Femtosecond laser-assisted flap creation in LASIK surgery has rapidly become popular with increasing safety and efficacy over time [1–4]. When using femtosecond laser for flap creation, each pulse of the laser causes the generation of a small amount of microplasma at its focal point in the corneal tissue leading to formation of microscopic gas bubbles and cavitations, which then dissipate into surrounding tissue [5, 6]. Compared with mechanical microkeratome for making flaps, a key benefit of femtosecond laser technology is to provide a more precision, predictable flap and less vision threatening flap complications [7, 8]. However, the femtosecond laser technique, as the resulting laser pattern may act as an optical grating, could be a major drawback. Previous studies indeed reported on perceptions of rainbow glare (RG) as a mild side effect of FS-LASIK and was verified on a model eye [9–11].

WaveLight FS200 femtosecond laser system (Alcon Laboratories Inc., Fort Worth, TX) is a high pulse frequency (repetition rate of 200 kHz) and comparatively lower pulse energy system that can produce flaps in a shorter period of time, around 6 s. Flaps created using FS200 demonstrated both precision and reproducibility with minimal side effects [12–14]. Only two previous studies reported two cases of unilateral RG following FS-LASIK with the WaveLight FS-200 femtosecond laser [15, 16]. To our knowledge, there is no published study on incidence rate and alleviating management of RG when using FS200 femtosecond laser. In this retrospective study, we reported a high incidence of RG after FS-LASIK using the upgraded FS200 and evaluated the alleviative effect of adjustment of flap cut parameter settings.

LASIK is the main stream refractive surgery procedure for correction of myopia and myopic astigmatism worldwide. The introduction of the femtosecond laser in LASIK enabled the creation of lamellar corneal flaps in a more accurate, more stable, evener and safer manner. Even though RG which was first reported in 2008 by Krueger et al. is a mild and temporary optical side effect of FS-LASIK [9], it may disturb patients’ night driving and decrease their satisfaction [9, 10]. The cause of RG was defined as the diffraction of light from the grating pattern created on the back surface of the LASIK flap after femtosecond laser exposure [17]. The first clinical study describing RG found an incidence rate of 19.07% with an older high pulse energy, low frequency (15 kHz) IntraLase femtosecond laser model [9]. A subsequent study reported an incidence of rainbow glare of 5.8% with a newer generation of lower pulse energy, higher frequency (60 kHz) IntraLase model [10].

Since low pulse energy and high frequency femtosecond laser system was applied, less incidence of RG has been reported. The first case of RG with WaveLight FS200 femtosecond laser platform, the lower pulse energy and higher frequency system, was reported by Gatinel D, et al. in 2013 [15]. We have used the WaveLight FS200 femtosecond laser platform (≤0.8 μJ pulse energy for flap, 200 kHz frequency) for performing FS-LASIK since 2011 with the recommended optimized cut settings of 0.80 μJ pulse energy, 8.00 μm spot and line separations for stromal bed cut and barely heard complaining of the RG symptom until the laser platform was upgraded to “Green” version in December, 2015. As we understood, “Green” brought only new features of software, such as patient data management, docking guidance system and flap position registration on the basis of pupil center for more convenient without changing any laser parameter settings. After the high incidence of RG was noted, we got a new recommendation of optimized cut settings of 0.60 μJ pulse energy, 6.00 μm spot and line separations, the tolerable arrangement of former version, for stromal bed cut. Since then, the incidence rate of RG had decreased to 1.57% among the following consecutive series cases.

Up to date, the reason why the incidence of RG significantly increased after machine being upgraded has still been unclear. The quality of the focused laser beam and numerical aperture of the focusing optics appears to be the most significant factor in minimizing the diffractive dispersion of light that leads to RG [9, 10]. Otherwise, the relationship between length of time between service calls and the incidence of RG emphasized the importance of a proper maintenance and alignment of the optical components used to focus the beam [10].

Gatinel D, et al. calculated a grating pattern spacing of 7.90μm was the cause of rainbow glare [15]. This value was very close to 8.0μm in group A of the present study. It might seem that reducing the spot/line separation could increase the angle of the spectral pattern sufficient enough to avoid photodetection by the retina altogether [17]. Correlative results in the present study showed that less laser pulse energy and narrower spot/line separations might play an important role in decreasing the incidence of RG. With the settings changed, the energy intensity and the flap cutting time (from 6 to 7 s to 12–13 s) increased, which may induce more corneal lamellar tissue reaction and should be observed for longer time.

With intensively in vivo confocal microscope scan, at around the 110 μm depth level from the anterior corneal surface of almost all the cases after FS-LASIK in this study, no matter whether RG existed or not, the hyper-reflective spots could be found in an equidistant grid-like pattern with HRT II confocal microscope. The spot pattern matched the laser spots/lines separation of 8.0μm or 6.0μm programmed in the FS200 femtosecond laser. There was a report of successful RG correction using undersurface ablation of the LASIK flap [16]. We considered that it was not necessary to do so if merely the symptom of RG existed, because the symptom of RG disappeared with time in all the subjects of the present study, even when the hyper-reflective spots were still visible under confocal microscopy. In this aspect, the patients’ individual variation of sensitivity and self-adaption might play a pivotal role.

Krueger et al. found an increased degree of myopic correction was associated with an increased incidence of RG [9]. However, the present study did not found correlation between initial SE or ablation depth and the incidence of RG, which was consistent with the study of Bamba et al [10]. The present study first found that older age was a correlative factor of RG. Only one previous report studied the relationship between age and the incidence of RG and found negative result [10]. Different race of subjects and femtosecond laser machine might lead to the reverse results. RG was subjective feeling, and older subjects might have more need in night vision, such as driving in the night, which might cause more older subjects felt and reported RG to the doctor. More prospective large-sample studies are needed in future to explore the risk or correlative factors of RG.

In conclusion, the upgrade of WaveLight FS200 femtosecond laser with original flap cut parameter settings could increase the incidence of RG. After the pulse energy, spot/line separations for stromal bed cut was adjusted, the incidence of RG decreased significantly. RG did not influence postoperative visual acuity or refractive results. It faded away as time went by. Narrower grating size and lower pulse energy could significantly ameliorate this side effect after FS-LASIK when using WaveLight FS200 femtosecond laser.