A brief introduction to the retinal photo
As part of your comprehensive eye exam at Vision Specialists, we take a picture of the back surface of the eye—called the retina—using a state-of-the-art camera known as the Optos machine. Viewing the retina gives us important information about the health of the eye, and can be used to identity and monitor many diseases that can effect your vision.
In this article we’ll go through the general anatomy of the retina and discuss some common medical signs that can appear by examining the retinal photo.
The developmental and evolutionary origins of the retina
The retina develops from a part of the embryonic tissue known as the diencephalon, which also gives way to parts of the mid-brain. For that reason, the retina is properly understood as an extension of the central nervous system, which also includes the brain and spinal cord. The retinal photo, then, is the only non-invasive technique that allows us to view this important part of the body.
We gain an intuitive understanding of the retina’s structure when we consider its evolutionary history. The earliest eyes were simply areas of exposed tissue containing photoreceptors: specialized cells which fire signals in response to certain wavelengths of light. These areas, called eyespots, were primitive, allowing the organism to respond to the presence of light without any indication of its direction or intensity. They can still be found in some species of flatworms and unicellular organisms.
Over the course of millions of years, a series of adaptive traits allows this crude system of light sensitivity to develop into an eye which gives a clear image of the surrounding environment. First, the tissue becomes indented, allowing the organism to determine the direction of the incoming light. This is how the retina becomes the posterior (meaning rear-end) of the eye. Next, this cavity develops so that only a small opening is left, causing light to be cast on the surface of the retina as a fully-resolved image, much like a pinhole camera.
Finally, the resulting chamber fills with fluid and a lens develops which further focuses the light onto the surface of the retina. This is the general structure of the vertebrate ‘eyeball’ that we’re all familiar with.
Some organisms develop further structures of the eye that reflect the needs of their environment. The octopus, for example, has a rectangular-shaped pupil which is kept horizontally-oriented by a reflex in order to separate out colors. Some species of birds have a thin membrane called a nictitating membrane, a transparent ‘third eyelid’ which replenishes the eye with moisture. Insects and crustaceans have compound eyes, which are capable of giving the organism nearly 360 degrees of vision.
As for the retina itself, the most notable evolutionary developments involve the diversification of opsins: light-sensitive proteins that cause photoreceptors to fire in response to light. The human retina contains three types of opsins: red-cone, green-cone, blue-cone, and rhodopsin. The first three give us the greatest color-vision acuity among primates, and it has been suggested that this developed due to our need to detect fruits and vegetables as well as notice social cues among ourselves.
Organization of the retina
The retina is a ‘sheet’ of cells on the inside surface of the eye. Its bottom-most layer is filled with photoreceptor cells, which translate incoming light into electrical signals to the brain. There are two kinds of photoreceptor cells: rods, which detect contrast, and cones, which detect color.
Just above the layer of photoreceptor cells are a series of cells including bipolar, horizontal, and amacrine cells. The purpose of these cells is to subtract and add the signals relayed by the photoreceptor cells. Interestingly, this means that some processing of visual information happens before the signal even reaches the brain.
Finally, the signal reaches the outermost layer of cells, known as ganglion cells. These transmit the signal from the retina to the brain, sending electrical impulses through their axons.
You might notice something counterintuitive about this set-up: it’s ‘inverted’. Light needs to pass all the way through a mess of ganglion and other cells before it hits the photoreceptors, only for the signal to move up towards the top layer again. The reason for this is that the photoreceptors need to be closest to the blood vessels at the bottom-most, pigmentary layer of the retina known as the pigment epithelium. Some species, like mollusks, have this scheme reversed, allowing them to see with even more clarity and resolution.
Major structures present in the retinal photo
To take the retinal photo, we invite our patients to look into the Optos machine with their eye wide open. They see a blue target, and when they get close enough to it, the target will turn green and flash a green flash of light.
