A guide to the many parts of the human eye and how they function.
The ability to see is dependent on the actions of several structures in and around the eyeball.
The graphic below lists many of the essential components of the eye's optical system.
When you look at an object, light rays are reflected from the object to the cornea, which is where the miracle begins. The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The lens' job is to make sure the rays come to a sharp focus on the retina. The resulting image on the retina is upside-down.
Here at the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position!
Think of the eye as a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract, or focus the light and a film (retina) on which to focus the rays. If any one or more of these components is not functioning correctly, the result is a poor picture. The retina represents the film in our camera.
It captures the image and sends it to the brain to be developed. The macula is the highly sensitive area of the retina.
The macula is responsible for our critical focusing vision. It is the part of the retina most used. We use our macula to read or to stare intently at an object.
Here you will find information about some common eye problems such as, Corneal Disease, Cataract, Glaucoma, Diabetic Retinopathy and Macular Degeneration constitute the bulk of sight threatening eye diseases the world over.
The fight for sight begins with awareness about these conditions
Your eyes are your body's most highly developed sensory organs. In fact, a far larger part of your brain is dedicated to the functions of eyesight than to those of hearing, taste, touch or smell. We tend to take eyesight for granted, yet when vision problems develop, most of us will do everything in our power to restore our eyesight to normal.
- The Eye's Vision
- Problems with Focus or Glare
- Problems with Integration
- What Causes Vision Problems?
- Problems with Image Detection
The eye produces vision in much the same manner as a camera taking a picture.
Light entering the eye is focused, first by the curved clear cornea (the watch crystal that covers the front of the eye) and then by the lens which is positioned just behind the iris. The iris has muscles which open and close the central hole in the iris, the pupil, to let in the correct amount of light (different on a bright, sunny day than indoors or at night).
The lens in the human eye is similar to the lens of a camera except that it is not glass or plastic, but is actually a bag of clear, gel protein lens Fibers. The lens is focused in the eye by a muscle, which pulls on the strands, or ligaments that suspend the lens behind the pupil to change its shape. Therefore the shape of the lens can be changed to focus the eye for objects on the horizon as well as objects right up under your nose!
The retina lies flat against the inside back wall of the eye; it is similar to the film in the camera in that it takes a picture or "sees" the image focused on it by the lens in the front of the eye. The retina in each eye sends the image it generates through the optic nerve to the brain where the images are combined into one image with the perception of depth.
In summary, the picture is taken of a scene by each eye (both eyes pointed at the same object!) and the brain interprets the image in depth, with feedback control of the position, focus, and pupil size for each eye.
What Causes Vision Problems?
When a person complains of vision problems, most often they fall into one of three general categories:
Problems with the pupil or focusing of one or both eyes that cause glare or blur.
Problems with the detection of the image by the retina or its transmission to the brain through the optic nerve that cause smudges in the field of vision.
3. Problems with the brain in its attempt to combine the images from the two eyes into one image with depth perception.
Problems with Focus or Glare
Focus problems cause the image to be blurred (fuzzy overall) and may result from the incorrect shape of either the cornea or lens (as in near-sightedness or far-sightedness) or astigmatism (where the elements may be slightly warped), all of which cause a poor focus on the retina. Most of these conditions can be improved with glasses or contact lenses that merely correct the focus of the eye. If the cornea or lens is sufficiently irregular that glasses or a contact lens cannot adequately correct the focus (and the image remains blurred), then surgery must be performed to correct the irregularity -- either corneal surgery (sometimes requiring transplantation) or lens surgery (removing the cataract or cloudy protein within the lens and implanting a small plastic Artificial transparent lens within the capsule to replace the protein that is removed).
Glare is created by the cornea or more often by the lens if it becomes cloudy. As we all grow older (chronologically gifted!), the clear gel protein in the lens becomes crystallized and cloudy resulting in glare -- which we experience as a haze that washes out the contrast of the image on bright sunny days, or as flare seen around headlights at night. Glare can be reduced with filtering lenses (yellow-amber lenses, orange "blue blockers", gray lenses that reduce the light level overall, or polarizing lenses that reduce the glare from reflective surfaces). When glare is not reduced adequately by such lenses, then surgery (most commonly cataract surgery) is performed to replace the cloudy lens in the eye with a clear plastic implanted lens.
Rarely, glare and focus problems are caused by an iris that cannot adequately close down the opening size of the pupil.
