A high myope who has already bought −11.00 D soft lenses online
A 24-year-old woman with spectacle refraction −11.00 DS in both eyes, worn at a 12 mm vertex distance, wants contact lenses for her wedding in ten days. Keratometry reads 7.55 mm and 7.70 mm in each eye. She has never worn lenses, works ten hours a day on a computer in an air-conditioned office and reports gritty eyes by evening. She also mentions occasional floaters and flashes over the past month. She has already ordered a box of −11.00 D monthly soft lenses online and brings them to the clinic, asking you to "just teach her how to put them in" so she can start using them this week. The doctor is in theatre and the OPD has 20 patients waiting.
1. Explain why −11.00 D is not her contact lens power, and calculate an approximate starting power.
Read the worked answer
The reason: a spectacle lens sits about 12 mm in front of the cornea; a contact lens sits on the cornea. For a strong minus lens, moving it closer to the eye makes it optically more powerful than needed, so the contact lens power must be reduced. The effect is negligible below about ±4 D and becomes clinically important above ±6 D - she is well past that.
The calculation: use Fc = Fg / (1 − d × Fg), with Fg = −11.00 D and d = 0.012 m.
- d × Fg = 0.012 × (−11.00) = −0.132
- 1 − (−0.132) = 1.132
- Fc = −11.00 / 1.132 = −9.72 D
So the starting trial power is about −9.75 D (some practitioners start at −9.50 D and refine). Wearing −11.00 D lenses would over-minus her by roughly 1.25 D, which typically causes blurred near vision, asthenopia and a tendency to push the lenses or rub the eyes.
Two caveats I must state: this calculation only predicts the spherical starting point. The final power comes from an over-refraction on the actual lens, because soft lens power also depends on base curve, lens thickness and any residual astigmatism. And her keratometry (7.55/7.70 mm, about 0.15 mm difference, roughly 0.75 D of corneal toricity) means a toric lens may be needed - a spherical lens may leave her dissatisfied.
2. Before any insertion training, what must be done and what do you tell her about the lenses she bought?
Read the worked answer
Clinical prerequisites - teaching insertion is the last step, not the first.
- Fundus examination with dilation: high myopia plus new floaters and flashes is a retinal tear or detachment warning. This outranks the wedding date. It needs a dilated fundus examination by the doctor, and I should say so plainly rather than let her postpone it.
- Ocular surface assessment: her gritty evenings, air-conditioned workplace and long screen hours suggest tear film instability. Note it, because it predicts dropout and complications; the doctor may advise a different modality or defer lenses.
- Keratometry and corneal assessment recorded, plus refraction, so the lens parameters are prescribed rather than guessed.
- A proper fitting: trial lenses, over-refraction, centration and movement assessment, then a follow-up visit at about one week.
About the online lenses: I would not teach her to insert a lens that has not been prescribed. Saying nothing implies approval. I would explain the over-correction in concrete terms ("these are about 1.25 D too strong for your eye, they will strain your focusing and blur your near work"), point out that an unverified lens also means an unverified base curve and material for her eye, and note that if she develops a problem there is no fitting record to work from. I would offer a practical route: keep the box aside, get the fitting done, and if the prescribed power matches she can use them - otherwise they should be returned or exchanged. If she insists on wearing them regardless, that is her decision, but it must be documented along with the advice given.
3. What should be recorded from this episode, and what follow-up instructions matter most?
Read the worked answer
Record, at minimum:
- Spectacle refraction with the vertex distance, and both corrected acuities.
- Keratometry readings and the date; anterior segment and tear film observations.
- The calculated contact lens power with the formula shown (−9.72 D, trial −9.75 D) so the arithmetic is auditable.
- That a dilated fundus examination was requested for floaters and flashes, and its result once available.
- The discussion about the unprescribed −11.00 D lenses, including that she was advised not to wear them before fitting, and her response - this is the entry that protects both her and the clinic.
- Fitting details when done: lens brand, base curve, diameter, power, over-refraction, centration and movement.
- Teaching session: insertion and removal demonstrated and returned by her, hygiene instructions, wearing schedule, and the follow-up appointment.
Follow-up instructions that matter most:
- Floaters and flashes: if they increase suddenly, or she notices a curtain or shadow across her vision, she must attend immediately, not at the next appointment - this is a retinal detachment warning in a high myope.
- Red eye, pain, photophobia or reduced vision while wearing lenses: remove the lens at once and attend the same day; do not sleep it off.
- No sleeping or swimming in lenses, no tap water, replace lenses and case per schedule.
- Overwear and dry eye: build wearing time gradually, use lubricants if advised, remove lenses for long screen sessions if symptoms build.
- Return date: about one week after starting wear, and before the wedding, so there is time to change the plan if the lenses are not working.