Peer Reviewed DIAGNOSING ACUTE BLINDNESS IN DOGS

Diagnosing Acute Blindness in Dogs

Caryn E. Plummer, DVM, Diplomate ACVO University of Florida

Vision loss can occur gradually or manifest quite different depending upon the onset and acutely in dogs, but acute and complete blindness duration of the deficits. can be particularly devastating. The abrupt The history should determine: nature of this blindness is very disconcerting • Whether vision loss is partial or complete. for all involved and pet owners may make hasty • Whether vision loss is acute or has been conclusions and decisions. developing gradually: Did the dog have The diagnostic approach to these patients functional vision yesterday, but blindness today should include: or has it been gradually losing vision and now is An Eye 1. Ophthalmic history identifying the onset and completely blind? Toward duration of blindness, degree of blindness (as • When signs of vision loss developed: Did signs of perceived by the owner), other signs of disease, vision loss manifest yesterday or 2 months ago? Anxiety and medication regimen • Whether the appearance of the eye has changed. 2. Vision assessments, including menace If so, when was this change noted? This When a dog is response, visual placing, and “maze” tests, that information is important because, in many presented for acute confirm whether the patient is blind instances, the physical appearance of the eye may blindness, or what 3. Causative lesion localization by pupillary change over time. is perceived as • Systemic signs of disease are present, or acute blindness, light reflex examination of the eye, potentially it is important with ocular ultrasound, blood pressure whether a systemic problem has been to proceed with measurement, electrophysiologic testing, previously diagnosed. Note that many systemic patience and specifically electroretinogram. diseases (eg, infectious disease, lymphoma, care because the hypertension) may initially be recognized by 1,2 patient is likely HISTORY their ophthalmic manifestations. anxious and upset, A thorough general and ophthalmic history • What medications the patient receives/has while its owner is is crucially important to the investigation of received, both chronically and more recently, often distraught. blindness because differential diagnoses can be including inadvertent administration/ingestion. Slow movements, For example, has the dog recently received or accompanied ingested ivermectin? by a calm voice, facilitate the VISION ASSESSMENT examination, may As the history is being gathered, confirmation soothe or allay of vision—or the lack thereof—should be some anxiety, and performed. Note that some patients—those make the clinician’s with neurologic disease and aged animals with presence known to cognitive dysfunction—may behave as if they the dog at all times. are visually impaired even though their visual systems are functional.

Menace Response Vision requires functioning central and FIGURE 1. Complete resting mydriasis in a peripheral ophthalmic systems, and may be young Chihuahua with bilateral optic neuritis. roughly assessed with a menace response. The Both direct and consensual PLRs were absent. menace response test is performed by making

18 TODAY’S VETERINARY PRACTICE | An Official Journal of the NAVC | November/December 2016 | tvpjournal.com DIAGNOSING ACUTE BLINDNESS IN DOGS Peer Reviewed

