Cataracts are a clouding of the lens inside the eye. They often develop slowly, but can eventually lead to vision problems. Treatment of cataracts is often a straightforward procedure which involves replacing your lens with a new prosthetic lens. Generally, your vision recovers well.…
What is vision loss?
Vision loss, sometimes called vision impairment, can occur for many reasons. In some cases, it is as simple as needing glasses or contact lenses, but in other cases it can be serious and irreversible.
Vision loss can vary from something that is largely unnoticeable by the person experiencing it, right up to blindness.
Blindness is often understood as complete loss of vision, but a person who is 'legally blind' may have what is called low vision. Low vision is when a person has a significant, permanent loss of vision that cannot be corrected with glasses, contact lenses or other treatments such as surgery. Estimates suggest that more than 1.3 million people in the US are legally blind, and another 7.3 million have low vision or a visual disability. People with low vision may, for example, have a severe loss of central vision and be unable to read or see faces clearly, but may have good peripheral (or side) vision. Alternatively, someone with low vision may be able to read, but have very restricted peripheral vision (sometimes called tunnel vision) and this means that they cannot drive vehicles and may have problems with their mobility.
Vision is a complex process. Light entering the front of the eye (the cornea) is focused by the lens on to the retina at the back of the eye, where it is converted to neural signals.
The optic nerve of each eye takes the neural signals from the retina and sends them, via a complex pathway, to a part of the brain at the back of the head (the occipital cortex), where the signals are processed.
Damage to any of the structures of the eye, the optic nerves or the brain that are involved in vision can lead to vision loss. Around the world, the most common causes of vision loss vary and include:
Macular degeneration (MD) is an eye condition in which the central retina is damaged. This can make it difficult to read and see fine detail. It is much more common in people over 50 years of age.
Cataracts are the clouding of the lens inside the eye. Most cataracts form as a normal part of getting older and can eventually cause problems with vision and tasks such as reading and driving.
Glaucoma is a group of eye conditions in which damage to the optic nerve leads to loss of vision. Because the vision loss is often slow and gradual and there are few other symptoms, many people in the early stages of glaucoma are unaware they have the condition. It is much more common in people 40 years of age and over.
Diabetes can cause damage to the small blood vessels in the retina, leading them to swell and leak fluid and blood (diabetic retinopathy). This can lead to irreversible scarring and damage to the retina, affecting both central and peripheral vision.
Uncorrected or undercorrected refractive error
In order for light to be focused clearly, the size and shape of the eye needs to be perfect. If this is not the case, there is a refractive error and this can lead to blurred vision. The most common types of refractive errors are myopia (short-sightedness), hyperopia (long-sightedness) and astigmatism. Presbyopia, which occurs because the lens inside the eye gradually loses the flexibility required to change focus, is a common cause of blurred vision in people over 40 years of age.
Most refractive error can be corrected by wearing glasses or contact lenses or, in some cases, surgery (most commonly laser eye surgery).
As refractive error tends to change over time, regular eye examinations can help to ensure that your prescription is up to date and your vision as clear as possible.
Eye trauma (from injury and surgery)
Damage to eye structures such as the cornea, iris, lens, retina and optic nerve can lead to vision loss. The extent and type of vision loss will depend on the type of injury.
Injury can occur because of:
- A direct impact or blunt force to the eye;
- Foreign bodies penetrating the eye;
- Chemicals damaging the surface of the eye, and;
- Exposure to radiation.
Situations that can increase the risk of eye injury include:
- Working with machinery, chemicals and radiation;
- Some sports, particularly ball sports (such as basketball, hockey, football and golf), gun sports and water sports, and;
- Assault with trauma to the eye or eye socket.
Trachoma is a form of chronic conjunctivitis that can occur when the eyes are repeatedly infected by Chlamydia trachomatis bacteria. It can lead to scarring of the conjunctiva, the mucous membrane that lines the surface of the eye and the inner eyelids. This can pull the edge of the eyelid inwards, causing the eyelashes to turn in and rub against the cornea, leading to pain, scarring and loss of vision.