Problems with Image Detection by the Retina and Transmission of the Image though the Optic Nerve.
Problems of the retina and optic nerve most often cause loss of color or loss of contrast. Most commonly the problem is not observed over the whole field of vision of the eye but rather is experienced in portions (causing smudges or "scotomas" in the visual field). Our ability to see textures and hues is based on the contrast sensitivity of the retina -- the ability of the film in the camera to produce a good, high-contrast picture with correct colors. It is also dependent upon the ability of the optic nerve to transmit that picture to the brain (and on the ability of the brain to "see" the image).
Blockages of blood vessels, or a "stroke" of the retina, optic nerve, or brain, can cause a defect in a portion of the visual field, blurring with loss of color and contrast sensitivity. Inflammation or degeneration of the retina can cause focal blur or loss of color and contrast. Leakage of blood vessels in the retina ( as in diabetes) or scarring of the retina not only causes smudge defects in the field of vision but also produces distortions of the retina, causing distortions in the field of vision (doors or windows have kinks or bumps).
For these conditions, focus is not the problem and glasses do not improve the vision, since the problem lies with the retina and its inability to detect and transmit the image. The problem becomes much worse under conditions of reduced lighting (i.e. the restaurant where the lighting is romantic but you can't read the menu!). Here the key to improving vision is to increase the lighting on the object with focal lights that don't reflect into the face and with filter lenses that reduce glare and improve contrast. Magnification devices that magnify the image and increase light also are of some help, provided the visual field defect is not large or severe, or if it is, then devices which assist the person in using the remainder "peripheral" vision may be of help.
Problems with Integration of the Images from the two eyes into one Image with Depth
If the brain cannot integrate the two images (one from each eye) into one image with depth perception, then the person has double vision (often noticed as a ghosting of the image or as two separate images) or problems with depth perception (pouring liquids outside of the cup, hitting the fender of the car against the garage door!). Double vision is most often observed only when both eyes are open (and disappears when one eye is covered) and occurs either because the eyes are not lined up to look at the same object, or because one of the images is distorted or of a different size than that in the other eye. These problems often require neurologic or retinal evaluation and sometimes require surgery to correct (or if impossible to correct, then permanent patching of one eye may be required to remove the distracting second image).
Patients with diabetes are more likely to develop eye problems such as cataracts and glaucoma (Increase pressure inside the eye) , but the disease's affect on the retina is the main threat to vision. Most patients develop diabetic changes in the retina after approximately 10 years. But if the patient is not controlling his Diabetes changes could occur as early as 5 years or even less.
The earliest phase of the disease is known as background diabetic retinopathy. In this stage, the arteries in the retina become weakened and leak, forming small, dot-like hemorrhages. These leaking vessels often lead to swelling or edema in the retina and decreased vision.
Proliferative diabetic retinopathy; in this stage circulation problems cause areas of the retina to become oxygen-deprived or ischemic. New, fragile, vessels develop as the circulatory system attempts to maintain adequate oxygen levels within the retina. This is called neovascularization. Unfortunately, these delicate vessels hemorrhage easily. Blood may leak into the retina and vitreous, causing spots or floaters, along with decreased vision.
In the latest stages of the disease, continued abnormal vessel growth and scar tissue may cause serious problems such as retinal detachment and glaucoma.
How do you notice it?
There's really no specific way to know, but most importantly are;
- Blurring of vision or deterioration of vision
- Floaters and flashes of light
- Sudden loss of vision
That's why it is recommended that Diabetic patients do routine visit for ophthalmologist for evaluating their Retina via dilated fundus examination. If everything is ok, fundus examination can be done on annual visits.
How do I treat it?
Treatment of Diabetic retinopathy requires both ophthalmologist and patient's cooperation. The most single effective treatment is controlling your blood sugar levels. And it is important to know that any surgical interference could lose its effect when the patient fails to lower his blood sugar to or near normal levels. Keep in mind that regular visits and early detection can save your vision.
1- Laser Photocoagulation using argon laser can help to destroy the new fragile blood vessels. Argon laser leads to reduction in VEG-F (Vascular Endothelia Growth Factor) which is responsible for these abnormal vessels growth. Mostly, laser is applied to the peripheries of the retina. Laser can not be applied to the center of the retina or the macula.
2- Intravitreal Injection; this popular technique provide the means of treating neovascularization located near the macula. Most widely used are Avastin and Leucentis. More detailed discussion about IV injections are found in Procedures/Intravitreal injection.
