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PLEASE READ THIS: The information contained herein is not intended as a substitute for medical advice and care from qualified, licensed health care providers. The reader should regularly consult a physician in matters relating to his or her health and particularly with respect to any symptoms that may require diagnosis or medical attention. The information is presented here to educate and inform and to guide you to an understanding of cause, prevention as well as treatment.
by E. Lawrence Bickford, O.D.
This report is an in-depth discussion of the structure and function of the human eye and vision system. Some topics are quite complete in scope and somewhat technical, while other issues are reviewed in a more cursory manner.
The EyeCare Connection contains a number of other related articles that discuss some of these issues in more detail. It is suggested that a labeled picture (click here) of the human eye be available as you read this report.
Introduction
The old analogy comparing the human vision system to a camera remains a good place to start this discussion. A camera consists of lenses to focus the image, a variable diaphragm to control the total illumination and depth of focus, and a chemically sensitive film to record the picture.
The eye is sometimes called the globe, a term representative of its shape. The globe is attached to muscles which are used to locate and position the image correctly into the eye. There are a series of lenses to focus the image, a diaphragm to control the intensity of the illumination and a receiving screen onto which the image is projected. The screen then converts the image by chemical reaction into electrical signals which are sent to the brain for processing. A more current version of the old analogy would compare the eye to a video camera; the final step is an electrical transmission, not a hard copy.
This report will discuss the eye muscles, the cornea and crystalline lens, the iris and pupil, the retina and the brain, and how these parts integrate in the complicated and amazing process that provides us with the remarkable sense of vision. We will also briefly examine the roles of the associated structures including the conjunctiva, lacrimal (tear producing) system, the aqueous and vitreous. You will learn how the eye works, what can go wrong and what can be done to fix it.
The Eye Muscles
There are six external ocular muscles that move the eye into what is called "the various positions of gaze". Two muscles have their primary function to turn the eye left and right. Two other muscles work the up and down positions. The last two muscles assist the other four to finely control positioning of oblique orientations such as to an angle up and left or down and right.
The eye muscle positioning system is important for a number of reasons. The eye muscles align the eyes so that each one "targets" the image exactly. They must align parallel to each other for distance viewing and turn in slightly, a process called convergence, for near vision tasks. They also compensate for head position and tilt.
In a perfect system, both eyes "target" the desired scene so that the focused image falls onto the retina at the back of the eye at "corresponding" locations. Each retinal receiving unit, called rods and cones, has a matching unit in the other eye. If the focused image falls on "corresponding" areas of each retina, the brain can use this information to perceive a stereoscopic, "3-D" image. This is part of the process which is called depth perception and provides us with an awareness of our relative position within our viewed world and knowledge of how objects in space are located relative to each other.
The angle to which the two eyes turn in (converge) signals another muscle inside the eye (ciliary muscle) to adjust the crystalline lens to focus for the target distance. For distance viewing, the eyes are aligned parallel to each other and the internal focusing system is at zero net power. When the eyes turn inwards to target an image at 40cm (16 inches), the focusing system "understands" that the degree of turning requires a focal power correction of 2.5 diopters. For you mathematicians, the focal power in diopters equals the target distance in centimeters divided into the reference number of 100cm. One of the nerves that innervates the external muscles is part of a feedback loop that communicates with the internal focusing muscles which control the crystalline lens. And the reverse is true.
This feedback loop is also responsible for a condition called accommodative esotropia or esophoria. Farsighted (hyperopic) eyes are deficient in focal power. To compensate, the ciliary muscle causes the lens to focus (called accomodate), supplying the additional necessary focal power. This innervation also results in the external ocular muscles turning the eyes inwards. If the deviation is small enough, the opposing eye muscles can re-align the eyes. But sometimes the eyes do over-converge and appear crossed. The remedy for accommodative esotropia/esophoria is often simply to fully correct the hyperopia. In addition, exercises may be used to help re-align the system.
Also occurring in this dynamic process is the constriction of the iris pupil upon near convergence. When you look at something up-close, your pupils gets smaller. This is yet another part of the intricate neurological feedback loops involved in maintaining the human vision system.
Some of the tests that your eye doctor performs during a comprehensive examination determine the "neurological integrity" of these feedback loops. An error in the system can indicate a disease process affecting the nerve connections in the eye and in the brain. Sometimes these defects may indicate a serious pathology, including tumors and neurological diseases.
