Progressive lens non-tolerance in a high myope: separating fitting error from adaptation
A 47-year-old accountant who spends seven hours a day at a computer collects new spectacles and returns three weeks later with a list of complaints: the floor appears to slope when he walks, his head feels 'tight' by late afternoon, the screen at 60 cm is blurred unless he lifts his chin, and oncoming headlights at night produce coloured fringes. His previous spectacles were single-vision distance lenses of −8.50/−1.00×180 in both eyes with separate +2.50 readers, worn for four years. The new pair are −9.00/−1.25×180 with a +2.00 addition, a progressive design with an 18 mm corridor, 1.67 index lenses, fitted into a shallow frame with a 27 mm lens depth and a measured fitting height of 16 mm. The frame was dispensed with 6° pantoscopic tilt; the old frame measured 12° tilt, and the new lenses sit about 4 mm closer to the eye than the previous pair. Refraction in the trial frame was accepted without complaint, and the PD was taken from the autorefractor printout.
1. Which of his complaints are explained by the measurements, and which are not? Show the reasoning with the numbers, including vertex distance, corridor and fitting height, and tilt.
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Explained by the measurements.
Near blur at 60 cm and chin lifting. An 18 mm-corridor progressive in a 27 mm-deep frame with a fitting height of only 16 mm means the near zone begins about a corridor length below the fitting cross, so most of it has been cut off by the lens edge. He lifts his chin to find the remaining sliver of add. The +2.00 is nominally right for 50 cm; at 60 cm he needs about +1.66 D, which a correctly positioned intermediate zone would have covered. This is a fitting error, not an adaptation problem.
Coloured fringes at night. 1.67 index has an Abbe number around 32, against roughly 58 for a conventional 1.50 lens. At −9.00 the lateral chromatic aberration is visible, worst for point sources at night and off-axis.
Sloping floor and 'tight' head. Moving from single vision to a progressive is a genuine adaptation task, but two changes were made at once. Vertex: −9.00 at 14 mm delivers about −7.99 D to the eye; the same nominal power at 10 mm delivers about −8.26 D - roughly 0.27 D of extra minus simply because the lens sits closer, on top of the small prescription increase. Magnification changes from about 0.888 to 0.917, a 3% enlargement of the retinal image, which drives the perceived slope.
Possible induced prism. An autorefractor PD is a starting value, not a dispensing measurement. At −9.00 a 2 mm centration error in each lens gives about 1.8 Δ per eye by Prentice's rule (P = cF), enough to cause asthenopia and a swimmy sensation.
Not explained, needs checking. Pathology - with high myopia, examine the fundus and macula; and establish how consistently he has actually worn the new pair.
2. What is your ordered plan to resolve this, cheapest and least disruptive first? What will you remake and what will you adjust?
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Free and reversible first.
- Verify the job. Neutralise the new lenses; measure monocular PDs properly (pupil centre, distance, frame as worn), fitting heights, vertex distance, pantoscopic tilt and face form; compare with what was ordered. This tells you whether you are dealing with an ordering error or a design error.
- Adjust the frame. Increase pantoscopic tilt back towards 10–12°, adjust the nose pads to bring the vertex distance close to the trial-frame value, correct face form. Let him wear them for 30 minutes in the practice and re-test.
- Over-refract with the glasses on, at distance and at 60 cm. If he is being over-minused by 0.25 D, that alone may explain the night symptoms.
Then remake, in order of preference.
- Change the design, not the patient. If the 16 mm fitting height is confirmed, an 18 mm corridor was the wrong choice: either fit a short-corridor progressive with a minimum fitting height of about 15–16 mm, or, better, move to a frame with at least 30 mm of depth and set the fitting cross at 20–21 mm.
- Drop the index to 1.60 (Abbe ≈ 42) or 1.50 if the cosmetic thickness allows, to remove the chromatic fringing he notices at night.
- If he remains intolerant, offer an occupational/office lens for the desk plus a separate distance pair, or distance single vision with +2.50 readers - the arrangement he used happily for four years. Non-tolerance of a progressive is a legitimate outcome, not a failure of the patient.
3. How do you handle the conversation about who pays for the remake, and how do you prevent a repeat?
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The conversation. Show him the measurements and name the problem plainly: 'The reading area of this lens sat below the bottom of the frame, so you have been tilting your head to find it. That is a fitting error on our side and we will put it right at no cost to you.' Patients accept errors that are named and corrected; they do not accept being told they simply need to persist.
Who pays. If the practice ordered an 18 mm corridor into a frame that could never accommodate it, or used an unverified autorefractor PD at −9.00, the practice bears the cost of the remake. If the patient insisted on the shallow frame after being shown the consequence, a shared arrangement is reasonable - but that must have been discussed and recorded at the time of order, not invented afterwards.
Prevention at the point of sale.
- Check lens depth against corridor length and minimum fitting height before the order is placed, and write the figures on the order form.
- Always measure monocular PDs and fitting height with the frame on the face, marked on the dummy lens.
- Record vertex distance, pantoscopic tilt and face form; if the new frame differs from the old by more than about 2 mm or 4° at high power, recalculate the effective power.
- Counsel every first-time progressive wearer about peripheral distortion, stair and kerb caution, and a realistic 2–3 week review.
- At high minus powers, default to the lowest index that satisfies the thickness requirement, then discuss the trade-off with the patient rather than deciding for them.