Ask the Vet: Why Do Dogs Go Blind Overnight?
Dr. Ivana Crnec
When dogs go blind overnight, it's important to determine the underlying cause. Dogs don't just go suddenly blind for no rhyme or reason. There may be several underlying causes and some of them may not even be related to their eyes. A thorough veterinary examination is therefore important. Veterinarian Dr. Ivana points out several reasons why dogs may go blind overnight.
A Dog's Sudden Loss of Vision
Blindness, particularly if sudden, is a distressing condition for both the dog and the dog parent. Dogs experiencing sudden blindness are likely to walk into walls and bump into furniture, showing confusion and obvious disorientation.
They are also reluctant to move and the eyes may or may not appear differently. Common changes include enlarged dog pupils, discolored or white pupils, abnormal redness or swollen appearance.
The reason causing sudden blindness can be located in the dog's eyeball or globe, the pupil (black pigmented center of the eye), the iris (colored portion of the eye), the lens (structure responsible for focusing light), the retina (sensory tissue of the eye responsible for processing light), the uvea (pigmented layer that protects the retina), the cornea (transparent layer covering the pupil and iris) and the optic nerve ( which connects the eye with the brain).
Why Do Dogs Go Blind Overnight?
The possible causes for a dog suddenly going blind overnight are several. Determining the exact underlying cause can sometimes require the intervention of a veterinarian. Following are several possibilities.
Car accidents, scratches on the face, falling from a height and exposure to smoke are examples of trauma that can cause damage to the eye, brain or optic nerve. The changes caused by the damage may or may not be reversible.
Certain systemic infections such as Toxoplasmosis or Blastomycosis can lead to sudden blindness. In such cases, the affected dog will show a plethora of additional signs and symptoms. If left untreated, the eye infections progress to ulcers. Once ulcers develop, the dog will start squinting, have visibly reddened and runny eyes.
Infections of different parts of the eye can trigger blindness. Common inflammations include neuritis (inflammation of the optic nerve), retinitis (inflammation of the retina) and uveitis (inflammation of the middle layer of the eye). Dogs with such inflammations are likely to exhibit squinting, redness and ocular discharge.
Acute blindness can be caused by brain tumors, nerve tumors, eye tumors and tumors located near the eye. Depending on the exact location of the tumor, the dog may or may not exhibit sighs other than blindness. For example, if the tumor is located on the brain the dog will show confusion, seizures and abnormal behavior.
Glaucoma occurs when the pressure inside the eye becomes too high. It can occur in one or both eyes at the same time. Dogs with glaucoma squint, have red and visibly enlarged or protruding eyes.
Certain brain and nerve diseases can cause blindness. In such cases, the dog will also show signs like confusion, seizures, mood changes, walking in circles and unusual behavior.
Toxins and medications
Overdosing with certain medications (such as the anti-parasitic drug ivermectin) may trigger sudden blindness. Common household products such as ethylene glycol (antifreeze) are powerful toxins and may also cause blindness. In both cases, the dog will manifest many other symptoms before becoming blind.
Cataract is a condition in which the transparent and clear lens becomes cloudy or even white. Based on the intensity of the discoloration and transparency loss, the dog may experience partial or complete blindness. Cataract occurs due to old age, genetics and diabetes mellitus.
Retinal detachment occurs when the sensory tissue loses contact with the rest of the eye. The condition is usually caused by trauma. Sometimes it can be caused by elevated blood pressure. A dog will retinal detachment will have pupils that remain enlarged regardless of the circumstances.
SARDS (Sudden Acquired Retinal Degeneration Syndrome)
Affecting around 4000 dogs each year, SARDS is a relatively rare condition. SARDS is more common in females than males and in middle-aged to old dogs than in young puppies. Both mixes and purebreds are equally susceptible to developing SARDS.
The underlying cause is not known and there is no suitable treatment. Two weeks before the acute blindness, affected dogs show excessive thirst, excessive urination, excessive hunger, weight gain and lethargy.
At the Vet's Office
As mentioned, finding the underlying cause for the blindness is important. In a dog gone blind overnight, the process of setting proper diagnosis includes taking an ophthalmic history , doing a vision assessment and located the causative lesions. Here's a close insight on the process.
The goal of the history gathering is to answer the following questions:
• Is the blindness partial or complete?
• Was the blindness onset gradual or acute?
• Was the vision loss initiated yesterday or two months ago?
• Is the overall appearance of the eye changed?
• Are there other systemic signs of disease?
• Is the dog on some medications or has it been exposed to certain toxins?
The vision is assessed through several tests:
Menace response – includes performing a menacing gesture towards the dog’s eyes and observing the response. A dog with proper vision will blink or move its head away from the threat.
Cotton ball test – performed by tossing a noiseless and scentless object such as a cotton ball in the dog’s visual field and checking its reaction. If the dog is not blind it will instinctively investigate the tossed object.
Visual placing reaction – suitable for small dogs. It is performed by holding the dog in the air and moving it towards a table. When close enough, a dog that is not blind will raise its legs to step onto the table. A blind dog will bump into the table’s edge.
Maze test – the dog and the owner are put on opposite ends of a maze and the dog is challenged to reach its owner.
You can read more about these tests: how to test your dog's vision at home.
Determining the exact location of the blindness causing lesion includes the following tests:
Pupillary light reflex (PLR) – performed in a dimly lit room by focusing a bright light source directly in the dog’s eye. Each eye is stimulated separately and two moments are evaluated – the reaction of the pupil and the presence of consensual response (the fellow eye should follow the movements performed by the stimulated eye). The reaction of the pupil is determined through three factors – how fast the pupil constricts, the extent of the constriction and how long the contraction lasts. If the PLR is normal, the lesion is probably located in the brain. If the PLR is abnormal, the lesion is located in the eye.
Ophthalmic examination – each layer and the fundus of the eye are examined with an ophthalmoscope.
Ocular ultrasound – performed when the fundus cannot be visualized due to hemorrhage or inflammation. This test is suitable for diagnosing retinal detachment and ocular neoplasia.
Blood pressure measurement – systemic hypertension may lead to fluid accumulation in certain structures of the eye which ultimately may cause retinal detachment.
Sudden blindness is considered an emergency and requires immediate veterinary attention. In some cases, depending on the underlying cause, instituting a prompt medical treatment may result in vision restoration. On the contrary, delayed diagnosing and treatment are associated with poorer prognosis.
About the Author
Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia.