SMILE effective to treat myopia without affecting visual image quality, finds Study
Myopia is the worldwide leading cause of vision impairment. It is increasing in prevalence throughout the world, affecting hundreds of millions of people. Various options for correcting myopia exist: spectacles and contact lenses are conservative methods of choice, but several refractive surgical procedures have been developed over the years; the most recently developed corneal refractive procedure, small-incision lenticule extraction (SMILE).
Similar to other refractive procedures, however, SMILE has shown to surgically induce higher-order aberrations (HOAs) with potential detrimental consequences for patients' optical quality and subjective satisfaction. Several studies have shown that HOAs interact in a complex manner because each individual aberration impacts visual quality differently and interacts with other aberrations.
"Following these observations, visual image quality metrics were developed to include not only the complex interaction between HOAs but also the neural processing of the visual system to provide a complete description of the optical quality of the eye. The logarithm of one of these visual image quality metrics known as the visual Strehl ratio (logVSX) has been proved to be well correlated with change in visual acuity independent of underlying pupil size and wavefront error, being predictive of subjective best focus, able to identify an optimal refraction (ie, a spherocylindrical correction), and useful for correcting highly aberrated eyes with custom made wavefront-guided contact lenses."
A high visual quality should be taken as the goal of a correction, so it is important to establish how the methods of correcting myopia affect the visual image quality; the resultant visual image quality from a correction is important for patient satisfaction.
Gyldenkerne et al performed a study to assess the influence of SMILE for high myopia on the visual image quality assessed by VSX and put this into a clinical context by comparing the postoperative result with values seen among similarly myopic controls corrected with spectacles or contact lenses.
The specific purpose of this study was to:
(1) investigate the influence of SMILE on VSX for eyes with a spherical equivalent of at least 6.00 diopters (D) of myopia,
(2) compare the postoperative VSX with the VSX seen in control myopic eyes corrected with either spectacles or contact lenses, and
(3) compare the VSX between the control myopic eyes corrected with either spectacles or contact lenses; the questions pertained to both the habitual VSX (ie, the VSX as it was measured directly for each condition) and the optimal VSX (the theoretically best-achievable VSX, calculated to see how much the VSX could theoretically improve).
This prospective study included patients receiving surgery at the Department of Ophthalmology, Aarhus University Hospital, Denmark, between February 2018 and November 2019. Patients with a myopic spherical equivalent of at least 6.00 diopters treated with SMILE aimed at emmetropia and correspondingly myopic controls corrected with spectacles and/or contact lenses were included. The logVSX calculation was divided into habitual logVSX based on the wavefront aberration measurement directly and optimal logVSX calculated in a theoretical through-focus experiment to obtain the best achievable logVSX.
The study included 117 eyes of 61 patients treated with SMILE and 64 eyes of 34 controls; 43 of the patients (38 right eyes and 33 left eyes) were in both the preoperative and postoperative groups and 21 controls (16 right and left eyes) were included in both the spectacle and contact lens group—all controls in the spectacle group were included in the contact lens group.
SMILE did not affect the habitual logVSX but worsened the optimal logVSX (P < .001). The postoperative habitual logVSX was statistically significantly worse compared with contact lenses (P = .002). The postoperative optimal logVSX was significantly worse compared with both spectacles (P < .01) and contact lenses (P = .003). There was no statistically significant difference in habitual or optimal logVSX between spectacles and contact lenses.
The discrepancy between habitual and optimal logVSX was seemingly because of the optimal logVSX being highly influenced by even small amounts of defocus.
The postoperative habitual logVSX, representing the combination of induced HOAs and the residual refraction postoperatively, was significantly worse compared with that of the control contact lens group but insignificant compared with that of the spectacle group. This statistically significant difference for the postoperative group in habitual logVSX compared with contact lenses group was mainly due to uncorrected residual refraction postoperatively because the difference in optimal logVSX (against both spectacles and contact lenses) was of much smaller magnitude than the difference in habitual logVSX; even so, the surgically induced HOAs caused a decrease in optimal logVSX for the postoperative group.
The logVSX values were equivalent between spectacles and soft contact lenses, indicating that the visual image quality is not affected by which of these modalities of conservative correction is used. The visual acuity and contrast sensitivity were also equivalent between the spectacle group and contact lens group, even though autorefraction and the applied corrections needed to obtain optimal logVSX suggested that the contact lens group was better corrected than the spectacle group; this was not surprising because disposable contact lenses are more readily updated than spectacles.
This study showed the following:
(1) SMILE for high myopia did not affect the habitual logVSX but has a small impact on the optimal logVSX;
(2) The postoperative habitual logVSX was worse than what is measured for contact lenses but not spectacles and that the postoperative optimal logVSX was worse than for both contact lenses and spectacles; and
(3) There was no statistically significant difference in neither habitual nor optimal logVSX between spectacles and contact lenses.
Furthermore, this study showed that the optimal VSX is dependent on the amount of HOAs, and that even small amounts of spherical defocus had a detrimental effect on the calculated VSX, which likely explains the different results concerning the habitual and optimal levels of VSX.
"Thus, SMILE effectively treats myopia with reassuringly clinically insignificant effects on the visual image quality compared with correction with spectacles or contact lenses; the visual image quality resulting from correction with either spectacles or contact lenses is equivalent."
Source: Anders Gyldenkerne, Anders Ivarsen, Ivan Nisted, Jesper Hjortdal; J Cataract Refract Surg 2021; 47:731–740