Sudden Acquired Retinal Degeneration Syndrome: An Overview

Sudden acquired retinal degeneration syndrome (SARDS) is a permanently blinding disease that occurs suddenly, as the name suggests. It is one of the leading causes of incurable canine vision loss diagnosed by veterinary ophthalmologists. Much research has been done to try and discover both the cause and a cure; however, as yet, neither has been found. This article provides an overview of the current knowledge about SARDS and the role of the veterinary nurse in identifying and managing these patients.

The predominant signalment of a SARDS patient is a middle-aged, spayed female, mixed-breed dog. Of the purebred dogs, small breeds are most commonly affected (BOX 1). The median age at diagnosis is 7 to 10 years. Often, the dog is moderately overweight.

History and Clinical Signs

Owners of dogs with SARDS present their pets with the primary complaint of sudden blindness. They often report that their dog bumps into stationary objects, becomes confused in corners, or seems lost in large, open spaces. In these situations, the dog may become anxious and begin vocalizing and/or panting excessively. Astute owners may notice the dog’s pupils are dilated.

During the history taking, the veterinary nurse should ask whether the owner has observed polyphagia, polydipsia, polyuria, and recent weight gain. Up to 85% of dogs with SARDS have these systemic signs.² One study reported that 26% of dogs diagnosed with SARDS also presented with conjunctival hyperemia.³ It is unclear whether this finding is related to SARDS, as conjunctival hyperemia has many nonspecific causes.

Although most SARDS patients have no neurologic abnormalities, loss of smell has been reported in a few cases.¹ It is unknown how this is linked to the loss of vision.

Patients that present with signs of hyperadrenocorticism should be tested with adrenocorticotropic hormone stimulation or low-dose dexamethasone suppression testing.

Ophthalmic Examination

After a thorough history is taken, and before the complete physical examination is performed, the ophthalmic examination should take place. It can begin with the veterinary nurse observing the patient moving around the examination room and noting if it bumps into stationary objects. If this is not obvious, a maze should be set up in the room and the dog encouraged to walk through it in both light and dark conditions.

Some acutely blind dogs have a very wide-eyed appearance, as if they are opening their eyes as much as possible trying to see (FIGURE 1). The veterinary nurse should discern if this is the case or if the patient is exophthalmic or buphthalmic. Next, the palpebral reflex should be evaluated to determine whether the patient can blink. Some dogs that do not blink during menace or dazzle testing are misdiagnosed as blind when they have facial nerve paralysis.

Figure 1. Classic appearance of a dog with SARDS. Note the wide eyes and dilated pupils. This dog is middle-aged and overweight.

After confirmation that the patient can blink, the menace reflex should be evaluated. To perform this test, the examiner covers one of the patient’s eyes with their hand and makes a menacing gesture toward the other eye with the other hand, taking care not to create an air current that could be felt by the patient. Dogs with SARDS do not react to the menacing motion (i.e., have a negative result).

The ability to track motion can be evaluated by dropping a cotton ball or tissue in front of the patient. The object used should have no smell and make no noise. Most animals that can see follow the dropped object, but occasionally they are not interested in it.

Using a transilluminator or other suitable bright, white light source, the veterinary nurse should begin to evaluate the pupils. The pupils should be symmetric in size. Almost all (90%) dogs with SARDS have mydriatic pupils with a slight pupillary light reflex—that is, the pupils constrict slightly in response to bright light.² This constriction is slow and incomplete. The same bright light source should also be used to evaluate the dazzle reflex. Dogs with SARDS do not have a dazzle reflex, meaning they do not blink or pull away in response to the bright light being shone in their eye. In these patients, the retina does not perceive the light.

Next, the anterior chamber should be evaluated. Other than the mydriatic pupils, the anterior segment examination should be normal in cases of SARDS. There should be no signs of inflammation or anything obscuring vision.

The posterior segment and fundus should be evaluated next. The vitreous should be clear, and there should be nothing blocking the examiner’s view of the fundus. The fundus itself should have no abnormalities. The exception is in patients that have been blind for several weeks. In these patients, fundic changes may be noted, such as tapetal hyperreflectivity, blood vessel attenuation, and optic disc atrophy.

Patients with dilated pupils should also have intraocular pressure measured to rule out glaucoma.

To confirm a diagnosis of SARDS versus a neurologic disconnection between the eyes and the occipital cortex, referral to an ophthalmologist for electroretinography is recommended. Referral to a neurologist to rule out other neurologic deficits or diseases that imitate SARDS may also be offered. The veterinary nurse can assist the client in scheduling these appointments.

