Refractive Conditions

Refractive Conditions

Myopia (near-sightedness)

Myopia or near-sightedness is where the power or refractive strength of the eye is too strong for its length. This means that light from a far distance will focus in front of the retina. Because of this a myopic person sees objects at a near distance better than they do at a far distance. For mild amounts of myopia this means that objects within arms length are in good focus, however, for high amounts of myopia only objects within inches of the eye can be seen clearly.

To correct for myopia a minus lens is placed in front of the eye to diverge light so that it enters the eye correctly to focus further back on the retina. Contact lenses do the same, but they are placed on the eye itself. LASIK removes tissue with an excimer laser on the cornea to flatten its shape and to also move the focal point further back.
Myopia Diagram
Myopic Glasses
Hyperopia (far-sightedness)

Hyperopia or far-sightedness is where the power or refractive strength of the eye is too weak for its length. This means that light from a far distance will focus behind the retina. Because of this it is easier to see at a far distance than at a near distance. Mild amounts of hyperopia at a young age can easily be overcome by the strong focal ability of the crystalline lens inside the eye. However, at an older age mild amounts of hyperopia cannot be overcome by the weak focal ability of the crystalline lens and this makes visual acuity blurry far away and even worse at near. High amounts of hyperopia at a young age may cause significant visual strain, headaches, avoidance of near work or an eye turn. At an older age high amounts of hyperopia cause very blurry distant visual acuity and extremely blurry near visual acuity.

To correct for hyperopia a plus lens is placed in front of the eye to converge light so that it enters the eye correctly to focus closer forward on the retina. Contact lenses do the same, but they are placed on the eye itself. LASIK removes tissue on the cornea to steepen its shape and moves the focal point further forward to focus properly on the retina.

Hyperopic Vision
Hyperopia Diagram
Astigmatism

Astigmatism is where the eye is shaped like a football, rather than a basketball. The eye has a short curvature in one axis and a longer one in the other. This causes light to focus in two different meridians that are perpendicular to each other. Astigmatism can cause blurry vision at far and near distances. It commonly causes eye strain, headaches, shadowing and glare.

To correct for astigmatism cylindrical eyeglass lenses are required that need to be oriented properly along a certain axis. If the astigmatism power is correct in a prescription, but the axis is off vision will be blurry. This is especially true in high amounts of astigmatism. With the proper lens power & orientation the focal point of both meridians will be at the same point. Contact lenses that correct for astigmatism need to be properly oriented on the eye also. This is usually done by having a weight on one portion of the lens to allow gravity to move it into the proper orientation. LASIK corrects for astigmatism by removing tissue in an elliptical fashion or more in one meridian than the other.
Astigmatism
Astigmatism
Presbyopia

Presbyopia is where an individual over the age of 40 years is unable to read up close as well with the proper correction on. They often have to hold material further away to read it.  As we continue to age it worsens and requires a higher add power to correct for it. It happens because the crystalline lens in the eye has grown and hardened to the point that it can no longer adjust its shape to focus at near.  

Presbyopia is corrected by the use of over-the-counter readers, lined bifocals or progressive lenses. Multifocal contact lenses have a gradual transition between the prescription for near and distance. They are also very dependent upon pupil size.  Unfortunately, LASIK doesn’t correct presbyopia well either. However, some patients can overcome this with LASIK by utilizing monovision where one eye is focused for distance vision and the other is focused for near vision. In order for this to work the brain has to be able to adapt to the eyes not being focused at the same point, which affects depth perception. Patients with a small amount of myopia are able to read up-close because they have a "built-in" bifocal. This makes it easy for them because all they have to do to read up-close is to take their glasses off. 
Presbyopia
Presbyopia

Double Vision


Double vision or diplopia is seeing two images.  It can occur with one eye open or with both open.  It can be very disorienting.  When it occurs with one eye open, it is because of a media opacity like a cataract.  To resolve this type of double vision the media opacity needs to be removed or corrected.  When diplopia happens with both eyes, it is because of a misalignment between the two eyes involving the extraocular muscles or the nerves that innervate them.  When double vision is a new concern, it should be thoroughly investigated for signs of disease.  With both eyes the underlying cause should be resolved, if possible.  Other options include surgery on the extraocular muscles or using prisms in a glasses prescription.  The prism compensates for the misalignment and optically places the images together so the brain can fuse them.

Double Vision or Diplopia
Prism
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