Blindness

Blindness is a frequent occurrence in diabetic dogs, less frequent in diabetic cats. Hyperglycemia in dogs causes both cataracts and retinopathy, either of which alone can cause blindness in a few days or weeks. Blood glucose levels as low as 250mg/dL (14.0 mmol/L) can cause blindness in dogs.

Cats are much more fortunate in this regard -- their visual systems seem to be resilient to high blood sugar, and it's unusual (but not unknown) for them to go blind from it, though retinopathy is a possibility. This source says that cats do not get cataracts as a result of diabetes^[1].

Diabetic retinopathy is a special case of neuropathy in the optic nerve and retina, and is caused by high blood sugar. Glucose toxicity damages blood vessels that nourish the retina at the back of the eye. This progressively results in blurred vision. Severe vision loss may be preventable if the diabetic retinopathy is detected and treated early and appropriately. Treatment may maintain vision, though it rarely restores it.

Mechanisms of hyperglycemic blindness in dogs[]

Cataracts in diabetics are caused by high blood sugar. A cataract is an opacity in the lens of the eye. The entire lens may be involved or just a part of it. The patient will not be able to see through the opacity.

The eye fluids normally contain some amount of glucose, which the lens uses to nourish its cells. When the eye fluids contain too much glucose, some is converted to sorbitol, which is absorbed by the lens but cannot be consumed^[2]. The lens becomes hypertonic and pulls extra water into the lens, which damages it and makes it lose transparency. This damage to the lens is permanent, though lens transplants are possible.

Retinopathy (see first section) is more common in dogs than cats.

It is fortunate that dogs rely so much on their senses of smell and hearing, and can therefore lead relatively good quality lives while blind.

Dogs with diabetes have more "at risk" eye problems, other than cataracts and blindness. They can be prone to recurring or nonhealing ulcers of the eyes. This 2003 study found that dogs with diabetes have significantly reduced sensitivity of their corneas. This means that the nerves of the corneas do not function normally. The study indicates this to be true no matter what the glycemic control is and length of time with diabetes^[3].

Blindness in diabetic cats[]

Hypoglycemia[]

Cats are sometimes rendered temporarily or permanently blind by severe hypoglycemic episodes. Hypoglycemia can cause severe brain damage and that may include blindness. See the discussion on FDMB^[4].

Hypertension[]

People (and possibly cats) with diabetes (and especially those with Chronic renal failure) are more prone to suffering from hypertension (high blood pressure) than their non-diabetic counterparts.

Abnormally dilated pupils^[5] over a few days may mean it's time for a blood pressure check at the vet's. High blood pressure (hypertension) in cats^[6] can cause blindness^[7]^[8], severe renal (kidney) damage, and other bodily damage^[9].

The problem with hypertension in animals, as in people, is that there are no overt symptoms--no signs of not feeling well--until the disease is far-advanced. According to Dr. Lisa Pierson, all cats with CRF should have their blood pressure monitored. Many vets do not have blood-pressure monitors, so check around.

See the Long Beach Animal Hospital^[10] link for the use of Doppler equipment for measuring feline blood pressure, and download a QuickTime movie of the Doppler unit in action.

References[]

[1] Cats, Diabetes & Cataracts

[2] Cataracts in Dogs-Drs. Foster & Smith Pet Education Library

[3] Canine Diabetes & Eye Problems

[4] Complications Discussion

[5] FDMB-Abnormally dilated pupils

[6] Feline Systemic Hypertension--WSAVA 2003

[7] Photo of Blindness Caused by Hypertension

[8] Blood Pressure Assessment in Healthy Cats and Cats with Hypertensive Retinopathy-American Journal of Veterinary Research-2004

[9] Feline Hypertension: Risks & Management--WSAVA 2005

[10] Long Beach Animal Hospital Feline Hypertension/Doppler Equipment

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