What is refraction?
Vision is a complex process that is performed by a very small organ. Vision depends on a simple physical phenomenon known as refraction which is the bending of light. But how can a small organ like the eye perform such a complicated task? The eye consists of numerous working layers. The cornea is the first layer encountered by light and forms the front of the eye. The cornea along with the lens refracts and focuses the incoming light rays on the retina. The lens is an elastic structure located behind the cornea that changes shape to adjust to the distance of the object. If the object is distant, the lens gets flattened. If the object is close, the lens gets curved. That way, the object’s image is focused on the retina, no matter its distance from the eye. Located at the back of the eye, the retina is stimulated by the refracted light rays and sends signals to the brain where they are translated into sharp images. To adjust the amount of light entering the eye, pupils constrict in bright surroundings and dilate in dim light to capture as much light as possible. A circular structure called the iris controls the size of the pupil.
What is an error refraction?
A refractive error (also called ametropia) happens when one or both eyes fail to clearly focus the image of surrounding objects, resulting in blurred vision. The eye in this case is called ametropic. Emmetropia on the other hand is the state in which the eye has no refractive error. Refractive errors are considered the most common eye disorder globally and a leading cause of vision loss.
What causes an error refraction?
Proper vision depends on the ability of the eye’s components to refract light effectively. Any abnormality in the pathway traveled by light through the eye can lead to an error of refraction. There are 3 primary causes for an eye to improperly refract or focus light:
Length of the eyeball
If the length of the eyeball from front to back is too long, images will be focused at a point in front of the retina and not on it. This is known as nearsightedness. If on the other hand the eyeball is too short, images are not focused enough before they reach the retina, causing farsightedness. Nearsightedness and farsightedness will be discussed later in details.
Curvature of the cornea
The cornea is the transparent spherical structure at the front of the eye. The cornea refracts light rays passing through it and contributes to the eye’s focusing ability. Irregularities in the shape of the cornea lead to improper focus of light creating distorted images. This condition is called astigmatism and will be discussed later.
Curvature of the lens
The lens is elastic and changes its curvature to adjust the focal distance allowing a sharp real image. This process is known as accommodation. With age, the elasticity of the lens declines, and the focal power of the lens decreases.
What are the types of errors of refraction?
There are 4 major types of errors of refraction:
Also known as nearsightedness, myopia is one of the most common types of errors of refraction and affects approximately 22% of the population. In patients with myopia, the refracted light is focused at a point in front of the retina because the distance between the cornea and the retina is too long. As a result, myopic patients will experience difficulty in reading distant road signs and seeing objects at a long distance. The myopic eye can often focus near objects normally, leading to normal or near-normal vision for close-up tasks such as reading and working on computers. Blurry vision as well as headaches and eye strain are common symptoms. Myopia can be diagnosed by routine eye examination and treated with eyewear or a refractive surgery (discussed later).
As opposed to myopia and hyperopia, astigmatism results from irregularities in the shape of the cornea and not the length of the eyeball. The cornea is a spherical structure that is symmetrically curved in all directions (like a basketball) allowing the light to be focused sharply on the retina. In patients with astigmatism, the cornea is irregularly curved, either steeply curved or flattened at one or more directions. This irregular curvature leads to improper light focus on the retina, hence distorted images. This type of astigmatism is called corneal astigmatism. Irregularities in the shape of the lens (that is also a curved structure) may cause lenticular astigmatism. In both types, the vision is blurry, and images are stretched out. Other symptoms include eye squinting in an attempt to focus better, headaches and eye strain and discomfort. Astigmatism can occur with or without other refractive errors, and some patients are born with it. It is diagnosed with a comprehensive eye examination and is also treated with eyewear or refractive surgery.
In patients with hyperopia, the refracted light is focused at a point behind the retina because the distance between the cornea and the retina is too short. The hyperopic eye is sometimes capable of seeing distant objects well, hence the name farsightedness. When focusing on near objects, patients with hyperopia will experience blurred vision, headaches and eye strain and fatigue. Hyperopia is also diagnosed by routine eye examination and treated with eyewear or refractive surgeries. Many children are born with farsightedness, and some outgrow it as the length of the eye increases with normal growth.
As discussed earlier, the lens has an elastic ability that allows it to change its degree of curvature to adjust to the distance of the objects from the eye. With age, the lens loses this ability gradually, and the accommodation process of the eyes declines. Presbyopia is a natural part of the aging process, and difficulty in reading or seeing typically starts in middle age. The degree however varies from one person to another, and presbyopia can occur with other refractive errors. Presbyopic patients will have a hard time focusing on near objects and reading small labels and will try adjusting the distance of the reading material manually to see clearly. Headaches and eye strain are also symptoms of presbyopia. Diagnosis of presbyopia is also made by routine eye examination, and eyewear is the typical treatment option.
What are the risk factors of errors of refraction?
- Genetics: The likelihood of developing a refractive error increases if one or both parents are affected.
- Age: Errors of refraction affect both children and adults. Some children are born with an error of refraction and might outgrow it. Presbyopia is more likely to develop in adults above 35.
How are errors of refraction diagnosed?
Errors of refraction are typically diagnosed with a routine eye examination performed by an ophthalmologist. During this examination, the patient is asked to read a vision chart while trying an assortment of lenses to adjust vision. Recently, more advanced devices can measure errors of refraction directly.
How are errors of refraction treated?
Typically, errors of refraction are treated with 3 options:
A frame with lenses is one of the ancient yet more common corrective options for a refractive error. After an eye examination, the ophthalmologist prescribes specific lens measurements suitable to correct the refractive error a patient has. A lensmeter can be used to check if the provided pair of glasses matches those specified measurements. The only disadvantage of eyeglasses is that clear vision is only achieved when the patient is wearing the eyeglasses. The patient is completely dependent on them.
A more cosmetically appealing option than eyeglasses. Instead of a frame, the lenses are placed directly on the eyes. Lens measurements are also prescribed by the ophthalmologist. Same with eyeglasses, clear vision is dependent on the patient wearing the lenses.
In recent years, technology has revolutionized the treatment of errors of refraction. Unlike eyewear, refractive surgery corrects vision permanently and eliminates the dependency on eyeglasses or contact lenses. The most common technique used in a refractive surgery is reshaping the cornea to correct vision using laser like in LASIK and others. Phakic intraocular lens implantation is another technique to correct vision by inserting a lens over the natural lens of the eye. The ophthalmologist decides on which technique is more suitable according to the condition of the patient.
What are the common symptoms of errors of refraction?
Symptoms of errors of refraction vary from one patient to another, and some patients exhibit no symptoms whatsoever. For that, it is highly recommended to have periodic eye examinations as a part of routine checkup. However, there are some common symptoms for errors of refraction:
- Blurred vision.
- Glare or halos around bright lights.
- Squinting (the attempt of focusing more to see better)
- Eye strain.
- Difficulty driving at night.
- Eye irritation, redness and itching.
If you are experiencing one or more of these symptoms, you should check with your ophthalmologist as you might have a refractive error. Contact us to book your appointment with Dr. Ramy Riad.