Types of Astigmatism.
Based on axis of the principal meridians
- Regular astigmatism – principal meridians are perpendicular
- With-the-rule astigmatism – the vertical meridian is steepest (an American football lying on its side).
- Against-the-rule astigmatism – the horizontal meridian is steepest (an American football standing on its end).
- Oblique astigmatism – the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.
- Irregular astigmatism – principal meridians are not perpendicular
In With-the-rule astigmatism a minus cylinder is placed in the horizontal axis to correct the refractive error. Adding a minus cylinder in the horizontal axis makes the horizontal axis “steeper” which makes both axes equally “steep.” In Against-the-rule astigmatism a plus cylinder is added in the horizontal axis (or a minus cylinder in the vertical axis).
Children tend to have With-the-rule astigmatism and elderly people tend to have Against-the-rule astigmatism.
Axis is always recorded as an angle in degrees, between 0 and 180 degrees in a counter-clockwise direction. 0 and 180 lie on a horizontal line at the level of the centre of the pupil, and as seen by an observer, 0 lies on the right of both eyes.
Based on focus of the principal meridians
With accommodation relaxed:
- Simple astigmatism
- Simple hyperopic astigmatism – first focal line is on retina while the second is located behind the retina
- Simple myopic astigmatism – first focal line is in front of the retina while the second is on the retina
- Compound astigmatism
- Compound hyperopic astigmatism – both focal lines are located behind the retina
- Compound myopic astigmatism – both focal lines are located in front of the retina
- Mixed astigmatism – focal lines are on both sides of the retina (straddling the retina)