TAILORED PRESBYLASIK

Utilising adaptive optics before surgery can help identify patient-specific presbyLASIK treatment parameters that improve visual outcomes, Pablo Artal PhD told Refractive Surgery Day at the 2014 American Academy of Ophthalmology annual meeting in Chicago.

PresbyLASIK addresses loss of accommodation by inducing corneal aberrations that increase depth of focus, usually by increasing negative asphericity, said Dr Artal, of Murcia University, Spain. This improves middle and near vision, enabling patients to see objects over a range of distances – but at the cost of reduced distance image quality. It is often used with monovision to further stretch functional vision range.

However, it is hard to predict how inducing a particular corneal aberration will affect vision because visual outcomes depend on many variables, he noted. Some of these are purely optical, such as the eye’s total spherical aberration, which may be anywhere from neutral to 0.2 microns. Also, neural responses to different amounts of spherical aberration vary from patient to patient. Therefore, every eye cannot be treated the same.

The key to a good visual outcome in presbyLASIK is to find the degree of corneal asphericity that provides adequate near vision without intolerable loss of distance vision, he explained.

“PresbyLASIK forces you to have a good compromise between the two factors, depth of focus and visual acuity. If you are able to find a good compromise, you will have a good visual outcome; if not it is going to be a problem,” said Dr Artal.

Adaptive optics

Adaptive optics make it possible to test how a patient responds to induced aberrations before surgery, making it possible to determine how specific aberrations affect depth of focus and visual acuity. These findings can then be used to adjust the amount of asphericity induced at LASIK surgery.

The adaptive optics device combines an aberrometer which objectively measures the optical properties of the eye, and spatial light modulators which can introduce different aberrations into projected images. The patient’s visual acuity at different distances can then be measured and graphed. Generally, distance visual acuity declines and near acuity increases as negative asphericity increases.

The point where the two lines cross is often close to where patients are most comfortable with the compromise, though some will prefer more distance and others more near vision, Dr Artal said.

“You can induce different amounts of spherical aberration in a non-invasive way and see how the patient responds,” he added.

Dr Artal and colleagues have demonstrated that customising LASIK-induced corneal asphericity based on adaptive optics analysis can improve presbyLASIK visual outcomes.

In a study involving 76 patients, the 37 treated with an optimal spherical aberration value as determined preoperatively using an adaptive optics instrument (AOneye, Voptica SL) gained a mean 0.93 +/- 0.5 dioptre of pseudo-accommodative capacity, significantly more than the 0.46 +/- 0.42 dioptre gained by those treated with a non-optimal degree of spherical aberration, and the 0.35 +/- 0.32 gained by controls receiving standard LASIK ablation (P<0.05).

The mean spherical aberration induced in the adaptive optics group was -0.18±0.13 microns at a pupil size of 4.5mm. The wide range of spherical adjustments reflects the variability of the optimal correction from patient to patient. All the study patients received distance correction in the dominant eye with a standard LASIK profile, and -0.75 monovision with increased depth of focus profile or standard profile in the non-dominant eye (Leray B et al. Ophthalmology. 2015 Feb;122(2):233-43).

Dr Artal has observed similar results inducing asphericity and monovision in the non-dominant eyes of patients implanted with the Light Adjustable Lens (Calhoun Vision). The system uses a special ultraviolet lamp to correct the IOL’s power and optical characteristics after it is implanted, allowing patients to “try out” the adjustments before they are locked in.

“It is possible to find a good compromise if you are customising for each patient. The solution is to use adaptive optics,” Dr Artal said.

Pablo Artal: pablo@um.es