Published by Rachel Davis, DVM, MS, DACVO August 2017
Publication: Veterinary Information Network (VIN)
Disease Description
Aqueous misdirection syndrome is an uncommon to rare primary bilateral ocular disease described in cats characterized by progressive anterior chamber shallowing, anterior iris and lens displacement and a narrowed approach to an open iridocorneal angle.1,2 Aqueous misdirection syndrome causes progressive, often blinding glaucoma over time. This has not been described in dogs.
Etiology
Aqueous misdirection syndrome is thought to be caused by “misdirection” of aqueous humor fluid into the vitreous space, thus displacing the ocular structures anterior to the vitreous forward.1 Normally, aqueous humor, an ultrafiltrate of plasma, is produced by the ciliary body and flows into the posterior chamber (the small space between the lens and posterior iris), through the pupil and out the iridocorneal drainage angle.3 In the case of aqueous misdirection syndrome, the aqueous is thought to be diverted pathologically into the vitreous based on ultrasonic and clinical findings of large “trapped” vitreal fluid pockets posterior to the lens. The underlying cause of the fluid diversion is not known in cats, but may be surgically induced in humans.4,5 Spontaneous aqueous misdirection is rare in humans, but may occur and the cause, as in cats, is not known.6.7
Diagnosis
Ophthalmic Examination Findings: In early cases of aqueous misdirection syndrome, subtle anterior displacement of the lens and iris diaphragm may be noted with mildly elevated intraocular pressure. Early cases may not be detected unless full slit-lamp biomicroscopy is performed by a veterinary ophthalmologist and early cases are not usually noted by the clients. Moderate to severe disease is typically easily diagnosed by the characteristic appearance of the eye/s caused by prominent anterior displacement of the iris and lens. These cats usually present with variably elevated intraocular pressure.1 The anterior displacement of the iris-lens diaphragm is most apparent based on examination of the patient with the observer lateral to the patient. From this position, the iris appears to “fill” the anterior chamber. This is especially noticeable in cats because they normally have a deep anterior chamber with the chamber appearing clear from the side. Variable hyperemia, corneal edema, mydriasis, decreased pupillary light response, vision loss or ocular pain may be present, depending on the level of intraocular pressure elevation.1,2 Aqueous misdirection syndrome is a bilateral disease, but may be disparate on initial presentation, with one eye being more severely affected.
Physical Examination Findings: Because aqueous misdirection syndrome is a primary ocular disease, no systemic findings are expected other than behavioral changes related to vision loss, if this is present.
Ocular Ultrasonographic Findings: Ocular ultrasound findings include variable anterior chamber shallowing, anterior lens displacement, a thickened vitreal face between the lens and ciliary body, a collapsed ciliary cleft and multifocal, variably sized hypoechoic mid to anterior vitreal spaces consistent with fluid pockets/cavitations.1
Disease Description in this Species
Signalment
Aqueous misdirection syndrome is seen in middle-aged cats (mean age 11.7yr).1 A higher prevalence has been reported in female cats. No breed predisposition has been reported.1,13
Clinical Signs
Depending on the stage of disease at presentation, clinical signs may range in severity and include mild to severe shallowing of the anterior chamber with forward displacement of the lens and iris, mydriasis, decreased iridal movement, corneal edema, vision loss and/or mildly to severely elevated intraocular pressure. Both eyes are typically affected, but often not equally. Cats may exhibit alterations in behavior at home caused by changes in vision and/or discomfort secondary to glaucoma.
Etiology
Genetic
Unknown
Inherited
Breed Predilection
None
Sex Predilection
Female
Age Predilection
Middle aged to older cats (mean 11.7yr, 12.9yr)
Diagnostic Procedures
The appearance of this condition is unique and diagnosis is usually straightforward. Slit-lamp biomicroscopy and ocular ultrasonography may be helpful to assess early disease and determine the severity of the disease. Tonometry is useful for identifying the presence and severity of secondary glaucoma as the end-result of aqueous misdirection syndrome is almost always glaucoma.
Images
Treatment/Management/Prognosis
Specific Therapy
Medical treatment is aimed at decreasing aqueous production, thereby decreasing intraocular pressure and the amount of aqueous being misdirected. Topical carbonic anhydrase inhibitors or CAI’s (e.g. dorzolamide, brinzolamide) have been shown to be effective for reducing aqueous humor production and flow in cats as well as decreasing intraocular pressure.8,9 Other topical anti-tensive medications (e.g. prostaglandin analogs, parasympathomimetics) are poorly or not effective at lowering intraocular pressure in cats and are not indicated.10-12 Topical CAI therapy often improves the appearance of the eye/s with return of the iris-lens diaphragm to a more normal location for some time while the patient is on topical therapy. This can be helpful for clients when having them monitor progression or worsening of the disease at home.
