Normal Visual Development
The Importance of Routine Eye Care
Early Detection is Key to Preventing Vision Loss
Finding an Eye Care Professional
Getting the Most From Eye Exams
Low Vision Resources
The Eye Care Page
Blindness in Depth
Sarah J. Blake is a person with a visual impairment who has also studied blindness and eye conditions. She is not an eye care professional, and information on this site is not a substitute for face-to-face consultation with an eye care professional. This site is designed solely to assist patients and doctors in building cooperative relationships.
Development of content for this site is supported by your contributions, the sale of products through various affiliate programs, and Sarah's work as a writer, speaker, and in other areas listed on her services page. If the material on this site has been helpful or encouraging to you, please consider shopping using the sites linked here--and return to do your shopping anytime using these links to continue supporting this site--or making a contribution using the Amazon Honor System.
Normally functioning vision depends on many structures within the eye as well as the brain. If one of those structures is damaged or functioning improperly, a visual impairment may result.
Each of the links below leads to a description of one of the structures involved in the pathway which light and images follow, allowing a person to see. Other parts of the eye provide various kinds of support to these structures. These are just as important, but because they are not directly involved in the process of seeing they are not discussed here.
The Cornea
The Aqueous Humour
The Pupil and the Iris
The Lens
The Vitreous Humour
The Retina
The Brain
The cornea is the clear "window" at the front of the eye. It is one of several structures which allow light to enter the eye and to be focused in a certain manner onto the retina. The way in which light rays enter the eye is controlled by both the shape and condition of the cornea. Disorders of the cornea may involve abnormal shape, swelling, scarring or injuries. Treatment for these disorders may involve a graft, or transplantation of new tissue to the cornea. In rare cases, the original cornea is completely replaced by a donor cornea. More information about grafts is available in the article about cornea transplant.
The cornea is also the structure of concern in refractive surgery. By changing the shape or structure of the cornea, refractive surgery enables light to be directed into the eye in a different manner, correcting errors in the way it converges upon the retina.
For more information about diseases and conditions which affect the cornea, visit the corneal diseases library.
The aqueous humor is a fluid at the front of the eye which helps to maintain the shape of the eye. Light also passes through this fluid on its way to the retina.
The part of the eye where the aqueous humor is located is called the anterior chamber. It is bounded at the front by the cornea and at the back by the lens. The shape of the cornea, size of the lens, and overall size of the eye determine the size of this chamber.
Aqueous is produced in the ciliary body. It drains out of the eye through an area called the trabecular meshwork. If problems with the trabecular meshwork prevent adequate drainage, the pressure inside the eye builds. This high pressure can damage the optic nerve, resulting in loss of night vision and peripheral vision. Peripheral vision is what allows a person to see things which are not directly in front of him. The condition resulting from high pressure when the aqueous does not drain out of the eye is called glaucoma. Small eyes may be at risk for glaucoma, as well as eyes affected by other conditions.
Once this kind of damage has occurred, it is not correctable. However, early detection and treatment of glaucoma can result in preservation of remaining vision.
For more information about glaucoma and its treatment, please visit the glaucoma links library.
When a normal eye encounters bright light, the pupil contracts to cause less light to enter the eye. The pupil is actually a hole in a structure called the iris, and it is the iris which causes the pupil to change shape. In dim light, the pupil gets bigger, or dilates, so that more light can come in.
Conditions affecting the iris may cause a person to experience photophobia, or sensitivity to bright light, or to have difficulty seeing at all if the pupil is obstructed. Some conditions, such as iritis, can be very painful.
The iris can be important in the treatment of other conditions as well. After a surgery, dilating drops may be prescribed for a few weeks so that the dilation and constriction of the pupil will not cause pain. During this time, the patient may see things differently from the way he/she will see them after the dilating drops are discontinued.
The lens is the structure in the eye which changes shape to allow objects to be seen clearly. This process is called accommodation. The lens is responsible for a great deal of the eye's ability to see clearly.
If the lens becomes cloudy, the person is said to have a cataract. In most cases, cataracts can be removed with very little risk. In fact, cataract surgery is an outpatient procedure. For more information about cataracts, visit the cataract links library.
People who have had cataract surgery may wear powerful glasses or contact lenses. If the other structures of the eye are in good condition, they will have fairly normal vision while wearing their corrective lenses. Most people also receive an intraocular lens (IOL), an artificial lens which is implanted in the eye after the cataract is removed.
Post-cataract patients may experience difficulty with glare in certain conditions. Sunglasses or tinted lenses may help to cut down on glare.
The vitreous humour is a gel-like substance behind the lens. It helps the eye to maintain its shape, and light passes through it on its way to the retina.
If the eye hemorrhages or the retina tears, debris can cloud the vitreous and result in vision loss. If the retina is partially detached, vitreous fluid can also get behind the retina and cause the detachment to become worse. These problems are often treated with a procedure called a vitrectomy
.At the back of the eye lies a layer of cells as thin as a piece of tissue paper. This layer of cells is called the retina and is responsible for converting patterns of light into images before sending them to the brain via the optic nerve.
The retina is made up of different types of cells which serve different purposes. The macula is the part of the retina which is responsible for central vision and discrimination of color and detail. It is made up of cells called cones. If this part of the retina is damaged, visual acuity is lost, and objects will not be seen clearly. The macula may be damaged by a number of conditions, including retinopathies, cone-rod dystrophy, and macular degeneration.
Rods are another type of cell in the retina. Rods are responsible for night vision and peripheral vision, or what is seen out of the corner of the eye. If the part of the retina where rods are located is damaged, these abilities will be lost. Conditions which can cause this kind of loss include retinitis pigmentosa, LCA, and rod-cone dystrophy.
All conditions that affect the retina are not degenerative. Some conditions require surgical treatment in order to preserve sight. A retinal detachment means that the retina is pulled away from the back of the eye and thus cannot send images to the brain. If a retinal detachment has occurred recently, surgery may be performed to repair it. Retinal reattachment surgeries have varying degrees of success depending on a number of factors. One type of reattachment procedure is the vitrectomy.
If you see floaters or flashing lights, or if you have blind spots in your field of vision, you should see an eye doctor as soon as possible. These things may indicate problems which are treatable.
The optic nerve is responsible for sending images from the retina to the brain for processing. Damage to the optic nerve can cause visual field loss, distortion of images, and problems with sensitivity to contrast.
There is no treatment for optic nerve damage. In the case of glaucoma, which can cause visual field loss by damaging the optic nerve, early detection and preventive therapy is the only course of action which can be taken. Existing damage cannot be repaired.
The brain is as important a factor in the way we see as is the eye. It is responsible for perceiving images correctly, for translating them into things we recognize, for understanding abstract representations such as print and pictures, and for many other processes which are necessary for a person to use the vision he/she has. When a portion of the brain is damaged, any aspect of visual processing controlled by that part of the brain can be damaged or lost. The result of this damage is that the affected person will be visually impaired in some way.
Sometimes this damage repairs itself, especially if the affected person is a young child. Other times, the damage can be overcome by using low vision devices, such as those used to help a person overcome a visual field loss caused by a stroke. In other cases, visual exercises can help a person who has experienced vision loss because of brain damage to learn to use his/her vision, such as in the case of cortical visual impairment.
The brain is a very complex organ, and understanding its role is important for helping people who have lost vision because of brain injuries or damage which is a part of another condition.
Sarah J. Blake