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What is a refractive error?

2018-04-09 08:00:52

There are two main types of refractive states of the eye. One is called metropia; One is called non-metropia, which is also called ametropia. There are three types of refractive errors: those whose focus is behind the retina become hyperopia; Those who focus on the front of the retina become short-sighted; If the size of the refractive power of the meridians of the eyes is different, a focal point cannot be formed, that is, astigmatic eyes.


1. The positions of the components of the refractive system are abnormal: (1) the anterior and posterior diameter of the eyeball, that is, the axial length of the eye, is too short, leading to farsightedness, and too long, leading to myopia; (2) Forward displacement of the lens leads to myopia, and backward displacement leads to farsightedness. 2. Abnormal refractive power: (1) the curvature of the cornea and lens is too small to form farsightedness or too large to form myopia; (2) Irregular curvature of the cornea and lens forms astigmatism. 3. Abnormal refractive index. 4. The absence of components in the refractive system, such as the absence of crystalline eyes.

Classification of disease

The most common eye diseases are as follows: myopia, farsightedness, astigmatism, presbyopia and anisometropia. The symptoms of myopia are the inability to see objects; The symptoms of hyperopia were mild hyperopia with good near and far vision, but medium and high hyperopia with bad near and far vision. Astigmatism Astigmatism is a state of eye refractive error, the cause of astigmatism is that the curvature of the cornea or lens surface is inconsistent, such as football shape, resulting in different refractive power of the two vertical meridians can not focus a point, the formation of unclear or overlapping images, see far and near are not clear; Presbyopia means that with the increase of age, the lens gradually stiffens and the elasticity decreases, which leads to the loss of the adjustment ability of the eye, resulting in difficulty in seeing. Anisometropia The dioptric difference between the two eyes, the difference between the spherical lens ≥1.50D, and the difference between the cylindrical lens ≥1.00D is called anisometropia, and anisometropia will cause the retinal image of both eyes to be unequal, and the visual center is difficult to fusion. (The diopter difference of both eyes is 1.00D, and the object size difference is 2%).


It means that when the eye is in a relaxed state, the parallel light from 5m away forms a focus in front of the retina after it passes through the refraction system of the eye, and the retina cannot form a clear image. If you want to get a clear image, there are two ways; One is to turn the parallel light into a spread light, and the target to be seen will move to a certain distance in front of the eye, and a clear image can be obtained. One is to place an appropriate degree of concave lens in front of the eye, and the force of the lens causes the parallel rays to spread out. 1. Axial myopia is the most common type of myopia, with a lengthening of the anterior and posterior diameters of the eye. The refractive power of each refractive system is the same as that of the metropic eye, and the cause of myopia is that the axis of the eye is too long so that the focus is in front of the retina. Most of the high myopia belongs to this type 2. Curvature myopia, the eye size is normal, but the refractive system such as the cornea or lens is too flexion, resulting in imaging in front of the retina. It is more common in the dilatation stage of old age cataract.


1. Genetic quality: myopia has a certain genetic tendency, which has been recognized, especially for high myopia. But for general myopia, this tendency is not very obvious. With genetic factors, the age of disease is earlier, more than 6.00D. But there are also people with high myopia, no family history. High myopia is autosomal recessive, general myopia is a multi-factor genetic disease. 2. Development factors: infants are farsighted due to small eyeballs, but with the growth of age, the eye axis is gradually lengthened, and the development is normal in adolescence. Such as excessive development, the formation of myopia, this myopia is called simple myopia, mostly at school age. They are generally below 6.00D. Development stops around the age of 20. If the progress is rapid in childhood, to 15 to 20 years old progress faster, later slowed down, this type of myopia is often higher than 6.00D, can be 20D to 25D or 30D. This type of myopia is called high myopia or progressive myopia or pathological myopia. This kind of myopia can be developed and degenerative in old age, so the vision can gradually decline, and the vision can not be corrected by the lens. It is rare to have myopia at birth, but very few are congenital. 3. Environmental factors: The role of environmental factors has become more and more important. The main reasons for the rising rate of myopia among teenagers are the heavy study load, the increase of close eye use and the decrease of outdoor activities caused by the popularization of television and computers. In reading and writing and other close-range work, the eyes need to use greater adjustment, if there is no long-term, a large number of close-range eye environment, myopia rate would not be so high. For example, during the Second World War, learning was greatly affected, and statistics show that the rate of myopia in all countries in the world has been greatly reduced. In recent decades, with the explosion of knowledge, the number of myopia has also increased rapidly. According to incomplete statistics, the myopia rate of primary school students in urban areas in our country is 20-30%, middle school students 40-50%, high school students as high as 70%, and college students as high as 80%. 4. Dietary factors: The lack of certain trace elements in the eyes caused by fine diet, partial diet and picky eating is also the cause of premature poor vision for some students. The lack of some or some important nutrients during the growth and development of the eyes makes the eyeball tissue more fragile, and most myopic children have the habit of eating snacks, picky food and partial food. They eat more refined grains and fast food, which lack nutrients or destroy more nutrients. At the same time, their diets lack high-quality protein foods and coarse grain foods such as milk, eggs, milk, fish, meat, and chicken. The more types of food that are lacking and the larger the total amount, the higher the incidence of myopia and the higher the degree of myopia.


