A Slipping Frame on a High-Plus Preschooler
A 5-year-old boy is brought in for his first pair of glasses after a cycloplegic refraction: OD +8.00 -1.00 x 180, OS +7.50 -1.50 x 010, with a 12 mm vertex distance noted by the prescriber. The optician chooses a 40-16 plastic frame with a saddle bridge because the child has a flat nasal bridge and a small face. The laboratory mounts the lenses with the optical centers at the pupil height measured while the child is seated. Within ten minutes of wear, the frame slides 6 mm down the nose and the child is looking over the top of the lenses. The lenses now sit about 18 mm from the corneal apex. The parent asks whether the prescription is wrong because the child says the floor 'looks funny.' The frame's temples are straight and the bridge is not adjustable.
1. Calculate the effective power at the eye at the prescribed 12 mm vertex distance and at the current 18 mm, using the sphere power. Is the child over- or under-plussed, and what does that mean clinically?
Read the worked answer
Use F_eff = F / (1 - dF), with d in metres.
- At 12 mm: 8.00 / (1 - 0.012 x 8.00) = 8.00 / 0.904 = 8.85 D.
- At 18 mm: 8.00 / (1 - 0.018 x 8.00) = 8.00 / 0.856 = 9.35 D.
The eye now receives about 0.50 D more plus than intended. For a hyperopic child this can blur distance vision and relax accommodation; if the glasses are for accommodative esotropia, the extra plus can disturb alignment control. The more obvious problem is mechanical: the frame has slid down, the child is looking over the top of the lenses, and the optical centres are no longer at the pupils. The 'funny floor' is consistent with looking through the wrong part of a high-plus lens.
2. List the frame and adjustment changes you would make to stabilize the fit and restore the intended vertex distance, and explain how you would verify the fit.
Read the worked answer
Stabilise the frame before changing any optics:
- Replace the frame with one that has a lower, deeper bridge and adjustable silicone nose pads, or add pad arms to the existing frame.
- Reduce eye size to cut weight, and use a frame with cable temples or a head strap.
- Adjust the temples so they hug the skull and the frame sits at the intended 12 mm vertex distance; check pantoscopic tilt (about 8-12 degrees) and that the lower rim clears the cheeks.
- Verify with a distometer or ruler, and mark pupil height with the child seated and looking straight ahead.
If the frame still cannot hold position, document the finding and contact the prescriber.
3. The parent asks you to 'just lower the prescription' to stop the slipping. How do you respond, and what is the appropriate communication with the prescriber?
Read the worked answer
Do not lower the prescription yourself. The optician is not the prescriber, and a power change made to compensate for a slipping frame could cause blur or, in accommodative esotropia, affect alignment. Explain that the problem is frame fit and vertex distance, not the prescription. Try the frame changes first. If the frame cannot be stabilised, contact the prescriber, describe the measured vertex distance and the slipping, and ask whether a compensated prescription or a different frame strategy is appropriate. Document the conversation and the measurements.