How to find Type of Astigmatism through Prescription?
To find out Type of Astigmatism, We first need to know the classification. There are many but the important ones are based on focal points and Principle meridians.
Based on the focal points astigmatism is divided into 5 types
Simple myopic:
Light focuses on two focal points one in front of the retina and the other on the retina
Simple hyperopic:
One focal point is behind the retina and the other on the retina
Compound myopic:
Both focal points focus in front of the retina
Compound hyperopic:
Both focal points focus behind the retina.
Mixed:
One focal point focuses in front of the retina and the other behind the retina.
How to identify the type of astigmatism in a given prescription?
- Check whether the prescription is in double cylinder format
- If it is in negative or positive cylinder format transpose it to double cylinder format.
after transposing if
| One Cylinder is | Other Cylinder is | Type of astigmatism |
|---|---|---|
| + | - | Mixed |
| _ | _ | Compound myopic |
| + | + | Compound hyperopic |
| + | Plano | Simple hyperopic |
| _ | Plano | Simple myopic |
EXAMPLES:
- -3.00DC× 90; +4.00DC× 180
Mixed astigmatism - Plano; -4.00DC ×180
Simple myopic - +3.00DC ×90; Plano
Simple hyperopic - +6.00DS/-3.00DC ×70(- Cylinder, convert to double cyl)
+3.00DS/+3.00DC ×160(+ Cylinder)
+3.00DC× 70; +6.00DC ×160(Double cylinder)
Compound hyperopic - -5.00DS/+3.00DC ×80(+Cylinder)
-2.00DS/-3.00DC× 170(- Cylinder)
-5.00DC ×170; -2.00DC× 80(Double cylinder)
Compound myopic - -3.00DS/-1.00DC ×20(- Cylinder)
-4.00DS/+1.00DC ×110(+ Cylinder)
-3.00DC × 110;-4.00DC ×20(Double cylinder)
Compound myopic - +6.00DS/-7.00DC ×30( - Cylinder)
-1.00DS/+7.00DC ×120(+ Cylinder)
+6.00DC × 120;-1.00DC ×30(Double cylinder)
Mixed astigmatism
Based on principle meridians Astigmatisms are of two types regular and irregular
- Regular astigmatism: Principal meridians are perpendicular to each other.
- With the rule astigmatism:
-Vertical meridian is steeper than horizontal meridian.
-Minus is placed in horizontal meridian (-cylinder×180)
-The greatest refractive power between 120 to 60
- Against the rule astigmatism:
-Horizontal meridian is steeper than vertical meridian
-Plus cylinder is placed in vertical meridian (+cyl×180 )
-The greatest refractive power present between 0 to 30 and 150 to 180.
- Oblique astigmatism:
-Steeper meridian is not in horizontal or vertical but they are perpendicular to each other.
-The steepest meridian lies in between 120 to 150 and 30 to 60.
- Irregular astigmatism:
Principal meridians are not 90 apart.
How to identify the type of astigmatism in a given prescription?
- ‘-‘ Power is steeper than ‘+’ power (‘+’ power is more flatter than - power.)
-If the prescription is in sphere cylinder format convert it into double cylinder format.
-Draw an optic cross.
-Identify the steeper meridian.
| If one cyl is | Other is | Then steeper one is |
|---|---|---|
| - | - | High - |
| - | + | - |
| + | + | Low + |
| +cylinder×180 | Plano | + flatter, other meridian will be steeper |
| +cylinder×90 | Plano | + flatter, other meridian will be steeper |
| -cylinder×180 | Plano | -steeper |
| -cylinder×90 | Plano | -steeper |
-Observe the axis of the steeper meridian
-If the steeper meridian is
120 to 60 - With the rule
150 to 30 - Against the rule
120 to 30 and 60 and 30- Oblique
EXAMPLES:
- + 3.00DC×90,+5.00DC×180
Both are ‘+’ powers. So less ‘+’ power would be steeper. Here +3.00 will be steeper. It is in 180, Hence it is against the rule astigmatism.
- -3.00DC×90, +5.00DC×180
When one cylinder is ‘+’ and the other is ‘-‘, The ‘-‘ power will be steeper. Here -3.00 is in 180. Hence it is against the rule.
- + 3.00DC×90,-5.00DC×180
When one cylinder is ‘+’ and the other is ‘-‘, The ‘-‘ power will be steeper. Here -5.00 is in 90. Hence it is with the rule.
- -2.00DC×60, -7.00DC×150
When both cylinders are ‘-‘, Then high ‘-‘ will be steeper. Here -7 .00 is in 60. Hence it is oblique astigmatism.
- -5.00DS , +8.00DC×170(-cylinder format ,convert to double cyl)
+3.00DS, -8.00DC × 80(- cylinder)
-5.00DC× 80; +3.00× 170(+ cylinder)
When one is ‘+’ and the other is ‘–’, The ‘-‘power will be steeper. Here -5 is in 170. Hence it is against the rule.
- -2.50DS; -0.75 DC × 80(- Cylinder format)
-3.25DS; +0.75DC × 170((+ Cylinder format)
-2.50DC× 170; -3.25DC× 80(Double cylinder)
When both are’-‘, The high ‘-‘ will be steeper. Here -3.25 will be steeper. It is against the rule astigmatism.