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The mis-positioning of the eyes by the external muscles is called strabismus. This can occur for a number of different reasons:
1. Cosmetic: The eyes look "wall-eyed" or "cross eyed"
2. Functional: Stereoscopic (3-D ) vision is missing or limited. The focusing system may be slow or inappropriate for the desired distance. Tracking errors may occur which in children may result in difficulty learning to read and other learning disabilities
3. Spatial: Limited depth perception causes incorrect spatial cues and may result in problems learning to walk, eye-hand coordination difficulties, general "clumsiness" and other perceptual problems
The body has many ways to compensate for these anomalies and many people learn to get around just fine with a poorly functioning system. Others desire correction of the problem solely for cosmetic reasons. Eye doctors generally try to intervene, especially with young children, so that the vision system and spatial orientation systems can develop normally.
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There are a number of remedies for strabismus, including surgery and eye muscle exercises and other "vision system therapies". In certain situations, there is a clear anatomical defect; the muscles are just made wrong. Other times the problem is neurological. Sometimes both remedies are combined in a treatment plan for the best outcome.
The eye surgeon can adjust the length and/or position of attachment of the external eye muscles so that they work correctly. The human vision system "learns" to use the alignment system in a process by which the brain "learns" to properly interpret the information. The earlier in the child's development that we correct the problem, the greater the likelihood of a functional correction, allowing not only cosmetic enhancement, but good depth perception. If the surgery is performed on an adult, although the cosmetic effect is often excellent, the functional result is often not.
A skilled surgeon can very often achieve the desired result of having the eyes appear aligned. The ability of the vision system to utilize and process the information from the now- straightened eyes varies. Sometimes vision system training exercises are used to stimulate the eye-brain connections to yield a fully functioning binocular system.
Vision training, consisting of neurological integration and muscle balancing, is often used instead of and along with surgery. If the eye muscles are significantly anatomically incorrect, surgery is often necessary to "normalize" the situation. If it is a minor error and/or associated with a poorly developed or incorrectly responding nervous system connection, exercises may be used to stimulate the system to develop and function correctly.
As the child ages, the success rates for a fully functioning, neurologically integrated system declines, and by adulthood, a truly functional correction is often difficult to achieve. There is great controversy around this issue among ophthalmologists and optometrists and other "wholistic" practitioners, some of whom subscribe to the strict anatomical model of correction and others who believe in the "behavioral" model. There are also, of course, various combinations of the two philosophies.
So which is the most appropriate treatment? We know that surgery can create proper alignment. We know that exercises can stimulate neurological connections and aid muscle control and coordination. The real issue is what kind of treatment yields the best outcomes and at what age the system resists modification. Although it is clear that our brains and nervous system become somewhat less adaptable as we age, there are many instances where "permanently paralyzed" people get up and walk and stroke victims recover lost function.
Clearly, the earlier the age at which we attempt to fix the problem, the better the outcome. This reality focuses on the need for early detection and treatment. Children should have an eye examination by age three or four and earlier if the parents notice an obvious dysfunction.
Additional report: Binocular Vision
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The Focusing System: The Cornea and Lens
The optical lenses of the human eye consist of the cornea, the clear window at the front of the eyeball, and the crystalline lens, located a few millimeters inside the eye and right behind the iris.
The Cornea
The cornea is a remarkable tissue. It has no blood supply, but is a rapidly reproducing and repairable structure. It is maintained in a perfectly transparent state with a constant curvature, providing the main focusing element of the eye. Microscopically, it has many layers and an active nerve supply. The most superficial layers are constantly being removed by blinking and replaced from deeper cell layers. A surface scratch, although often quite painful, can be completely healed in a matter of a few hours. Even a deep cut can repair itself within a day. It is very difficult to damage the cornea.
Chemical exposure and burns, deep cuts and some (uncommon) infections can cause scars which can interfere with vision. Almost any problem that has the potential for causing permanent damage will be accompanied by significant symptoms of pain, light sensitivity and tearing. If you experience such trauma, immediate medical attention is crucial. Exposure to chemical trauma requires dilution (rinse it out, at least 15 minutes with water, or preferably sterile saline) and sometimes medical treatment. A foreign body which cuts the cornea must be removed and infections properly diagnosed and treated.
Improperly fitted contact lenses, which rest upon the cornea, can affect corneal health and structure. Sometimes cornea sensitivity to pain is reduced and there may not be the usual warning signs. The remedy is refitting. At least annual contact lenses check-ups are vital to assure the health of your eyes. Never wear a contact lens which causes pain or affects vision while in the eye or after removal.
The Crystalline Lens and Ciliary Body
After the cornea focuses the image, the light then passes through the crystalline lens. This tissue is designed to "fine tune" the focus and correct the focal power for viewing of near objects. The emmetropic (optically perfect) eye is focused for infinity--a far point in the distance. For most eyes, this point is approximately 6 meters (20 feet), the standardized vision testing distance. When we observe an object at a point closer than infinity, the lens must adjust and focus the image clearly upon the retina. The tiny muscles of the ciliary body (a part of the uvea) contract to exert force on the lens, changing its shape to affect a change in focal power.
