A normal eye is defined as being emmetropic.
In an emmetropic eye, images are focused directly onto the retina.
The shape of a myopic eye (the eyeball is longer than a normal eyeball and/or the power of the cornea and the lens may be too strong) is such that light rays focus in front of the retina instead of on it. Myopic patients are unable to see distant objects clearly.
Myopia usually appears at a young age and tends to stabilise when the child stops growing; in rare cases (degenerative or progressive myopia) it may continue to evolve over the whole lifetime of the patient.
As they age, myopic patients tend to suffer a greater incidence of retinal disorders and glaucoma than the rest of the population.
Myopia can be corrected with glasses, contact lenses or refractive surgery.
The shape of the hyperopic eye (the eyeball is shorter than a normal eyeball and/or the power of the cornea and the lens may be too weak) is such that light rays focus behind the retina instead of on it.
In young patients, the lens is very elastic and may partially or fully compensate for the anomaly, maintaining the images on the retina by "accommodation".
In some cases, the younger patient is able to compensate for this error perfectly.
In other patients, this subconscious ACCOMMODATIVE EFFORT may cause eyestrain, headaches, lacrimation, ocular hyperemia or even accommodative strabismus (see orthoptics and Strabismus Surgery).
In patients with more serious hypermetropia the compensating mechanism may not be sufficient and the vision poor. In all hypermetropic subjects the lens becomes less elastic over the years and is no longer able to compensate for the anomaly in the shape of the eyeball. The degree of latent hypermetropia decreases (where initially it was compensated) and the degree of manifest hypermetropia increases (no longer compensated), almost like the principal of communicating vessels.
Hypermetropic patients notice a decrease in the quality of vision, most critically after the age of 40, associated with the need to progressively increase the strength of the lenses used and to start using other lenses for reading.
Hypermetropia may be corrected with mono or multi-focal lenses, sometimes with contact lenses (hypermetropic patients are often unable to tolerate contact lenses) or refractive surgery.
The corneal curvature of an astigmatic eye is different in the two main meridians: it curves in one direction than in the other. The cornea therefore can no longer be compared to a section of a sphere like a normal cornea can, but to a section of an ellipse (rugby ball).
The light rays are refracted and focussed on the retina at two different points, perpendicular to each other (in the case of normal astigmatism), causing confused vision of both near and distant objects.
Astigmatism can be associated with myopia or with hypermetropia , adding to the penalising effects for the patient’s vision. Astigmatism can be corrected with glasses, contact lenses or refractive surgery.
Anisometrope patients have a different refractive power in the two eyes as the eyes are different shapes.
If this difference is moderate and if the disorder is corrected early, glasses can be easily tolerated and guarantee good visual and binocular function.
When there is a large difference between the two eyes, the disorder is corrected later or arises in adult age (post-trauma), correction with glasses is not sufficient to guarantee binocular vision. It is therefore necessary to use contact lenses and/or refractive surgery, together with binocular re-education.
This is the progressive inability to focus on close-up images caused when the lens loses elasticity. It starts from the age of 40-45 and continues to develop until the age of 60.
This functional deficit complicates the pre-existing refractive state and a different method has to be used to correct it:
- Myopic patients must use lower strength lenses for reading than those used for seeing distant objects; in the case of slight myopia patients may be able to read without glasses.
- Hypermetropic patients use stronger lenses for seeing near objects
- Astigmatic patients use different lenses for near and distant objects
Visual defects can be corrected using glasses, contact lenses or with refractive surgery.
Negative lenses are used for myopia, positive ones for hypermetropia, cylindrical ones for astigmatism. By moving the focal point of light backwards or forwards, the too strong, too weak, or irregular ocular refractive power can be neutralised. The same result can be obtained with refractive surgery.
With an excimer laser it is the curvature of the cornea that is modified: flattening it in the case of myopia, increasing it in the case of hypermetropia, making it more rounded in the case of astigmatism.
Intraocular lens implants modify the focal point of light inside the eyeball.