprost may alter this process. Methods: High resolution (20 MHz) ultrasound images of the anterior segment of 6 dogs with various forms of spontaneous PACG were collected prior to, and 1-2 hours following topical application of 1 drop of 0.005% latanoprost. IOP and resting pupil diameter (PD) were also noted. In 3 dogs with acute PACG the normotensive, but at-risk, fellow eyes (ACG suspects) were also imaged. Results: 5 dogs had acute PACG at the time of imaging. These dogs had a mean IOP of 58.4 mm Hg (range 37-80 mm Hg) and a moderately dilated (8-9 mm) pupil. The sixth dog had intermittent ACG with an IOP of 12 mm Hg and PD = 13 mm when imaged. In acute PACG the mechanism appeared to be a variation of reverse pupillary block in which the iris assumed a sigmoidal shape. The pupillary border of the iris was flattened and in close apposition to the anterior lens capsule, the midperipheral iris was concave, and the peripheral iris was rotated anteriorly and convex. This configuration was associated with gonioscopic angle closure, a relatively shallow posterior chamber, and collapse of the ciliary cleft. Similar, but less dramatic alterations were present in the eye with intermittent ACG, and in ACG suspect eyes. In the latter, however, the ciliary cleft was still present in 2 of 3 dogs even though the angle was gonioscopically narrowed to closed. Latanoprost alleviated the attack in 2 dogs with acute PACG, and reduced IOP to 6 mm Hg in the dog with intermittent ACG. In all 3 dogs the pupil became markedly miotic (2-3.5 mm), the iris assumed a more planar to uniformly convex configuration, the posterior chamber deepened, and the anterior chamber became shallower. In the 3 dogs with acute PACG in which IOP remained elevated following latanoprost, the pupil did not become miotic and the original configuration remained. Conclusions: A form of reverse pupillary block is one mechanism by which PACG occurs in dogs. The ability of latanoprost to rapidly alleviate an attack appears to be associated with its ability to induce miosis.
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