Clinical manifestations (1) The clinical manifestations of myopia are diverse: mild myopia is more accustomed to fuzzy distant images, and because the vision is very clear, they can adapt to daily life, study and work, and do not feel limited. It is detected only when there is a need for far-sightedness, or when compared to a person with normal vision, or during a healthy physical examination. General complaints of blurred vision or direct complaints of "myopia", such as can not see the blackboard, dividing unknown road signs and so on. And once I put on my corrective glasses, I was amazed to see another world before my eyes. Some people who have myopia at an early age, because of the obvious low vision, usually prefer to live in the room, activities alone, so that the personality is more introverted. (2) In order to reduce the hazy image formed by the diffuse aperture of the eye, many nearsighted people improve their eyesight by reducing the eyelid crack and increasing the depth of field, which is often manifested as habitual squinting. The appearance of myopia is usually characterized by large, full, protruding eyeballs. When the eyeball is extremely inwardly turned, the equatorial part can appear in the palpebral fissure area, and this phenomenon is more obvious in the monocular high myopia. (3) myopsis or myiodesopsia are common complaints of high myopia. This is due to high myopia complicated by vitreous degeneration, liquefaction, turbidity of the formation of fine flotsam, projected on the retina, causing the phenomenon of black shadow fluttering in front of the eyes. Due to different parts, sizes, quantities and forms. It may appear as dots, lines, networks, or clouds, as if there were insects or flies flying in front of the eyes. The quantity is different, hidden and present, the density is uneven, there are light and strong. Can be seen in all types of myopia, appear early or late. It generally increases slightly with age. When attention is distracted, or over time due to adaptation and habit, floaters (flies) do not notice. It usually does not affect vision, but some patients are very sensitive to it and often worry about the abnormal phenomena in front of them. Moderate and mild myopia mostly without this symptom. (4) Usually myopia after excessive use of the eyes can appear some abnormal sensation and visual fatigue phenomenon. It is more common when there is astigmatism, anisometropia, or poor general condition. Such as deformities, double vision, small vision (especially when wearing high diopter glasses), flashing, discoloration, photophobia, dry eyes, eye itching, eye foreign body sensation, heavy eyelids, eye swelling pain, headache and inability to read for a long time. The possible causes of these phenomena are as follows: (1) the relationship between the adjustment and the aggregation function of myopia is dysfunctional, and the adjustment tension and strabismus appear; (2) The adjustment range of high myopia is very small, and it is difficult to adapt to the change in distance when reading close; (3) improper glasses (such as refractive correction, mirror frame is too large, pupil distance error, etc.), (5) the formation of amblyopia, more in childhood onset of high myopia, treatment to wear a mirror, as long as the fundus is normal most of the effect is very good. (6) fundus lesions; It is more common in people with high myopia, manifested as the degeneration of the leopard stripe fundus and even the peripheral omentum, and is more likely to lead to retinal detachment when suffering trauma.


