1.How is vision formed?
After a clear image is formed on the retina, light-sensitive cells convert light signals into electrical signals that are transmitted to the brain through the optic nerve, forming a virtual space in the visual center that reflects the real objects in the outside world. This is the formation process of vision.
2. How do both eyes see together? What are the conditions required?
After the electrical signal from binocular imaging enters the visual center, the two images with subtle differences are displayed simultaneously through the processing of the central system, and the two images are fused into one, and then the tiny differences taken from different angles are processed into a space comparable to the natural world. It is required that both eyes are imaged with the same basic degree of clarity, and the size and position are basically equivalent.
3.What is the ocular surface?
The mucosa, epithelium, lining and glands between the upper and lower lid margins are collectively referred to as the ocular surface.
4. What tissue structures are included in the ocular surface?
The tear film, conjunctival epithelium, corneal epithelium; the lid glands, primary and secondary lacrimal glands and the lid margin.
5.What does ocular surface abnormality include?
Ocular surface abnormalities include ocular surface instability and ocular surface diseases.
6.What is ocular surface instability?
Ocular surface instability includes: decreased tear production, altered tear composition, decreased lipids leading to excessive evaporation, decreased mucus leading to shortened tear break-up time (BUT), altered eyelid movement such as decreased transient or increased incomplete transient, abnormal eyelid structure such as rabbit eye, lid inversion, etc., incomplete corneal and conjunctival epithelium, etc.
7. What are the ocular surface diseases?
Ocular surface diseases include dry eye disease (in the narrow sense) and superficial conjunctival disease and ocular surface tear disease (in the broad sense).
8.What is the tear film? How thick is the tear film? What does it consist of? How does it come about? What is the main function?
The tear film is the layer of tear fluid that coats the surface of the eye.
The tear film is about 35-45 μm thick.
It consists of a lipid layer, an aqueous layer and a mucin layer.
The lipid layer is secreted by the lid glands and functions as an anti-evaporation agent. The aqueous layer is secreted by the primary and secondary lacrimal glands and is the main component of tears with moistening and oxygen transfer and scavenging functions.
The mucin layer is secreted by cupped cells and keratoconjunctival epithelial cells and is mainly responsible for sticking and balancing colloid osmotic pressure.
The functions of the tear film include constituting a hydrophilic interface, wetting and sliding the ocular surface, antimicrobial action (mechanical flushing, antimicrobial component), nutritional repair and optical action.
9.What is myopia?
A refractive abnormality in which a beam of parallel light converges in front of the retina in a state of ciliary muscle relaxation.
Since the near light can converge on the retina, it can be clear to see near; when looking far away, there is no clear imaging on the retina, so it is not clear to see far away.
10.What types of myopia are there?
By pathology.
Simple myopia.
Pathological myopia.
By degree.
Low myopia <-3.00Ds.
Moderate myopia -3.00Ds to 6.00Ds.
High myopia >-6.00Ds.
By refractive nature.
Axial myopia.
Curvature myopia.
Mixed myopia.
Age of onset.
Early-onset myopia.
Late-onset myopia.
11. How does myopia come about? What are the causes?
In a famous experiment, monkeys were divided into three groups, the first group was allowed to live in a wildlife zoo, the second group was allowed to live in an animal laboratory, the size of their animal laboratory is similar to our classroom, and the third group was kept in small cages. It was found that the monkeys in the first group had no myopia at all, a few monkeys in the second group had mild myopia, and all monkeys in the third group got high myopia. This simple experiment can give us an idea that a small living environment can lead to myopia. Of course, such an experiment is too expensive to repeat and cannot be used as an animal model.
The laboratory uses the following four main aspects of experimental models. The experimental animal is a chick that has just emerged from its shell, because the visual development of chickens has a similar process to that of humans, and the visual development process that humans complete between 0-12 years old can be completed by chickens within 3 months. These animal experimental mapping types can provide some evidence on the causative factors of myopia onset and development.
①Masking experiment
One eye of the newly fledged chicks was sutured and fed normally, and after 3 months of examination and photometry, it could be found that the refractive state of most of the chicks’ masked eyes had developed myopia. This phenomenon can also be seen in children with congenital cataracts. Maintaining a good gaze environment during the formation of children’s vision is also necessary to prevent myopia.