Surgical therapy with phacoemulsification and posterior capsulotomy with our without anterior vitrectomy and/or endocyclophotocoagulation, especially when medical therapy fails or vision is compromised, has been shown to be an effective treatment with all cats being visual a year after treatment and 7/9 cats having controlled intraocular pressure.13
For clients pursuing long term vision and/or therapy for this condition, referral to a board-certified veterinary ophthalmologist should be considered. Enucleation should be considered for cats with buphthalmos, chronic blindness, severe disease and/or ongoing glaucomatous pain or in cases of uncontrolled disease for which referral is not an option.14,15
Supportive Therapy
Cats that are visual impaired or blind should be kept inside or in an enclosed space with a consistent environment. Clients should be educated as to the painful nature of the disease when glaucoma develops, even if the patient is not perceived to be experiencing pain. Enucleation should be pursued for cats that will not tolerate topical medications or treatment and have ongoing elevated intraocular pressure.
Monitoring and Prognosis
Some patients respond well initially to topical therapy (CAI’s TID-BID) and may be maintained on this for some time. Periodic pressure monitoring should be instituted every 1-2 months with surgical intervention when medical therapy fails. Surgical therapy carries a good prognosis and has been shown to decrease the need for medical therapy and to maintain controlled intraocular pressure and vision in affected patients. Referral to a veterinary ophthalmologist should be considered for management and treatment of aqueous misdirection syndrome long term.
Differential Diagnosis
Uveitis
Primary glaucoma
Developmental disease
References
1. Czederpiltz JM, La Croix NC, van der Woerdt A, Bentley E, Dubielzig RR, Murphy CJ, Miller PE. Putative aqueous humor misdirection syndrome as a cause of glaucoma in cats: 32 cases (1997-2003). J Am Vet Med Assoc. 2005 Nov 1;227(9):1434-41.
2. Sapienza JS. Feline lens disorders. Clin Tech Small Anim Pract. 2005 May;20(2):102-7.
3. Glenwood GG, Gelatt KN, Esson DW. Physiology of the Eye. In Gelatt KN (ed): Veterinary Ophthalmology 4th ed. Pg 160-2. Blackwell Publishing, Ames IA
4. Dave P, Rao A, Senthil S, Choudhari NS. Recurrence of aqueous misdirection following pars plana vitrectomy in pseudophakic eyes. BMJ Case Rep. 2015 Apr 21;2015.
5. Greenfield DS, Tello C, Budenz DL, Liebmann JM, Ritch R. Aqueous misdirection after glaucoma drainage device implantation. Ophthalmol. 1999 May;106(5):1035-40.
6. Feng L, Roufas A, Healey PR, White AJ. Bilateral spontaneous aqueous misdirection: it can happen! Clin Exp Ophthalmol. 2015 Nov;43(8):771-3.
7. Jarade EF, Dirani A, Jabbour E, Antoun J, Tomey KF. Spontaneous simultaneous bilateral malignant glaucoma of a patient with no antecedent history of medical or surgical eye diseases. Clin Ophthalmol. 2014 May 27;8:1047-50.
8. Rankin AJ, Crumley WR, Allbaugh RA. Effects of ocular administration of ophthalmic 2% dorzolamide hydrochloride solution on aqueous humor flow rate and intraocular pressure in clinically normal cats. Am J Vet Res. 2012 Jul;73(7):1074-8.
9. Rainbow ME, Dziezyc J. Effects of twice daily application of 2% dorzolamide on intraocular pressure in normal cats. Vet Ophthalmol. 2003 Jun;6(2):147-50.
10. Studer ME, Martin CL, Stiles J. Effects of 0.005% latanoprost solution on intraocular pressure in healthy dogs and cats. Am J Vet Res. 2000 Oct;61(10):1220-4.
11. McDonald JE, Kiland JA, Kaufman PL, Bentley E, Ellinwood NM, McLellan GJ. Effect of topical latanoprost 0.005% on intraocular pressure and pupil diameter in normal and glaucomatous cats. Vet Ophthalmol. 2016 Jul;19 Suppl 1:13-23.
12. Wilkie DA, Latimer CA. Effects of topical administration of 2.0% pilocarpine on intraocular pressure and pupil size in cats. Am J Vet Res. 1991 Mar;52(3):441-4.
13. Atkins RM, Armour MD, Hyman JA. Surgical outcome of cats treated for aqueous humor misdirection syndrome: a case series. Vet Ophthalmol. 2016 Jul;19 Suppl 1:136-142.
14. Dietrich U. Feline glaucomas. Clin Tech Small Anim Pract. 2005 May;20(2):108-16.
15. T. Blocker and A. van der Woerdt. The feline glaucomas: 82 cases (1995–1999). Vet Ophthalmol. 2001; 4, 2, 81–85.
