You should use an auxiliary cylinder lens. A phoropter holds a fixed set of lenses on wheels, so its built-in cylinder runs out at about 6 diopters. An auxiliary lens is an extra cylinder lens mounted in front of the opening. It adds power beyond the built-in range, so you can keep refining instead of guessing.
Keratoconus thins the cornea and lets it bulge into a cone shape. A cone is not evenly curved, so these patients often carry very high, irregular astigmatism. Astigmatism means the cornea is curved more in one meridian than another, like a football. Cylinder power is the part of the prescription that corrects it, and the cross cylinder is the tool used to refine its axis and its power. If the phoropter cannot reach the power the patient needs, the refraction stops short and the recorded number is too low. The other way around the problem is a trial frame, which lets you stack lenses by hand from a trial lens set. A trial frame also lets the patient look around in a natural way. Either method gets you past the built-in limit of the wheels.
When an instrument runs out of range, add lens power rather than settle for the limit. Record what the patient really needs, not what the phoropter can reach. Handling high astigmatism during refraction is a core part of the COA refraction content area.
Incorrect Answers
- A. Stopping at the limit records a number you already know is too low. The patient told you the correction is not finished.
- B. The spherical equivalent is a sphere-only average. It leaves the astigmatism uncorrected, which is the exact problem in keratoconus.
- D. Interpupillary distance centers the lenses over the pupils. It adds no cylinder power and cannot measure astigmatism.
Testing Pearls
- Keratoconus is a thinning cornea that takes a cone shape and blurs vision.
- Most phoropters top out near 6 diopters of cylinder.
- A trial frame and trial lens set let you insert lenses by hand when the phoropter cannot.
- The cross cylinder refines cylinder axis first, then cylinder power.
- Mild cones are often managed with contact lenses; severe cones may need a corneal transplant.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 5, Optics and Refractive States of the Eye.
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 3, Abnormalities of the Eye.