Treatment method 1. Ophthalmic treatment includes frame glasses and contact lenses (contact lenses). Frame glasses are the oldest, safe and effective way to correct myopia, and are currently the most widely used around the world. It is best to use this method for young students. Although the contact lens has its own advantages, its lens cleaning and eye care are more complicated, so it is not the best choice for young myopic students with heavy learning tasks and poor self-living ability. Some special professionals such as actors, athletes and so on can be worn for a short time. Second, surgical treatment of the corneal structure is like a camera lens, its refraction rate is +43D, accounting for 2/3 of the total refractive index of the eye, so changing the corneal curvature is the main choice for the treatment of ametropia, has been carried out to treat the surgical method of refractive error is mainly to artificially change the curvature of the cornea to achieve the purpose of treating ametropia. The refractive surgical methods in China include lamellar keratoplasty (ALK), manual lamellar keratectomy (MLK), laser thermokeratoplasty (LTK), radial keratotomy (RK), excimer laser refractive keratectomy (PRK), excimer laser in situ keratomileusis (LASIK) and so on. Surgical treatment of myopia is currently the only effective way to correct myopia (removing glasses). From the current medical development, excimer laser in situ keratomileusis (LASIK) is the safest and most effective. However, this kind of surgery is generally only suitable for myopia patients over 18 years old and stable in the past two years, and the corneal thickness can not be too thin. Third, Orthokeratology treatment Simply put, orthokeratology is a special semi-rigid contact lens that improves vision by reshaping the cornea. However, this method only changes the time of wearing a mirror in disguise, and is not the treatment of myopia, and it is only a clinical trial in the experimental stage, and there are many complications, which must be carried out under the guidance of experienced doctors. From the current medical development, there is no drug that can clearly treat true myopia or prevent the development of myopia. For false myopia, some cyclomuscle relaxants can be used to relax the spastic cyclomuscle, thereby promoting the recovery of vision. However, it should be noted that false myopia is not true myopia, and proper eye use and rest can restore normal vision.


When the eye is in a relaxed state of regulation, parallel light rays from 5m away form a focus behind the retina after passing through the refraction system of the eye, and the retina cannot form a clear object like farsightedness. 1. Axial hyperopia The anterior and posterior diameter of the eyeball, or the short axis of the eye, is the most common cause of hyperopia. It's also part of normal development. The newborn is +2.50D- +3.00D. The eyeballs grow with age, and some people still have some degree of farsightedness into adulthood. (2) Curvature hyperopia, the curvature of any flexion of the eye can be reduced to produce hyperopia, but the common corneal curvature, such as flat cornea.


Clinical manifestations 1. Visual acuity The visual acuity of the far-sighted eye is determined by the level of the far-sighted diopter and the strength of the adjustment force. Mild hyperopia, with a small amount of adjustment can be overcome, far and near vision can be normal, generally asymptomatic. This kind of hyperopia is called recessive hyperopia. Slightly heavy farsightedness or lack of adjustment, so far and near vision are not good. Because the long-sighted eye is in a state of adjustment tension for a long time, it is easy to have eye fatigue symptoms. High hypermetropia in childhood can lead to amblyopia and strabismus, which should be paid great attention to. Moderate hyperopia is easy to weariness and inattention due to near visual discomfort, and is often mistaken for naughty and ignored. Youth because of the need to use the adjustment force to move the focus behind the retina to the retina, so the eye is often in a state of adjustment, than myopia, more prone to eye fatigue, such as writing, reading, watching TV and other long-term vision, will feel heavy eyelids, eyes dry, eyeball acid, swelling, pain, and then blurred vision, dizziness, head distension, headache. In the middle and old age, because the adjustment force is insufficient, the performance of presbyopia is advanced, and the elasticity of the crystal is gradually lost, and the final sight is not clear. 2. Eye position: medium and high hyperopia overuse adjustment, the corresponding set of contradictions, prone to esotropia or esotropia, strabismus occurs in the eyes with higher degrees of hyperopia, often amblyopia occurs. 3. Others: medium and high farsightedness, short eye axis, small cornea, obvious changes in the fundus, small papillae, flush color, blurred edges slightly elevated, false papillitis. In addition, it is often accompanied by conjunctivitis, blepharitis or blepharitis. Angle-closure glaucoma may occur due to the anatomical characteristics of far-sighted eyes.


