Mini-monovision with an EDF lens costs contrast, and buys less than expected

Design

Prospective, randomised, patient- and examiner-masked single-centre trial

Population

55 patients completing follow-up after bilateral cataract surgery with a refractive extended-depth-of-field intraocular lens

Primary outcome

Binocular visual acuity at distance, intermediate and near, low-contrast acuity, stereopsis and patient-reported outcomes at three months

Effect

No significant acuity differences; low-contrast acuity better with emmetropia by 0.10 logMAR at 5% (95% CI 0.04-0.16, P < 0.001) and 1.25% contrast (0.03-0.17, P = 0.01); more glare reported with mini-monovision

Sixty patients having bilateral cataract surgery received the same refractive extended-depth-of-field intraocular lens and were randomised, patient- and examiner-masked, to a mini-monovision target of minus 0.75 dioptres in the non-dominant eye or to bilateral emmetropia. Fifty-five completed three-month follow-up.

On acuity, the two strategies were indistinguishable. Binocular uncorrected distance acuity was minus 0.02 logMAR with monovision and minus 0.05 with emmetropia (mean difference 0.03, 95% CI minus 0.01 to 0.07, P = 0.14). Intermediate (0.06 against 0.10) and near acuity (0.20 against 0.23) also failed to separate, and binocular defocus curves were comparable after correction for multiple comparisons. Stereopsis was preserved in both.

Where they did differ is contrast. Low-contrast acuity was better with emmetropia at both 5% and 1.25% contrast, by 0.10 logMAR in each case, and these differences survived Holm-Bonferroni correction. Patient-reported outcomes followed the same shape: both groups were highly satisfied with low spectacle dependence at far and intermediate distance, but monovision bought slightly less near spectacle dependence at the price of more glare.

That is a trade worth naming in the consent conversation. With this lens, offsetting the non-dominant eye does not buy a meaningful acuity gain at any distance; it buys a small reduction in reading glasses use, and it costs contrast sensitivity - which is what a patient notices driving at night, not on a letter chart.

• With a refractive EDF lens, emmetropia in both eyes is a reasonable default rather than a missed opportunity
• Warn specifically about glare if you offset the non-dominant eye
• Low-contrast acuity, not letter acuity, is where the difference between the strategies appeared
• Stereopsis was preserved at minus 0.75 D, so depth perception is not the argument against mini-monovision here
• Findings apply to this specific lens; do not transfer them to diffractive multifocals

Why it matters

Mini-monovision is often offered as a free upgrade on top of an EDF lens, and here it is not free.

Don't overread it

Three months of follow-up in 55 patients, with one specific lens model - not a general verdict on monovision.

The statistics, in plain English

Several comparisons here have P values between 0.06 and 0.18, which with 55 patients means underpowered rather than equal - the corrected distance acuity difference at P = 0.06 could well be real. The contrast findings are the ones to trust, because they survived Holm-Bonferroni correction, a conservative adjustment for having made many comparisons. A 0.10 logMAR difference is about half a line on a chart: small, but measured at low contrast, where small differences correspond to the conditions patients complain about. Three months is also early for a lens; neuroadaptation to monovision continues beyond that.

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