Mechanisms for Retinal Detachment Development
It is speculated that specific damage caused by corneal refractive surgery in an eye with an underlying predisposition, particularly in myopia, may result in retinal detachment.15 The possible mechanisms by which corneal refractive surgery is believed to produce damage include trauma caused by the suction ring, the shock-wave effect associated with the excimer laser, and a thermic effect.16
Biomechanical Changes Caused by the Suction Ring. Biomechanical stress caused by the suction ring of the microkeratome or femtosecond laser is suggested as the principal causative factor in vitreoretinal pathologies associated with corneal refractive surgery.11 It is known that the intraocular pressure (IOP) generated by placing a manual microkeratome suction ring might reach levels of 150 mm Hg.17 Mostafavi et al.18 showed that application of the suction ring resulted in a shortening of the axial length of porcine eyes ranging from 0.38 to 1.48 mm (mean: 0.67 mm). This change in axial length could generate transverse vector forces at the vitreoretinal interface and provoke posterior vitreous detachment (PVD) and the development of retinal tears or retinal detachment. On the other hand, in an experimental model based on human eyes obtained from an eye bank, the application of the suction ring resulted in increasing the axial length of the eye by approximately 1.125 mm, whereas the anterior chamber depth remained stable.19 In this model, anterior movement of the vitreous base could presumably provoke development of vitreoretinal pathology.20 Finally, in a study by Mirshahi and Kohnen,3 the anterior chamber depth and axial length did not change during microkeratome suction, but a decrease in lens thickness and an increase in vitreous distance was observed. Similarly, such accelerated vitreous detachment could increase traction at the vitreous base.
Arevalo et al.21 suggested that the temporal location of the retinal breaks in patients with retinal detachment might be associated with the location of the microkeratome handle, putting extra pressure on that side of the eye. An example of the intensity of mechanical forces applied within the procedure and affecting the posterior segment of the eye is a report of bilateral macular hemorrhage following LASIK in a patient with high myopia.11
It should be underscored that the biomechanics of the vitreous was assessed by Mazur et al.20 in 69,039 ocular pneumoplethysmography tests. Ocular pneumoplethysmography is non-invasive technique applied to evaluate suspected carotid arterial lesions in patients having concomitant ocular diseases.22 During the procedure, typical vacuum levels reach 300 mm Hg, whereas the IOP might exceed 100 mm Hg. Nevertheless, during the procedure, using B-scan ultrasonography, no motions or disturbances of vitreous structures were detected in myopic patients.20 It should be noted that the rapidity of IOP changes generated by the suction ring is greater than that during pneumoplethysmography, where a progressive IOP decay lasts approximately 30 seconds.23 Yet, the IOP changes related solely to the use of the suction rings could result in mechanical stress on the eye.
Excimer Laser Shock-wave. Pulsed laser ablation of any kind of materials results in the creation of a stress wave at the object's surface that spreads into the bulk.24 The stress wave peak amplitude is greater for large spot diameters and correlates linearly with the applied fluence.25 Spot diameters of 1 to 6.5 mm are used for tissue ablation in currently used lasers. For spot diameters up to 3 mm, the stress wave amplitude significantly decreases with distance from the endothelium, whereas in diameters larger than 3 mm, the peak pressure is localized 6 to 7 mm behind the endothelium.25 The critical volume where the side effects are to be expected is located near the lens–vitreous boundary; however, the negative pressure wave may also reach the retina.
It was speculated that excimer laser shock waves might be an iatrogenic factor generating stress on the posterior segment of the eye.9 This would result mainly in cellular alterations and structural damage to the adjacent collagen layer.9 Particularly, myopic eyes would be predisposed to such pathology because they have extremely fragile structures (ie, the submacular vessels).10 Ozdamar et al.5 suggested a combined mechanism of a sudden increase in IOP exerting mechanical stress on the vitreous along with mechanical stress induced by laser shock waves.
Use of the Femtosecond Laser. In femtosecond laser surgery, slightly less vacuum is necessary for suction cup placement than with manual microkeratomes; however, the time needed to create a flap is significantly longer.11 Nevertheless, strong deformation of the eye during applanation results in IOP elevation up to 90 mm Hg above baseline. The IOP rise during the procedure is higher with curved contact lens interfaces than with a liquid optical immersion interface.26 A macular hemorrhage was reported after femtosecond laser–assisted LASIK without any risk factors for such a pathology.11 Moreover, studies on animals presented that the femtosecond laser itself is potentially capable of causing retinal injuries.27
Factors Influencing the Retinal Detachment Risk After Corneal Refractive Surgery
Presence of Peripheral Retinal Lesions—Should They Be Treated? LASIK does not appear to lead to progressive retinal lesions in asymptomatic patients.28 Moreover, the efficacy of prophylactic treatment in such conditions is doubtful. For example, in a study by Ruiz-Moreno et al.,29 patients with predisposing lesions previously treated with laser photocoagulation had a higher retinal detachment rate compared to the untreated group (0.92% vs 0.15%, respectively). In a study by Chan et al.,30 prophylactic treatment of vitreoretinal pathology such as lattice degeneration or retinal breaks did not guarantee that vitreoretinal complications following LASIK will not occur. In their study, 88.2% of patients who developed retinal detachment had retinal breaks adjacent to lesions found before LASIK, whereas retinal breaks were noted in other quadrants than those found before LASIK in 29.4% of patients. In a survey by Chan et al.,28 a large percentage of eyes with substantial myopia manifested retinal breaks after LASIK, but were comparable with results found in unoperated eyes with high myopia. Some authors have suggested that patients undergoing corneal refractive surgery should receive more aggressive prophylactic retinal therapy.31 Nevertheless, it is not possible to scientifically determine whether peripheral retinal lesions should be treated differently from standard practice simply because a patient is to undergo LASIK. Close postoperative monitoring of patients with high myopia to prevent potential vitreoretinal complications could be suggested.
