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Common eye problems in students

1. 3.1
   Objective: To describe and identify the common eye problems in students – refractive errors, strabismus and amblyopia 
   

   By completing this chapter, you will learn the following list of topics： Three types of refractive errors – nearsightedness, farsightedness and astigmatism Formation of strabismus and amblyopia

1. 3.2
   Refractive errors
   

   Nearsightedness, farsightedness and astigmatism are the types of refractive errors among school children in Hong Kong. 

   Nearsightedness	Farsightedness	Astigmatism

1. 3.3
   Nearsightedness (Myopia)
   

   Nearsightedness (Myopia) is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it. This causes the image that one sees when looking at a distant object to be out of focus, but in focus when looking at a close object.

    

    

   Nearsightedness (Myopia) Video

    

   	Nearsightedness is the commonest refractive errors, and affecting around 40% of primary school children in Hong Kong.Fan et al, IOVS 2004

    

   Hong Kong has one of the highest prevalence of myopia in the world. Our prevalence was three times that of the United States, more than 10 times that reported from the Middle East, and near twice that in South America. Fan et al, IOVS 2004

    

   Heredity	Reading in Dim Light	Excessive Nearsighted Work

    

   While the exact cause of nearsightedness remains unknown, it is believed that an interaction of heredity and environment may play a role. It tends to run in families. Environmental factors such as reading in dim light or doing excessive amount of nearsighted work may contribute to nearsightedness.

    

   A common sign of nearsightedness is difficulty with the clarity of distant objects like a movie, TV screen or the chalkboard in school. Nearsightedness can be corrected by spectacles with concave lens.

1. 3.4
   Farsightedness (Hyperopia)
   

   Farsightedness (Hyperopia) is a condition of the eye where the light that comes in focus behind the retina. This occurs if eyeball is too short or the cornea has too little curvature, so light entering your eye is not focused correctly, causing difficulty focusing on near objects, and in extreme cases causing difficulty to focus on objects at any distance.

    

    

   Farsightedness (Hyperopia) Video

    

   Common signs of farsightedness include difficulty in concentrating and maintaining a clear focus on near objects, eye strain, fatigue and/or headaches after close work, aching or burning eyes, irritability or nervousness after sustained concentration. Farsightedness can be corrected by spectacles with convex lens.

    

    Farsightedness can be corrected by spectacles with convex lens.

1. 3.5
   Astigmatism 
   

   Astigmatism is a condition where the light comes into the eye but do not focus on a single point of the retina. It is caused by the irregular shape of the cornea or sometimes the unequal curvature of the lens inside the eye. An irregular shaped cornea or lens prevents light from focusing properly on the retina, and thus, the vision becomes blurred at any distance.
   Astigmatism is a very common vision condition. Most people have some degree of astigmatism. Slight amounts of astigmatism usually do not affect vision and do not require treatment. However, larger amounts cause distorted or blurred vision, eye discomfort and headaches. Astigmatism can be corrected by spectacles with cylindrical lens.

    

    Astigmatism Video 

2. 3.6
   Strabismus
   

   Strabismus is a condition in which both eyes do not look at the same place at the same time. It occurs when an eye turns in, out, up or down and is usually caused by poor eye muscle control.

    

    

    Strabismus Video 

    

   There are six muscles attached to each eye that control how it moves. The muscles receive signals from the brain that direct their movements. Normally, the eyes work together so they both point at the same place.

   When problems develop with eye movement control, an eye may turn in, out, up or down. The eye turning may be evident all the time or may appear only at certain times such as when the person is tired, ill, or has done a lot of reading or close work. In some cases, the same eye may turn each time, while in other cases, the eyes may alternate turning.

    

   	Maintaining proper eye alignment is important to avoid seeing double vision, to good depth perception, and to prevent the development of poor vision in the turned eye.
   	When the eyes are misaligned, the brain receives two different images. At first, this may create double vision and confusion, but over time the brain will learn to ignore the image from the turned eye. If the eye turning becomes constant and is not treated, it can lead to permanent reduction of vision in one eye, a condition called amblyopia or lazy eye.

    

   Esotropia		Exotropia
   Hypertropia		Hypotropia

   In Hong Kong, exotropia is the most common form of strabismus. Treatment for strabismus may include eyeglasses, prisms, vision therapy, or eye muscle surgery.

    

3. 3.7
   Amblyopia
   

   Amblyopia results from an early disruption of the normal development of vision, and as such usually occurs before the age of eight.

   Amblyopia may develop in young children who receive visual information from one eye that is blurred or conflicts with information from the other eye. Consider that the brain is receiving two stimuli for each visual event: one from a visually aligned (fixing) eye and one from an "abnormal" eye (vision burred or eye misaligned on another target).

   The child's brain selects the better image and suppresses the blurred or conflicting image, which results in the faulty development of vision in the amblyopic eye. It can be caused by the presence of strabismus, unequal refractive error (eg. myopia or hyperopia), or a physical obstruction of vision (eg. cataract).

   The vision of the amblyopic eye becomes weaker from disuse. A child may exhibit noticeable favoring of one eye and may have a tendency to bump into objects on one side. Amblyopia can be treated effectively by occlusion of the unaffected eye. The child wears an adhesive patch over the good eye, forcing the brain to utilize the previously suppressed eye.

1. 3.8
   Summary
   

   Congratulation! You have completed Chapter 3. You should now be able to distinguish the three types of refractive errors.