As we age, the lens stiffens, becoming less flexible. Also, the ciliary muscle becomes weaker and the system fails to adequately focus the image. This is called presbyopia, a condition which many people notice as they reach their fourth decade of life. Treatment for presbyopia includes, in the early stages, exercises to improve muscle strength and possibly to increase flexibility of the lens. The more common remedy is the use of near or "reading" eyeglasses (or bifocals if there is a distance correction as well) to add additional focal power. This compensates for that which is lost from the lens. Bifocal and multifocal contact lenses are also available.
Cataract is the term for the loss of light transmission through the lens. Another age-related condition, cataracts can also be caused by certain medications (notably long term use of corticosteroids), metabolic disorders such as diabetes, or can result from trauma to the eye. As the lens becomes less transparent, its optics become irregular and vision is affected. Sometimes this is more noticeable at distance or near. Most often the change occurs very gradually and does not seriously impact a person's lifestyle for some time. When it does, the standard treatment is to remove the old lens and replace it with a brand new plastic replacement part, called a lens implant. Recently, there has been some evidence that there is sometimes a nutritional component to this condition. Perhaps some antioxidants (vitamins and minerals) and amino acids may help to mitigate certain types of age-related cataracts.
Additional reports:
Presbyopia,
The Aging Eye,
Cataracts
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The Conjunctiva And Lacrimal System
The conjunctiva is the thin membrane that covers the sclera (the white-colored outer "shell" of the eyeball) and lines the inside of the eyelids. Inflammation of this tissue is called conjunctivitis. The conjunctiva can become damaged, callused-appearing (pinguecula, pterigium) or inflamed from allergy, bacteria, virus or chemical or physical trauma. Signs of dis-ease include: redness (from dilated blood vessels), tearing (from excessive tear production or a clogged drainage system), and discharge (mucus or cell debris and puss from bacteria or sometimes virus). Symptoms range from itching and watering (allergy, sometimes virus), to burning (bacteria, sometimes virus) and pain and grittiness (virus, sometimes bacteria).
Conjunctivitis persisting for more than three days, or with rapidly increasing signs and symptoms, when accompanied by fever, pain or blurred vision, or occurring in immuno-compromised people----requires prompt medical attention. The conjunctiva is constantly exposed to the environment, toxins and pathogens. The eye does an excellent job of eliminating insults to its function. But when it doesn't, medical treatment with antibiotics and anti-viral agents is available.
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The Lacrimal (tear-producing) System
Tears are more than just water. Medically known as the pre-corneal tear film, tears consist of water (produced by the lacrimal gland and other glands), oil and mucus (produced by a number of different glands in the eyelids), and components designed to disarm invading pathogens like viruses, bacteria and even pollen.
The tears are the first line of defense against environmental toxins and debris, washing them away and utilizing a targeted defense system against pathogens (bacteria, viruses, allergens). At the same time, the tears provide moisture and lubrication for the cornea, conjunctiva and sclera.
Deficiency of tears causes immediate sensation of dryness, and tired, heavy lids. Allowed to continue, the surface of the cornea develops dry spots and areas of dying cells, causing a gritty feeling, pain and blurred vision. As the ocular surface is increasingly compromised in function it becomes open to increased inflammation and infection.
The treatment for tear deficiency depends upon the cause, as sometimes dry eyes are a symptom of some other underlying systemic disease (Sjorgens, lupus, and others) or metabolic disorder (diabetes, thyroid dysfunction), or a side effect of medication.
Symptomatic treatment consists of artificial tear replacement drops and blocking of the drainage system to allow increased tear contact time.
Persistent dry eye problems should be properly diagnosed by a doctor.
Additional reports: The Dry Eye, Pingucula and Pterigium
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The Iris and Uvea
The iris is a disk shaped muscle with a hole (the pupil) in the center. It is responsible for adjusting the amount of light entering the eye. There are, in addition, optical effects created by the diameter of the pupil opening. The muscle fibers of the iris contain various amounts of melanin pigment deposited on the back (inside) surface and on the front. The purpose of the brown pigment is to absorb reflected light scatter inside the eye. A minimal amount of pigment on the back surface results in the appearance of "blue eyes", a larger amount of brown pigment on the rear makes for "hazel eyes". Add some pigment to the front and you get "green eyes" and more dense pigment on the front yields "brown eyes".
