Jesper Hjortdal MD
The goal of providing refractive surgery patients with un-aberrated vision with almost superhuman visual acuity is neither currently feasible nor desirable; instead the aim should be to emulate the compromises that the normal human eye has adopted throughout the millennia in response to human visual needs, said Jesper Hjortdal MD, Aarhus University Hospital, Denmark.
“Surgical efforts towards normalisation of refractive errors and age-related changes in our visual functions is the realistic main aim for the near future,” Dr Hjortdal told the 38th Congress of the ESCRS.
At the beginning of this century, wavefront-guided corneal refractive surgery became available, offering the prospect of patients achieving visual acuity of better than 20/20, perhaps up to the limit of the retina’s capabilities, with the elimination of all natural sources of optical blur.
However, the technologies available to date have fallen short of those aims. Moreover, a better understanding of the way the human visual system processes aberrations now calls into question the value of an aberration-free retinal image, he said.
Pitfalls in the quest for super vision
Dr Hjortdal noted that in several randomised trials, wavefront-guided refractive surgery has performed no better than wavefront-optimised refractive surgery in terms of visual outcomes. He added that in a series of 51 eyes treated at his centre, best spectacle-corrected vision (BSCVA) becomes slightly worse after SMILE with an ablation profile corrected for aberrations measured using adaptive optics.
Part of the loss in BSCVA in such cases may result from "neural insensitivity", which can cause a person’s visual system to perform less well when presented with a diffraction-limited retinal image or an image blurred by an unfamiliar point-spread function. Natural optical aberrations in the normal human eye can compensate for the limits to the sharpness of a retinal image by providing the anti-aliasing needed to generate a softer view with blurred edges and lower contrast.
Aberrations can also reduce the effects of presbyopia onset. Many refractive surgery treatments for presbyopia are designed to add aberrations to increase the depth of field. Removing the cornea’s natural aberrations can make the loss in image quality with near focus more precipitous when the eye’s ability to accommodate begins to wane with age, he noted.
“Visual quality is much more than visual acuity. We want glare-free vision, normal peripheral vision, normal contrast vision, normal night vision, normal colour vision and most important of all, particularly in older patients, normal uncorrected visual acuity at all distances. And we need technology to achieve this somehow in the future,” Dr Hjortdal concluded.
Jesper Hjortdal: jesper.hjortdal@dadlnet.dk