a menacing gesture with the hand toward the LESION LOCALIZATION patient’s eye. Take care not to touch the vibrissae The next step in the evaluation of the blind patient Eye or cause excessive air currents, both of which is to determine where the causative lesion is located. stimulate the sensation of touch rather than Is the patient blind because something is obscuring Evaluation sight, potentially inducing a false-positive result.2 the visual axis, such as pigmentary keratitis, corneal For the menace If the animal can see, it should blink or move its edema, or a cataract? Or is the patient blind because response, cotton head away from the stimulus. the retina or central nervous system is at fault? ball test, and visual placing reaction, Cotton Ball Test Pupillary Light Reflex each eye should The patient’s vision can be further evaluated by The size of the pupils and the direct and consensual be evaluated noting its response to cotton balls (or some such (response in the fellow, non-stimulated eye) separately by noiseless, scentless object) tossed into the visual field pupillary light reflexes (PLRs) are very important covering each in or observing the visual placing reaction. for lesion localization (Figure 1).2 These turn and testing the assessments should be performed with a bright light exposed eye. Visual Placing Reaction in a dimly lit room. Visual placing is assessed most easily in small The PLR evaluates: patients that are able to be held. For this test, the • Rapidity of pupillary light response patient is held in the examiner’s arms so that the • Extent of miosis Dark forelimbs dangle freely. The dog is moved slowly • Ability to maintain miosis to constant light adaption toward a table or other elevated flat surface. stimulation. As the limbs approach the edge of the table/ The consensual pupillary reflex is normally equal to is the adjustment flat surface, if visual and able to respond, the the direct. of the eye to low dog will raise its limbs in order to step onto the The PLRs require integrity of retinal neural light intensities, table.2 If the dog does not see the table, it will cells, optic nerves, optic chiasm, optic tracts, involving reflex not raise its limbs, allowing them to bump into midbrain (Edinger-Westphal nucleus), and dilation of the pupil the edge. parasympathetic fibers via the oculomotor and activation of nerve, ciliary ganglia, and the iridal sphincter the rod cells in Maze Test musculature, but integrity of the cerebral preference to the In patients with suspected blindness, an obstacle cortex is not required (Figure 2). The reflex is, cone cells. course or “maze test” may be used to determine whether vision is present. Traffic cones, foam cylinders, or even examination room furniture, such as chairs and waste cans, suffice, although elaborate mazes may be constructed for standardized testing. The dog should be placed at the opposite end of the maze from its human companion, who is asked to call the dog’s name only once, which keeps the dog from following voice cues in order to maneuver. Vision should be evaluated in normal light and then dim light (after dark adaption) and obstacles should be adjusted between tests to avoid memorization and mapping. To assess night vision, dim the ambient illumination until you can barely distinguish the room furniture and maze course obstacles. Normally sighted dogs have better developed night vision than humans; therefore, the patient should be able to see the maze obstacles better than FIGURE 2. The pathway of the pupillary light reflex. the examiner.3

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in a focused (non-scattered) manner TABLE 1. • Lack of response by the retina to light stimulation Acute Blindness & Pupillary Light Reflexes • Inability of the optic nerve and tracts to transmit VISION PLR LESION LOCALIZATION the electrical response of the retina RESULTS • Inability of the occipital cortex to process the Normal No lesion Visual information supplied by the eyes and optic pathways. Visual Abnormal Lesion in efferent pathway Lesions obscuring the visual axis are often dictating pupillary constriction (iris sphincter relatively easy to diagnose and may develop slowly muscle atrophy, lesion in or rapidly. Table 3, page 22, provides a list of ocular oculomotor cranial nerve III) lesions that may present as an acute onset of vision Avisual Normal Lesion obscuring visual axis loss. (corneal pigment or edema, Because vision can potentially be restored with cataract) or interfering with cortical processing of visual appropriate management, rapidly developing information (brain or central cataracts, corneal ulcers, severe uveitis, intraocular nervous system disease) hemorrhage, and anterior lens luxation should Avisual Abnormal Lesion in retina or optic be considered urgent or emergent depending nerve of affected eye on severity. Acute glaucoma should always be considered an emergency. therefore, subcortical and should be considered A thorough fundic examination is the next step an evaluation of the integrity of the retina and optic if the: tracts, not of vision.2 • Anterior segment (ie, cornea, anterior chamber, In general, in dogs with vision impairment, when and lens) appears normal PLRs are (Table 1): • Anterior segment abnormalities found are not • Absent or diminished: The lesion is likely located severe enough to account for the degree of vision in the retina or optic nerve. loss noted. • Intact: The lesion often obscures the visual axis Often, if the retina or optic nerve is affected, or interferes with the cortical processing of the pupil is dilated, making pharmacologic visual information. mydriasis unnecessary.

