Glaucoma is less commonly recognized in cats than in dogs. As in dogs and humans, however, when glaucoma occurs in the cat, it frequently results in blindness. Glaucomas are ophthalmic diseases that involve progressive death of retinal ganglion cells, often resulting from increases in intraocular pressure (IOP) of various etiologies. While canine glaucomas may be primary, in cats, a species in which glaucoma tends to have more subtle clinical signs, glaucoma is almost always secondary and may be triggered by anterior uveitis, intraocular neoplasia, trauma, intraocular hemorrhage, lens-associated etiologies, space-occupying lesions in the orbit, or feline aqueous humor misdirection syndrome (FAHMS).
FAHMS is an uncommon, idiopathic, bilateral ocular disease in cats. There is no breed predilection, but middle aged to older cats (mean age 11.7-12.9 years), usually females, are most often affected. Normally, when aqueous humor is produced by the ciliary body of the eye, it initially flows into the posterior chamber, a small space between the posterior iris and the lens, and then through the pupil into the anterior chamber. In FAHMS, the aqueous humor is misdirected posteriorly into the vitreous space, and this inappropriate accumulation of fluid in the vitreous space pushes the lens and iris anteriorly and results in increased vitreal pressure as well as narrowing of the anterior chamber, which is normally quite deep in cats. In some cases the fluid may accumulate between the retina and vitreous as well.
Initial clinical signs of FAHMS are often subtle with normal to slightly increased IOP. Slit-lamp biomicroscopy performed by a veterinary ophthalmologist may be necessary to detect early FAHMS. Although FAHMS is a bilateral disease, it may be asymmetrical in severity. As the disease progresses, the anterior chamber(s) will appear shallow for a cat, especially when viewed by an observer lateral to the animal, and the iris and lens will be displaced anteriorly; the pupil may become mydriatic due to a decrease in iris mobility, and the IOP may slowly increase. When IOP is markedly increased, corneal edema, vision loss and ocular pain are likely.
Cats with FAHMS are usually treated medically with topical carbonic anhydrase inhibitors, which reduce the production of aqueous humor, thereby reducing IOP. When medical therapy fails to ameliorate the condition and/or vision is threatened, surgery needs to be considered. Once glaucoma and attendant pain becomes uncontrolled, the only option is enucleation of the affected eye(s). This retrospective study of seven cats (nine eyes) evaluates the clinical outcome of cats treated surgically for FAHMS.
Six of the seven patients were female, and five of the seven had bilateral FAHMS. All cats were mixed breed. Three groups of eyes were established based on the type of surgery performed. Group 1 eyes (one eye) had phacoemulsification and posterior capsulotomy only; group 2 eyes (four eyes) were treated with phacoemulsification, posterior capsulotomy, and anterior vitrectomy; group 3 eyes (four eyes) had phacoemulsification, posterior capsulotomy, anterior vitrectomy, and endocyclophotocoagulation. None of the patients received an intraocular lens during surgery. Severity of disease, lack of response to medical therapy, and increasing IOP despite medical therapy were the reasons for surgery; however, no records were available correlating the type of surgery performed with the reasons for that specific surgery. The intent of all surgeries for FAHMS is to create a direct connection between the anterior chamber and vitreous space, allowing the free flow of aqueous humor into the anterior chamber.
All of the eyes in this case series maintained vision and had controlled intraocular inflammation at one year post-surgery, and also maintained vision for the duration of follow-up (mean follow-up time, 21 months). Two of the nine eyes had elevated IOP (>25 mm Hg) within the first 6 months of surgery and one of the nine eyes had elevated IOP one year postoperatively. All of the surgical patients continued on at least one topical anti-glaucoma medication and one topical anti-inflammatory medication postoperatively, but the mean number of drops required to treat the eye(s) decreased from 3.9 drops/day prior to surgery to 2.2 drops/day postoperatively.
The authors conclude that there may be a subset of cats with FAHMS that require surgery in order to save vision. There is very little published literature on this syndrome, and additional studies evaluating response to medical therapy, imaging of the anterior and posterior segments of affected cats, gonioscopy, intraocular lens implantation following phacoemulsifcation and long-term use of topical ophthalmic cortiocosteroids (which can themselves increase IOP) in cats are recommended by these researchers. [PJS]