SMALL ANIMAL | OPHTHALMOLOGY ONLINE EDITION Glaucoma in dogs – (1) cause, clinical signs and diagnosis Glaucomas refer to a large group of neuro-degenerative diseases caused by persistent and ongoing elevation of intraocular pressure. Eventually this will lead to permanent retinal degeneration and blindness owing to loss of the retinal ganglion cells and optic nerve damage (Pizzirani, 2015). Andra-Elena Enache Dr med vet MVM MRCVS On the other hand, elevated intraocular pressure is termed ocular hypertension and should be distinguished from glaucoma. The final outcome is irreversible blindness After graduating with a Masters Degree from the Faculty of and, in some primary glaucoma cases, bilateral enucleation is required. Veterinary Medicine, Bucharest, Andra spent a short time Although elevated intraocular in general practice before pressure is one of the most relocating to the UK. Here, she consistent risk factors for enjoyed working in general glaucomas, development in practice for four years before dogs (Pizzirani, 2015) genetics, completing an ophthalmology ageing, vascular, immunologic internship at the Animal Health and environmental factors Trust, Newmarket. all contribute to the optic nerve degeneration. Andra began her studies toward a European College According to the European of Veterinary Ophthalmology College of Veterinary Diploma at the Royal Veterinary Ophthalmologists Manual, ‘It College in July 2017. is characterised by an elevation of intraocular pressure (IOP) which, when sustained, Outside of work, she enjoys Figures 1 & 2. Intraocular pressure – can be measured using a tonometer. travelling and spending time results in destruction of with her husband and their cat. intraocular structure and function, resulting in blindness. small, rounded probe on to overextension or flexion of The elevated intraocular the cornea – the higher the the neck. When using the pressure occurs mainly with intraocular pressure, the Tonopen, care should be taken developmental abnormalities quicker the probe returns to to gently flatten the cornea or disease processes affecting the instrument (Figures 1 & 2). without causing an obvious the intraocular circulation indentation – false high IOPs and especially the drainage When measuring the IOP, are easily obtained if the tip of aqueous humour from the one should ensure there is no of the Tonopen is pushed eye through the irido-corneal pressure on or underneath with force against the cornea. angle (ICA). the neck of the patient – Blepharospasm may also for example, tight collars, falsely elevate the IOP owing ‘Diagnosis and classification of harnesses, owners holding to eyelid spasm. glaucoma requires measurement – or on the eye globe itself. of the IOP (tonometry) and When opening the eyelids, the Repeated measurements examination of the iridocorneal fingers should rest on the bony should be taken. If in doubt, angle (gonioscopy). DNA- orbital rims, not on the eye; admit the dog for an IOP tests for primary open angle and it is important to avoid curve (measured every three 1.0 glaucoma (POAG) in specific overstretching the eyelids hours) and monitor in hospital hours* breeds are available.’ as well. (Sanchez et al, 2017). ™ Measuring intraocular The dog should be in a sitting Diagnosis pressure or standing position with It is important to take a *Suggested Personal & Professional This can be performed using the neck straight, and no full history and carry out a ™ Development (PPD) different devices: commonly either by an applanation tonometer (Tonopen) that ™ measures the force to flatten “The final outcome is irreversible blindness a distinct area on the cornea; and, in some primary glaucoma cases, OPHTHALMOLOGY or a rebound tonometer ™ (TonoVet) that expels a bilateral enucleation is required” 24 VETERINARY PRACTICE TODAY | VOLUME SEVEN | ISSUE ONE| 2019 WWW.VETCOMMUNITY.COM | ONLINE EDITION ™ ™ ™ ONLINE EDITION OPHTHALMOLOGY | SMALL ANIMAL Figures 3-7. Acute onset signs in primary glaucoma cases. thorough general physical Acute onset glaucoma optic nerve head (atrophy), secondary, depending on examination, looking in This is more common in primary attenuation of the retinal their cause. particular for evidence of glaucoma cases (Figures 3-7) as vasculature, cupped optic previous trauma and to rule out evidenced by: nerve head (retinal vessels Congenital glaucoma underlying systemic diseases, severe ocular and head pain stop at the rim of the optic This is rare in the dog. It such as lymphoma. This should lethargy, reduced appetite nerve, no vessels are seen develops within the first few be followed by a full ophthalmic diffuse corneal oedema crossing the optic nerve), months of life (three to six examination: including visual blepharospasm hyperreflectivity