The flash of light goes through the pupil to the back of the eye and gives a 270 degree image of the retinal surface. It is shown to the screener, who checks it for any blur or lid obstruction. The retinal photo — also called the ‘fundus’ photo, Latin for ‘chamber’ — is then read by the optometrist.
A good retinal photo is clear, and free of lids or lash obstruction. Here’s what a healthy retina looks like:
The most obvious structure present in the retinal photo is the optic disc: the yellow-green circle near the middle of the image. This is the point at which all of the retina’s ganglion cell axons bundle together and leave the eye as a nerve tract to the brain. Because this area contains no photoreceptor cells, light that hits it does not elicit a response, and this area of your vision corresponds to your blind spot.
To observe the blind spot for yourself, cover your left eye and look at the dot above. Start some distance away from the screen, and slowly move in. Eventually, the cross will disappear. This happens when the light coming off of the cross hits the exact spot of your optic disc, where no visual processing can occur.
The macula can be seen just to the left of the optic disc in this photo. It is the area of the retinal surface which receives direct light: for that reason, it’s often called the ‘island of 20/20 vision’. The macula has more cones — color-sensing photoreceptors — than rods. You can also observe this for yourself. If you’re ever trying to look at something in the dark, try looking right next to it: this will actually allow you to see it better.
The retina is variously pigmented: in this photo, we notice red and green coloration, which is due to the retinal pigment epithelium. These pigments help to absorb as much light as possible. You can also see blood vessels, which emanate from the optic disc and supply the cells of the retina with oxygen and nutrients.
Reading the photo
The optometrist typically starts by noting the cup/disc ratio, which refers to the relative sizes of the optic cup and its disc. The optic cup is a smaller, lightly-colored region in the middle of the optic disc whose size varies widely among individuals. An enlarged, oblong or tilted disc can be an indication of high intraocular pressure, which is associated with glaucoma.
Secondly, the optometrist notes the ratio of arteries to veins, known as the AV ratio, and checks the vessels for tortuosity, or twistedness. Tortuous vessels can be an indicator of high blood pressure. In addition, atherosclerotic changes can be noted in the vessels. This refers to the build-up of plaque in the vessel walls.
The optometrist also checks for the presence of a foveal reflex, a sparkle of light which appears around the fovea caused by the flash of the photo.
Common disorders and diagnoses
The most obvious abnormalities visible in the retinal photo are direct damage to the retinal surface. These can include retinal tears, detachments, and hemorrhages. Tears occur when the vitreous humor that fills the eyeball pulls out part of the retina. When fluid leaks into the tear, this can cause a detachment. Hemorrhages can be caused by trauma to the eye.
Macular degeneration is a common disorder which can cause dramatic loss of vision. It is the leading cause of vision loss in those 50 years or older. Drusen, or white and yellow specs on the retina, are usually benign but can sometimes be an indication of macular degeneration. The exact causes of macular degeneration aren’t currently understood, but age and smoking status are both correlated with its onset.
Diabetes can cause damage to the retinal vessels in a condition known as diabetic retinopathy. This is visible in the retinal photo as small hemes or abnormally-looking blood vessels.
When atherosclerotic changes are noted in the vessels, this can be an indication of high cholesterol, which leads to an increased chance of developing cardiovascular disease. Plaque build up can cause an occlusion, which can cause the vessels to die or travel to the brain and cause a stroke.
Diabetes and hypertension can also cause a finding called cotton-wool spots, caused by damage to the nerve fibers of retinal ganglion cells. These white and yellow deposits are the contents of the cell’s cytoplasm.
Conclusion
The retina is a remarkable tissue: the product of millions of years of evolution and biological fine-tuning. The retinal photo gives us an important look into the health of the eye and an indication of other systemic health issues. Optos is a particularly effective machine which takes the photo in less than a second and does not require dilation of the patient’s eyes. The retinal photo is just one important part of your comprehensive eye exam at Vision Specialists.