Examination of the Eye Optic Nerve Because the eye can often be visualized to the level The optic nerve should be thoroughly examined for of the posterior segment (in its normal state), a evidence of disease or inflammation. complete ophthalmic examination can provide a rapid and accurate diagnosis for many ophthalmic diseases (Table 2). During the ophthalmic examination, keep in mind the general causes of vision loss: • Lesions that prevent light from reaching the retina

Keep in Mind Remember, PLRs are affected by the psychological state of the animal, room During PLR illumination, age, many topical and systemic drugs, and the intensity of the light stimulus. If an animal is highly nervous or frightened, the pupils may be dilated and respond poorly to low-intensity light. However, with acclimation or a strong light source, this effect is minimized. Older animals may exhibit slow and incomplete FIGURE 3. Iris atrophy in a Yorkshire terrier has PLRs resulting from atrophy of the iris sphincter resulted in mydriasis and an incomplete pupillary muscle. This response is common in small dogs, light reflex. Note the scalloping of the pupil especially poodles. The pupil margin may have an margin and the holes in the temporal iris stroma. irregular or scalloped appearance or an irregular This aged individual also has an immature pupil shape, referred to as dyscoria (Figure 3). cataract.

20 TODAY’S VETERINARY PRACTICE | An Official Journal of the NAVC | November/December 2016 | tvpjournal.com DIAGNOSING ACUTE BLINDNESS IN DOGS Peer Reviewed

TABLE 2. Acute Blindness: Diagnostic Approach DIAGNOSIS VISION LOSS PLR VISUAL AXIS FUNDUS OTHER DIAGNOSTICS Anterior lens Acute or Impaired, depending on Obscured Visualization of fundus Measure IOP; consider ocular luxation chronic position of lens and IOP may be limited ultrasound Chorioretinal Acute or Normal or abnormal, Variably affected, Tapetal hyporeflectivity Pursue systemic inflammatory/ inflammation chronic depending on degree anterior uveitis neoplastic disease workup of involvement and may be present severity concurrently

Cortical Acute or Normal Normal Normal Pursue electrophysiologic testing, MRI/ disease chronic CT, CSF analysis, systemic inflammato- ry/neoplastic disease workup

Corneal Acute Normal or abnormal, Obscured Visualization of fundus Consider corneal culture and cytology; ulcers/ depending on position may be limited evaluate for concurrent anterior uveitis perforation of iris and degree of anterior uveitis present

Diabetic Acute or Normal Obscured Visualization of fundus Measure IOP; evaluate for lens-induced cataracts chronic may be limited anterior uveitis Glaucoma Acute Abnormal (mydriatic) Obscured Visualization of fundus Evaluate for concurrent ocular conditions (acute) may be limited (eg, uveitis, lens luxation) that would indicate that glaucoma is secondary

Glaucoma Acute or Abnormal (mydriatic) Variably affected Optic nerve recessed or Evaluate for concurrent ocular conditions (chronic) chronic atrophic (eg, uveitis, lens luxation) that would indicate that glaucoma is secondary

Intraocular Acute or Normal or abnormal, Obscured Visualization of fundus Consider systemic blood pressure and hemorrhage chronic depending on etiology may be limited ocular ultrasound Ocular Acute or Normal or abnormal Variably affected Visualization of fundus Consider ocular ultrasound and neoplasia chronic may be limited measure IOP Optic neuritis Acute Abnormal Usually normal, Optic nerve raised, swol- Pursue MRI/CT, CSF analysis, systemic (acute) unless there len, or hemorrhagic (optic inflammatory/neoplastic disease is concurrent disc); may be unremark- workup + neurologic examination anterior uveitis able if retrobulbar optic nerve is solely affected

Optic neuritis Acute or Abnormal Usually normal Optic nerve recessed or Pursue electrophysiologic testing, MRI/ (chronic) chronic atrophic CT, CSF analysis, systemic inflammato- ry/neoplastic disease workup

Progressive Gradual onset Normal or abnormal Usually normal; Tapetal hyperreflectivity Consider genetic testing in purebred retinal cataracts often dogs atrophy develop over time Retinal Gradual onset Abnormal Usually normal Tapetal hyperreflectivity, Complete ophthalmic examination and degeneration (usually) retinal vascular history (Toxins? Antibiotics? Medica- attenuation tions? Historical ophthalmic disease?)

