This article is a continuation of Part 1: The Blood Supply
Part 2 – The Optic Nerve Head
My previous post discussed Retinal Imaging and how it can help us identify problems with the body’s blood supply. Detecting such problems early is a critically important way to reduce the risk of vision loss (or worse) from problems such as: diabetes, hypertension, or stroke. Retinal imaging is also vital for discovering similar threats to the central nervous system that appear on the optic nerve head.
The optic nerve head is the most prominent landmark inside the eye. When your family doctor or the actor on your favourite medical TV show shines a black flashlight (called an ophthalmoscope) near the eye and looks in, she is attempting to view this structure. The optic nerve head represents the ending of the optic nerve.
This is the inside of my right eye. The red squiggly lines are blood vessels. The large yellow ring to the right of centre is the optic nerve head. Changes in the appearance of the nerve head can be indicative of eye disease or certain neurological conditions.
Strictly speaking, the optic nerve is not actually a nerve at all. It is an extension of the brain that pushes into the back of the eye. It is responsible for transporting information that is collected by tens of millions of cells and microscopic nerve fibres that line the retina. The collected information is carried from the front of the brain to the back for processing in an area called the Occipital Cortex.
Because the optic nerve is actually brain tissue and because it travels a relatively long distance, disease that impacts the brain or central nervous system can cause changes to the appearance of the optic nerve head.
I saw one such example in practice two years ago. A fifteen year old patient, TM presented with no specific visual complaints. He mentioned recent headaches that had been investigated at the local children’s hospital and attributed to migraines. When I looked at his optic nerve heads, I was surprised by what I saw:
Both optic nerve heads are cloudy, white, and swollen. There are hemorrhaging blood vessels around the edges of the nerve heads that look like spokes on a bike tire. This young man had a condition called papilledema that is caused by a buildup of fluid in the brain. His vision was normal during our appointment, so it was important for us to utilize retinal imaging to help come to a proper diagnosis. Left untreated, papilledema can cause permanent vision loss, neurological damage, or even death.
Similar to papilledema, a condition called optic neuritis causes swelling of the nerve head and is associated with neurological problems. In fact, 66% of people with Multiple Sclerosis (MS) develop optic neuritis at some point and it is often the first sign that leads to eventual diagnosis.
Sometimes nerve swelling can be subtle and difficult to detect, even with retinal imaging. For your optometrist, it is always easier to tell if a nerve is swollen when they can use an older picture of the same nerve for comparison.
Optic Neuritis in figure A and partial resolution in figure C (B and D are normal). Sometimes optic neuritis can be subtle and diagnosis is easier if we have pictures of the nerve heads in their healthy state for comparison. Images courtesy of: Karli SZ, Liao SD, Carey AR, Lam BL, Wester ST, Society for Clinical Ophthalmology (Great Britain), Volume 2014:8 Pages 2171—2175.
The most common disease that we monitor with optic nerve head imaging is glaucoma. In this condition, the light coloured depressed area in the centre of the optic nerve, known as the cup, grows over time because of a loss of nerve tissue. In the early stages of glaucoma, there is no noticeable loss of vision and most types of glaucoma are painless. One of the ways your optometrist tests for glaucoma (besides blowing the annoying air puff into the eye) is by looking at the optic nerve head appearance. Again, comparing images that were taken consistently over a long period of time can help us determine when subtle changes appear. This allows us to order more tests and begin treatment if needed. Glaucoma is the second leading cause of blindness in the world, affecting over 60 million people worldwide.
Progression of optic nerve head cupping. Notice the light coloured tissue in the centre of the nerve head growing larger in each image. Having retinal photos of the optic nerve head makes it easier to detect change over time.
When optic nerve head cupping changes, it is vital for us to detect it early. That growing crater in the centre of the nerve represents dying tissue that cannot be restored. If we know the tissue is degenerating, we can provide eye drops that help slow the degeneration. For more information about glaucoma, read this great post from fellow eyeDOC Serge Fauchon.
The inside of your eye is a very unique and complex place and your optometrist has to determine if the structures of the eye are healthy and stable. Retinal Imaging allows us to confirm good health on the day of your appointment while creating a record for future reference. This is why Retinal Imaging is an indispensable tool in maintaining good eye health.
About the Author
Dr. Shawn Charland
Dr. Shawn Charland graduated from the University of Waterloo, School of Optometry in 1999. He has been practicing optometry since and joined the eyeDOCS team in 2006 and became a partner in 2007. For more information about Dr. Charland and his specialized interests relating to optometry, click here.