Responding to the Reader
What is ocular stoke?
Story by Connie Bryson/Photo by iStockphoto
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Most of us know about stroke—an obstruction of blood flow leading to part of the brain, and one of the most prevalent causes of death and disability—but ocular stroke is a different category of stroke. “I prefer to use the term ‘ocular vascular events’ to describe the variety of syndromes that affect blood flow to the eye and, therefore, affect vision,” explains Dr. Naeem Dean, director of the Stroke Program at Edmonton’s Royal Alexandra Hospital. “Ocular stroke makes it sound as if there’s only one disorder and only one cause. It’s much more complicated than that.”
While there are many types of ocular vascular events, Dr. Dean identifies five main ones.
Amaurosis fugax is transient loss of vision in one eye due to a temporary lack of blood flow to the retina. It is similar to a transient ischemic attack of the brain (TIA or mini-stroke) and may be a sign of an impending stroke. Amaurosis fugax happens when a small clot breaks off from a main artery or the heart and lodges in the vessels of the eye. When the clot breaks up, blood flow resumes and vision returns.
Retinal artery occlusion occurs when a clot lodges in the main retinal artery and does not break up (as in amaurosis fugax). The lack of blood supply will damage the retina, and vision will remain impaired.
Retinal vein occlusion is the blockage of the central retinal vein or one of its branches, which may impair vision, often permanently. The central retinal vein drains blood away from the retina. The problem is usually caused by underlying vascular disease, not by clots breaking off arteries or the heart.
Anterior ischemic optic neuropathy involves loss of vision due to damage to the optic nerve from insufficient blood supply. Vision loss is usually sudden and is often permanent, with some recovery possibly occurring within the first weeks or months. There are two major categories of ischemic optic neuropathy: non-arteritic ischemic optic neuropathy, which is assumed to be the result of underlying cardiovascular risk factors, and arteritic ischemic optic neuropathy, which is caused by the inflammation of vessels supplying blood to the optic nerve.
Ocular migraine causes vision loss or impairment in one eye in comparison to classical migraines, which cause visual impairment in both eyes. Ocular migraines are reoccurring and temporary; their exact cause is unknown.
Although ocular vascular events have different causes, in most cases there are cardiovascular risk factors common to all types. These include high blood pressure, diabetes, high cholesterol, sedentary lifestyle, and smoking. In Edmonton, most of the patients diagnosed with an ocular vascular event are sent to Dr. Dean’s stroke prevention clinic at the Royal Alexandra Hospital for assessment of their risk factors.
“Most of the patients we see in the clinic have modifiable risk factors that increase their risk of not only another ocular vascular event but also a brain stroke or a heart attack,” notes Dr. Dean. “As a stroke physician, an important part of my job is to educate patients about how they can monitor their vascular risk factors to prevent stroke.”
Over the past seven years, Dr. Dean and his team have amassed a database on approximately 1,500 patients who have had an ocular vascular event. The database contains information about diagnosis and associated risk factors. The team won a 2010 Award for Excellence in Ophthalmic Research from the Canadian Ophthalmological Society for their work on the database.
“We started the database because we wanted to understand more about risk factors and ocular vascular events,” says Dr. Dean. “There’s still much more to learn. For example, conventional thinking holds that individuals who have had transient visual loss (amaurosis fugax) are at lower risk for a brain stroke than those who have had a TIA. We are skeptical about this because of our experience in the ocular stroke clinic. We hope that long-term follow-up of the patients in our database will provide definitive evidence as to whether or not this hypothesis is correct.”