Retinal Acute Abnormal Usually normal, Retina edematous, Consider ocular ultrasound; measure detachment unless there is displaced anteriorly; systemic blood pressure; consider hemorrhage or hemorrhage may be systemic/vascular/inflammatory/ anterior segment present; visualization of neoplastic disease workup involvement fundus may be limited

Retinitis Acute Abnormal Usually normal Normal Pursue electrophysiologic testing SARDS Acute Normal or abnormal Normal Initially normal; tapetal Pursue electrophysiologic testing hyperreflectivity and vascular attenuation develop over time

Uveitis Acute Abnormal (miotic) Obscured Visualization of fundus Evaluate for presence of concurrent (severe) may be limited ophthalmic disease (cataract); measure IOP; consider systemic inflammatory/ neoplastic disease workup CSF = cerebrospinal fluid; CT = computed tomography; IOP = intraocular pressure; MRI = magnetic resonance imaging; PLR = pupillary light reflex; SARDS = sudden acquired retinal degeneration syndrome

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first, followed by decreasing day vision and then TABLE 3. complete blindness. In both advanced PRA and Lesions Obscuring the Visual Axis & SARDS, vascular attenuation and diffuse tapetal Resulting in Acute Blindness hypereflectivity develop; a dog with SARDS, though, Anterior lens luxation Corneal ulcers or perforation has a normal fundus initially and becomes acutely Diabetic cataracts blind. Physical evidence of retinal degeneration a Glaucoma (tapetal hyperreflectivity and vascular attenuation) Intraocular hemorrhage Severe uveitisb develops over time in SARDS patients. a. Generally manifests with episcleral injection, corneal Tapetal hyporeflectivity (dull tapetal appearance) is edema, and mydriasis seen with chorioretinal inflammation. Since much of b. Recognized by presence of episcleral injection, corneal edema, aqueous flare, fibrin, and miosis the retina must be inflamed for obvious vision loss to be present, diagnosis is often easily made by fundus If the optic nerve is raised, swollen, or examination. hemorrhagic, optic neuritis is a likely diagnosis. A variety of infectious or inflammatory diseases and Ocular Ultrasound neoplastic processes may result in optic neuritis, If the fundus cannot be visualized due to vitreal which usually manifests with an acute onset of hemorrhage or inflammation, an ocular ultrasound marked vision loss. Once diagnosed, neurologic should be performed to assess for conditions that evaluation should be pursued as other neurologic result in ocular hemorrhage: retinal detachment and deficits often are present concurrently. ocular neoplasia. Conversely, if the optic nerve is recessed or atrophic, the changes present are chronic. This Blood Pressure Measurement condition can occur due to damage from chronic If retinal detachment is identified—either by fundus glaucoma, retinal degeneration, or chronic optic examination or by ultrasound—or if posterior neuritis. Optic nerve disease carries a guarded to poor segment hemorrhage is present, systemic blood prognosis for vision return. pressure should be measured. As a result of increased hydrostatic pressure in the vasculature, systemic Retina hypertension can cause fluid accumulation in the Examination of the retina should include both subretinal space, which displaces the retina into the the tapetal and non-tapetal regions and the retinal vitreal chamber, resulting in retinal detachment.1,4 vasculature. The appearance of the tapetal fundus Retinal detachments can also occur: should be carefully evaluated, particularly its • With congenital or developmental disorders (eg, reflectivity. collie eye anomaly) Generalized tapetal hyperreflectivity (excessively • Secondary to inflammatory disease (eg, shiny tapetal appearance) indicates retinal thinning chorioretinal inflammation), vitreal degeneration, and degeneration, which can be associated with: cataracts, or lens luxation • Gradual onset of vision loss, as found with • As a complication of intraocular surgery (eg, progressive retinal atrophy (PRA) following cataract surgery). • Acute onset of vision loss, as found with sudden acquired retinal degeneration syndrome (SARDS). Electrophysiologic Testing Dogs with PRA generally lose night vision If the fundus appears normal and the animal is avisual, especially if vision loss was acute, consider CARYN E. PLUMMER the following retinal conditions: Caryn E. Plummer, DVM, Diplomate ACVO, is an associate professor • SARDS (as hyperreflectivity occurs over time) in comparative ophthalmology at University of Florida College of • Retinitis (typically immune-mediated) Veterinary Medicine, where she also serves as chief of the compar- ative ophthalmology service. Her research interests include corneal • Cortical disease wound healing and glaucoma, and she has lectured extensively— • Retrobulbar optic neuritis (optic disc appears both nationally and internationally—on many topics associated with normal but retrobulbar optic nerve is affected). clinical veterinary ophthalmology and animal models of ophthalmic disease, especially glaucoma. Dr. Plummer received her DVM from These conditions can usually be differentiated with University of Florida; then she completed an internship in small an- PLRs and electrophysiologic testing. Abnormal PLRs imal medicine and surgery at Michigan State University and a resi- are typically associated with retinal or optic nerve dency in comparative ophthalmology at University of Florida. disease and normal PLRs with cerebral cortical disease.

