The Eye
The features of this incredible organ are amazing. Approximately 80% of perception in human beings is through the eyes. 70% of sensory receptors of the body are in the retina layer of the eye. The human eye operates with an incredible sensitivity provided by its 120,000,000 black & white and 6,500,000 color receptors.
How do we see?
The eye functions like a camera or more like a video camera. Light rays coming from the objects arrive on the cornea first. The cornea provides 70% of the refractive power of the eye. After refracting the light rays, it directs them to the lens through an aperture called the pupil. The lens bends, in turn, refracts the rays again. It makes a fine adjustment to focus the light rays on the retina. This adjustment is similar to the auto-focus function of cameras. This superb, silent and continuous auto-focus function is due to the lens' ability to change its refractive power by altering its form and thickness. This function is called accommodation. In a non-accommodating young eye the refractive power of the lens is below 20 diopters but with maximum accommodation it increases to more than 30 diopters, which is nearly equal to an 8.5 diopters increase in the eye's total refractive power. Because of this ability the young eye can see both near and far objects well.
After the light rays are bent at the lens, they pass through the vitreous, the jelly-like substance filling the back part of the eyeball, and reach the retina. The retina, which makes up the inner lining of the eyeball functions like a camera film, or a CCD of a digital video camera. Then light rays are converted into electrical impulses by the retina and carried to the brain by the optic nerve. These impulses are processed and perceived as images by the brain.
Refractive Errors
An eye which does not have any refractive errors and has the ability to see the far and near and distant objects clearly is called an emmetropic (normal) eye. A refractive error means that the shape of your eye does not bend (refract) the light rayscorrectly. In other words, the eye can not focus images properly, resulting in a blurred image. In an emmetropic eye parallel rays of light from a distant object and divergent rays of light from a near object focus on the retina after being refracted by the cornea and the lens. The eyes, having refractive errors have problems with focusing light rays on the retina. Myopia (nearsightedness), hyperopia (farsightedness), astigmatism and presbyopia are names given to different refractive errors.
The refractive errors are quantified with the power of the lens that corrects the refractive error. The unit of the lens power is measured in diopters. 1 Diopter (D) is equal to the optical power of the lens which has a focusing distance of 1 meter. A lens with 1 diopter power focuses parallel light rays at 1 meter while a lens with 2 diopters power focuses parallel light rays at 0.5 (1/2) meter.
Total focusing power of the eye is between 58 and 71 diopters. Due to this amazing focusing ability the eye can see images from 25 cm to infinity by focusing on a tiny screen (retina), which is only 17 mm away from its lens.What is my refractive error?
Myopia (nearsightedness) is the inability to see distant objects well. Hyperopia (farsightedness) is the difficulty to see close objects clearly. If near vision difficulty occurs in a normal-sighted person over 40 years, then it is called presbyopia. Sometimes both close and distant objects are seen blurred or ghosted. This might be caused by astigmatism (very severe myopia or hyperopia may cause blurred vision for both distances). See your prescription if you're unsure of your refractive error.
On your prescription, you will notice 2 main columns: OD or R (RE) which means your right eye and OS or L (LE) means your left eye. Under this OD or R title you will see 3 columns which are labelled Sph. (spherical), Cyl (cylindrical) and Axis. Sph values show the magnitude of the lens you need to put on in order to compansate for your hyperopia or myopia. The unit of the lens magnitude or power is diopter (D). It shows the focusing power of the lens: 1 D means the lens focuses at 1 meter, 2 D means ½ meters, 4 D means ¼ meters. If there is + sign before that value, it means that you are hyperopic, if it is a - value, then you are myopic. If you are over 40, there may be three lines under the sph column, and on one line you might be myopic but at the others you may be hyperopic. Due to aging, the eye can not accommodate for close distance (presbyopia). This extra line is for "near / add" or "reading glasses". The other columns Cyl and Axis describes the lens you need to neutralise your astigmatism. Cyl shows the magnitude and axis is the orientation of the cylindrical lens in degrees.
Myopia
(Nearsightedness)
[Myop: seeing with closed eyes]
Myopia is known as nearsightedness. 22% of people in general are myopic. In myopia, light rays from a distant object converge too soon and focus in front of the retina. Diverging rays from a near object focus on the retina. Distant objects seem blurry while near objects seem clear.
Emmetropic (normal eye) people require reading glasses around the age of 40 whereas myopic people might not need them even with advanced age. In other words, people over the age of 40 with mild myopia are usually able to read by taking their glasses off. On the other hand, people with moderate to pathological myopia do not have such an advantage. In other words, they don't have a satisfactory near vision, but some can read by bringing the reading materials to such a distance as to touch their nose.
