Putative aqueous humor misdirection syndrome as a cause of glaucoma in cats: 32 Cases (1997-2003)

Objective - To characterize the clinical and morphologic aspects of aqueous humor misdirection syndrome (AHMS) in cats and provide a hypothesis regarding its pathogenesis on the basis of detailed analysis of affected cats. Design - Retrospective study. Animals - 32 cats (40 eyes). Procedure - Medical records of cats in which AHMS was diagnosed from July 1997 to August 2003 were reviewed. In certain cats, results of additional diagnostic testing were also obtained, including A-scan, B-scan, and high-resolution ultrasonography; streak retinoscopy; video keratometry; and infrared neutralizing videoretinoscopy as well as results of analysis of flash-frozen sections and histologic examination of enucleated globes. Results - Cats had a uniformly shallow anterior chamber, intact lens zonules, and a narrowed approach to an open iridocorneal angle. Mean age of affected cats was 11.7 years (range, 4 to 16 years), and female cats were significantly more often affected than male cats. Clinical signs included mydriasis, decreased pupillary light reflex, decreased menace response, and blindness. Glaucomatous changes to the optic nerve, incipient cataracts, and eventual blindness were seen. Intraocular pressure was ≥ 20 mm Hg (range, 12 to 58 mm Hg) in 32 of 40 eyes. Ultrasonography and histologic examination revealed a thickened anterior vitreal face interposed between the lens and ciliary body, partial ciliary cleft collapse, and cavitated vitreal regions. Various treatment modalities were used. Conclusions and Clinical Relevance - AHMS affects older cats, especially females, and may result in glaucoma, vision loss, and signs of ocular pain. Topical administration of carbonic anhydrase inhibitors decreased intraocular pressure.