Furthermore, Sinha et al.32 reported that LASIK for correcting postoperative refractive errors may be performed safely after retinal detachment surgery. In this case, none of the 8 eyes having undergone LASIK had postoperative retinal complications. The only difficulty encountered in these eyes was conjunctival scarring, which influenced the suction of the microkeratome. Thus, the surgery could not be completed in two eyes.
Preoperative Refractive Error. Lee et al.33 reported a greater level of myopia in patients who developed retinal detachment after corneal refractive surgery than in the overall cohort undergoing surgery. Myopic individuals with retinal detachment had a refractive error of −8.84 ± 2.92 D, whereas, in general, myopic patients undergoing corneal refractive surgery had a refractive error of −5.94 ± 2.75 D. Similar results were presented in a study by Faghihi et al.34; the mean refractive error in eyes that developed retinal detachment was −8.60 ± 3.90 D, whereas it was −6.10 ± 3.50 D in the general myopic cohort undergoing corneal refractive surgery (P < .001). It should be highlighted that the general risk for retinal detachment is greater in eyes with a higher refractive error.35 We suggest that when considering corneal refractive surgery in individuals with greater refractive errors, the treatment should be performed with caution. With that, patients should be informed that corneal refractive surgery reduces the refractive error but does not eliminate the risks related to myopia itself.
Older Age. Myopic patients who developed retinal detachment after corneal refractive surgery were significantly older than the general cohort that underwent corneal refractive surgery; the mean age was 46.4 ± 11.2 and 33.41 ± 7.64 years, respectively.33 In another study, patients who developed retinal detachment were 38.2 ± 11.2 years old, whereas the mean patient age of those undergoing corneal refractive surgery was 30.7 ± 8.5 years.34 According to Faghihi et al.,34 the association between age and retinal detachment risk is not strong, with an adjusted odds ratio of 1.08. However, in a general population there is a significant association between age and retinal detachment incidence rates.36 One possible reason is the PVD, because retinal detachment is commonly associated with symptomatic PVD.37 There is a correlation between PVD and presence of horseshoe-shaped tears at the central border of the vitreous base.36
Surgical Approach: PRK, LASIK, and SMILE. The idea that the forces applied by the suction result in a risk of retinal detachment should manifest in the presence of a dissimilarity of retinal detachment rates between LASIK and PRK. Nevertheless, such a difference was never proven, and the differences between procedures were not statistically significant in studies that assessed outcomes of PRK and LASIK.16,33 Notably, within several of the analyzed studies, patients undergoing LASIK had a greater refractive error than those undergoing PRK, which could also be a confounding factor.16,33 Because SMILE is a relatively new technique, no study was found that analyzed the long-term risk of developing retinal detachment after SMILE. Nevertheless, it could be similar to that of studies of other techniques.
PVD. One of the mechanisms by which shock waves, IOP alterations, and biomechanical changes caused by the suction ring could influence the posterior segment is by the development of PVD. An association between microkeratome-assisted LASIK and PVD was reported with an overall incidence of new PVD after LASIK, reaching 9.5% to 21.4% of eyes.3,38 In a study by Luna et al.,39 PVD developed more frequently in eyes with high myopia than in those with low myopia.
Theoretically, because the pressure alterations within the eye are lower in femtosecond laser–assisted LASIK than in manual microkeratome LASIK, the PVD development may also be less common. However, in a study by Osman et al.,40 PVD was detected in 85% of cases (17 eyes) after femtosecond laser–assisted LASIK, whereas only 20% of cases (4 eyes) developed PVD after LASIK performed with a manual microkeratome. The authors concluded that this may be caused by the longer suction time in femtosecond laser–assisted LASIK. In another study, the development of PVD after femtosecond laser–assisted LASIK was reported in 16% of myopic eyes.41 A disadvantage of all of the aforementioned studies is a relatively small sample size.
Other Factors. Faghihi et al.34 noted that although male patients contributed to 39.2% of LASIK patients, 71.4% of the cohort that developed retinal detachment following LASIK were male (adjusted OR was 3.16 for male gender). On the other hand, even males who have not undergone surgery are more susceptible to developing retinal detachment.36,42 For example, in the Dutch Rhegmatogenous Retinal Detachment Study,36 the male-to-female ratio was 1.3:1. It was speculated that this could be attributed to a higher risk of ocular trauma in males than in females. Although PVD is more common in females than in males,43,44 in males, it is more often complicated by the development of retinal tears.37