22 TODAY’S VETERINARY PRACTICE | An Official Journal of the NAVC | November/December 2016 | tvpjournal.com DIAGNOSING ACUTE BLINDNESS IN DOGS

Electroretinogram Sometimes a normal PLR is present with retina and optic nerve disease, even if the animal is avisual. An electroretinogram (ERG) can be used to differentiate vision loss due to retinal disease (abnormal ERG result) from vision loss due to disease of the optic nerve or cerebral cortex (normal ERG result).3

Further Diagnostic Testing Animals with cortical or optic nerve blindness should be evaluated with: • Magnetic resonance imaging or computed tomography • Cerebrospinal fluid analysis • Generalized workup for systemic inflammatory or neoplastic diseases. In addition, a normal ophthalmic examination should prompt consideration of the central nervous system as the primary site of disease, especially when the PLRs are normal.

IN SUMMARY Acute vision loss in the dog is generally considered an emergency and warrants prompt evaluation by a veterinarian to confirm vision loss, localize the causative lesion, and institute therapy. In some cases, prompt medical treatment will result in return of vision. Delayed care carries a poor prognosis for sight and may delay diagnosis of a significant systemic condition.

ERG = electroretinogram; PLR = pupillary light reflex; PRA = progressive retinal atrophy; SARDS = sudden acquired retinal degeneration syndrome

References 1. Webb AA, Cullen CL. Neuro-ophthalmology. In Gelatt KN (ed): Veterinary Ophthalmology, 5th ed. Ames, IA: Wiley-Blackwell, 2013, pp 1820-1896. 2. Webb AA, Cullen CL. Ocular manifestations of systemic disease: The dog. In Gelatt KN (ed): Veterinary Ophthalmology, 5th ed. Ames, IA: Wiley- Blackwell, 2013, pp 1897-1977. 3. Ekesten B. Ophthalmic examination and diagnostics: Electrodiagnostic evaluation of vision. In Gelatt KN (ed): Veterinary Ophthalmology, 5th ed. Ames, IA: Wiley-Blackwell, 2013, pp 684-702. 4. Ofri R. Retina. In Maggs DJ, Miller PE, Ofri R (eds): Slatter’s Fundamentals of Veterinary Ophthalmology, 4th ed. St. Louis: Saunders, 2008, pp 285-317.

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