This study observed that triple-DSAEK surgery induced the postoperative flattening of anterior keratometry due to corneal edema resolution after surgery. These changes in postoperative anterior keratometry alter IOL calculations and cause a hyperopic shift.
A hyperopic shift after endothelial keratoplasty is well-known and is commonly explained by changes in the posterior corneal surface. After DSAEK, the concave shape of the donor lenticule induced a negative RE by diminishing the total refractive corneal power.8,9,11,15 However, only 2/3 of the RE can be attributed to the shape and size of the donor lenticule.11 After DMEK, the thin donor tissue had no refractive power, so it was unable to justify the hyperopic shift. Even so, it has been demonstrated that postoperative changes in curvature and asphericity of the posterior cornea surface occur after DMEK and reduce the total refractive cornea power.2,3,12,16,17 However, the changes found in the posterior corneal surface appeared insufficient to justify the reported total hyperopic shift.
In 2011, Hamm et al. suggested for the first time that FED corneal edema flattened the cornea posterior surface, increased the total refractive corneal power and induced myopia. They also stated that postoperative edema resolution made posterior surface steeper by balancing out the preoperative changes induced by FED corneal edema. Hamm et al. concluded that standard IOL calculations can be applied to cataract surgery performed before or during DMEK.
Recently, some authors have observed that patients with a more marked hyperopic shift after triple-DMEK showed similar preoperative corneal features: corneal posterior surface keratometry was flatter, posterior asphericity was positive and pachymetry was thicker than in normal corneas.5,6,17 Furthermore, all authors agree that the changes observed before and after transplant are secondary to changes in the degree of corneal hydration. Thus corneal changes due to edema in FED may determine IOL calculation accuracy in procedures that combine endothelial transplant, phacoemulsification and IOL-implant (triple procedures).3,16
The main IOL calculation devices based on partial coherence interferometry, such as IOL-Master, cannot measure the posterior cornea surface. These devices base their results on third-generation formulas in which the main variables are anterior cornea keratometry and AXL. So if anterior cornea keratometry is not altered by corneal hydration as some authors suggest,3,12,16,18 there should be no RE after edema resolution in triple-DMEK. Despite published reports documenting major differences in their surgery results, they all show a clear hyperopic RE trend. 7 So possibly changes in the anterior corneal surface occur that some IOL calculation devices cannot consider3 and they could play an important role in the RE observed after DSAEK and DMEK, as Clemmensen et al. suggested in 2015.
In our study, the same device was used to measure the preoperative and postoperative keratometries (KpreDSAEK and KpostDSAEK, respectively). IOL Master 500 showed statistically significant keratometric flattening of approximately 0.60D after edema resolution, which is similar to previous data published by other authors.2–4, 19 Therefore, it could be assumed that the corneal edema present in patients with FED overestimates keratometry.
In order to verify the effect of corneal edema on IOL-power selection, we performed IOL calculation twice: once using the initial preoperative keratometry (with corneal edema) and a second with the postoperative keratometry (after edema resolution). We performed these two IOL-power calculations in the same theorical model eye to eliminate other causes of error in IOL calculations (such as effective lens position, AXL or formula). Thus, we can verify the direct relation between keratometric change (variable EK) and the error in IOL-power selection (EIOL).
Our results showed that, after edema resolution, IOL-power (IOLpostDSAEK) was approximately 1D higher than that calculated with corneal edema (IOLpreDSAEK). This difference in IOL calculation occurred because the overestimated preoperative keratometry (KpreDSAEK) conditioned insufficient IOL-power (IOLpreDSAEK). Finally, the underestimated IOL (IOLpreDSAEK) generated a mean theorical hyperopic shift of + 0.45D after corneal edema resolution (with KpostDSAEK). Curiously, the hyperopic error obtained herein was similar to previously published data in a report by the American Academy of Ophthalmology after DMEK (+ 0.31D).7 It should be considered that this hyperopic shift is calculated for a theoretical eye with an AXL of 24 mm. Thus, shorter eyes with higher IOL-power will show greater hyperopic errors, while we observed the opposite in longer eyes.