There are multiple factors causing myopia. The main factor is heredity. Myopia usually starts between the age of 5 and puberty. It increases during adolescence in accordance with the development of the whole body. 1mm increase in the length of the eye causes 3 diopters of myopia. That is why new glasses with higher diopters are more necessary in a growing child. Around the age of 20, the increase of myopia usually ceases.
The main complaint of myopic people is not being able to see into the distance. Myopics may squint in order to see well. Some don't realize that they have a refractive error because they think it is natural and logical not to see distant objects that well.
For children and adolescents seeing the distance is especially important for their psychological and social development. If their distant vision is low, they live in a limited environment. If they don't see the board at school, this might affect their success at school.
Similar to other refractive errors, definitive treatment of myopia is mainly by laser vision correction. To compensate myopia, concave lenses are used. It is written with a minus sign in front of the dioptric power.
Mild myopia <3.00 diopters
Moderate myopia 3.00 - 6.00 diopters
Severe myopia 6.00 - 9.00 diopters
Pathological (Extreme) myopia >9.00 diopters
People with severe or extreme myopia might have retinal degenerations or detachments. The probability of having glaucoma is higher in this group as well. Because of these serious problems they should have regular yearly eye examinations and be aware of their conditions.
There are 3 types of myopia.
1. Axial myopia
This is the most frequent type. Refractive power of the eye is normal but the length of the eye is longer than normal. Because of that light rays coming from distant objects focus in front of the retina.
2. Curvature myopia
The length of the eye is normal but increase in the curvature of the cornea or the lens causes the light rays to be refracted more and focused in front of the retina.
3. Refractive myopia
The refractive power of the lens might increase due to senile cataract and this might cause myopia. Elderly people with cataracts might notice an improvement in their near vision. Other than cataracts diabetes might cause myopia by increasing the sugar level in the fluids of the eye. People with diabetes should have their eyes examined when their blood sugar levels are regulated.
Hyperopia
(Farsightedness)
Hyperopia is usually inherited. Approximately 25% of the general population is hyperopic. In hyperopia, light rays from a distant object converge and focus on the retina. But diverging rays from a near object can't converge sufficiently and therefore they focus beyond the retina. The hyperopic eye can see distant objects well, but near objects blurry.
In theory, hyperopia is the inability to focus and see the close objects clearly, but in practice many young hyperopics can compensate the weakness of their focusing ability by excessive use of the accommodation functions of their eyes. But older hyperopics are not as lucky as them. By aging, accommodation range diminishes and for older hyperopics seeing close objects becomes an impossible mission. Also for people with advanced hyperopia, including young people, the accommodation mechanism is not sufficient and they need spectacles, or a laser vision correction procedure. Unfortunately, people with advanced hyperopia may even suffer from both close and distant vision problems. Types of hyperopia:
1. Axial hyperopia
This is the most frequent type. Refractive power of the eye is normal, but the length of the eye is shorter than normal. Because of that light rays coming from close objects focus behind the retina. If you compare this to a movie theater, the projection screen is too close and the projected image is not in focus.
2. Curvature hyperopia
The length of the eye is normal, but the decrease in the curvature of the cornea, or the lens, causes the light rays to be refracted insufficiently and focused behind the retina.3. Refractive hyperopia
In refractive hyperopia the size of the eye is normal, but the refractive power of the lens is less than normal. Generally this occurs with aging. Normally babies and young children are naturally mild hyperopics but with the unbelievable elasticity of their lenses, they can easily compensate, in other words, accommodate their vision. Natural hyperopia diminishes and disappears with age.
Hyperopics may frequently feel headaches, eye strain while using a computer, reading or continuous TV watching. They may also complain of sensitivity to bright lights, difficulty of reading small prints and even blurry distant vision. The reason for headaches and eye strain is continuous stretching and use of accommodation muscles (tiny muscles inside the eye that thicken the lens).
Definitive treatment of hyperopia is laser vision correction (LASIK, LASEK or PRK). To compensate hyperopia, thin edged convex lenses (looks like a loop) are used. The dioptric power of these convex lenses are marked as (+) plus. The severity of hyperopia is classified as follows:
Mild Hyperopia < +2.00 diopters
Moderate Hyperopia +2.00 to +4.00 diopters
Severe Hyperopia +4.00 to +6.00 diopters
Extreme Hyperopia > +6.00 diopters
At EyeSTAR LASIK Institute, Istanbul, we are even able to correct extremely hyperopic eyes with our cutting-edge technology.
Astigmatism
[a-not + stigma-, dot]
Astigmatism is a Greek word meaning "not like a dot". The name of this refractive error comes from the fact that astigmatic people can see dots like a line. This is true for moderate to severe astigmatism, since a survey of normal eyes shows that almost every human eye has a baseline corneal astigmatism of at least 0.25 to 0.50 diopters. Astigmatism exceeding 1 diopter is about 20% among the general population.
