REFRACTIVE ERRORS


Near sighted? Far sighted? Astigmatism? What’s the difference?

These are all known as refractive errors and no doubt you’ve heard these terms before. However, these terms are very misleading and can be hard to understand but generally speaking the terms describe what the patient can do. For example a nearsighted individual can see near but not far away.

Emmetropia

This is when a patient has perfect distance vision. They can see great far away because the curvature of the front of their eye matches the back of their eye. For these individuals they do not require glasses for far away or up close in most cases.

Myopia (Nearsightedness)

This is a common reason why a person may wear a pair of glasses. It occurs when the length of the eyeball does not match the curvature of the front of the eye. Either the eye is too long compared to the front of the eye (aka axial myopia) or the front of the eye is too curved compared to the back of the eye (aka refractive myopia). Generally speaking these patients can see up close; however they may still have reading problems because of a number of causes. Myopic patients can often be good candidates for LASIK procedures.

Hyperopia (Farsightedness)

This is another common cause for a patient to be wearing spectacles. This condition can occur in any combination of two ways. First the front of the eye is not curved enough (is too flat) compared to the length of the eye or the eye is too short with a normal curvature at the front of the eye. Patient’s that are farsighted can often see far but not near. However, in young children a mechanism called accommodation can allow a child to see both far and close quite clearly but a child may still need glasses for farsightedness even if their vision is 20/20.

Presbyopia

This is a condition that affects people in their 4th decade of life, There is a process called accommodation that helps us focus in on a target that is an arms length away and closer. However, as more birthdays go by we are less and less able to focus in on this material. Everyone becomes presbyopic as they get older but depending on their refractive condition they will have different options in order to read up close. If a patient is myopic they may remove their glasses to read up close, because their eyes are often good for the near point. If patients are hyperopic they will need either a multifocal lens (a progressive, bifocal or trifocal) or a separate pair of reading glasses. If someone has no distance prescription then the patient will often just need a pair of reading glasses.

Astigmatism

This is a condition where there are two different prescriptions, and this is caused by the cornea not being spherical like a soccer ball but more of an oval shape like a football. Astigmatism can occur with a patient that is myopic or hyperopic. Astigmatism can make vision blurry for both up close and far away.