This commentary is a further discussion of results from the FORSYN study recently published in Scientific Reports [1]. In the FORSYN study 10,350 people selected by Statistics Denmark to represent the adult Danish population with respect to age, sex, and socio-economical parameters were invited for a non-cycloplegic examination at the Department of Ophthalmology, Aarhus University Hospital.

Approximately one third of the invited participated in the examination, and demographic and socio-economic parameters were used to calculate weights for extrapolation of data to the invited population to reduce the risk of selection bias.

The purpose of the study was to report proportions of optical aids used and identify characteristics of people who would benefit from a change of prescription of optical aids.

In the adult population refractive aids were used by 72.6% of the total population and the proportion increased with increasing age. The type of refractive aids used differed between age groups with a stepwise decrease in use of single vision distance correction from 23.9% for citizens aged 18-39 years to 9.6% in 60–79-year-old individuals and a stepwise increase in use of progressive addition lenses from 1.1% to 63.5% in the same age groups.

These trends differed for the 80+ years age group of which only 52.2% used progressive addition spectacle lenses while 24.5% used reading glasses which may partially be due to cataract surgery targeted at spectacle free distance vision reducing need for multifocal optical corrections. While the use of bifocals was infrequent (<1%) for citizens below the age of 80 years, it was used by 9.7% of individuals aged 80+ years. This may be explained by the fact that bifocals were the optical solution of choice when they acquired their first spectacle correction for presbyopia and that some were reluctant to change type of optical correction on subsequent visits to the dispensing optometrist. However, progressive addition spectacle lenses appear to increase risk of fall due to visual distortion and reduced awareness of blurred distance objects in the lower visual field [2,3], which may also explain the frequent use of bifocals and reading glasses in this age group. Prospective observational studies with long-term follow up are needed to fully elucidate whether progressive addition spectacle lenses increase risk of falls.

Comparison of visual acuity with habitual correction and optimal subjective refraction using Early Treatment Diabetic Retinopathy Study (ETDRS) chart showed improvement in visual acuity of 2.99 and 10.3 letters per dioptre of uncorrected hyperopia and myopia, respectively. The lower improvement with hyperopic correction is most likely explained by younger individuals’ ability to compensate for hyperopia by use of accommodation. This is supported by the observed improvement in visual acuity per dioptre increased with age.

The improvement in visual acuity with myopic correction was lower than previously reported. In a review Smith [4] reported approximately 65% larger improvement than in the present study, but this difference may be due to methodological differences. All previous studies used cardboard charts instead of electronic flat panels. The latter provides higher contrast and thereby more likely better visual acuity under suboptimal conditions reducing the difference between habitual and optimal refraction. Furthermore, the larger improvement in visual acuity in previous studies may also be partially caused by visual acuity assessment under dimly lit conditions resulting in increased pupil size which has been shown to increase the improvement in visual acuity when correcting myopia [5]. Finally, some of the previous studies used change in spherical component of the refraction instead of spherical equivalent which may also influence results. We performed multiple linear regression and found no effect of the power of astigmatism on visual acuity change. This analysis did however show clinically and statistically significant improvement in visual acuity with optimal refraction when time since last visit to dispensing optometrist exceeded three years for citizens using progressive addition lenses and myopic individuals using single vision lenses (p<0.01 for both comparisons). Since there is no guideline for the frequency of eye and vision examinations currently, a check-up every three years can be used as a general recommendation for patients without self-perceived visual symptoms. For 2.4% of the population, visual acuity was reduced by 5 EDTRS letters or more, indicating a need for change in prescription.

Although non-cycloplegic refraction increases risk of underestimation of hyperopia, previous studies have found the difference between cycloplegic and non-cycloplegic refraction to be minute even for the youngest patients in the included age range [6] and negligible when fogging with plus lenses is used to relax accommodation [7].

In conclusion, refractive aids were used by almost 3 out of 4 citizens which underscores the importance for optimizing prescription to improve quality of vision and prevent falls. Prescriptions should be checked when time since the last visit with a dispensing optometrist exceeds three years.