In principle, the treatment method is corrected with convex lenses according to the original degree. Children and adolescents should be in the paralytic ciliary muscle post-examination optometry (general use of atropine), low degree of hyperopia, if there are no symptoms, do not wear glasses, with the development of the eye can become orthotropia. If there are symptoms, especially when accompanied by strabismus, it is necessary to get an eyeglass. For adults with medium and high hypermetropia, it is generally not easy to accept the first time, and the degree can be appropriately reduced and corrected, and the reduced degree does not exceed one-third of the original degree. In order to avoid the effect of hyperfarsightedness lenses, it is best to wear contact lenses for monocular hyperfarsightedness or aphakia eyes.


Astigmatism is a state of eye refractive error, the cause of astigmatism is that the curvature of the cornea or lens surface is inconsistent, such as football shape, resulting in different refractive power of the two vertical meridians can not focus a point, the formation of unclear or overlapping images, see far and near are not clear. Clinically, there are two types: regular astigmatism and irregular astigmatism. 1. Regular astigmatism where the main meridians of maximum and minimum refractive power are perpendicular to each other is regular astigmatism. Regular astigmatism can be divided into normal astigmatism, inverse astigmatism and oblique astigmatism. According to the dioptric state of each meridian, regular astigmatism can be divided into simple myopic astigmatism, simple faropic astigmatism, compound myopic astigmatism, compound faropic astigmatism and mixed astigmatism. 2. Irregular astigmatism The refractive state of the eye is not only the refractive power of each diameter line is not the same, the refractive power of each part of the same diameter line is also different, there is no rule to follow, can not form two focal lines before and after, and can not be corrected with column lenses, commonly seen in keratoconus, corneal cloud or lens disease caused by cornea or lens bending surface irregularities.


Etiology Astigmatism is often aggravated by poor eye posture. For example, lying on the stomach reading, or even squinting or squinting, will cause improper eyelid oppression of the eyeball, and affect its normal, so abstain from bad habits is to prevent astigmatism, eliminate myopia. And these habits are often the cause of myopia, so some people think that myopia will cause astigmatism, in fact, the two have no relationship. The parallel light rays entering the eyeball are called astigmatism when the focus of each meridian is not on the same cross section. Ordinary lens can only correct regular astigmatism, and irregular astigmatism can not be corrected with it, the best way is to use a hard contact lens to correct, the effect is better. The effect of regular astigmatism correction with ordinary lenses is not the same, the greater the degree of astigmatism, the worse the effect, below the moderate astigmatism can generally be corrected to normal values, and high astigmatism is more difficult to correct to good results. Especially high farsighted astigmatism, see far and near can not rely on the adjustment of the object, if not corrected as early as possible in early childhood, visual function development will be inhibited to a certain extent, to grow up to wear corrective glasses, corrected vision will be worse, often form moderate amblyopia. High myopia astigmatism is better, although it is not clear to see far, but it is better to see near, visual function can also be exercised and not completely suppressed, and corrected vision is generally better than high farsightedness astigmatism. But it also needs to be corrected as soon as possible. Regardless of the height of the astigmatism, the earlier the correction, the better the effect. Because the correction is early, it can not only promote the development of visual function, but also the error response of object deformation and spatial positioning is small, and the degree of tolerance of patients is also large.


Symptoms (1) Astigmatism Simple myopic astigmatism: imaging of a main meridian parallel rays on the retina, and it is perpendicular to the other meridian parallel rays in front of the retina focused imaging. (2) Simple hypermyopic astigmatism: imaging of parallel rays on one main meridian on the retina, and it is perpendicular to the other meridian parallel rays on the retina behind the focus imaging. (3) Multiple myopic astigmatism: Two perpendicular main meridians, parallel rays are imaging in front of the retina, but their refractive power is not equal. (4) Multiple hypertrophiy astigmatism: Two perpendicular main meridians, parallel rays are imaging behind the retina, but their refractive power is not equal. (5) Mixed astigmatism of eye symptoms: on two perpendicular main meridians, parallel rays are imaged in front of the retina, and parallel rays on the other meridian are imaged in focus behind the retina.


