Page 20 - VETcpd - Vol 2 - Issue 1, 2015

Obvious ophthalmology:canine glaucoma

Dr David L Williams MA VetMD PhD CertVOphthal CertWEL FRCVS

David qualified from Cambridge in 1988, aiming to devote his professional life to veterinary ophthalmology. Having worked at the Animal Health Trust and Royal Veterinary College, gaining his CertVOphthal and PhD, he returned to Cambridge, studied for his FRCVS, CertWEL and VetMD and now teaches ophthalmology there as well as exotic animal medicine, animal welfare and ethics.His latest foray into postnomials is a Masters in Education to improve his teaching skills.Fellow and Director of Studies, Veterinary Medicine and Pathology St John’s College, Cambridge CB2 1TPAssociate Lecturer in Veterinary Ophthalmology Department of Veterinary Medicine, Madingley Road, Cambridge CB3 0ESTel: 07939074682E: doctordlwilliams@aol.comwww.davidlwilliams.org.uk

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

For Ophthalmology Referrals in your area: vetindex.co.uk/eyes

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IntroductionGlaucoma is one of the most taxing of ophthalmic diseases in the dog, and for different reasons, perhaps also in man.

In humans with a condition such as pri-mary open angle glaucoma the problem is often that early diagnosis is difficult, with loss of visual fields occurring for a significant period before a diagnosis is made and treatment instigated. In the dog, early signs may also be missed, but the key feature of glaucoma in many dog breeds is that of a sudden onset red eye with pain and blindness, manifested by an acutely painful blind red eye with a high intraocular pressure (Figure 1).

In humans glaucoma is now defined as an optic neuropathy in which raised intraocular pressure is just one of the risk factors of optic nerve pathology, albeit an exceptionally important one. At present we do not recognise “low tension glaucoma” in the dog, but then the same could be said of human ophthalmology twenty years ago.

A key problem in both humans and dogs is establishing treatments that will provide a low enough pressure to avoid optic nerve damage for a prolonged period. The difference in aqueous outflow anatomy between the canine and human eye renders trabeculectomy, which has for many years been a standard treatment for many human glaucoma patients, inappropriate for canine cases of the condition. A similarity between the species however, is the improvement in medical management of glaucoma with the advent of drugs such as topical carbonic anhydrase inhibitors and prostaglandin analogues. Advances in the future, just appearing on the horizon now, include the use of intraocular “endolaser”

techniques for the surgical management of increased intraocular pressure and the possibility of neuroprotective therapy to ameliorate the optic neuropathy that is at the heart of vision loss in glaucoma.

Causes of canine glaucoma: similarities and differences from the human disease• Human eyeThe aqueous outflow pathway in the human eye involves the trabecular meshwork lying, as it does, on the corneal side of the iridocorneal angle. Thus we have “open angle glaucoma” in which proteoglycan deposition in the trabecular meshwork precludes aqueous outflow, while “closed angle glaucoma” involves the iris abutting against the trabecular meshwork, also stopping outflow.

• Canine eye In the dog, however, the trabecular meshwork occupies the apex of the iridocorneal angle (Figures 2a - 2c).

Primary glaucoma This condition normally involves closure, or rather occlusion of the iridocorneal angle in the majority of cases, with only a small number of cases such as animals from the colony of beagles in Gainesville, Florida, having a truly open angle glaucoma. A significant number of dog breeds have an inherited dysplasia of the trabecular meshwork with aberrant tissue occluding aqueous outflow. The iridocorneal angle thus appears closed since the trabecular meshwork is occluded by aberrant tissue either throughout its circumference or for a proportion of it (Figure 2c). The situation is complicated by the fact that any glaucoma eventually involves collapse of the iridociliary angle and thus closure of the iridocorneal angle when observed gonioscopically.

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Figure 1: The key feature of glaucoma in many dog breeds is that of a sudden onset red eye with pain and blindness.

Figure 3 (above): The glaucomatous eye is red primarily because of episcleral venous congestion with white sclera between the engorged red vessels

Figure 4 (below): Schiotz tonometer in use - restraint to ensure a horizontal ocular surface can be difficult.

Secondary Glaucoma Glaucoma can occur secondary to uveitis, neoplasia or lens luxation.

• Uveitis can result in posterior synechiae that adhere the pupil margin to the anterior lens capsule with resultant iris bombe and pupil block glaucoma, or give rise to peripheral anterior synechiae with closure of the iridocorneal angle.

• Neoplasia can block the angle through solid tissue invasion or blockage of the trabeuclar meshwork by neoplastic cells shed from the iris surface.

• Lens luxation: partial or total lens luxation, displacing the lens anteriorly, 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 eyeOne key issue with glaucoma is that, in the dog certainly, the eye with an acutely high intraocular pressure presents as a painful, potentially blind red eye. This might seem an easy diagnosis, but the problem is that an acute uveitis, with intraocular inflammation and a lowered intraocular pressure, can present in a similar manner but needs diametrically different treatment – specifically dilation of the pupil in uveitis which is contraindicated in glaucoma.

Uveitis: the red eye with uveitis, or indeed conjunctivitis, has a redness in the episclera which covers the whole white of the eye, as it is caused by hyperaemia related to the diffusion of inflammatory cytokines through the sclera.

Glaucoma: the glaucomatous eye however is red primarily because of episcleral venous congestion with white sclera between the engorged red vessels (Figure 3).

Figure 2b: Histological cross-section and scanning electronmicroscopic view of the normal canine eye

Figure 2c: Histological cross-section and scanning electronmicroscopic view of a glaucomatous eye

The trabecular meshwork occupies the apex of the iridocorneal angle

Dysplastic tissue blocking the iridocorneal angle

Normal appearance of the trabecular meshwork

Sheet of dysplastic tissue obscuring the iridocorneal angle

Cornea

Iris

Cornea

Ciliary body

Anterior chamber

Posterior chamber

Iris

The trabecular meshwork occupies the apex of the iridocorneal angle.

Anterior chamber

Ciliary body

Things are not always so simple though as glaucoma and uveitis can occur concur-rently, and as described above, uveitis can precipitate glaucoma. Differentiation of these conditions requires measurement of the intraocular pressure, which is elevated in glaucoma and depressed in uveitis.

Diagnostic problems: measuring intraocular pressureMore than a century ago in 1905 the Norwegian ophthalmologist, mathemati-cian and inventor Hjalmar August Schiøtz first demonstrated his simple and effective device for measuring intraocular pressure (IOP), an “indentation tonometer”. The Schiotz tonometer measures the IOP by determining the degree to which a given weight, normally 5 grams, indents the central cornea (Figure 4). That does mean that the corneal surface has to be horizon-tal, easy to accomplish in a prone human but less so in a dog or cat. The other problem is that the greater the indentation the lower the IOP, so one has to use a

Figure 2a: Schematic diagram of the front of the canine eye.

Aqueous humour produced by the ciliary body flows from the posterior chamber through the iris into the anterior chamber (purple arrows) and is drained by the trabecular meshwork into the episcleral veins. The aqueous humour supplies nutrition and oxygen to the avascular tissues of the eye

Lens