Visual acuity and lens power
Research type
Research Study
Full title
Visual and stereoacuity: average paraxial power and spherical equivalent
IRAS ID
339361
Contact name
Stephen Kaye
Contact email
Sponsor organisation
Royal Liverpool University Hospital
Duration of Study in the UK
1 years, 0 months, 1 days
Research summary
The wearing of glasses (spectacles) to correct a person’s refractive error i.e., short-sightedness (myopia) longsightedness (hypermetropia), astigmatism (different degrees of myopia and hypermetropia in different meridians) or combination of these, is very common with over 70% of the UK population wearing glasses. The prescription for the spherical errors of myopia and hypermetropia is a single number or scalar number that is, without direction. For the correction of astigmatism (aspherical or a toric prescription) which affects 33% of the global population, the prescription requires a value or number with direction e.g., a vector.
Some people find it difficult to tolerate the spectacle prescription to correct their astigmatism and in such cases an attempt is made to reduce or balance the astigmatic correction with a spherical (scalar) correction. A commonly used approach for this is to use the formula for a spherical equivalent (SE) which uses half the power of the cylinder. This formula, however, excludes the presence of oblique rays which contribute to vision. As such a more recently derived formula to reduce astigmatism to a single scalar value has been developed and uses a quarter of the power of the cylinder. It is known as the average paraxial lens power (ApP).
It has recently been shown in an experimental group of subjects with induced astigmatic refractive errors containing cylinders at 90 and 180 degrees, that the A_p P was also associated with better visual acuity that the SE and maybe of more clinical use.
The aim of this study is to investigate whether the ApP outperforms the SE in terms of distance visual acuity and stereoacuity (depth perception) correction in patients with distance refractive errors; particularly in cases with high astigmatic errors where conventional measures might be inadequate, or subjects are unable to tolerate their toric prescription.REC name
South West - Frenchay Research Ethics Committee
REC reference
24/SW/0110
Date of REC Opinion
6 Nov 2024
REC opinion
Further Information Favourable Opinion