Start with a soft spherical contact lens of -1.25 sphere. When you fit a spherical lens, you choose the power that equals the spherical equivalent of the refraction. Take the sphere and add half the cylinder. The sphere is -1.00 and the cylinder is -0.50. Half of -0.50 is -0.25. So -1.00 plus -0.25 gives -1.25. This patient has a low prescription with low astigmatism, so a plain spherical lens is a fair first try.
A toric lens carries sphere, cylinder, and axis, so it can correct astigmatism. It also has to stay rotated in the right position on the eye, which makes fitting harder. Toric lenses usually cost more too. With only 0.50 diopter of cylinder, most patients see well with the spherical equivalent. After the lens settles, you refract over it and check the vision. If the patient is happy, you stay with the spherical lens. If the over-refraction shows a clear gain from the cylinder, then a toric lens is worth trying. Note that the toric choice offered here also lists -0.75 cylinder, which is more cylinder than this patient has. The textbook makes the same split. A contact lens specification lists the spherical equivalent as the power, or sphere, cylinder, and axis when a toric lens is needed.
Follow the same order each time. Work out the spherical equivalent, try the simplest lens first, then check the result on the eye. A lens that is easy to fit and easy to replace keeps the patient wearing it. Choosing a sensible starting lens is a core part of the COA contact lens content area.
Incorrect Answers
- A. -1.00 is the sphere with no cylinder added, so it under-corrects this patient by 0.25 diopter.
- B. A toric lens is harder to fit and costs more, and this one's -0.75 cylinder does not match the patient's -0.50.
- D. -1.50 adds the full cylinder instead of half, so it over-corrects and can blur distance vision.
Testing Pearls
- For a soft spherical lens, start at the spherical equivalent of the refraction.
- Spherical equivalent = sphere + half the cylinder.
- A toric (spherocylindrical) lens lists sphere, cylinder, and axis and must hold its rotation.
- Low cylinder, around 0.50 diopter, often does fine in a spherical lens.
- Always check the result with an over-refraction before deciding.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 14, Principles and Problems of Contact Lenses.