Classification according to the causes of classification: 1) corneal astigmatism: the curvature of the meridians on the front surface of the cornea is different, the most common is that the vertical curvature is larger than the horizontal one (and the eyelid is often compressed), so its flexion force is also stronger than the horizontal meridian, the difference value is about 0.25D, which is physiological and biological astigmatism. Acquired astigmatism can be caused by keratopathy (such as keratoconus, keratitis, etc.) or after surgery, most of which are irregular astigmatism. 2) Residual astigmatism: It can be caused by other refractive factors, such as abnormal lens bending, tilted position, and inconsistent refractive index of each part. 3) Total astigmatism: the sum of the above corneal astigmatism and residual astigmatism. According to whether the strong and weak main meridians intersect vertically (can be corrected by lenses) classification: 1) irregular astigmatism: each meridian refractive power is different, there is no certain rule, even if the same meridian because of its distortion is not correct, refractive index is different, its refractive power is different. Therefore, this kind of astigmatism can not be quantified by degrees. It is usually caused by keratopathy and cannot be corrected with lenses. 2) Regular astigmatism: The two main meridians (that is, the meridians with the greatest refractive power and the least refractive power) are directly intersected with each other, and the astigmatism that can be corrected by lenses is called regular astigmatism. Regular astigmatism can be divided according to the direction of the strong main meridian: normal astigmatism (normal astigmatism, normal astigmatism) : The strong main meridian is located in the vertical direction. Inverse astigmatism (inverse astigmatism, inverse astigmatism) : the strong main meridian is located in the horizontal direction. Oblique astigmatism: The strong main meridian is located in the oblique direction.


The incidence of disease was generally calculated with a fully automatic starting point of 0.50D astigmatism, and there was not much difference between men and women. Most astigmatism is combined with farsightedness and nearsightedness, and the higher the degree of farsightedness and nearsightedness, the more likely it is to occur. Farsightedness combined with astigmatism accounted for 40% of the population. Myopia combined with astigmatism accounted for 50 ~ 60%, and some survey proportion is also high. The degree of astigmatism is generally between 0.50D and 1.50D, and astigmatism below 2.00D accounts for more than 90%.


The main symptoms of astigmatism are fatigue, headache, blurred vision and so on. Because the retina can not form a clear image, so patients in order to see more clearly, often narrow the eyes into a thin slit. Many patients do not understand the reason, often complain that their glasses are low. Experts remind: if the vision is reduced, and there are symptoms of visual fatigue, should be mydriatic optometry. Through the examination, you can accurately know whether the existence of astigmatism, the degree of astigmatism, but also check whether there is a combination of nearsightedness or farsightedness, and can wear glasses according to the results of optometry correction. Nearsightedness or farsightedness can be corrected at the same time as astigmatism. At present, the most effective solution is timely prescription correction to solve the problem of astigmatism. (1) Those who are slightly heavier, whether they see far objects or near objects, all feel blurred, and patients often have the habit of half-closing their eyelids into gaps in an attempt to make objects see more clearly. (2) Vision loss: astigmatic eye away from the retina of the focal line direction of the visual label color lightening, the edge of the blurred, not easy to identify, vision decline, heavy people produce double vision. In addition to physiological astigmatism, all kinds of astigmatism are easy to cause vision loss. Far-sighted astigmatism is better than nearsighted astigmatism due to its adjustment. For example, the same is 1.00 degrees astigmatism, far-sighted astigmatism vision is generally about 0.8; The vision of myopic and astigmatic eyes is generally about 0.5; The visual icon is prone to distortion when viewed with oblique astigmatism. "Long hair" phenomenon, often resulting in reduced vision due to deformation. Therefore, it is very important to correct the axis of oblique astigmatism. (3) Visual fatigue; Because of the different refractive power on each meridian, the parallel light refraction cannot form a focus but two focal lines, so the brain is easy to produce a selective reading of the image. The reason is that astigmatic eyes tend to produce scenes in the direction of the focal line near the retina; In addition, in order to see the scene clearly, the astigmatic eye should reduce the size of the dispersion circle as much as possible to improve the quality of the image; High astigmatism is easy to cause headache if it is not corrected properly or without wearing a mirror. Visual fatigue and other symptoms, so astigmatic eyes are easy to produce visual fatigue. (4) amblyopia: it is more common in high astigmatism, especially far-sighted astigmatism, because it is not clear to see far and near, and the vision can not be exercised. It is prone to amblyopia, followed by a tendency to strabismus.


