DIAGNOSTIC OPHTHALMOLOGY

History and clinical signs

A 7-yeafigr-old, spayed female terrier-cross was referred to the ophthalmology service at the Atlantic Veterinary College (AVC) with a history of sudden onset of blindness. The owners had noticed that the dog was polyuric, polydypsic, lethargic, and had developed a ravenous appetite 2 wk prior to becoming blind. Physical examination was unremarkable, except for the ophthalmic system. The dog was unable to negotiate an obstacle course in photopic and scotopic conditions, and failed to demonstrate a menace response in either eye. The pupils were dilated, and direct and consensual pupillary light reflexes were sluggish bilaterally. The palpebral and oculocephalic reflexes were present. Schirmer tear test (Schirmer Tear Test Strips; Alcon Canada, Mississauga, Ontario) values were within normal reference ranges. The result of fluorescein dye (Fluor-I-Strip AT; Ayerst Laboratories, St. Laurent, Quebec) staining was negative. Applanation tonometry (Tonopen XL; Biorad Ophthalmic Division, Santa Clara, California, USA) revealed intraocular pressures of 17 mmHg and 19 mmHg in the right and left eyes, respectively. The use of a transilluminator and slit lamp biomicroscope (Kowa SL-14; Kowa, Tokyo, Japan) did not reveal any abnormalities within the anterior segments of either eye. Topical application of 0.5% tropicamide (Diotrope; Dioptic Laboratories, Markham, Ontario) caused complete dilation of the pupils, and indirect ophthalmoscopy (Keeler All Pupil Indirect; Keeler Instruments, Broomall, Pennsylvania, USA) revealed similarities in both fundi (Figure 1).

Figure 1. Photograph of the left fundus of a 7-year-old, spayed, female terrier-cross presenting with acute blindness.

Discussion

Our diagnosis was acute prechiasmal blindness. Our differential diagnoses included sudden acquired retinal degeneration syndrome (SARDS), bilateral optic neuritis (retrobulbar or intracranial, but not intraocular, as the optic discs appeared normal on indirect ophthalmoscopy), or optic chiasmal neoplasia, granuloma, compression, or trauma. Electroretinography, with or without advanced imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), and analysis of cerebrospinal fluid are diagnostic tests to identify the lesion. An electroretinogram (ERG) was completed. The dog was dark-adapted for 10 min, and a platinum reference electrode (Platinum Subdermal Electrodes; Grass Instruments, Quincy, Massachusetts, USA) was positioned, subdermally, approximately 1 cm caudal to the lateral canthus of the right eye. A platinum ground electrode was placed subdermally overlying the occipital tuberosity. Both eyes were anesthetized by using topical proparacaine (Alcaine; Alcon Canada). A gold-plated corneal contact lens (ERG-jet; Universo SA, La Chaux-de-Fonds, Switzerland) was placed on the right cornea by using a viscous tear supplement (Tear Gel; CIBA Vision Canada, Mississauga, Ontario). A light stimulator (Grass PS33 Photic Stimulator; Grass Instruments) was held 10 cm from the right eye and flashed. The ERG waveform (Nicolet, Spirit; Roxon, Montreal, Quebec), an average of 10 responses, was recorded. The ERG was repeated in the left eye. The ERG waveforms were extinguished (flatline) in both eyes (Figure 2a) as compared with those obtained from a normally functioning retina (Figure 2b). The flatline responses documented in this case confirmed a loss of retinal function in both eyes. The acute blindness in this dog, the normal fundic appearance of both eyes, and the flatline ERG responses confirmed a diagnosis of SARDS.

Figure 2a. Photopic electroretinogram response for the left fundus of the 7-year-old, terrier-cross. Note the completely extinquished a- and b-waves (flatline response).

Figure 2b. Photopic electroretinogram response for the left fundus of a 5-year-old German shepherd-cross with normal retinal function. Note the large negative deflection (a-wave) and larger positive deflection (b-wave) waveforms.

Sudden acquired retinal degeneration syndrome is an idiopathic, permanently blinding, bilateral canine retinal disease (1,2,3,4,5,6). The vision loss in dogs with SARDS is rapid and, generally, complete blindness occurs over a period of 24 h to 4 wk (1,2,3,4,5,6). The diagnosis of SARDS is based on normal appearing fundi on ophthalmoscopy, and an extinguished ERG, confirming loss of retinal photoreceptor cell function. Affected dogs are middle- to old-aged; and females are predisposed (1,2,3,4,5,6). There is no breed predilection. The primary presenting complaint is acute blindness. Owners often report polyuria and polydipsia, polyphagia, weight gain, and lethargy (1,2,3,4,5,6). Abnormal laboratory data may accompany the SARDS-affected patient. In particular, previous studies have reported elevations in serum alkaline phosphatase, serum alanine aminotransferase, serum cholesterol, and bilirubin levels (1,2,3,4,5,6). These serum biochemical panel abnormalities are suggestive of hyperadrenocorticism; although confirmatory endocrine test results (adrenocorticotropic hormone response test; low-dose dexamethasone suppression test) are often lacking in many dogs with SARDS.

In the early stages of SARDS, both fundi appear normal (Figure 1). Several weeks to months following the onset of blindness, the fundi of dogs with SARDS will develop generalized tapetal hyperreflectivity and attenuated or diminished retinal vessels; and their clinical appearance will be indistinguishable from those of dogs with chronic retinal degeneration due to other causes (genetic: progressive retinal atrophy; nutritional: vitamin A deficiency).

The etiology of SARDS remains unknown. There are several hypotheses as to its pathogenesis. Results from earlier studies suggested the involvement of defective lipid metabolism (7) or an autoimmune mechanism (8), in that 5/5 dogs with SARDS had circulating antiretinal antibodies. More recently, studies have focused on a possible toxic etiology with extracellular glutamate potentially playing a role in its pathogenesis (5). Apoptosis has also been reported as one mechanism of photoreceptor cell death in SARDS-affected dogs (5).

It is important to distinguish SARDS from other causes of acute blindness in dogs, because certain acutely blinding conditions may respond to treatment (optic neuritis). Unfortunately, there is no effective treatment for SARDS. However, SARDS is an ocular disease that does not result in concurrent life-threatening systemic illness. Most SARDS-affected dogs adapt to the blindness and are functional, happy pets.