Astigmatism is a refraction anomaly. Normally cornea has a spherical form, its refracting power in a vertical and horizontal planes are equal. With astigmatism the refracting power in these planes is different; for example the cornea refracts down stronger than across. In the result objects both in the near and in the distance are seen with distortions.
Astigmatism is divided on myopic, hypermetropic and mixed. There are three levels of astigmatism:
- Weak – up to 2 diopters
- Middle – up to 3 diopters
- High – 4 and more diopters
Apart from congenital astigmatism there is astigmatism appearing after an eye operation, after a cornea disease or a trauma. Astigmatism, especially of high level, is difficult to correct with glasses or contact lenses.