The principle message of our study is that Viscoat provides better protection to corneal endothelium in comparison with Visthesia during uneventful phacoemulsification cataract surgery. This is apparent, because Visthesia caused a larger endothelial cell loss, as well as more corneal edema than Viscoat. Nevertheless, the two OVDs had no significantly difference as far as the BCVA acquired postoperatively.
The main reason for using OVDs in cataract surgery is to prevent damage of the corneal endothelium. Dispersive OVDs, such as Viscoat, are considered to protect the endothelium better than cohesive OVDs, because of their lower specific surface properties . However, they need longer aspiration time to be removed from the anterior chamber and it may damage the corneal endothelium in this way [2–9, 15]. In our study, there was a significant difference in the duration of the operation, noting that Viscoat group needed more time for the surgery, although the duration of the ultrasound application did not differ between the two groups. On the other hand, cohesive OVDs, like Visthesia, are more effective in keeping a deep anterior chamber than the dispersive one. They also tend to escape from the anterior chamber, leaving endothelial cells without protection [6, 16]. In addition to this, there are studies showing that Visthesia could be harmful to the corneal endothelium [17, 18].
The damage of the corneal endothelium can be evaluated by measuring the endothelial cell decrease after surgery . Adult human corneal endothelium is considered a non-replicative tissue and there is a natural decrease in endothelial cell density by age . Dispersive OVDs are expected to cause less endothelial cell loss, as they protect better the corneal endothelium. This is in line with Glasser et al. who observed less endothelial cell loss in eyes receiving Viscoat than in those receiving 1% sodium hyaluronate (Healon® OVD, AMO), a cohesive OVD . On the contrary, Holzer et al. suggested that 2.3% sodium hyaluronate (Healon® 5, OVD, AMO) had lower mean endothelial cell loss in comparison with Viscoat, while Lane et al. found similar amounts of endothelial cell loss in eyes receiving cohesive and dispersive OVDs [5, 21]. In our study, there is a statistically significant difference in endothelial cell loss between the two groups. The mean endothelial cell loss was for Visthesia 9.6% and for Viscoat 1.2%. Of note, the endothelial cell decreases for several OVDs are between 0.3% and 20.32%. Our results are comparable with those in other investigations published in the literature [3, 5–9, 16, 20–25].
Another sign of functional damage of the corneal endothelium is the CCT. Corneal thickness increases when the pump and barrier functions of the endothelium are damaged, affecting the clarity of the cornea [24, 26]. If there is a certain decrease in endothelial cells and an increase in corneal thickness, corneal edema appears . There are several studies reporting an acute reversible increase in CCT after phacoemulsification cataract surgery [3, 15, 24, 27]. Kiss et al. found no significantly difference in corneal edema and endothelial cell loss, when comparing Viscoat and 2% hydroxypropyl methylcellulose (Ocucoat®, Bausch & Lomb) . On the contrary, Koch et al. observed better endothelial protection with Viscoat vs. Healon , in parallel with Storr-Paulsen et al. who stated that dispersive OVDs are more protective for the corneal endothelium . The latter is in accordance with our results showing that Viscoat differ significantly in CCT and endothelial cell loss in comparison with Visthesia, supporting the fact that Viscoat provides more protection to the corneal endothelium. Concerning Visthesia, in line with our results, Valimaki et al. noted that Visthesia presented an increased risk for postoperative corneal edema .
Furthermore, an interesting finding of our study was that macular thickness was significantly higher in Visthesia group than in Viscoat one on postoperative day 28. This is the first study examining the possible effect of OVDs on macular thickness. It is well established that foveal thickness could be increased postoperatively [28, 29]. However, according to Johansson et al., intracameral lidocaine did not produce more pronounced macular edema than other methods of anesthesia . In our study, there was a little increase (4%) in macular thickness in Visthesia group, while in Viscoat group macular thickness was lower postoperatively (3% decrease). Nevertheless, macular thickness of both groups was normal and cystoid macular edema was not developed, except for one patient in Visthesia group who was excluded.
Noticeably, postoperative BCVA did not differ between the two groups, although there were differences in ECC, CCT and macular thickness. A possible explanation is that the aforementioned parameters reached normal ranges postoperatively in both groups and therefore did not affect negatively BCVA. It is worthy to say that all patients presented with excellent BCVA on postoperative day 28, suggesting that uneventful phacoemulsification cataract surgery is beneficial for patients' visual acuity.
Concerning intraoperative pain, Visthesia group exhibited less intense pain than patients in Viscoat group. Combining an OVD with an anaesthetic agent is very important, as this viscoelastic concept provides comfort to both patients and surgeons during the operation [9, 31]. However, lidocaine hydrochloride may be the responsible agent for the differences in ECC, CCT and macular thickness between the two groups. Nevertheless, experimental works in the rabbits showed that intracameral use of lidocaine 2% induces few ultrastructure alterations in the corneal endothelial cells . Also clinical studies demonstrated that intracameral lidocaine does not induce significant macular edema than the standard regimen of topical mydriatics plus intracameral lidocaine .
A meaningful limitation of this study pertains to the underlying nature of the comparison presented herein. Specifically, we compared two OVDs with different physicochemical and rheological properties, one with and the other without lidocaine. Therefore, we have reached conclusions encompassing potential confounders i.e., between the presence of lidocaine in Visthesia and the different properties of the two OVDs. In addition, the possibility of recall bias interfering with intraocular pain may not be ruled out, but has essentially been minimized as patients were asked to rate their intraoperative pain immediately after surgery.