Diagnosis

Electroretinography tests the electrical function of the retina. The resulting chart, or electroretinogram (ERG), is similar to an electrocardiography (ECG) or electroencephalography (EEG). Every ophthalmology practice has its own set protocol and baseline normal values for the ERG system and software it uses. In the author’s practice, electroretinography is performed on awake animals after 20 minutes of dark adaption and pharmacologic pupil dilation. Dark adaption allows for complete relaxation of the rods and cones so that maximal response is recorded. Three electrodes—ground, reference, and active—are attached to the patient. A bright white light is then flashed into the eye, and the active electrode picks up the electrical response of the retina, which is transferred to the computer and reported in a graph measured in microvolts. In dogs with SARDS, no electrical response from the retina is seen. This is reported as a flat ERG (FIGURE 2). A flat ERG distinguishes SARDS from optic neuritis and, in addition to the classic signalment and history, is enough for a diagnosis of SARDS.

Another test that can be used to diagnose SARDS, the chromatic pupillary light reflex, uses a special instrument that emits red and blue light at specific wavelengths. In one study, healthy eyes responded to low intensities of both red and blue light, but in dogs with SARDS the pupil responded only to high-intensity blue light.⁴ This is due to stimulation of a photosensitive pigment called melanopsin.

Etiology and Pathogenesis

SARDS is caused by a widespread loss of function of the photoreceptors in the retina, but researchers have been unable to discover the cause of this loss. Most research leans toward an autoimmune disease, although a neuroendocrine disease process has not been completely ruled out. Some researchers believe that SARDS is used to describe a multidisease process.

One study reported that more than 90% of SARDS patients have increased adrenal sex hormone and/or cortisol serum concentrations.⁵ Another article reported that in Canada, the most common laboratory abnormality is elevated liver enzymes and hypercholesterolemia.⁶

Some veterinarians recommend pituitary and adrenal gland imaging. This can help determine if there is a neuroendocrine component.

A very recent study evaluated optical coherence tomography (OCT) in dogs diagnosed with SARDS.⁷ It found that 38% of the patients in the study had a <1 mm wide retinal detachment that was not diagnosed during the ocular examination. This study also looked at microarray and histology analysis of enucleated eyes diagnosed with SARDS and found molecular changes that were suggestive of immune-mediated disease. This study concludes that these observations are highly suggestive of an immune-mediated cause of SARDS.

Treatment

Unfortunately, there is no treatment for SARDS. Various treatments have been tried to restore vision and have failed. One study evaluated the use of mycophenolate mofetil, a strong immunosuppressive medication with minimal systemic side effects, in 10 dogs diagnosed with SARDS.⁸ After 6 weeks of treatment, no increase in vision or ERG was observed. Other areas of interest in ongoing research include a canine-specific intravenous immunoglobulin (IVIg) and treatment for atypical hyperadrenocorticism.

For the past 12 years, Dr. Sinisa Grozdanic has been testing the use of human IVIg in SARDS patients. Dr. Grozdanic’s work is based on the theory that many SARDS patients have a history of autoimmune disease and that there is clinical evidence confirming that SARDS is an immune-mediated disease. He has identified 4 stages of SARDS and reported success with early treatment with a combination of systemic immunosuppressive therapy and IVIg.⁹ In these studies, positive vision is considered being able to pass a maze test.¹⁰ However, these findings are still considered controversial and have yet to be peer reviewed.

Prognosis and Client Education and Communication

Blind dogs can live very happy lives. Owners of dogs with SARDS report that if anything, their relationship with their pet improved after the sudden blindness.¹¹ It does take some time—up to a couple of months—for the dog to adjust to being blind, and adjustment is harder with sudden vision loss than with gradual blindness. However, dogs have cognitive mapping skills and can easily memorize their home environment. They may still become disoriented outside of their home or yard.

A few accommodations may need to be made for safety reasons. Blind dogs should be kept in a fenced yard or on a leash when outside. Access to bodies of water should be restricted. Even the best swimmers may become lost in the water and unable to find shore or the stairs to get out of a pool. Owners should avoid moving furniture as much as possible to avoid confusing the dog’s mental map of the environment and may want to consider padding sharp corners to prevent injuries.

Other changes can help dogs with their mental map. For example, different textured surfaces can be used to indicate food, water, door, stairs, walls, or fences. Placing rocks 6 to 12 inches inside the yard fence can serve as a warning track that the fence is near.

Several websites provide advice for living with blind dogs, such as providing toys that make noise or have a strong scent (BOX 2).

The veterinary nurse should discuss learning to live with the concurrent systemic signs (e.g., polyphagia) with the owner. It has been noted that polyphagia is the only associated systemic sign to increase in severity over time.¹¹

Conclusion

Blindness is not a reason for euthanasia. Hopefully, ongoing research and clinical trials will someday reveal a treatment or cure for SARDS. In the meantime, the veterinary nurse can provide counseling and training to help the owner and patient return to the happy life they had before blindness occurred, keeping in mind the transition is harder on the owner emotionally than it is for the dog.