The uvea is the name given to the extension of the iris tissue to the lining of the inside of the eye. The iris is the anterior part of the uveal tract. The function and disorders of the posterior uvea are beyond the scope of this report. In general, infection of one part of the uvea affects, to some degree, the entire tissue and treatments are similar.
As the pupil diameter decreases, the total amount of light entering the eye is decreased and the depth of focus is increased---precisely the same effect as increasing the F-stop number on a camera. Through a smaller pupil, the apparent range of clear vision is increased and peripheral distortions are decreased. Through a larger pupil, the reverse is true.
Traumatic damage to the iris can cause the tissue to swell and/or disrupt pupil constriction and dilation. The swelling can block the aqueous drainage causing angle-closure glaucoma. Treatment requires the use of drugs (eye drops) to force the iris to constrict. Disease of the iris is rare and when it occurs is often quite serious and difficult to treat. Iritis or anterior uveitis is the name given to inflammation of this tissue. Iritis can be caused by internal infection or as a sign of other systemic diseases Some of the more common systemic disorders affecting the iris include inflammatory non-rheumatoid arthritis conditions such as Ankylosing spondylitis, Reiters syndrome and psoriasis, and diabetes.
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The Fluid-filled Eyeball: The Aqueous and Vitreous
The front chamber of the eye, from the cornea to the lens, is filled with a fluid called the aqueous. This fluid is constantly being produced and drained from the eye. Over production of fluid or obstruction of the outflow drainage system causes fluid pressure to rise, similar to over-inflating a ball. But unlike a flexible ball, the eyeball is fairly rigid. The increasing pressure inside the eye at some point begins to damage the nerves of the retina. This is called glaucoma.
There are a number of different root casues of glaucoma, some of which are understood and others not. Treatment of glaucoma depends upon the cause(s) of the disorder and may consist of oral medications, topically applied drugs (eye drops), laser surgery and combinations of the above.
The other clear, fluid-filled cavity, the vitreous is a gel-like substance and fills the largest area of the inside of the eyeball. As we age, the vitreous gel sometimes develops strands and fibers and may allow small fragments of other tissue to move in the more liquefied portion of the cavity. We sometimes can see these as "floaters". Extreme contraction of the vitreous is a more serious matter, as it may damage the retina to which it is attached. Symptoms are more obvious and include a rapid onset of sometimes large floaters, flashing lights and other visual disturbances, including loss of parts of the visual field. These symptoms require medical diagnosis and treatment.
Additional reports: Glaucoma, Flashes and Floaters
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The Retina
The rods and cones contain chemicals based on beta carotene, the natural, unmodified form of vitamin A. As light strikes the photoreceptor, there occurs a chemical reaction which releases an electrical charge. The chemical reaction continues and the molecule is restored to its original condition---until the next impulse of light energy hits and the photoreceptor sends out yet another electrical charge, over and over again, millions of times a second in a few hundred thousand locations on the retina.
Disorders of the retina can be quite serious and vision threatening. Primary retinal disease includes retina detachment, macula degeneration and others. Systemic diseases which affect the retina include diabetes, hypertension (high blood pressure), and artherosclerosis (hardening of the arteries). Any disturbance to the retina causing visual symptoms requires medical diagnosis. There is treatment for many of these disorders, including medicines and surgery. Symptoms include "flashes and floaters", visual field defects (loss of areas of the peripheral vision) and distorted or missing central vision.
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The Optic Nerve and the Brain
The eye has done its part and the information is on its way to the brain for final processing. What happens here is largely speculation, as the final conversion of electrical impulses into the sense of vision is not so clearly understood. Discussion of the knowledge and theories on this topic is too complex for this report.
Damage to the optic nerve or the brain can interfere with this process. Symptoms include loss of vision, especially in part of the visual field. There are a number of causes, including Multiple Sclerosis, primary neurological diseases and space occupying tumors affecting the optic nerve or in the brain. Again, medical diagnosis is critical, as certain conditions are temporary and others can be repaired.
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Conclusion
You have now followed the path of an image as its light reaches the eye, is focused by the cornea, its illumination adjusted by the iris, then additionally focused by the lens, received by the retina, converted to electrical signals by the rods and cones, and then is finally sent to the brain which magically lets us experience the sense of vision.
The eye is a remarkably durable and well-protected part of our bodies. Many disorders which occur can be fixed. Regular eye examinations are important to help detect abnormal conditions and potential problems so that an effective treatment plan can be initiated. Do seek professional diagnosis and treatment if you have concerns about your vision or eye health. This report is meant to provide you with information to help you understand the incredibly intricate workings of the human vision system. It is not meant to be a substitute for diagnosis and treatment by a qualified health professional.
Additional reports: The Eye Examination, Children's Vision and the Pediatric Eye Examination
