Types of Vision Loss
There are broad spectrums of eye conditions which can lead to vision impairment, many resulting in blindness and permanent vision loss. This page describes some of the common types of vision loss and outlines the definition of vision impairment.
Definition of vision loss
Vision impairment is defined as the limitation of actions and functions of the visual system, which places the individual in a position which inhibits their ability to function in the standard manner to that of other human beings.
The U.S. National Eye Institute defines low vision as “a visual impairment not correctable by standard glasses, contact lenses, medication or surgery that interferes with the ability to perform activities of daily living.”
Types of vision loss
According to the the World Health Organization:
- Visual acuity is the measure of how well the eyes can see objects from a set distance.
- For example, 6/60 describes the ability to see objects only at a distance of six metres, while a normal eye can see the same object at 60 metres.
- Normal visual acuity is 6/6 (20/20 in the imperial measure of feet).
- The World Health Organization defines blindness as a visual acuity of less than 3/60 (or equivalent).
Types of Vision
Accommodation is the process by which the human eye changes optical power to maintain a clear focus on an object
- This is mediated by ciliary muscles, which contract to change the aperture of the lens (and thus adjust focal distance)
- Normally, the light from an object will be focused on the centre of the retina (at the fovea) in a healthy person
- However some individuals may be short / near-sighted (myopic) or long / far-sighted (hyperopic)
Short Sightedness
- Light is focused in front of the retina, causing distant objects to appear blurry (close objects still appear focused)
- Commonly develops with age (the ciliary muscles start to lose tensile strength, resulting in the widening of the lens)
- Most common corrective measure is the use of concave lenses to refocus the incoming light
Long Sightedness
- Light is focused on a point behind the retina, causing nearby objects to appear blurry (distant objects still appear focused)
- It is a type of refractive error caused by imperfections in the eye (e.g. the eyeball may be too short)
- Most common corrective measure is the use of convex lenses to refocus the incoming light
Three Common Vision Problems
Most people who start needing glasses or contacts while they’re young have at least one of three common vision problems: myopia, hyperopia, and astigmatism.
These are all refractive errors, which means they’re problems with the way the eyes focus light, rather than an eye disease. Refractive errors have to do with the physical shape of our eyes, so let’s take a closer look!
Myopia: What’s Right In Front Of You
Myopia is the technical term for nearsightedness, meaning that you can see clearly up close but distant objects are blurred. This happens when the eyeball itself is too long, or else when the cornea (the clear front part of the eye) is too curved. That additional curvature or length causes light to focus in front of the retina instead of on it, which makes the resulting images look fuzzy.
The way glasses or contacts correct myopia is by compensating for this error to extend the light’s focus onto the retina where it belongs. These lenses are concave (thinner in the middle), and always have a negative prescription.
Hyperopia: Gazing Into The Distance
Hyperopia, better known as farsightedness, means that you can see distant objects clearly, but everything up close is blurry. Hyperopia happens for the opposite reasons that myopia does. Instead of being too long, the eyeball is too short, or else the cornea is too flat. This causes light to focus behind the retina, making near images fuzzy.
In order to correct hyperopia, corrective lenses must be convex (thicker in the middle) and have a positive prescription. The larger the number, the stronger the prescription.
Astigmatism: A Warped Perspective
The third common refractive error people experience is astigmatism, and it’s a little different from the other two. A normal cornea is uniformly curved so that there is a single focal point. A cornea with astigmatism is more football shaped, creating multiple focal points, which makes things appear blurry at any distance and bends their images.
Astigmatism is often paired with one of the other refractive errors, and it requires more complex lenses to correct than they do. Typically, the lens will be somewhat cylindrical rather than spherical.
Common Eye Conditions
- Age-related Macular Degeneration (AMD) is a degenerative condition affecting central vision. Reading and crossing roads safely may be difficult. Early detection can slow down progression of the disease and improve treatment outcomes.
- Cataract is a clouding of the lens, resulting in blurry vision and faded colours. This leads to glare-sensitivity and increases the risk of trips and falls. Cataracts are usually safely treated by surgery. A cataract is a cloudiness that forms in the lens of the eye – the part that allows a focused image to be transmitted onto the retina. This cloudiness creates blurring, affecting both near and distance vision. Cataracts generally result from the ageing process, though they can also develop from other causes such as congenital disability (from birth) or trauma to the eye. Cataracts due to ageing usually develop slowly and affect both eyes at different rates.
- Glaucoma is an eye disease that damages the fine nerves connecting the eye to the brain. Glaucoma can result in tunnel vision and even total blindness. Early detection is vital.
- Diabetic Retinopathy is caused by diabetes and result in increasingly blurred, patchy and fluctuating vision. Regular eye checks are crucial to reducing acuity.
- Neurological Vision Impairment (NVI) can result from an acquired brain injury. Glasses may not improve vision. Homonymous Hemanopia is a common type of NVI.
These conditions can involve loss of central vision or side vision or indeed the whole field of vision. Sometimes people can see both in front and to the sides but their vision is blurry. Other people may see well in daylight but experience great difficulty indoors or at night.
It is important to understand that different people are affected in different ways by their changing vision. Vision impairment can have an impact on a person’s functioning in their home, workplace, school, and so on.
No matter what the vision impairment, there are a range of services and mobility aids that can provide assistance for someone that has problems getting around safely and independently.
- Guide Dogs provide advice and training in orientation and mobility, enabling people with vision impairment to move around safely and confidently. As well as guide dogs, their broad range of mobility services includes training with mobility aids such as canes, low vision aids or electronic devices.
Symptoms of Cataracts
Could you have cataracts?
You use the lens of your eye every day, for everything from reading to driving to bird watching. With age, the proteins inside your lens can clump together turning the lens from clear to cloudy. Certain behaviors can put you at a higher risk for getting a cataract. These include:
- Too much time in the sun without eye protection
- Smoking
- High blood sugar
- Using steroid medications
- Exposure to radiation
But you aren’t alone. Over 20 million Americans over the age of 40 have cataractsTrusted Source in one or both eyes, and 6 million have had corrective surgery. If you have any of the following symptoms, talk to your eye doctor soon.
Cloudy days
Cataracts start small and initially may have little effect on your vision. Things might seem a little blurry — like looking at an impressionist painting. This effect usually increases over time. The world will seem cloudy, blurry, or dim.
There are three main types of cataracts, affecting different parts of the lens:
- Posterior subcapsular cataracts
- Nuclear cataracts in the center of the lens
- Cortical cataracts on the side of the lens, which appear as small streaks
Those with nuclear cataracts may briefly see their vision improve. This sensation is sometimes called “second sight.”
No more wild nights
As cataracts become more advanced, they begin to darken with a yellow or brown tinge.
This begins to affect night vision and makes certain nighttime activities, such as driving, more difficult. In fact, a study from Curtin University in Australia found that treating cataracts reduced the risk of car accidents by 13 percent.
If you suspect you have cataracts, be very careful at night and don’t drive when your vision is compromised.