In order to have clear vision, the cornea has to have a smooth surface and curvature. In astigmatism natural corneal curvature is distorted in a meridian (egg shape). This distorted corneal meridian(s) scatter(s) light rays. In other words, an astigmatic eye bends light rays different at different meridians, resulting in many images with many focal points (inability to focus on a single plane). This causes blurry, ghosted, or smudged vision. If you examine the image focused by an astigmatic eye on its retina you may notice many superposed off-focus images on a well-focused image. As you can guess, people with astigmatism have a constant blur for both near and distant vision.
Sometimes astigmatism may not stem from the cornea but from the lens or from other refractive media in the eye.
The severity of astigmatism can be classified as follows:Mild Astigmatism < 1.00 diopter
Moderate Astigmatism 1.00 to 2.00 diopters
Severe Astigmatism 2.00 to 3.00 diopters
Extreme Astigmatism > 3.00 diopters
Astigmatic people may suffer from headaches, tired eyes and sensitivity to bright lights. Sometimes, like nearsighted people, they may squint in order to improve their vision.
Classifications of Astigmatism
Astigmatism has many classifications, but in general there are two types of astigmatism: regular astigmatism and irregular astigmatism. If a dot is seen like two perpendicular lines, it is regular astigmatism. These lines are produced by the steepest and flattest meridians of the cornea. Cylindrical lenses are used to compensate for regular astigmatism. On the other hand, if the astigmatic eye sees many lines, it is irregular astigmatism. This means that the cornea has many irregularly shaped meridians at different angles. Unfortunately, people with irregular astigmatism cannot see well with glasses, but they can benefit from hard contact lenses.
Your eye doctor may tell you that the condition of your eyes is not merely astigmatism, but a combination of refractive errors. The following list will provide you with a more detailed explanation about classes of astigmatism to help you understand your condition better. Regular astigmatism may have 5 sub-classes.
1. Simple hyperopic astigmatism: One meridian is emmetropic (normal), while the other is hyperopic.
2. Simple myopic astigmatism: One meridian is emmetropic (normal), while the other is myopic.
3. Compound hyperopic astigmatism: Both meridians are hyperopic but one is more severe.
4. Compound myopic astigmatism: Both meridians are myopic but one is more severe.
5. Mixed astigmatism: One meridian is myopic, while the other one is hyperopic.Astigmatism can be treated with LASIK. Sometimes when the astigmatism is severe, astigmatic keratotomy (AK) is performed after LASIK treatment.
For irregular astigmatism, it's better to measure the eyes' refractive power point-by-point with aberrometers (wavefront devices) rather than relying only on the conventional refractive measurements. Custom LASIK treatment of irregular astigmatism with wavefront and corneal topography data is a quantum leap in the correction of the refractive errors.
Presbyopia
(Near vision difficulty due to aging)
(Root words presbus=old man, opi=to see)
The presbyopic eye can't focus on close objects: the result is blurred near vision. Presbyopia is a natural vision problem seen after the age of 40. The aging process decreases the accommodation ability (thickening of the lens) of the eye resulting in a decrease in the focal range. Even accommodation power of a twenty-year-old person is less than that of a-ten-year-old. If this accommodation power drops 3 diopters in range presbyopia becomes apparent. When presbyopia starts, the person tries to read by holding the reading materials as far as possible. Since presbyopic eye can't focus the divergent rays coming from close objects, presbyopic people try to move away these objects in order to bring them into their focal range. Presbyopics need reading glasses, these are the same lenses used by hyperopics (+ signed, thin edged, loop like). The following shows the average magnitude (dioptric power) of reading glasses throughout the age groups.
Aging and addition in glasses
Age
Diopters
40-45 yrs
+1.25
45-50 yrs
+1.50
50-55 yrs
+1.75
55-60 yrs
+2.00
60-65 yrs
+2.25
65yrs+
+2.50
As you can deduce from this table, the older you are, a more powerful prescription you need. Please also note that presbyopia is not just for people with refractive errors. People (emmetropic) with perfect vision will also suffer from presbyopia. Only mild myopics may have satisfactory near vision when they get older. Their extremely superior near vision ability decreases to a normal level after the age of 40. Monovision, which is a solution for presbyopia, is based on the natural advantage of mild myopia.
Unfortunately, presbyopia is like destiny, and according to latest studies, there are more than 590 million of presbyopic people living on earth now. But thanks to the advances in science and technology, there are solutions like SEB (Scleral Expansion Bands; Presby), progressive LASIK, LASIK for monovision and Multifocal Intraocular Lenses.
