High Myope, New Frame, and a Tight Vertex Distance
Marcus, 29, works 10-hour days in front of a trading screen and plays competitive basketball twice weekly. His current spectacles, worn at a 13 mm vertex distance, are OD -9.75 DS and OS -9.25 -1.00 x 175. He has chosen a low-profile frame that sits only 6 mm from his cornea and insists the old prescription be glazed into it because "the numbers are the same." Best-corrected acuity is 6/5 in each eye; cover test is orthophoric, and anterior segment, fundus, and visual fields are normal. Keratometry is 43.50/44.00 OD and 43.75/44.25 OS. He is highly motivated for soft contact lenses but has never worn them and has a basketball tournament in four weeks. He reports that his current glasses make the floor appear slightly farther away than it is, which worries him when rebounding. He asks whether the new frame will blur his vision and whether contacts can be ready in time.
1. Calculate the vertex-corrected spectacle prescription at the new 6 mm distance for each principal meridian. Show the formula, give values to 0.25 D, and state the prescription you would actually send to the lab.
Read the worked answer
Use the vertex formula F_new = F_old / [1 + (d_new - d_old) F_old], with d in metres and the sign convention that a lens closer to the eye has a smaller d. Here d_new - d_old = 0.006 - 0.013 = -0.007 m.
- OD sphere: F_new = -9.75 / [1 + (-0.007)(-9.75)] = -9.75 / 1.06825 = -9.13 D. To the nearest quarter dioptre, -9.00 D is the practical choice; -9.25 D would over-minus slightly.
- OS sphere: -9.25 / [1 + (-0.007)(-9.25)] = -9.25 / 1.06475 = -8.69 D, so -8.75 D.
- OS cylinder: -1.00 / [1 + (-0.007)(-1.00)] = -1.00 / 1.007 = -0.99 D, which remains -1.00 D; axis unchanged at 175 degrees.
I would send OD -9.00 DS and OS -8.75 -1.00 x 175, then verify with a trial frame at the actual 6 mm vertex and check acuity, comfort, and prismatic effect. The calculation is a starting point, not a substitute for over-refraction in the chosen frame.
2. Explain why keeping the old numbers in the new frame can degrade vision and comfort even though the refractive error has not changed. Include the effect on spectacle magnification and whether aniseikonia is likely.
Read the worked answer
The refraction has not changed, but the effective power at the cornea has. Moving a minus lens from 13 mm to 6 mm from the eye increases its effective minus power if the same number is kept: the old -9.75 D lens at 6 mm would act like about -9.21 D at the corneal plane, whereas at 13 mm it acted like about -8.65 D. That is roughly 0.55 D of over-minus, enough to cause blurred distance vision, asthenopia, or adaptation by accommodation. The old cylinder would also change slightly, though less noticeably.
Spectacle magnification for a minus lens is approximately M = 1/(1 - dF). At 13 mm and -9.75 D, M is about 0.887; at 6 mm with the corrected -9.13 D, M is about 0.948. The retinal image becomes about 7% larger. Because both eyes change similarly, aniseikonia is unlikely to be a major problem, but if only one eye had a vertex change or if the anisometropia were greater, aniseikonia and spatial distortion would need consideration. His report that the floor looks farther away is consistent with minification, and reducing that minification may change his sense of distance in sports.
3. The patient asks about soft contact lenses for basketball. Using the compensated spectacle refraction, estimate the necessary soft lens power and list two reasons a trial is still required.
Read the worked answer
The ocular refraction at the corneal plane is the old effective power: OD about -8.65 D, OS about -8.26 -0.99 x 175. A soft lens power is usually chosen close to this ocular refraction, so a starting estimate is OD -8.50 D and OS -8.25 -1.00 x 175. The final power must come from trial and over-refraction.
A trial is still required because soft lenses do not sit exactly at the corneal plane, and lens flexure, base curve, water content, centre thickness, tear lens, and residual astigmatism all alter the result. A 1.00 D cylinder may also need a toric design rather than a spherical lens. For basketball, a daily disposable silicone hydrogel with good oxygen transmission and a stable fit is sensible. Four weeks is enough for fitting and adaptation, but he should not wear lenses for the tournament without a completed fit, hygiene instruction, and a spare pair of spectacles.