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Manifest refraction shown superior to autorefraction in keratoconus
Manifest refraction leads to better corrected distance visual acuity results than autorefraction in eyes with keratoconus and is, therefore, preferable as a method to prescribe spectacles or contact lenses, according to a study.
Ninety eyes with keratoconus at various stages were analyzed. Sixty-six of them were treated with crosslinking more than 6 months before inclusion in the study.
All monocular manifest refractions were performed by the same optometrist in dim light using an automated phoropter (CV3000, Topcon) and a visual acuity screen (CC100P, Topcon), based on the Donders refraction technique. A keratorefractometer (KR8800, Topcon) was used for autorefraction, providing one mean refraction based on three measurements. Cone location, keratometry and higher-order aberrations were determined by Pentacam Scheimpflug tomography.
A superior increase in corrected distance visual acuity was achieved with manifest refraction as compared with autorefraction, with a mean dioptric difference of 4.83 D between the two methods. A correlation was found between steeper Kmax (ie, more advanced keratoconus) and higher discrepancy.
According to the authors, there are several possible explanations for the discrepancy between the two methods. Most likely, the corneal ectasia deforms the image, resulting in aberrations at the retinal plane, which interfere with autorefraction measurements. Therefore, although repeatability of manifest refractions in patients with keratoconus is commonly thought to be weak, “further investigations should be performed considering the better visual acuity found in our study compared to autorefraction,” the authors wrote. – by Michela Cimberle
Disclosure: The authors reported no relevant financial disclosures.