VETcpd - Ophthalmology Peer Reviewed Obvious ophthalmology: canine glaucoma Canine glaucoma is a frustrating disease in many ways. Its presentation as a red painful eye can be confused with that of uveitis, but requiring a very different treatment regime. Its diagnosis is relatively simple as long as one has a tonometer, but made difficult by the price of the equipment required to measure intraocular pressure. Treatment can be similarly costly – and while it may be effective in the short term, preservation of vision and ocular comfort is rarely completely successful in the long term. Key words: eye, glaucoma, intraocular pressure, pain, blindness, tonometry Introduction Glaucoma is one of techniques for the surgical management the most taxing of of increased intraocular pressure and the ophthalmic diseases possibility of neuroprotective therapy to in the dog, and for ameliorate the optic neuropathy that is at different reasons, the heart of vision loss in glaucoma. perhaps also in man. Dr David L Williams MA In humans with a condition such as pri- Causes of canine glaucoma: VetMD PhD CertVOphthal mary open angle glaucoma the problem similarities and differences CertWEL FRCVS is often that early diagnosis is difficult, from the human disease with loss of visual fields occurring for David qualified from Cambridge in a significant period before a diagnosis • Human eye 1988, aiming to devote his professional is made and treatment instigated. In the The aqueous outflow pathway in the life to veterinary ophthalmology. dog, early signs may also be missed, but human eye involves the trabecular Having worked at the Animal Health the key feature of glaucoma in many dog meshwork lying, as it does, on the corneal Trust and Royal Veterinary College, breeds is that of a sudden onset red eye side of the iridocorneal angle. Thus we gaining his CertVOphthal and PhD, he with pain and blindness, manifested by an have “open angle glaucoma” in which returned to Cambridge, studied for his proteoglycan deposition in the trabecular FRCVS, CertWEL and VetMD and now acutely painful blind red eye with a high intraocular pressure (Figure 1). meshwork precludes aqueous outflow, teaches ophthalmology there as well as while “closed angle glaucoma” involves exotic animal medicine, animal welfare In humans glaucoma is now defined the iris abutting against the trabecular and ethics. as an optic neuropathy in which raised meshwork, also stopping outflow. His latest foray into postnomials is a intraocular pressure is just one of the Masters in Education to improve his risk factors of optic nerve pathology, • Canine eye teaching skills. albeit an exceptionally important one. At In the dog, however, the trabecular Fellow and Director of Studies, present we do not recognise “low tension meshwork occupies the apex of the Veterinary Medicine and Pathology glaucoma” in the dog, but then the same iridocorneal angle (Figures 2a - 2c). St John’s College, Cambridge CB2 1TP could be said of human ophthalmology Primary glaucoma Associate Lecturer in Veterinary twenty years ago. This condition normally involves closure, Ophthalmology Department of Veterinary Medicine, Madingley Road, A key problem in both humans and or rather occlusion of the iridocorneal Cambridge CB3 0ES dogs is establishing treatments that will angle in the majority of cases, with provide a low enough pressure to avoid only a small number of cases such as Tel: 07939074682 optic nerve damage for a prolonged animals from the colony of beagles in E: [email protected] period. The difference in aqueous Gainesville, Florida, having a truly open www.davidlwilliams.org.uk outflow anatomy between the canine angle glaucoma. A significant number of and human eye renders trabeculectomy, dog breeds have an inherited dysplasia of which has for many years been a standard the trabecular meshwork with aberrant treatment for many human glaucoma tissue occluding aqueous outflow. The 16th Edition VetIndex 2015 patients, inappropriate for canine cases iridocorneal angle thus appears closed 21st Edition ® since the trabecular meshwork is occluded THE of the condition. A similarity between A - Z DIRECTORY the species however, is the improvement by aberrant tissue either throughout its OF VETERINARY in medical management of glaucoma circumference or for a proportion of it PRODUCTS , with the advent of drugs such as topical (Figure 2c). The situation is complicated SUPPLIES OF VETERINARY A-Z DIRECTORY 2015THE AND SERVICES SUPPLIES AND PRODUCTS , by the fact that any glaucoma eventually SERVICES carbonic anhydrase inhibitors and VE Tcp d - Vo l 1 - I ss ue 3 VETc pd - V VE ol 1 Tc pd - Issue 3 Peer Reviewed - V Vet Vol 1 - Issue 4 CPD ol 1 VET d cpd - A eviewe Vet CPD Journal: Peer R pri l 2 For Ophthalmology 3 01 - Issue al 4 Vet Vol 1 CPD D Journ VET Vet CP cpd site D esign | - Web Peer Reviewed VetInde x 2015 Vet CPD Journal VET Vol 1 - July 2014 Peer Reviewed cpd - 4, 2014 Vol 1 - Issues 1 VETVet Reprints from CPD