It is pretty difficult to explain (and also to treat) astigmatism which is one of the most widespread causes of poor vision. Very often astigmatism goes hand in hand with myopia (myopic astigmatism) or hyperopia (hypermetropic astigmatism).
Astigmatism in Latin means absence of a focal point. This ailment comes as a result of an incorrect (non-spherical) shape of the cornea (rarely of the crystalline lens). In a normal state the cornea and the crystalline lens of a healthy eye have an even spherical surface. And astigmatism distorts it. The spherical surface displays uneven curvature in various directions. Accordingly, astigmatism introduces different refractive power in different meridians and the image is distorted when light pass through such a cornea. Some parts of the image may be focused on the retina, while others are focused either before or after the retina (there may be more complicated cases). The result is that instead of a normal image a person sees a distorted one in which some lines are sharp while other lines are blurred. One can get an idea of this by looking at the reflection in an oval-shaped teaspoon. A similar image distortion on retina occurs in case of astigmatism.
Specialists distinguish corneal astigmatism and lenticular astigmatism. The corneal type has greater impact on vision than the lenticular one since the cornea has greater refractive power. The difference in the refractive strength of the strongest and the weakest meridians defines the value of astigmatism in diopters. The direction of the meridians is defines by the astigmatism axis expressed in degrees.
Grades of astigmatism
Specialists identify three grades of astigmatism.
- light astigmatism – up to 3.0 D
- medium astigmatism – from 3.0 D to 6.0 D
- high astigmatism – over 6.0 D
Types of astigmatism
In terms of its nature, astigmatism is either congenital or acquired
- Congenital astigmatism of up to 0.5 D is found in most children and is identified as "functional", i.e. such astigmatism does not affect sharpness of sight and the development of binocular vision. However, if astigmatism exceeds 1.0 D and more, it reduces the vision substantially and requires treatment using glasses.
- Acquired astigmatism results from scars on the cornea due to injury or surgery.
There are three methods for correcting astigmatism – glasses, contact lens, and excimer laser correction.
Astigmatism correction with glasses
Astigmatic patients are usually prescribed special "intricate" glasses with special cylindrical lenses. Specialists note that wearing such glasses may cause unpleasant symptoms for patients with high astigmatism – e.g. giddiness, eye gripes, and vision discomfort. In contrast to standard glasses, prescriptions for "intricate" glasses for astigmatism patients include data on the cylinder and its axis. It is very important to conduct a very detailed diagnosis prior to selection of glasses. Quite often people with an astigmatis have to change their glasses several times.
Contact lenses for astigmatic patients
Speaking of contact lens correction for astigmatism, it is important to note that until recently the only method to correct astigmatism was to use hard contact lenses. These lenses not only caused discomfort to patients but also had a negative effect on the cornea. But medical science is constantly in progress and special toric contact lenses are used today.
- Following prescription of glasses or contact lenses, it is essential to be under constant observation of an ophthalmologist to ensure timely replacement of the glasses or contact lens for more powerful or less powerful ones.
- Glasses and contact lens are not the ultimate solution to the problem of astigmatism – they are merely an instrument for temporary correction of vision. Only surgery allows full elimination of astigmatism.
Excimer laser correction of astigmatism.
Recently, excimer laser correction has been usually applied for the treatment of astigmatism (up to ±3.0 D). Laser correction based on the LASIK can hardly be classified as surgery. This procedure lasts 10–15 minutes with local drop anaesthesia and the laser works does only 30-40 seconds depending on the complexity of the case.
During the LASIK procedure, a special microkeratom instrument is used to separate 130–150 microns flap of the cornea surface layer to open the path to the laser beam into deeper layers of the cornea. The laser then evaporates a part of the cornea. The flap is then replaced and fixed by the own collagen of the cornea. No suturing is required since there is self-restoration of the epithelial tissue along the edges of the flap. LASIK vision correction requires only a short period of rehabilitation. A patient has good vision within 1 to 2 hours after the treatment and his vision is fully restored within one week.
Dangers of astigmatism
If astigmatism has not been taken care of, it may lead to strabismus and a acute regression of vision. Without proper correction astigmatism may cause headaches and eye gripes. For this reason it is important to visit an ophthalmologist regularly.