This study is the first one to establish a direct relation between hyperopic shift and the degree of preoperative corneal edema in patients with FED. To date, different authors have theorized that the corneal changes responsible for hyperopic shifts are secondary to variations in the corneal hydration level. However, none has measured or quantified the preoperative corneal edema level. To date, authors have only related changes after endothelial keratoplasty to pachymetric changes 5,20, variations in corneal densitometry 13 or preoperative positive asphericity on the posterior corneal surface 6. In the present study, we divided our sample in two groups according to the degree of preoperative corneal edema observed by slit lamp (both groups were similar in number, age and preoperative keratometry terms). Data demonstrate that different degrees of preoperative corneal edema induce statistically distinct results.
Specifically, the patients undergoing triple-DSAEK with moderate-to-severe preoperative edema showed more marked keratometry flattening after edema resolution than those with mild preoperative edema. Consequently, the hyperopic shift in the patients with more corneal edema was + 0.52D greater (+ 0.71 ± 0.52D vs + 0.19 ± 0.37D, respectively). Therefore, we suggest that the RE related to corneal edema should be considered in IOL-power calculations. Increasing the negative target of IOL according to the preoperative degree of edema could diminish postoperative refractive surprises after triple procedures.
Most surgeons choose a negative IOL target to compensate the hyperopic shift; from − 0.50D to -1.00D in triple-DMEK3,5,6,21−23 and from − 0.8D to -1.25D in triple-DSAEK. 24–26 According to our results, we propose selecting the − 0.19D IOL target in the patients with mild edema and one of -0.71D IOL in those patients with moderate-to-severe edema in triple-DMEK. In triple-DSAEK, an additional myopic target from − 0.62D to -0.97D, secondary to the effect of the donor lenticule, must be added.10,11 So in triple-DSAEK, we propose selecting an IOL target from − 0.81D to -1.16D and from − 1.33D to -1.68D in the patients with mild edema and with moderate-to-severe edema, respectively.
When the degree of preoperative corneal edema is not evaluated, choosing a -0.45D myopic target could compensate an average error induced by corneal edema resolution, but with wider variability in the results. This average adjustment in IOL calculation agrees with that suggested by other authors. 6,20.
Fritz et al. found that those patients who showed preoperative positive asphericity on the posterior corneal surface presented a higher hyperopic shift after DMEK.6 These authors suggest that the oblate shape of the posterior corneal surface (positive asphericity) is an indicator of corneal edema worsening in FED progression. So they propose adding − 0.50D of target in those eyes with positive posterior cornea surface asphericity preoperatively. Nevertheless, corneal opacity induced by edema complicates acquiring accurate posterior corneal surface data. This has been demonstrated in corneas with pachymetry over 650 µm measured by Pentacam Scheimpflug imaging.6,27 In these cases, slit lamp examination is the quickest, easiest and most reliable method for corneal edema assessments.
As corneal central thickness increases with greater edema, Bae et al. found a higher hyperopic shift after DMEK in thicker corneas.20 These authors propose adding − 0.50D of myopic adjustment of the target in those eyes with a preoperative corneal central thickness of 640 µm or more. With the proposed adjustment, their mean postoperative refractive surprise dropped from + 1.20 ± 0.92 to + 0.70 ± 0.92 D. However, corneal central thickness is extremely variable between patients and may not be universally valid for defining the IOL-power target.17,28,29 So once again, corneal transparency evaluation by slit lamp could be more reliable for establishing the degree of corneal edema and for setting the IOL target.
Some limitations in the current study should be taken into account. The subjective classification of the preoperative degree of corneal edema may reduce the repeatability of our results. However, we currently have no effective objective systems for grading corneal edema. To reduce the variability of our classification, only two well-differentiated groups were recorded: a mild corneal edema group including clear corneas and moderate-to-severe corneal edema included unclear corneas that do not allow details of anterior chamber to be seen. Besides, corneal irregularities secondary to FED severity could make keratometry acquisition difficult. 29 In our study, only repeatable keratometries were recorded. IOL-Master 500 performs keratometries in 2.3 mm of the central cornea.30 Thus, peripheral corneal irregularities secondary to severe edema should not affect (or less affect) the recorded keratometry and our results.
In summary, corneal edema resolution after a triple-DSAEK procedure induces an anterior keratometry flattening of -0.64 D. This corneal flattening induces an error in IOL calculations, which is the cause of an average hyperopic shift of + 0.45D. Moderate-to-severe preoperative corneal edema presented a greater hyperopic shift. Thus, we advocate adding − 0.50D more myopic IOL target in the patients with moderate-to-severe preoperative corneal edema than in those with preoperative mild edema.