②Defocusing experiment
The refractive state of both eyes of newly emerged chicks were sewn with different degrees of contact lenses in each eye, and the refractive state of the chicks changed positively with the reading of the sewn contact lenses after 3 months. This experiment can also explain the phenomenon of myopia deepening induced by inappropriate lens wear. Due to the inadequacy of some optometry facilities and methods, most of the results are large, and these large results are likely to produce a defocusing effect. Accurate prescription can prevent the further development of myopia.
③Environmental restriction experiment
Chicks that have just emerged from their shells are grown in an environment with a visual distance of less than 30 cm. Optometry after 3 months will reveal a significant increase in the chance of myopia. The higher incidence of myopia in urban children than in rural children can also be explained by this experiment. Giving children a good environment for visual development will reduce the occurrence of myopia.
④Dysregulation experiment
Chicks that had just emerged from their shells were placed in a vertically oriented environment with thin stripes, and then these stripes were allowed to vibrate in a horizontal direction to prevent these chicks from seeing easily. Most of the chicks that grew up in this environment developed myopia. Many of these conditions can be encountered during a child’s visual development, such as prolonged gazing during shaking, reading while walking or riding in a car, and so on. Astigmatism is caused by the inconsistency of refractive power in two directions, and this inconsistency can also cause adjustment disorders. If astigmatism is not corrected for a long time or if spherical lenses are used instead of cylindrical lenses for the convenience of the prescription, these conditions can accelerate myopia development. The development of myopia can also be slowed down by avoiding the development of regulatory disorders.
In addition to the four experiments, there are many other factors that may aggravate myopia progressively. For example, genetic factors, there is a pathological myopia, and genetic factors have a close relationship. It can be said that the mobilization of genes or the regulation of the orthokeratology process are genetically related. Some hereditary myopia may be caused by genetic mutations, and some may be caused by the accumulation of several generations. Genetic mutations are not very much related to eye use; such mutated genes can be passed on to the next generation along with chromosomes and there is generally no way to prevent and control them. The other is a product of human adaptation to the environment in the process of natural selection. With the development of modern civilization, there are more and more opportunities to use the eyes at close range, and each generation is choosing the refractive state suitable for the environment, and the end of the process of orthokeratology is myopia.
There are also drug-induced myopia, diabetic myopia, acute myopia induced by ciliary muscle spasm, idiopathic lower lid impingement, high intraocular pressure, and so on that can induce or aggravate myopia through various pathways.
12.What is eye regulation? How does the human eye perform its regulation function?
The process by which the eye changes its own diopter with the distance of the point of gaze is called accommodation. The regulation of the human eye is a physiological process that occurs late in development, and is not perfect until after the age of 3. The action of the regulating muscle differs from that of other muscles in that its contraction does not strain the suspensory ligament that is pulled in the direction of its primary function, but rather relaxes it. At this time the lens retracts by its own tension to increase its refractive power, a state suitable for near vision. When the ciliary muscle is relaxed, the tension of the suspensory ligament increases due to the rebound of the muscle, and the lens becomes flattened by the tension, and the refractive power decreases, which is suitable for distance viewing. A normal child’s eye can produce about 14D of accommodation. Such a large range of adjustment is difficult to achieve with any autofocus camera. The main direction of the adjustment function is to increase the refractive power and allow the human eye to see near targets. Clinically, we can sometimes see that in traumatic or pathological lens subluxation, part of the suspensory ligament is broken, the pull on the crystal is reduced, the refractive index of the crystal surface is increased, and the refractive power of the eye is significantly increased, which is equivalent to the contraction of the ciliary muscle and relaxation of the suspensory ligament. At this point, it is easy to see clearly up close, but not so easy to see far away. There is also a portion of the accommodation function that allows for a reduction in the refractive power of the lens, known as negative relative accommodation. This part of regulation accounts for a small percentage of the regulation volume, and whether it can be strengthened by exercise remains to be studied.
13.What are the clinical manifestations of myopia?
When reading and writing, myopic eyes do not feel different from normal people. Only when looking at a distance is it not so clear. Even if there is 0.2 vision left, sitting in the first 3 rows of the classroom will not affect the ability to see the blackboard. This will not attract the attention of parents. When vision continues to decline, myopia has increased year after year, even toward progressive myopia.