astigmatism
Introduction
Introduction to astigmatism If the refractive state or diopter of the eyeball is different on different meridians, especially if the radius of curvature of the corneal surface is not a spherical surface, then the eye is in an unregulated state, and the equal light outside 5 meters cannot be inflected after the different meridians of the eyeball. The retina forms a focus and forms a focal line, so the image on the retina is blurred. This ametropia is called astigmatism. Astigmatism can be divided into irregular astigmatism and regular astigmatism. basic knowledge The proportion of the disease: 3-5% (the incidence of this disease in myopia is about 3-5%) Susceptible people: no special people Mode of infection: non-infectious Complications: myopia, presbyopia
Cause
Astigmatism
Curvature astigmatism (15%):
If the degree of astigmatism is high, it often occurs in the cornea. This astigmatism is usually congenital. It can be found that the normal human eye has mild astigmatism measured by the keratometer, which is almost inevitable. The most common astigmatism is vertical bending. The degree is larger than the level, generally about 0.2D, and the mild astigmatism can be considered physiological. It is caused by the frequent compression of the upper and lower eyelids. With the increase of age, this physiological defect has a slight increase. tendency.
Acquired astigmatism (15%):
Can be caused by corneal lesions (the most prominent example is keratoconus), eye trauma involving the cornea (such as surgery after eye surgery, especially corneal incision) can also produce the same result, after eye muscle ablation, can also cause mild astigmatism Corneal astigmatism can also be caused by oppression of the eyelid tumor. Under normal circumstances, when the eyeball is pressed with a finger, the contraction of the eyelid or the action of the extraocular muscle can produce a temporary change in the shape of the eyeball, resulting in different degrees of astigmatism. Lens curvature astigmatism is not uncommon, but the incidence of this case is relatively light, and the astigmatism caused by the cone lens can reach a very obvious degree, but it is extremely rare.
Optical center deviation astigmatism (18%):
It is not uncommon for the position of the lens to be slightly skewed or to leave the axis of the optical system, but this congenital defect is often very slight and neglected. The subluxation of the lens caused by trauma causes the optical properties to change little, but The result is very obvious.
Exponential astigmatism (18%):
This is due to a small difference in the refractive index of different regions of the lens, which is physiological. This degree of astigmatism is slight and has no practical significance. However, the change of refractive medium caused by cataract has obvious influence and can produce various A kind of astigmatism, visual distortion and polyopia.
Pathogenesis
The astigmatism of the eye mainly comes from corneal astigmatism. Therefore, the cornea is used as the refractive surface of the eye. After the cornea passes through the cornea, two focal lines are formed, which are called the front focal line F1 and the back focal line F2, respectively, with the two focal lines as the boundary and parallel rays. After the corneal refraction, a cone-shaped astigmatism cone is formed, which is called Sturm conoid. The shape of the cone section can be seen from Fig. 1B, and the astigmatisms of different shapes are formed due to different positions. A little before the front focal line F1, because the light of the AB main meridian is close to focusing, and the light of the CD weak main meridian is far from the focal line, the light is widely dispersed, and the light cone cross section is horizontally elliptical (1), AB The warp line forms a horizontal line (2) in the front of the F1. After that, the AB warp line is scattered, the CD warp is not yet focused, the light cone section forms a horizontal ellipse (3) smaller than (1), and then the AB warp is spread and the CD warp. The amount of collected rays is equal, and the cross section of the light cone forms a circular circle of dispersion (4), which is called the minimum dispersion circle or the circle of least diffusion. Although the image is not clear here, the deformation is the lightest. Later, because the vertical warp rays spread out, the horizontal warp rays are close to the poly The focal section of the light cone is vertically elliptical (5). After that, the horizontal plane, the warp CD is focused on F2, forming a vertical back focal line (6), and then the vertical and horizontal meridians are spread out, and the cone of light is vertically elliptical. Shape (7), it is impossible to form a clear optical focus in the light cones of the F1 and F2 two focal lines, so all the images are ambiguous.
Prevention
Astigmatism prevention
1. Pay attention to the observation and find that there is a problem and bring it to the eye hospital for examination. It is best to do the first full eye examination at the age of 3-4, and regular eye examinations 1-2 times in the following year.
2. Instruct young children to recognize games and toys that are dangerous to reduce eye injuries. Instruct young children to develop good hygiene habits, do not casually touch the eyes with their hands or their belongings to avoid infectious eye diseases, and try to reduce the incidence when eye diseases are infected.
3, when reading the book, the light should be sufficient, the light is best from the left rear; the reading position should be correct, and keep the distance between 30 cm and 40 cm. Don't read a book in a rocking car, and don't lie down and read a book. When choosing a reading, the font should be clear and not too small.
4, the TV placement height is a little below the parallel line of the eye, there are 5-7 times the diagonal of the TV screen. Do not read more than one hour in a row, and rest for 5 minutes in 30 minutes.
5, nutrition should be balanced. Go to the countryside to see more and more in the green wilderness. If you need to wear glasses, the doctor should check the glasses. Take the initiative to get in touch with the school and actively cooperate.
