Phacoemulsification has become the most commonly used technique in cataract surgery. Although both the safety and surgical outcome of phacoemulsification have dramatically improved owing to recent technical advances, phacoemulsification-associated corneal injury still remains an important complication. Several factors have been postulated to cause the injury, including excessive duration of the phacoemulsification, trauma due to the impact of the lens nuclear fragments, air bubbles, and mechanical and heat damage. In addition, it has been well established that ultrasound oscillations in water induce acoustic cavitations, which lead to the thermal dissociation of water vapors into hydroxyl radicals and hydrogen atoms.¹ In fact, some experimental studies in ophthalmology have shown that phacoemulsification results in the formation of free radical species.^2-4 Using electron spin resonance analysis, we have also demonstrated the formation of hydroxyl radicals in the anterior chamber during phacoemulsification.⁵ In addition, there are some recent studies that have shown that the phacoemulsification-associated endothelial injury may be caused by free radicals.^6-9 Based on these findings, it seems likely that the corneal endothelium can be damaged by free radicals during phacoemulsification because free radical species, hydroxyl radicals in particular, are highly reactive and can cause lethal injuries to cells.¹⁰ The purpose of this study was to investigate the effects of free radicals on the corneal endothelium with special emphasis on histologic and immunohistochemical analyses after performing ultrasound oscillation in the anterior chambers of rabbit eyes.

Hydrogen (H2) has been reported to scavenge free radicals, particularly the hydroxyl radical (·OH). Ultrasound oscillation in an aqueous solution produces ·OH. Our recent study demonstrated that H2 dissolved in an irrigation solution prevented corneal endothelial damage during phacoemulsification in an animal model. We examined the effects of H2 during clinical phacoemulsification. A single-center, prospective, randomized, double-masked clinical trial. Thirty-two patients who had cataracts of similar nucleus hardness in both eyes (age: 75.4±7.68 years; 17 males, 15 females) were recruited. Phacoemulsification was performed using a solution of dissolved H2 in one eye, and a conventional solution in the contralateral eye. Endothelial cell density (ECD) at the center of the cornea was measured using noncontact specular microscopy preoperatively and at 1 day, 1 week, and 3 weeks postoperatively. Reduction rates of ECD (mean ± standard deviation) were 16.0%±15.7% at 1 day, 15.4%±16.1% at 1 week, and 18.4%±14.9% at 3 weeks in the control group, compared to 6.5%±8.7% at 1 day (P = .003), 9.3%±11.0% at 1 week (P = .039), and 8.5%±10.5% at 3 weeks (P = .004) in the H2 groups. These rates were significantly smaller in the H2 group at all time points. H2 dissolved in irrigation solution reduced corneal endothelial damage during phacoemulsification. This suggests that a considerable part of the corneal endothelial damage during phacoemulsification is caused by oxidative stress, and that H2 is useful in clinical phacoemulsification.

In phacoemulsification, ultrasound induces hydroxyl radical (·OH) formation, damaging corneal endothelium. Whether H₂ can prevent such oxidative damage in phacoemulsification was examined by in vitro and in vivo studies. H₂ was dissolved in a commercial irrigating solution. The effects of H₂ against ·OH generation were first confirmed in vitro by electron-spin resonance (ESR) and hydroxyphenyl fluorescein (HPF). ESR showed a significantly decreased signal magnitude and fluorescence intensity by oxidized HPF was significantly less in the H₂-dissolved solution. The effects of H₂ in phacoemulsification were evaluated in rabbits, comparing H₂-dissolved and control solutions. Five hours after the procedure, the whole cornea was excised and subjected to image analysis for corneal edema, real-time semiquantitative PCR (qPCR) for heme oxygenase (HO)-1, catalase (CAT), superoxide dismutase 1 (SOD1) and SOD2 mRNA and immunohistochemistry. Corneal edema was significantly less and the increases in anti-oxidative HO-1, CAT and SOD2 mRNA expressions were significantly suppressed in the H₂ group. In addition, corneal endothelial cell expressions of two oxidative stress markers, 4-HNE and 8-OHdG, were significantly lower in the H₂ group. In conclusion, H₂ dissolved in the ocular irrigating solution protected corneal endothelial cells from phacoemulsification-induced oxidative stress and damage.