of the months) and is a consequence reflexes (‘menace’ response, epiphora tapetum fundus. of extensive goniodysgenesis dazzle and pupillary light marked conjunctival or trabecular meshwork reflexes), pupil size (miotic/ hyperaemia and, sometimes, Chronic glaucoma maldevelopment. Different mydriatic), fluorescein test, mucoid discharge that can be Signs of chronic structural genes have been tonometry (TonoPen/TonoVet), misdiagnosed as conjunctivitis glaucoma include: associated with the trabecular funduscopy and gonioscopy. third eyelid protrusion Haab striae – breaks in meshwork malformation. marked episcleral Descemet membranes Ocular ultrasonography vascular congestion secondary to stretching of Primary and secondary is useful in order to rule mid- to large dilated and the globe glaucomas can be difficult to out intraocular neoplasia, unresponsive pupil buphthalmos – a permanently distinguish clinically. intraocular haemorrhage, deep perilimbal enlarged eye caused vitriitis/vitreous membranes, corneal vascularisation by chronically elevated Primary glaucoma and retinal detachment. mild aqueous flare or intraocular pressure Primary glaucoma is a pigment dispersion in the scleral thinning and bilateral disease that occurs Depending on the stage of the anterior chamber visualisation of the underlying spontaneously with no evidence disease, glaucomas can be changes to appearance of choroid owing to of trauma or other intraocular classified as acute or chronic. the optic nerve – pale or dark globe enlargement disease. There is an increased phthisis bulbi – in advanced incidence of the condition with cases, when the ciliary age and this could be the result of body stops producing changes in the iridocorneal angle “Glaucomas, however, can easily mask aqueous humour. seen with age (Palko et al, 2016). underlying systemic diseases, such as Glaucomas can be classified Primary glaucomas are infectious uveitis and neoplasia” into congenital, primary and classified on the gonioscopic WWW.VETCOMMUNITY.COM | ONLINE EDITION @VPTODAY | WWW.VETERINARYPRACTICETODAY.COM 25 SMALL ANIMAL | OPHTHALMOLOGY ONLINE EDITION appearance as open-angle retriever, Siberian husky, glaucoma – seen particularly cocker spaniel, English in the beagle, petit basset springer spaniel and Welsh griffon vendeen, basset springer spaniel. hound, shar pei, Norwegian elkhound – and closed-angle Primary closed-angle glaucoma which is seen in glaucoma (PCAG) the majority of breeds, for This is the most common example, the cocker spaniel form of primary glaucoma in IRIS and Welsh springer spaniel. dogs. There is a strong breed predisposition – American ICA The ICA can be assessed by cocker spaniel, basset hound, gonioscopy and an abnormal chow chow, Welsh springer ENDOTHELIUM appearance is termed spaniel, miniature and toy goniodysgenesis. This occurs poodles, great Dane – as well KOEPPE LENS owing to a defect in the as a genetic influence. In most development of the angle, breeds, there is a significant Figure 8. In order to perform gonioscopy, a special contact lens which can have a decreased female predisposition and (goniolense, such as the Koeppe lens) is placed on the surface of the width or malformation of the most dogs are middle- to cornea to change the refraction angle of the incoming light. pectinate ligament (Miller and older-aged. Bentley, 2015). A dog can be considered to In order to perform gonioscopy, have PCAG, if it is presented a special contact lens with clinical signs of glaucoma (goniolense, such as the Koeppe (mydriasis, blepharospasm, lens) is placed on the surface elevated IOP), with absence of the cornea to change the of other intraocular diseases refraction angle of the incoming (neoplasia, chronic uveitis light (Figure 8). The ICA with iris bombe, lens can then be visualised using luxation), and abnormal a slit lamp biomicroscope. ICA at gonioscopy of the The pectinate ligament (thin contralateral unaffected eye fibres from the iris base to its (Miller and Bentley, 2015). insertion at the cornea) and the iridocorneal angle width An acute presentation are evaluated by gonioscopy of glaucoma – with no – the latter to determine its other intraocular findings proportion of 360 degrees. A in a dog included on grading scheme of the changes the list of predisposed visualised at gonioscopy is in breeds for PCAG – should place to classify the degree raise the suspicions of of goniodysgenesis – open, primary glaucoma and narrow, closed, closed angle. prophylactic treatment of the contralateral eye should The degree of goniodysgenesis be considered. also changes with age. Evaluation of the ICA in the Early diagnosis and treatment affected glaucomatous eye is essential, because may be difficult because of the normalising the intraocular