Diagnosis point 1. Blurred vision, far and near vision are poor, and its degree is proportional to the size of astigmatism. 2. Eye fatigue, far-sightedness and astigmatism are more obvious. May have eye distension, fear of light, tears, headache, severe cases may be accompanied by nausea and vomiting. 3. Corneal curvature was inconsistent by Placido disk or window mapping. 4. Fundus: if the degree of astigmatism is large, the visual nipple can be elliptical. 5. High astigmatism, often squint to play the role of cracks or pinholes.


Astigmatism meter optometry: It is an effective method to check the axial direction of astigmatism with an astigmatism meter. The commonly used astigmatism table is shown in Figure 2. When applying, cover one eye and let the tested eye look at the astigmatism table in front of it. If the black lines on the astigmatism table are thick and thin, it means that there is no astigmatism or only very little astigmatism (about 0.25D ~ 0.5D); Uneven shades suggest astigmatism. The strongest longitude line on the astigmatism table is the direction of astigmatism, and the astigmatism axis is perpendicular to this (at right angles). Another way to determine the astigmatism axis is to place a slit lens (slit width 1 mm) on the test frame and rotate the lens slowly along the diameters until the concentration of the diameters on the astigmatism table is the same. The axis of the astigmatism is consistent with the direction indicated by the fissure. For example, if the fissure is 180°, the astigmatism axis is also 180. Cylindrical mirror method: The cylindrical mirror with the same symbol as the spherical mirror is usually used, that is, the concave spherical mirror is also used. First, the 0.5D cylindrical mirror and the spherical mirror are added to one eye at the same time, and the cylindrical mirror is slowly rotated 180°, which can be repeatedly rotated several times. When rotating the cylindrical mirror, ask the patient to pay attention to whether the clarity of the smaller characters on the eye chart has changed (whether the size, distance, etc.). If the cylindrical mirror is repeatedly rotated, the clarity of the astigmatism must be changed, sometimes clear, sometimes fuzzy. When the vision is the clearest, the axis of the cylindrical mirror is the axis of the cylindrical mirror, that is, the axis of the astigmatism. If the astigmatism axis is determined to the right, the degree of astigmatism can be further determined.


Treatment 1. Mild asymptomatic astigmatism does not require correction. 2. Mild astigmatism, high astigmatism or visual impairment with symptoms should be corrected with a cylindrical mirror. 3. Irregular astigmatism can be corrected with contact lenses. Astigmatism can be corrected by surgery, depending on the specific degree, the average person's astigmatism is not too high, it does not have to be considered, and individual people can consider surgery if the degree is very high! Most astigmatic eyes can be corrected with glasses. Astigmatic eyes can also be corrected with special contact lenses and refractive surgery that have recently been developed. Astigmatism affects vision in the same way as nearsightedness and farsightedness, but it is much more complex because it has problems with both diopter and direction. Under normal circumstances, the degree of astigmatism is relatively stable, and generally does not increase with age. Therefore, mild astigmatism (within 50 degrees), the average person does not need to be corrected. However, those who are engaged in precision work or need to read small text for a long time can consider wearing glasses. Moderate and high astigmatism, as well as farsighted astigmatism or myopia astigmatism, are usually corrected by wearing glasses. In the early stage of conical cornea, when corneal irregular astigmatism is not obvious, frame glasses or general soft contact lenses can achieve better corrected vision. With the progression of the lesion, when frame glasses or soft contact lenses cannot obtain satisfactory corrected vision, rigid contact lenses can be used. However, if the cornea is cloudy or cracked due to corneal injury, it cannot be corrected. So it's important to protect the cornea. Children under the age of six who suffer from astigmatism should be checked early. If astigmatism is found during the vision screening process, they should wear glasses to treat astigmatism as soon as possible. More than two astigmatism will cause symptoms of amblyopia. If they have passed the golden period of treatment between the ages of six and nine, even if they wear glasses in the future, they will not be able to treat the amblyopia problems later on. Easy to confuse sickness and vomiting symptoms.