Other important causes of vision loss include:
- Degenerative retinal conditions such as retinitis pigmentosa;
- Amblyopia (lazy eye) - a developmental condition that arises in children in which the brain suppresses vision from one eye;
- Complications of premature birth (retinopathy of prematurity);
- Stroke, when parts of the brain involved in vision are damaged;
- Optic nerve inflammation (optic neuritis);
- Tumors of the eye (including retinoblastoma in children, melanoma of the eye and optic glioma);
- Blockage of the blood vessels that supply blood to the retina, and;
Because there are many different causes of vision loss, there are many risk factors.
The risk of vision loss increases significantly as people get older. It is estimated that around 40% of people over the age of 90 have low vision  .
Lifestyle factors such as smoking, diet, sun exposure and doing activities that increase the risk of eye injury can increase the risk of vision loss.
Signs and symptoms
Vision loss will depend on the condition causing it. Some conditions cause very gradual vision loss that may be difficult to notice, while in others changes can occur very quickly. Most forms of vision loss are not associated with any pain.
Because some forms of vision loss are gradual and difficult to notice in their early stages, a regular eye test can also help to detect any problems early.
Sudden changes of vision can be a sign that something is wrong. Having your eyes checked (by an ophthalmologist or optometrist) can detect problems as early as possible, which may mean that treatment will be more effective.
In some conditions, vision loss may occur temporarily and then recover, but still require immediate investigation. For example, vision may become temporarily blurred during a transient ischemic attack (TIA). A TIA can be an early warning sign of a stroke.
Other symptoms that can accompany vision loss include:
- Blurred, fuzzy or cloudy vision;
- Sensitivity to glare;
- Increased difficulty seeing at night;
- Dark spots or blank patches in the vision;
- Tunnel vision, in which the peripheral or side vision is very restricted, and;
- Loss of vision on the same side in both eyes.
Bright flashing lights or sparks, particularly in peripheral vision, can be a symptom of retinal detachment, a condition that can occur after a blunt blow to the eye, or spontaneously (without obvious cause). This can lead to significant vision loss, but early treatment may help to limit the loss.
Methods for diagnosis
Tests for vision loss will vary, according to the type of vision loss and the condition suspected of causing it. They can include:
Visual acuity tests usually involve reading letters off a chart (to test distance vision) or a card (to test near vision).
The best prescription for your glasses or contact lenses is determined by a process called refraction. An initial indication of the prescription can be given by a machine called an autorefractor, but the prescription is usually determined using a phoropter, or a special pair of glasses in which lenses can be added and removed, called a trial frame.
Ophthalmoscopy is used to view the back of the eye, in particular the optic disc (the end of the optic nerve) and retina. Eye drops may be used to dilate the pupils to improve the view. These drops can blur vision for a couple of hours after the exam.
Tonometry is used to measure the pressure inside the eye (intraocular pressure). There are different types of tonometers; some touch the surface of the eye (anesthetic drops are used to numb the cornea), while others blow a puff of air.
Visual field testing
A visual field test can be used to assess the extent of vision field loss caused by eye or brain conditions.
Slit-lamp testing (biomicroscopy)
A slit lamp is an instrument similar to a microscope that allows examination of the front part of the eye, including the cornea, iris and lens.
Types of treatment
Treatment will depend on the condition causing the vision loss. Please see relevant reports for treatments of specific conditions.
Significant vision loss can have a profound impact on a person's life, making it more difficult to perform many daily activities that most people with normal sight take for granted. Blindness and low-vision services can provide advice and support on how to adapt and live independently. There are many products available to help with daily activities, mobility, travelling, reading, writing and accessing computers. For further information on these resources, please contact the American Foundation for the Blind.
Prognosis will depend on the condition causing the vision loss. Unfortunately, once significant damage is done to some tissues and structures in the eye or brain, there may be no way to repair it and the vision loss will be permanent. For some eye conditions, regular monitoring by an eye-health professional can help to make sure that treatment is most effective.
While it is not possible to prevent some of the eye and brain conditions that cause vision loss, things you can to do to help keep your eyes healthy include:
- Having regular eye exams so that eye conditions are detected early;
- Making sure you follow appropriate safety precautions when using machinery or hazardous chemicals, or participating in sports;
- Avoiding overexposure to the sun (ultraviolet light) by wearing brimmed hats and sunglasses;
- Eating a healthy diet, high in fruit and vegetables;
- Stopping smoking, as smoking can increase the risk of several eye conditions that can cause vision loss, and;
- Taking measures to prevent development of diabetes, which increases the risk of a number of eye conditions including cataracts and glaucoma.