Irksome Intraocular Problems Rachel Allbaugh, DVM, MS, Diplomate American College of Veterinary Ophthalmologists Iowa State University Department of Veterinary Clinical Sciences, Ames, IA

Intraocular problems may include inflammatory diseases, glaucoma, lens abnormalities or fundic lesions. A thorough ocular examination is necessary to identify abnormalities so that an accurate diagnosis can be made and appropriate management planned.

Examination Techniques When performing an ophthalmic evaluation the examination room should be quiet to prevent the animal from being distracted, which makes the examination difficult, and all necessary equipment should be readily accessible. The room should also be able to be darkened to allow greater visualization of intraocular structures. Prior to detailed ophthalmic examination make preliminary patient observations regarding visual behavior, facial symmetry and note obvious ophthalmic abnormalities for closer evaluation. Vision assessment should be done prior to examination of the ocular structures. This may be accomplished by observing the animal as it walks to or in the room. The use of obstacles will help when setting up a maze test. The lights should initially be on and then turned off to determine if there is a change in behavior. Once the animal is on a table for examination a menace response should be elicited, being careful not to stimulate the cornea with air currents or touch facial hairs. Other cranial nerve reflexes should be assessed as well. Examine the animal at eye level (being seated on an adjustable chair/stool is ideal) with an assistant gently presenting the head for ocular examination. Retroillumination is the technique of aligning an examiner’s gaze with a light shone into the patient’s eyes from approximately 3 feet away to result in a fundic reflex. This can be done prior to dilation to help assess pupil size, symmetry, and general optical axis clarity but should also be done after dilation to help identify keratic precipitates, cataracts, vitreal opacities, or other subtle imperfections which may be more easily highlighted. The direct ophthalmoscope set on the large circle beam and held up to the examiner’s eye is useful for this technique. The smallest circle beam on the direct ophthalmoscope is a valuable setting which can be used as an illuminator to highlight changes in shape or location of structures. Holding the bright, focal ophthalmoscope beam very close to the patient’s eye (25 mmHg) that results in retinal and optic nerve damage. Clinical signs of acute glaucoma may include signs of pain (though not always), injected episcleral blood vessels, corneal edema, mydriasis, blindness, possibly other changes and is an ocular emergency! Patients with chronic glaucoma may have globe enlargement (buphthalmos), lens luxation or subluxation and retinal degeneration. Primary glaucoma is caused by an inherited malformation in the iridocorneal drainage angle which predisposes both eyes to glaucoma, while secondary glaucoma may be due to uveitis, hyphema, intraocular neoplasia, or lens luxation/subluxation. Distinguishing between acute and chronic glaucoma as well as primary and secondary glaucoma is important for determining the potential for vision and guiding treatment. Immediate emergency treatment and/or referral to a veterinary ophthalmologist is imperative for potentially visual eyes. Therapy for dogs with primary glaucoma may include the following: a topical prostaglandin analogue (latanoprost or

travoprost) SID-BID, topical carbonic anhydrase inhibitor (dorzolamide or brinzolamide) BIDTID, beta-blocker (timolol or betaxolol) BID, or a parasympathomimetic (compounded demecarium bromide) BID. Some therapies are contraindicated in certain patients (e.g. do not use timolol in patients with cardiac disease) or with some of the secondary glaucomas (e.g. do not use a prostaglandin analogue if an anterior lens luxation is present as miosis will worsen the situation). In addition to the therapies mentioned, emergency treatment with a hyperosmotic agent (mannitol IV or glycerin PO) may be needed in an acute case to rapidly reduce pressure or aqueocentesis can be performed if a patient is refractory to emergency medical therapy. Endstage surgery (enucleation, evisceration, or chemical ablation) is appropriate for irreversibly blind, painful eyes.

Cataracts Inherited cataracts are the most common cause of vision threatening lens opacities in dogs, affecting certain purebred dogs more than others. Diabetes mellitus is another common cause with 80% of dogs developing cataracts due to the osmotic pull of water by sorbitol locked in the lens. Currently there is no medical therapy proven to prevent or significantly reduce cataract formation and surgical removal remains the only way to restore vision to dogs blinded by cataracts. Pupil dilation with tropicamide is needed to allow thorough lens examination and monitoring of cataract progression. Early referral to a veterinary ophthalmologist is best for any clients potentially interested in cataract surgery as cataract surgery success is greater when operated at the immature stage and also because advancing cataracts may result in lensinduced uveitis, glaucoma, and/or retinal disease that could prevent the option for surgery. Diabetic patients, those with hypermature cataracts, or any patient with apparent lens-induced uveitis should be managed long term with a topical ophthalmic anti-inflammatory drug (e.g. diclofenac). Patients with active uveitis will need additional treatment with a topical corticosteroid until the inflammation is under control. Eyes with cataracts should be examined regularly for cataract progression, signs of intraocular inflammation and IOP should also be monitored if possible.

Lens Luxation/Subluxation Lens luxation (complete movement) or subluxation (partial shifting) may occur due to a primary abnormality in the supporting lens zonules affecting a number of terrier breeds. Partial lens shifting may manifest as asymmetric anterior chamber depth, an aphakic crescent or pupil abnormalities. Complete luxation that is posterior is not the surgical emergency that an anterior lens luxation is due to the lens blocking aqueous humor flow in the eye and causing secondary

glaucoma. Anterior lens luxation patients should be referred to a veterinary ophthalmologist immediately for lens removal surgery or trans-corneal reduction can be attempted. If IOP is elevated carbonic anhydrase inhibitors and mannitol can be administered. If a lens is subluxated or posteriorly luxated surgical lens removal can be considered or topical medications that cause long-acting miosis can be used BID to help keep the lens from shifting anteriorly (e.g. demecarium bromide or a prostaglandin analogue). If one eye is affected in a breed with the confirmed/presumed genetic zonular mutation the other eye should be monitored and possibly prophylactically treated as well. If lens luxation or subluxation occurs due to chronic glaucoma lens removal is not recommended, and many of these eyes are already irreversibly blind due the prolonged high pressure causing irreversible retinal/optic nerve damage while resulting in buphthalmos and subsequent zonular rupture.

Blindness General causes of blindness include anything that obscures the optical axis (e.g. severe corneal pigmentation, complete cataract, intraocular hemorrhage, etc.), retinal dysfunction, optic nerve dysfunction or cortical disease affecting the occipital lobe of the brain. If the fundus can be visualized the retina should be evaluated for appropriate location, blood vessel caliber/extent/branching, optic nerve head appearance, and tapetal reflectivity. Signs of retinal degeneration include vascular attenuation, optic nerve head pallor and tapetal hyperreflectivity. A common cause of retinal degeneration in dogs is progressive retinal atrophy (PRA), an inherited condition that manifests first as difficulty seeing in dim light. There is no proven therapy for this condition and vision loss is expected to progress to complete blindness but nutritional supplementation (lutein or OcuGLO® PO SID) can be attempted to possibly halt the disease. Sudden acquired retinal degeneration syndrome (SARDS) is another disease that causes blindness due to retinal dysfunction but occurs rapidly and initially without observable signs of retinal degeneration. Diagnosis is made by documenting no retinal activity on electroretinogram. Experimental therapy with intraocular injections may restore some degree of vision in a minority of patients treated early, but most dogs adapt well to blindness within a few months with ongoing good quality of life. Blindness not due to observable optical axis impediment or retinal disease may be due to optic nerve or central nervous system inflammatory disease, neoplasia, vascular accidents, or toxicity and may require more advanced testing for a diagnosis.