In this study, the post-LASIK group underwent earlier PE. Compared to a control group and adjusting confounding factors for cataractogenesis such as comorbidities, axial length, CDVA before PE, and cataract grading, the patients with a history of LASIK underwent PE 7 years earlier on average. There was a strong association between LASIK and early PE ≤ 55 years. These results replicate previous studies by Yesilirmak et al [14]. that reported PE 9 years earlier in the LASIK group and by Iijima et al.15 that demonstrate a 10-year earlier PE in patients with LASIK, however, this is the largest cohort and the only one within Hispanic population to report this association.
LASIK is associated with cataractogenesis both in animal models and in clinical settings [7, 8, 14–16]. Several possible mechanisms might explain cataractogenesis after microkeratome-assisted LASIK, including photooxidative stress caused by secondary radiation of ArF excimer laser, shockwaves generated by photoablation, and intraocular structural alterations after microkeratome vacuum.
Photooxidative stress
It has been estimated that approximately 10− 5 of the total energy used on the cornea after excimer ArF irradiation is transformed into UV radiation with a wavelength ranging from 260–350 nm [8, 16]. In the aqueous humor, UV radiation promotes the formation of reactive oxygen species (ROS) that induce oxidative stress, known to induce UV-related cataracts in experimental models [17–19]. The aqueous humor has biochemical buffers including enzymatic (superoxide dismutase, catalase, and glutathione peroxidase) and non-enzymatic mechanisms that might reduce the oxidative damage in the lens [20]. However, the aqueous humor has a 78% transmittance rate of UV radiation, that reaches the lens and induces biochemical alterations [10]. Furthermore, the biochemical buffers in the aqueous are eventually surpassed, leading to a loss of homeostasis, lipid peroxidation, and cell membrane damage in the lens [21, 22].
Shockwaves after photoablation
Photoablation of the cornea results in the expulsion of tissue fragments occurring in nanoseconds at a speed of 400 m/s [23]. This process generates reactive stress shockwaves with amplitudes ranging from 80 to 150 atm [24, 25] Krueger et al. [26] have described that in the human cornea, the effect of these shockwaves behind the cornea is linear to the fluence used, and peaks around 100 atm at 7 to 8 mm behind the corneal endothelium. With this amplitude, shockwaves reach the corneal endothelium, lens, and anterior vitreous with enough energy to induce corneal endothelial cell loss and lens protein alterations [26–29].
Microkeratome-induced changes
The microkeratome creates a vacuum during LASIK to generate a corneal flap that might exceed 90 mmHg [30]. The vacuum generated to maintain the eye steady and create a corneal flap generates compression-decompression changes in the intraocular structures [11, 12]. The latter might cause anterior traction of ocular structures that could explain the observed transitory reduction in lens thickness during microkeratome vacuum [31].
Wachtlin et al. [32] studied the concentration of malondialdehyde (MDA), a marker of oxidative stress and cataractogenesis [33], in the aqueous humor in murine models of photorefractive keratectomy (PRK) and LASIK plus flap suturing. In the LASIK group, MDA levels were two-to-three times higher than PRK and control groups. These findings indicate that increased inflammation after a microkeratome flap might induce greater free-radical-induced tissue damage than secondary radiation from an excimer laser alone [32].
Several limitations aside from the retrospective nature of the study should be addressed. First, LASIK surgery data was not available for study. Differences in excimer laser platform, fluence, ablation depth, optical zone, microkeratome used, and planned flap thickness may play a role in cataractogenesis and were not studied. Also, LASIK induces corneal aberrations [15, 34] that might generate visual dissatisfaction and reduced visual quality that could lead to earlier cataract surgery. Aberrometry analysis was not performed before PE to study this variable. LASIK patients might be more particular about their vision and seek surgery at earlier stages of cataract development; the higher percentage of multifocal IOLs in the LASIK group is consistent with this theory—they want spectacle independence and thereby undergo surgery at an earlier age, perhaps in part due to ametropia. In the exploratory analysis, we found no difference between the grading of cataract when matching by age and AL. In addition, during the duration of the study, our center faced a lower volume of PE due to COVID-19 [35]. This study was performed within a third-level high-resource setting, where patients usually have a better understanding of cataracts, their impact on vision, and the benefits of cataract surgery. This awareness might lead to an earlier decision to seek treatment. Finally, other corneal refractive surgeries such as PRK and femtosecond-assisted LASIK were not included due to the low number of cases in our search for patients and the relative novelty of femtosecond laser technology.
This study suggests that LASIK may be associated with early cataract development. We suggest preoperative counseling for patients undergoing LASIK to address visual demands regarding future earlier cataract surgery and IOL selection. However, prospective studies including patients with PRK and FS-LASIK with long follow-up may further elucidate cataractogenesis after excimer laser photoablation.