Corneal cylinder affects visual acuity in RGP lens wearers

Rigid gas-permeable (RGP) contact lens corrected visual acuities are affected by corneal cylinder and Sim K values, claims a study in the Journal of the British Contact Lens Association.

Dr Cirous Dehgani et al., Poostchi Ophthalmology Research Centre, Department of Ophthalmology, Shiraz University of Medical Sciences, Shiraz, Iran, performed a prospective study on 156 eyes from 134 keratoconus patients divided into central and paracentral cone groups.

Best-spectacle corrected visual acuity (BSCVA), residual astigmatism (RA) and best-corrected RGP visual acuities (RGPVA) were measured. Corneal topography and slit-lamp biomicroscopy was completed. Flat Sim K readings and 'three-point touch' fittings were used to determine which trial RGP lenses to use on the patients.

In the central cone group there were 76 eyes and in the paracentral cone group there were 80 eyes. Before the RGP lenses were fitted mean subjective refraction spherical equivalent (SRSE), subjective refraction astigmatism (SRAST) and BSCVA (logMAR) were −5.04 ± 2.27 D, −3.51 ± 1.68 D and 0.34 ± 0.14, respectively.

Statistically significant differences were found between central and paracentral cone groups with regards to SRSE, SRAST, flat meridian, steep meridian and mean K and corneal cylinder values.

Vision improved 0.3 logMAR and there were no statistically significant differences between RGPVAs and RAs of central and paracentral cone groups.

RGP-corrected visual acuities and residual astigmatism are not affected by cone location in keratoconus patients. However, RGP-corrected visual acuities are influenced by corneal cylinder and Sim K values.