Errors of refraction, also known as refractive errors, are vision problems that occur when the shape of the eye prevents light from focusing directly on the retina. They are the most common cause of vision problems. The primary types of refractive errors are:
- Myopia (nearsightedness): This is a condition where objects close to the eye are clear, but objects farther away appear blurry. Myopia occurs if the eyeball is too long or the cornea (the clear front cover of the eye) is too curved. As a result, the light entering the eye isn’t focused correctly, and distant objects look blurred.
- Hyperopia (farsightedness): This is a condition where distant objects may be seen more clearly than objects that are near. However, people experience hyperopia differently. Some people may not notice any problems with their vision, especially when they are young. For people with significant hyperopia, vision can be blurry for objects at any distance, near or far. It occurs if your eyeball is too short or the cornea is too flat.
- Astigmatism: This is a condition where vision is blurred due to an irregular shape of the cornea or the lens inside the eye. Instead of being a perfect sphere, like a ball bearing, an eye with astigmatism is more like a rugby ball with one meridian being significantly more curved than the meridian perpendicular to it. This causes light to be focused unevenly, resulting in blurred or distorted vision at all distances.
- Presbyopia: This is an age-related condition in which the ability to focus up close becomes more difficult. As the eye ages, the lens can no longer change shape enough to allow the eye to focus close objects clearly.
Each of these conditions can be corrected with eyeglasses, contact lenses, or surgery.
Causes
Refractive errors are typically caused by the physical characteristics of the eye that affect how it focuses light. These characteristics can be due to genetic factors or changes in the eye that occur over time. Here are the causes for each type of refractive error:
- Myopia (nearsightedness): Myopia occurs when the eyeball is too long relative to the focusing power of the cornea and lens. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. It can also be caused by the cornea and/or lens being too curved for the length of the eyeball. In many cases, myopia is inherited and is often discovered in children when they are between ages 8 and 12 years old.
- Hyperopia (farsightedness): Hyperopia can occur if your eyeball is too short or the cornea has too little curvature, so light entering your eye is not focused correctly. This condition is usually present at birth and tends to run in families.
- Astigmatism: Astigmatism usually is caused by an irregularly shaped cornea. Instead of the cornea having a symmetrically round shape (like a baseball), it is shaped more like a football, with one meridian being significantly more curved than the meridian perpendicular to it. This irregular shape causes light rays to focus at more than one point in the eye, resulting in blurry or distorted vision.
- Presbyopia: Presbyopia is caused by natural aging of the eye. As we age, the lens of our eye becomes less flexible, making it harder to focus on close objects. This usually becomes noticeable in the early to mid-40s and continues to progress until around the age of 60.
Other factors such as certain diseases (like diabetes), medications, and prolonged visual tasks (like reading or working at a computer) can also cause or worsen refractive errors. It’s important to have regular eye exams to detect these errors early and take appropriate corrective measures.
Types of Myopia or nearsightedness
Myopia, also known as nearsightedness, is a condition of the eyes in which close objects appear clearly, but distant objects appear blurry. It’s a common type of refractive error. Myopia occurs when the eye grows too long from front to back. Instead of focusing images on the retina—the light-sensitive tissue in the back of the eye—the lens of the eye focuses the image in front of the retina.
There are several types of myopia, including:
- Simple Myopia: The most common form of myopia, simple myopia, is when the eyeball is slightly longer than normal or the cornea is too steep, causing light rays to focus in front of the retina. It typically develops in childhood and may progressively worsen as the person grows, stabilizing in early adulthood.
- High Myopia: This is a more severe form of the condition in which nearsightedness continues to worsen over time. People with high myopia have a greater risk of complications, such as retinal detachment or glaucoma.
- Degenerative (or Pathological or Malignant) Myopia: This is a rare type of progressive myopia that, unlike simple myopia, continues to worsen with age. It is characterized by rapid progression and can lead to serious complications affecting the retina and the macula, leading to vision loss.