Journal Includes 5 hours Journalcpd of FREE CPD! prostaglandin analogues. Advances in the involves collapse of the iridociliary angle 5 hours FREE See inside for journal with further details!!! The peer reviewed clinical l with ducational inical journa online exams to turn your e CPD!! reviewed cl ational CPD Referrals in your area: The peer your educ reading into documented ams to turn ted CPD journal with 18:05 rnal with The peer reviewed clinical online ex documen 07/09/2014 ducational reading into 5 hours ed clinical jou tional online exams to turn your e The peer review certified CPD urn your educa 1 14.indd reading into 5 hours of Cover_20 FREE online exams to t ocumented CPD 11/06/2014 15:16 CPD!! reading into d Cover_2014.indd 1 future, just appearing on the horizon now, and thus closure of the iridocorneal angle www.vetindex.co.uk vetindex.co.uk/eyes 27/02/2015 10:00 when observed gonioscopically. Cover 2015_mod_futura.indd 1 include the use of intraocular “endolaser” Page 20 - VETcpd - Vol 2 - Issue 1, 2015 VETcpd - Ophthalmology Anterior chamber Cornea Figure 2a: Schematic diagram of the front of the canine eye. Iris Aqueous humour produced Posterior by the ciliary body flows chamber from the posterior chamber through the iris into the anterior chamber (purple arrows) and is drained by the The trabecular Lens trabecular meshwork into the meshwork occupies episcleral veins. The aqueous the apex of the iridocorneal angle. humour supplies nutrition and oxygen to the avascular Ciliary body tissues of the eye Figure 1: The key feature of glaucoma in many dog breeds is that of a sudden onset red eye with pain and Cornea blindness. The trabecular Secondary Glaucoma meshwork Dysplastic Anterior occupies the tissue Glaucoma can occur secondary to uveitis, chamber apex of the blocking the neoplasia or lens luxation. iridocorneal angle iridocorneal angle • Uveitis can result in posterior synechiae Ciliary body that adhere the pupil margin to the Iris anterior lens capsule with resultant iris bombe and pupil block glaucoma, or give Figure 2b: Histological cross-section Figure 2c: Histological cross-section and scanning rise to peripheral anterior synechiae with and scanning electronmicroscopic view electronmicroscopic view of a glaucomatous eye closure of the iridocorneal angle. of the normal canine eye • Neoplasia can block the angle through Sheet of solid tissue invasion or blockage of the dysplastic trabeuclar meshwork by neoplastic cells tissue obscuring the shed from the iris surface. Normal iridocorneal • Lens luxation: partial or total lens appearance of angle luxation, displacing the lens anteriorly, the trabecular meshwork can cause the lens to plug and acutely block the pupil, preventing aqueous fluid movement within the eye. Diagnostic problems: differential diagnosis of the red eye Things are not always so simple though as glaucoma and uveitis can occur concur- One key issue with glaucoma is that, in rently, and as described above, uveitis can the dog certainly, the eye with an acutely precipitate glaucoma. Differentiation of high intraocular pressure presents as a these conditions requires measurement of painful, potentially blind red eye. This the intraocular pressure, which is elevated might seem an easy diagnosis, but the in glaucoma and depressed in uveitis. problem is that an acute uveitis, with intraocular inflammation and a lowered Diagnostic problems: intraocular pressure, can present in a measuring intraocular pressure Figure 3 (above): The glaucomatous eye is red similar manner but needs diametrically primarily because of episcleral venous congestion with different treatment – specifically More than a century ago in 1905 the white sclera between the engorged red vessels dilation of the pupil in uveitis which is Norwegian ophthalmologist, mathemati- Figure 4 (below): Schiotz tonometer in use - restraint contraindicated in glaucoma. cian and inventor Hjalmar August Schiøtz first demonstrated his simple and effective to ensure a horizontal ocular surface can be difficult. Uveitis: the red eye with uveitis, or device for measuring intraocular pressure indeed conjunctivitis, has a redness in the (IOP), an “indentation tonometer”. The episclera which covers the whole white Schiotz tonometer measures the IOP by of the eye, as it is caused by hyperaemia determining the degree to which a given related to the diffusion of inflammatory weight, normally 5 grams, indents the cytokines through the sclera. central cornea (Figure 4). That does mean Glaucoma: the glaucomatous eye that the corneal surface has to be horizon- however is red primarily because of tal, easy to accomplish in a prone human episcleral venous congestion with white but less so in a dog or cat. The other sclera between the engorged red vessels problem is that the greater the indentation (Figure 3). the lower the IOP, so one has to use a Full article available for purchase at www.vetcpd.co.uk/modules/ VETcpd - Vol 2 - Issue 1, 2015 - Page 21.