Eye Disease and Cell Damage


Image: Over 8 million Canadians are affected by vision loss.

Blinding eye disease can occur in many different parts of the eye. Here are some of the most common. Many of these conditions are studied at the Donald K Johnson Eye Institute.

Corneal Disease

The clear round dome at the front of the eye is called the cornea. It protects the inner eye from damage. It has no blood vessels but is instead nourished and cleaned by tears and by the layer of clear fluid (the aqueous humor) behind it. The cornea self-repairs scratches and other damage, but severe injuries and infections can damage the cornea causing blindness. Dr. Allan Slomovic, an ophthalmologist at the Donald K. Johnson Eye Institute, has begun performing stem cell transplants that can restore a badly damaged cornea.

Institute scientist, Dr. Jeremy Sivak, also studies the cornea, working to understand the essential mechanisms that keep the cornea clear, and the interplay between blood vessels and inflammation that can lead to disease.

Cataracts

Once light passes through the cornea, it is focused by the lens of the eye. As a person ages, this lens can become cloudy, obscuring fine vision. In Ontario, surgery to replace a cloudy lens with a new synthetic one is common, and is an effective treatment for sight-obscuring cataracts.

Age-Related Macular Degeneration/Retinal Dystrophies

Light is captured in the eye by specialized nerve cells called photoreceptors. A damaged or destroyed photoreceptor cannot naturally be replaced or repaired by the body. Research to replace damaged photoreceptors is a major focus of research at the Donald K Johnson Eye Institute, led by Dr. Valerie Wallace. Dr. Wallace’s work focused on using stem cell transplants to replace damaged cone photoreceptors.

Age-related macular degeneration (AMD) occurs when the cone photoreceptors are damaged. These cells are found in the centre of the retina (the macula) and are responsible for fine vision and for colour vision. There are two forms of the disease: wet AMD and dry AMD.

  • In Dry AMD, waste products slowly build up under the retinal pigmented epithelium, below the macula where cone photoreceptors are clustered. The photoreceptors rely on the retinal pigmented epithelium for nourishment and support, as damage occurs they are less able to function. Vision loss due to dry AMD is usually slow, and many people with dry AMD never progress to serious vision loss.
  • Wet AMD is due to abnormal blood vessel growth underneath the retinal pigmented epithelium, likely a response to the damaged caused by dry AMD. At the moment there is no established way to predict which patients will progress from the dry to wet form of AMD.  Vision loss can be sudden and severe, however drugs have recently been developed to help control abnormal blood vessel growth, preventing photoreceptor damage.

With treatments available for wet AMD, many cases of blindness are being avoided. It is increasingly important to understand and monitor blood flow in the eye to facilitate prompt diagnosis and ongoing monitoring of treatment for AMD and for conditions like diabetic retinopathy, which also affects the blood flow in the eye. Dr. Chris Hudson of the Donald K Johnson Eye Institute, is working to develop new tools for monitoring ocular blood flow.

Despite advances in preventing AMD, once a photoreceptor is destroyed, vision loss is permanent. These are the patients that might be helped by transplants of cone photoreceptors. Research to replace cone photoreceptors may also benefit people with inherited genetic conditions that affect cone cells, and ultimately may suggest approaches for rod photoreceptors as well.

Genetic conditions that affect rod photoreceptors typically affect peripheral and night vision first, progressing slowly over many years until the cone photoreceptors are also affected. Retinitis pigmentosa is one example of a blinding condition that works in this way.  Donald K Johnson Eye Institute scientist Dr. Philippe Monnier, is leading research to block programmed cell death in vision cells, work intended to slow the progress of these conditions.

Learn more about Age-Related Macular Degeneration, Retinitis Pigmentosa and other retinal dystrophies that affect cone and/or rod photoreceptors from the Foundation Fighting Blindness.

Diabetic Retinopathy

Insulin is a hormone that controls the amount of sugar in the blood. When a person’s body is not able to produce enough insulin or able to respond properly to the insulin that is produced, they develop diabetes. Diabetes is one of the most common chronic diseases in Canada, an estimated 2.4 million Canadians are living with the disease. Diabetes can cause ill-effects and complications throughout the body, including serious damage to the retina of the eye.

The vision cells of the retina rely on a network of tiny blood vessels beneath the retina to supply them with nutrients and oxygen. The retina of the eye is the most metabolically active part of the body processing nutrients and oxygen at a tremendous rate, thus the functioning of these tiny blood vessels is tremendously important. Increased levels of sugar in the blood can cause blockages in these tiny vessels and damage the walls of the blood vessels. It may cause small vessels to rupture and leak fluid. This causes swelling in the retina and can damage vision. Because diabetes is relatively common, diabetic retinopathy is the most common cause of vision loss in working age Canadians.

The macula is the centre of the retina, where most of a person’s fine detailed vision happens. When high blood sugar damages the vessels in this area and causes swelling, the condition is called diabetic macular edema (DME). DME as considered a consequence of more generalized diabetic retinopathy, and may cause severe vision loss without rapid treatment.

Visit the Canadian Diabetes Association to learn more about diabetic retinopathy.

Glaucoma

Glaucoma occurs when the retinal ganglion cells at the front of the retina are damaged. Once these cells with their long slender axons are injured, visual information can no longer travel from the eye to the brain and vision is lost. Unfortunately, permanent damage can occur before a person has symptoms of vision loss.  Improved testing and screening for glaucoma to prevent permanent damage, is an ongoing focus of research for Dr. Graham Trope.

Although increased eye pressure is an important risk factor for glaucoma, not all patients exhibit this indicator, and the underlying causes of the disease are not well understood. Nerve damage is triggered by messages sent between cells, but a better understanding of the origins of these messages is needed. Dr. Jeremy Sivak studies cell communications and works to identify ways to block destructive messages and protect vulnerable cells.

Both glaucoma and serious injuries (traumatic damage) can cause currently irreversible damage to the optic nerve.  However, Dr. Philippe Monnier is studying the early development of retinal ganglion cells and how their long axons are generated. The ultimate goal of this work is to understand how retinal nerve cells could be regenerated, and guided to make connections with other cells restoring the flow of visual information.

Visit the Glaucoma Research Foundation of Canada to learn more about glaucoma.

Eye Movement Disorders

When light passes through the lens of the eye, it is processed across the retina, however the highest resolution images are processed in the fovea at the centre of the macula. Human eyes make rapid unconscious movements to allow different parts of our environment to be captured by the fovea. The unconscious movements of both eyes are coordinated and are critical to normal vision.

Eye movements are governed by the brain. Dr. Agnes Wong studies eye movement disorders that may appear in young children, such as strabismus (eye misalignment), and amblyopia (lazy eye), and the brain mechanisms that underlie them. Her work is helping to identify better methods of treatment and rehabilitation, including changes in surgical practice. Learn more about strabismus and amblyopia from the Canadian Ophthalmological Society.

Although eye movements are governed by the brain, conditions that damage the eye may also change eye movement as the brain attempts to compensate. Dr. Martin Steinbach studies the changes in eye movement and reading behaviour associated with age-related macular degeneration and glaucoma, in order to identify more effective ways to help people living with these conditions adapt to their vision loss.

New technology is also being developed to make more detailed study of eye movements. Donald K Johnson Eye Institute Adjunct scientist Moshe Eizenman is an electrical engineer working to improve these tools. This equipment has many potential applications to improve the diagnoses of eye diseases and also to diagnose and monitor other conditions that affect the brain such as Alzheimer disease and eating disorders.