[Popular Science of Optometry] Why does astigmatism have a direction? How is it formed?

▼Do you really understand astigmatism?

Astigmatism refers to the refractive state of the eyeball in which the refractive power is different on different meridians, forming two focal lines and the minimum diffuse spot. In layman's terms, our eyeball is a sphere when viewed from the front. For a patient with refractive error, if the refractive power of this sphere in each direction (horizontal, vertical, oblique, etc.) is the same, it is simple myopia or hyperopia. If the refractive power in different directions is different, it is called astigmatism, which is actually the same as myopia and hyperopia, except that the degree in each direction is different.

Astigmatism Perspective

▼Astigmatism can be divided into regular astigmatism and irregular astigmatism

Regular astigmatism means that the direction of maximum refraction is exactly perpendicular to the direction of minimum refraction. Regular astigmatism is relatively easy to solve, and frame glasses, corneal contact lenses, laser surgery, etc. can all achieve better vision.

You Changtao, deputy director of Zhengzhou Aier Eye Hospital and director of the refractive department, said: Irregular astigmatism means that the refractive power in each direction is different, the direction of the maximum refractive power is not perpendicular to the direction of the minimum refractive power, or the refractive power at different points in each direction is different, and the cornea is irregular. Irregular astigmatism cannot be corrected by frame glasses or soft contact lenses, but hard contact lenses can correct some irregular astigmatism.

▼Why does astigmatism occur? How to correct astigmatism?

Astigmatism is often congenital, mainly determined by genes, and occasionally caused by corneal dysplasia during development. Factors that cause changes in the shape of the cornea, such as inflammation, tumors, trauma, or corneal diseases (such as keratoconus) can also cause astigmatism. People with astigmatism find it difficult to see objects in detail. In most cases, astigmatism does not occur alone, and the patient's eyes are usually accompanied by myopia or hyperopia.

In clinical practice, regular astigmatism is corrected by cylindrical lenses (frame glasses), but when the astigmatism is too high or irregular, frame glasses cannot achieve a good correction effect or even cannot correct it. At this time, contact lenses, especially specially designed contact lenses, can correct these abnormal astigmatism. High astigmatism also requires rigid gas permeable corneal contact lenses (RGP) to correct.

▼Some symptoms of astigmatism

Patients with astigmatism may experience decreased vision, distorted vision, double vision in one eye, worse night vision than daytime vision (different from night blindness), a tendency to tilt the head and neck when seeing, habitual squinting, visual fatigue, etc. When checking the E-letter sight mark in the same direction on the eye chart, they can recognize it in one direction, but not in the perpendicular direction, and their vision is uneven.

For patients with high astigmatism, they often only have mild or rare symptoms such as blurred vision, headache or visual fatigue, but when the high astigmatism is close to full correction, these symptoms may appear instead, which is due to the remaining uncorrected mild astigmatism. In addition, they often have to squint their eyes to reduce the blurriness of the image and move the object closer to obtain a larger retinal image.

For myopic astigmatism, although the distance vision is very poor, the near vision is still normal or close to normal. In addition, the near vision is often used continuously in daily life and work, which provides an opportunity for the myopia function to exercise. Therefore, myopic astigmatism rarely develops disuse amblyopia.

▼ Steps for self-testing astigmatism

1. The subject's line of sight is parallel to the screen, about 1 meter away.

2. If you normally wear frame glasses or contact lenses, please take them off during the test. Those who wear frame glasses can take the test directly after taking them off; those who wear contact lenses need to rest for 20 minutes after taking them off before taking the test.

3. If some of the lines in the graph are clear or darker, and some are blurry or whiter, it means you may have astigmatism. Please go to a professional eye examination medical department for a detailed examination.

4. Please do not do this test when your eyes are tired or sick to avoid poor results.