Myopia typically begins early in life. People with this condition can usually see near objects, but they struggle to see distant objects. In myopia, the anatomy of the eyeball is longer than normal. This causes the light to focus in front of the retina, blurring the distance vision. Myopia is corrected with glasses and contact lenses, or with laser vision correction, all of which move the focal point of incoming light backwards to the retina. Laser vision correction is only recommended for people over 18 years old, when the eye has finished growing to adult size. High myopic prescriptions may not stabilize until the late twenties.
To correct the symptoms of myopia with glasses, lenses are used that are thicker on the edges and thinner in the middle. This is known as a concave lens, which can be cosmetically improved in higher prescriptions with a high index lens material and aspheric lens designs.
Moderate to high myopes are also at increased risk of retinal detachment due to the elongated eye. The signs and symptoms of a retinal detachment are flashing lights, persistent black floaters, or a curtain over the vision from any direction.
People with hyperopia are able to focus on objects that are further away, but have difficulty focusing on objects which are very close. This is because the eyeball is shorter than normal, which focuses light behind the retina. Unlike myopia, where clear vision can only be achieved with lenses, hyperopes can achieve clear vision by activating their reading muscles (the ciliary body). This creates a burden of additional work, however, which scales with the amount of hyperopia. By using part of their limited muscular ability to see far away, hyperopes have reduced capacity to see clearly up close. This can be particularly confusing with children, who have very strong reading muscles (see Accommodative Disorders below) and may have normal distance vision despite a strong hyperopic prescription.
Babies are often born with hyperopia but they can usually outgrow the condition as their eyes develop into the correct shape.
Hyperopia can be corrected with eyeglasses or contact lenses. There are also new surgical procedures that can correct hyperopia.
Sometimes the cornea is irregularly shaped, causing the eye to focus an object on two different areas of the retina. This is known as astigmatism. For the cornea to bend light correctly, it should be spherically-shaped, like a basketball. Astigmatic corneas are shaped more like an egg. This causes a distorted view when looking at objects both close-up and far away.
Astigmatism is often associated with myopia or hyperopia, and it usually is present from birth. It may be hereditary, or it may be caused by factors such as pressure on the cornea, incorrect posture, or increased use of the eyes for near activity.
Eyeglasses, contact lenses, or refractive surgery can correct moderate to high degrees of astigmatism. People with low amounts of astigmatism typically wear glasses for driving at night, watching television, or extended near activity.
As people get older, usually in their early forties, a condition called presbyopia sets in. Presbyopia is the inability to focus on objects near the eye. One usually notices that it is harder to read or use the computer. Bifocals or reading glasses are a way to remedy this condition; there are also surgical options like corneal inlays and multifocal intraocular lenses.
Presbyopia is a natural consequence of the aging process. There is no way to avoid it and no known cure, though researchers are constantly looking for one. Even if someone has never had vision problems before, he/she can still develop presbyopia. It may seem to occur suddenly, but it actually occurs over a long period of time. Symptoms include having to hold things at arm’s length to see them clearly, eye strain, fatigue, and headaches from near work.
Hyperopes will notice reading difficulty at a slightly earlier age, myopes may not complain as early due to the ability to read effectively without their glasses.
Amblyopia is a problem with the brain as a result of other problems with the eye. Commonly referred to as a "lazy eye" by practitioners (which can be confusing because lay people often use that term to describe a wandering eye, aka strabismus), amblyopia typically results in one eye that sees worse than the other eye, despite wearing the appropriate correction and having no other medical conditions. Two particular factors that can cause amblyopia are a large difference in prescription between the two eyes, and strabismus. When the brain receives two images that it cannot put together to create the 3D picture that we normally see (because one is clear and one is too blurry or because they don't align properly), the brain is forced to ignore one of those images in order to make sense of the data. As the brain develops, the processing area that should have been split 50/50 between the two eyes is instead predominantly used for the preferred eye. Later, when the blurred vision and/or misalignment is corrected, the brain is accustomed to ignored the non-preferred eye and has less processing power to devote to it.
If amblyopia is discovered at a young age - typically under 10 years old - the brain can reconfigure itself and restore binocular vision with some training and therapy. This is much less likely to happen in later years. Many amblyopes will have one normal eye with good vision and never complain about any problems. This is one of the many reasons annual exams are recommended for all school-age children, even those who don't complain about their vision.
The ability of the eye to change its point of focus from far to near and vice versa is called accommodation. This involves activating a muscle (the ciliary body) and changing the shape of the lens inside the eye. The ability to use that muscle is very high in young children and steadily decreases with age, ultimately resulting in presbyopia. While children and young adults have a large capacity for accommodation, they do not necessarily have good control of the muscles. Sometimes they have a tendency to accommodate too much, sometimes too little, sometimes they cannot turn it on and off as quickly as needed (like when taking notes in class).
Accommodative disorders are treated with reading glasses and/or vision therapy. Often, children will outgrow the disorder; just because they need reading glasses one year does not mean they will have to use them forever.
Humans have two eyes that are horizontally offset. In order to look exactly at an object with both eyes and create a stereoscopic image, our eyes must converge or diverge a given amount. If they do not, the brain receives images that don't align, and we see double vision (diplopia). The eyes can converge too much, too little, or not at all. They may converge different amounts at far and near, creating a problem at one or both distances. Only severe convergence issues result in diplopia. Prior to that, you may experience sleepiness when reading, headaches, eye strain, dry eyes, blurred vision, a "pulling" sensation, or general discomfort.
Convergence disorders are variously treated with reading glasses, prism lenses, and vision therapy.