Complication
Astigmatism complications Complications, myopia, presbyopia
Blurred vision, refractive error amblyopia.
Symptom
Astigmatism symptoms common symptoms disuse amblyopia fatigue myopia astigmatism amblyopia crowded strabismus
1. The object is blurred, far and near vision are poor, and there seems to be a ghost.
2. Easy eye fatigue, eye pain, headache, tears, nausea, vomiting.
3. Vision: Low astigmatism, far and near vision is generally normal, but eye fatigue is easy to occur when working with fine eyesight; people with high astigmatism often have blurred vision or blurred eyes.
4. The fundus optic disc is vertically elliptical with blurred edges, and the fundus cannot be clearly seen with glasses.
5. The keratometer measurement and PlAcido disc examination can be found that the corneal curvature is inconsistent.
Examine
Astigmatism check
Subjective inspection
(1) Observation of astigmatism: Subjective examination of astigmatic eyes can be observed with an astigmatism table to initially understand the shape of the image of the astigmatic meridian retina on the eye to be examined.
The astigmatism observation shows the nature of astigmatism and the nature of the astigmatism, the refractive properties, the adjustment of the functional state, the distance and shape of the gaze target, and various changes. Therefore, from the shape and nature of the image of the retina, the nature of the astigmatism can be understood during the examination. Degree, regular astigmatism can be used with a 5m distance astigmatism table. With subjective test, the index below the astigmatism table can be rotated. According to the astigmatism, the line clarity and tone intensity and orientation of the astigmatism table can be observed. It is possible to know whether there is astigmatism and its strength. The position of the main meridian, such as simple hyperopic astigmatism (regularity), the vertical meridian forms a clear horizontal anterior focal line on the retina, while the horizontal meridian forms a vertical posterior focal line behind the retina, thus presenting horizontal lines when looking at the astigmatism Clear, thick color, blurred vertical lines, light color, renaturation hyperopic astigmatism, astigmatism table showing vertical and horizontal lines are not clear, but in contrast, can indicate the difference in the degree of blur or which direction line color is thicker, Reflecting the difference between the two focal lines before and after the retina, the astigmatic myopic astigmatism is similar to the above, it should be noted that The refractive error of the two main meridians of the mixed astigmatism is basically equal, forming a circular circle. When looking at the astigmatism table, the vertical and horizontal lines should be unclear, the color tone is similar, and the error is considered to be astigmatism.
(2) Subjective audition optometry: Subjective audition optometry is generally performed after objective optometry, aiming at two points: First, subjective determination of the accuracy of monocular corrective lenses, Jackson cross cylindrical correction of astigmatism axial and astigmatism is important The role of to achieve the best visual acuity and the most comfortable visual effects; Second, binocular visual balance test, including the dual visual inspection of ordinary optotypes, red and green optotypes, stereo optotypes, etc., to achieve better binocular Visual effects, especially in the case where both eyes require astigmatism correction, the objective optometry astigmatism axis is not in the vertical or horizontal position, and the effect is good in the monocular test, but in the binocular visual test, the object may be deformed and tilted, visually optically For spatial distortion, the cylindrical position of the cylinder must be adjusted to eliminate this phenomenon. Some people think that for the astigmatism axis with small angles of both eyes, the cylindrical axis is better adjusted to the adjacent horizontal or vertical position.
2. Objective inspection
(1) corneal astigmatism examination:
1 corneal dispersal disc (Placido disc): The original method of corneal examination is the Placido disc, which is magnetic white, with several black concentric rings on one side and a lens of about +8.0D at the center of the peephole for observation. The inspected person faces away from the light source, and the examiner stands in front of his eyes, holds the handle of the disk, faces the ring to the cornea of the eye to be inspected, and the distance is about 12 cm. The corneal reflex ring image of the eye to be inspected is observed with a lens close to the peephole. To determine corneal astigmatism, the ring image shows a higher curvature with a dense meridian, and the density of the thinner meridian indicates a lower curvature, that is, the base arc meridian. The reflective corneal astigmatism mirror developed in 1993 in China uses the principle of a semi-reverse semi-lens. The illumination source and the observation system are integrated into one body, and the direct ophthalmoscope is similarly used, and semi-quantitative examination of corneal astigmatism can be performed in any body position or semi-dark room.
2 keratometer: keratometer measured corneal curvature is based on the measurement of the height of the anterior surface of the cornea (Purkinje image), the curvature radius r of the anterior surface is measured, and then the curvature of the front surface is converted by the formula, where n is the value 1.3375, is the refractive index of the folded cornea obtained by considering the curvature of the posterior surface of the cornea. Because the Purkinje image measurement is paraxial optics, the curvature of the anterior surface cornea measured by the curvature meter is also paraxial, generally refers to the diameter of the pupil 3 mm. Optical zone.
The basic methods of measurement and records are as follows:
A. Focus: When observing the eye by eye through the eyepiece, turn the handle to move the mirror up and down to the central area of the cornea of the eye to be inspected, and push forward and backward with the handle to find the coincidence point of the corneal reflection ring.
