Astigmatism
= refractive error present due to irregularities in the interface (shape) of cornea/lens thus convergence is not proper on one focus but have several focal lines
- 1. Regular astigmatism (where the when refractive power changes uniformly so not many shapes )
- a) Corneal → abnormalities of curvature of cornea (most common)
- b) Lenticular → curvature abnormality, positional changes (subluxation), index differences at different meridian
- c) Retinal→ due to oblique placement of macula
Each of the above has the following type:
- a) With the rule astigmatism =when vertical meridian >curved than horizontal
- b) Against the rule = when horizontal meridian >curved ; a&b meridians are at 90 angle
- c) Oblique =two principle meridian are not horizontal and vertical but at right angles
- d) Bioblique = two principal meridian are not at right angle
Refractive types of astigmatism of regular
Simple → rays are focused on one meridian ; but in the other myopic or hypermetropic
Compound → rays in both meridia are focused myopically or hypermetropically
Mixed → one myopic but in the other meridian hypermetropic
Symptoms defective vision, blurring , may appear elongated , asthenopic
Signs: different power meridian, oval/tilted optic disk , head tilt (compensatory to bring axes nearer) half closure of the lid to achieve greater clarity seen in stenopaeic vision
Investigations: retinoscopy (different power), keratometry (curvature) , astigmatic fan test
Treatment: appropriate cylindrical lens, spectacles contact lenses, surgical correction
2. Irregular astigmatism : irregular change in refractive power in different meridian
Etiology : extensive corneal scars or keratoconus ; symptoms: defective vision, polyopia, distortion of objections , Tests→ disc test , photokerotoscopy, treated with lens or PTK