3- Vitrectomy; commonly needed for diabetic patients who suffer a vitreous hemorrhage (bleeding in the gel-like substance that fills the center of the eye). During a vitrectomy, the retina surgeon carefully removes blood and vitreous from the eye, and replaces it with clear salt solution (saline). At the same time, the surgeon may also gently cut strands of vitreous attached to the retina that create traction and could lead to retinal detachment or tears. But the need for this surgery has become less after the use of IV injections.
4- Surgery For Retinal Detachment; Retinal detachment requires surgical treatment to reattach the retina to the back of the eye. The prognosis for visual recovery is dependent on the severity of the detachment.
5- Controlling Diabetes. Helps progression and preventions.
What is Glaucoma?
There has been many definitions for glaucoma but the most accepted one is; it's a combination of Increase IOP, Visual Field changes, and Optic Disk cupping. One of the leading causes for blindness worldwide, and a silent killer of the optic nerve.
Aqueous Humor Cycle:
Aqueous is the clear fluid filling the anterior chamber of the eye. It is produced by the cilliary body into the posterior chamber, flows through the pupil to the anterior chamber then to the Irido-Corneal angle, where it's absorbed via the trabecular meshwork.
Any thing that disrupt absorption or increase production can lead to increase trapping of aqueous with in the eye and increase in IOP (Intraocular Pressure). The normal IOP is ranging from 10-21 mmHg.
Consequences of high IOP:
Increase IOP can put pressure back on the retina causing direct damage to the sensitive light layers of Rods and Cones, it can lead to damage indirectly through pressing on the blood vessels that supply the retina and thus causing ischemic damage to the layers of retina.
Most Glaucoma treatments are directed toward lowering IOP.
Types and causes of Glaucoma:
Glaucoma is broadly classified into Open-Angle and Angle-Closure, Primary and Secondary.
The list of risk factors is wide; Age over 40, family history, steroids induced IOP elevation, diabetes, and myopia (short-sightedness).
While secondary glaucoma can be caused by conditions like Pseudoexfoliation syndrome, Pigmentary dispersion syndrome, Neovascular glaucoma (angle is closed by formation of new vessels as in diabetes).
The glaucoma is commonly silent with no symptoms, yet it may present with headaches, pain in the eye, deterioration of vision. Then, if the physician suspect it he must do the following tests:
- Measuring the IOP.
- Visual Field exam.
- Fundus examination.
IOP measurement is routine on ophthalmic examinations.
REMEMBER.. once you are diagnosed to have glaucoma, it is extremely important to stay on medication and frequent visits to check you IOP. What is lost can not be replaced!
1- Medical Treatment with drops that lower you IOP. A combination of more than one drop is possible to keep you IOP low.
2- YAG-Laser iridotomy; an opening in the iris in those with shallow anterior chamber to establish a communication between Anterior and posterior chamber in order to facilitate aqueous drainage.
3- Trabeculotomy; is a surgical procedure to create a valve like mechanisim that assist in filtering the excess aqueous humor. Is generally performed if medical treatment is insufficient to keep IOP low.
4- Ahmed Valve implantation; is very effective way to terminate the existence of increased IOP. Ahmed valve is placed on the sclera and under a conjunctival pocket. More details regarding the valve are found in the Procedures section of the website.
In general, refractive surgery is any procedure designed to reshape the eye and reduce or eliminate the need for contacts and glasses. Laser vision correction, most commonly in the form of LASIK or PRK, is the most well-known and popular form of refractive surgery. LASIK and PRK are common procedures, applicable to most people wearing corrective lenses, but there have also been some new and exciting surgical advances, including corneal rings , Laser Thermal Keratoplasty, refractive lensectomy, and intraocular lenses . Radial Keratotomy, the first well-known type of refractive surgery, has now become obsolete due to the advancements in laser technology.
- Radial Keratotomy (RK)
- Phakic Intraocular Lenses
- Intrastromal Corneal Ring Segments (INTACS)
- Laser Assisted In-Situ Keratomileusis (LASK)
- Photo Refractive Keratectomy (PRK)
- Laser Assisted Sub_Epithelial Keratomileusis(LASEK)
- Custom Lasik
- Automated Lamellar Keratectomy (ALK)
- Phototherapeutic Keratectomy (PTK)
- Laser Thermal Keratoplasty (LTK)