B. Fixed axis: Rotate the mirror body so that the misaligned ring image is aligned with the elliptical direction.
C. Measuring curvature: Rotate the left side measuring wheel to make the horizontal vertical cursor overlap, and then turn the right side wheel to make the vertical horizontal cursor overlap. At this time, the measurement is completed.
D. Reading value: read the rotation scale (0-180o) on the lens barrel, which is the main meridian orientation of the curvature, and then read the curvature scale on the left and right wheels respectively. The scale of the left wheel is the horizontal range of the cornea. Curvature, the right wheel scale is the corneal curvature of the vertical range.
For example: the barrel rotation scale 10, the left rotation wheel scale is 42.0D, the right rotation wheel scale is 43.0D, then the record is: or 42.0D position 10o/43.0D position 100o, for the regular corneal astigmatism 1.0DC, there are also Recorded as 42.0D/43.0D bit 100o, omitting another meridian with a phase difference of 90o, indicating that the astigmatism axis is at 10o.
3 corneal topographer: With the development of modern computer information science, the measurement technique of corneal curvature has also been rapidly developed, from the paraxial (central) measurement of the curvature meter to a comprehensive measurement, capturing the Purkinje ring image. Thousands of points of information are used for image height calculation and analysis to form a curvature profile of the anterior surface of the cornea, ie, a corneal topography. The corneal topographer is more convenient to measure the corneal topography, as long as the measuring head is used. Focus on the front surface of the cornea, a clear corneal ring image can be seen on the display. Once measured, the computer can provide a variety of corneal topographic information, mainly:
A. Simulated corneal curvature The Simk value is the average of many corneal curvatures within the 3 mm diameter of the center of the cornea.
B. The surface regular index (SRI) is an indicator of the regularity of the corneal surface. The normal value is 0.2 to 0.3. The more the value is 0, the more regular the surface is.
C. The surface asymmetric index (SAI) is an indicator of the symmetry of the corneal surface. The smaller the SAI value, the higher the symmetry.
D. The shape factor (sf) of the corneal surface is the shape of the tangential surface of the cornea, that is, the deviation from the spherical shape q = 1 - p (Fig. 7).
The normal person's q value is greater than 0, less than 1, that is, the curvature of the cornea from the center of the cornea to the periphery has a tendency to gradually decrease, that is, the surface optics of aspherical aberration.
The corneal topographer provides omnidirectional morphological information on the corneal surface. There is also a corneal topographer that involves the corneal optical section scan technique, which provides stereoscopic information about the cornea from a three-dimensional orientation.
(2) Eye astigmatism examination: The objective measurement of eye astigmatism is also the measurement of ametropia of the eye, so-called objective optometry. The most commonly used objective optometry in clinical practice is computer optometry optometry and retinoscopy optometry.
1 computer optometry: computer optometry is the use of computer to automatically measure the eye far point technology, easy to operate, but because of the computer optometry instrumentic myopia effect, the optometry results are not perfect, it is often easy to make children The result of refractive error is biased toward myopia, but it is more accurate for astigmatic axis measurement, especially for optometry after atropine or adult optometry.
2 Retinoscopy: The technique of using the retinoscopy to detect ametropia is called retinoscopy. It is a fairly accurate and reliable technique in clinical practice because it can make the adjustment of the eye to be as slack as possible. In addition, the shadowing sensitivity observed by the retinoscopy method is strong, and the positioning of the far point is relatively accurate and reliable. The determination of the astigmatism axis position by the cylinder retinoscopy is also very clear, and it should be promoted and promoted clinically, especially for children. Adolescent ametropia examination, of course, it has higher technical requirements for the examiner. Cylindrical retinoscopy using the principle of oblique angle crossing of small angle cylinders (1985) can achieve axial determination of astigmatism and estimation of astigmatism. More accurate astigmatism results.
Diagnosis
Astigmatism diagnosis
Diagnose based on
1. Low astigmatism is generally normal for near and near vision.
2. High astigmatism is far and near vision, blurred vision, easy to see fatigue.
3. A variety of astigmatism can be found by examining the PlAcido disk and the keratometer.
4. Try to wear various types of astigmatism to improve vision.
Differential diagnosis
1. Myopia: Adolescents with mild myopia, due to excessive reading and writing, such as reading and writing, sometimes causing abnormal tension and contraction of the ciliary muscles, leading to the occurrence of pseudo-myopia. At this time, the distance vision is reduced, and the correction by concave lens can improve vision. And astigmatism can only use the lens to improve vision.
2. Presbyopia: Hyperopia and presbyopia are two different refractive states, but because they are corrected with convex lenses, the distance vision is good, and the two are often confused. Hyperopia is a kind of refractive error, which can be seen after wearing a convex lens. Qingyuan side can also see the near side, and the old-fashioned only because of the weakening of the adjustment power, the near-target is not clear, of course, it is a physiological obstacle. Although the convex lens can be seen, the near-target can be seen. , newspaper), but can not use this mirror to see distant objects at the same time, the use of cylindrical correction is invalid or even cause vision loss can be identified.