Phacoemulsification and aspiration (PEA) has become the most popular cataract surgery, due to the establishment of safe surgical techniques and development of associated instruments. However, corneal endothelial damage still represents a serious complication, as excessive damage can lead to irreversible bullous keratopathy. In addition to causes such as mechanical or heat injuries, free radical formation due to ultrasound has been posited as another cause of corneal endothelium damage in PEA. Ultrasound in aqueous solution induces cavitation, directly causing water molecule disintegration and resulting in the formation of hydroxylradicals, the most potent of the reactive oxygen species. Considering the oxidative insult to endothelial cells caused by free radicals, their presence in the anterior chamber may represent one of the most harmful factors during these procedures. Indeed, some researchers have recently started to evaluate PEA from the perspective of oxidative stress. Conversely, the major ingredient in ophthalmic viscosurgical devices (OVDs), which are indispensable for maintaining the anterior chamber in PEA surgery, is sodium hyaluronate, a known free radical scavenger. OVDs can thus be expected to provide some anti-free radical effect during PEA procedures. In addition, since commercially available OVDs display different properties regarding retention in the anterior chamber during PEA, the anti-free radical effect of OVDs is likely to depend on behavior during irrigation and aspiration. The present study followed standard PEA procedures in an eye model and measured hydroxylradicals in the anterior chamber using electron spin resonance. The kinetics of free radical intensity and effects of several OVDs during clinical PEA were also demonstrated. These studies may be of significance in re-evaluating OVDs as a chemical protectant for corneal endothelium, since the OVD has thus far only been regarded as a physical barrier. In addition, many reports about corneal endothelium damage during PEA have been published, but objective evaluation of various damaging factors has been difficult. The present assay of free radicals in a simulation of clinical PEA offers the first method to quantitatively assess stress on the corneal endothelium.

PHACOEMULSIFICATION and aspiration (PEA) has become the most popular cataract surgery owing to the establishment of safe surgical techniques and the development of associated instruments. Corneal endothelial damage, however, can still be a serious complication because excessive damage can lead to irreversible bullous keratopathy. Surgeons need to be aware of this particular problem to prevent endothelial damage, especially when functional reservoirs are poor. Several causes that lead to damage have been documented, and include items such as mechanical or heat injuries.

We investigated the effect of a peroxisome proliferator-activated receptor α (PPARα) agonist after corneal alkali injury. Fenofibrate 0.05% (PPARα agonist group) or vehicle (Vehicle group) was topically instilled onto the rat cornea after injury. Histological, immunohistochemical, and real-time reverse transcription PCR analyses were performed. PPARα-positive cells were observed among basal cells of the corneal epithelium in normal and alkali-burned corneas. The number of infiltrating neutrophils and macrophages at the corneal limbus was lower in the PPARα agonist group. Interleukin-1β (IL-1β), IL-6, IL-8, monocyte chemoattractant protein-1 (MCP-1), and vascular endothelial growth factor-An mRNA expression was suppressed in the PPARα agonist group compared to the Vehicle group. mRNA levels of nuclear factor kappa B (NF-κB) in corneal tissue were not different. However, NF-κB was expressed in the cytoplasm of basal cells in the PPARα agonist group and in the nucleus in the Vehicle group. MCP-1 was more weakly expressed in the PPARα agonist group. The PPARα agonist inhibited inflammation during the early phase after injury. Anti-inflammatory effects of the PPARα agonist included prevention of up-regulation of proinflammatory cytokines and MCP-1, and prevention of inflammatory cell infiltration into the injured cornea. Thus, a PPARα agonist may be a promising treatment for corneal injury.