This prescription is written in plus cylinder notation and it describes with-the-rule astigmatism. Look at the middle number, the cylinder. Here it is +2.00, a positive number, so the office is using the plus cylinder form. Now look at the last number, the axis. It is 090, which sits in the vertical direction. A correcting plus cylinder at or near 90 degrees is the pattern called with-the-rule.
A spherocylinder lens has two principal meridians that lie 90 degrees apart. One is the steep meridian and one is the flat meridian. The written prescription has three parts: the sphere, the cylinder, and the axis. In astigmatism the eye needs different power in each meridian, so the cylinder supplies that difference. Most ophthalmology offices write the cylinder as a plus number. Optometrists and opticians usually write it as a minus number. Switching between the two forms is called transposition. You add the cylinder to the sphere, flip the sign of the cylinder, and shift the axis by 90 degrees, keeping the axis at 180 or less. Transposing this lens gives -3.00 -2.00 x 180, which is the same lens written the other way.
Read a prescription in two steps. The sign of the cylinder tells you the notation. The axis tells you the pattern: near 90 is with-the-rule, near 180 is against-the-rule, and anything far from both is oblique. Reading and transposing spectacle prescriptions correctly is part of the COA keratometry and refraction content area.
Incorrect Answers
- A. The cylinder here is +2.00, a plus number, so this is not minus cylinder notation. Minus form would show a negative cylinder.
- B. This choice misses the notation for the same reason, and the axis of 090 is not against-the-rule. A negative cylinder is nowhere in this prescription.
- D. The notation is right, but against-the-rule goes with a plus cylinder axis near 180. This axis is 090, which is vertical, not horizontal.
Testing Pearls
- A positive cylinder number means plus cylinder notation; a negative one means minus cylinder notation.
- Transposition: add cylinder to sphere, reverse the cylinder sign, then shift the axis by 90 degrees.
- Keep every recorded axis between 1 and 180 degrees.
- Most ophthalmology offices use plus cylinder form; opticians and optometrists often use minus form.
- The two principal meridians of a spherocylinder always lie 90 degrees apart.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 5, Optics and Refractive States of the Eye.