- Nocturnal Myopia: This type of myopia, also known as night myopia, is characterized by difficulty seeing in low light or at night. It is thought to be caused by the eye’s tendency to over-accommodate (or over-focus) in dim light.
- Pseudomyopia: This is a temporary form of myopia caused by increased eye strain, often from doing close work for extended periods, like reading or using a computer.
- Induced Myopia: Certain conditions or factors such as an increase in blood sugar levels (in diabetes), nuclear cataract, or from certain medications (like sulfa drugs) can temporarily create a myopic shift.
It’s important to note that myopia can be corrected with prescription glasses, contact lenses, or refractive surgery. Regular eye examinations are crucial, especially during childhood, to detect any progression in myopia and adjust prescriptions as needed.
Types of hyperopia or farsightedness
Hyperopia, also known as farsightedness, is a common type of refractive error where distant objects can be seen more clearly than near objects. This occurs when the eye is shorter than normal or has a cornea that is too flat, causing light to focus behind the retina rather than directly on it.
Similar to myopia, hyperopia can also be classified based on severity and other characteristics. Here are some of the types of hyperopia:
- Simple Hyperopia: This is the most common form of hyperopia and is usually present from birth. It often runs in families and can affect both children and adults. The eye is smaller than normal, which prevents light from focusing directly on the retina.
- Pathological Hyperopia: This type of farsightedness is due to disease, trauma, or abnormal development of the eye, leading to an unusually short eye length.
- Functional Hyperopia: This type of hyperopia occurs when the lens or cornea has lost its ability to refract light properly, despite normal eye length. This can occur due to diseases or injuries that affect the eye’s natural lens.
- Latent Hyperopia: This is a form of hyperopia that is not readily apparent because the eye can still focus on objects up close. It becomes more noticeable as the eye’s focusing ability decreases with age, and it may contribute to the development of presbyopia.
It’s important to note that hyperopia can also be corrected with glasses, contact lenses, or refractive surgery. In some cases, especially in young children, the eye can temporarily adjust the focus of light to see near objects clearly, a process known as accommodation. However, this can lead to eye strain and headaches. Regular eye examinations are necessary to monitor the condition and adjust treatment as needed.
Types of Astigmatism
Astigmatism is a type of refractive error where the eye does not focus light evenly onto the retina, the light-sensitive tissue at the back of the eye. This can cause images to appear blurry and stretched out. Astigmatism occurs due to an irregular shape of the cornea or the lens inside the eye.
There are several types of astigmatism, including:
- Regular Astigmatism: This is the most common type of astigmatism. In regular astigmatism, the principal meridians of the eye (the directions along which the curvature of the eye’s lens is steepest and least steep) are perpendicular to each other. It’s as if the cornea is shaped more like a rugby ball or an American football. Regular astigmatism can be further classified into two types:
- With-the-rule Astigmatism: The vertical meridian (up and down direction) is steeper than the horizontal meridian. This is the most common form of astigmatism.
- Against-the-rule Astigmatism: The horizontal meridian (left and right direction) is steeper than the vertical meridian. This form of astigmatism becomes more common with age.
- Irregular Astigmatism: This form of astigmatism is less common and occurs when the principal meridians are not perpendicular. Irregular astigmatism is often due to an injury that has caused scarring on the cornea, surgery, or certain eye conditions like keratoconus, a disease that causes a gradual thinning of the cornea.
- Oblique Astigmatism: In this type, the principal meridians are not vertical or horizontal but are oblique (at an angle).
- Lenticular Astigmatism: This form of astigmatism occurs when the lens inside the eye is distorted, rather than the cornea. The cornea maintains a normal shape.
Astigmatism can often occur in combination with other refractive errors like myopia and hyperopia. It can be corrected with eyeglasses, contact lenses (including special toric contact lenses designed for astigmatism), or refractive surgery. Regular eye examinations can help detect astigmatism early.