The chemical composition of the aqueous humor is determined by three active mechanisms: diffusion, ultrafiltration, and active secretion by the ciliary epithelium lining the ciliary processes.1 Human aqueous humor is a hyperosmotic fluid mainly composed of water, proteins, glucose, electrolytes such as chloride, sodium, and bicarbonate ions, and byproducts of the coagulation and anticoagulation pathways. The aqueous humor pH in the human eye ranges from 7.32 to 7.60 according to different studies2,3 and its rate of production is 1.5 to 4.5 µL/min.
The aqueous humor biochemical composition may change under different circumstances. Specifically, higher protein levels have been found in the presence of pathological conditions, such as cataract4 and glaucoma,5 as a consequence of the increased leakage of lens proteins through the capsule or the abnormal permeability of ocular capillaries. The aqueous humor composition may also change after application of topical medications6,7 or an increased temperature during a surgical intraocular procedure. It has been found that conventional phacoemulsification may potentially induce thermal damage8 and thus a higher temperature in the anterior chamber that is proportional to the amount of the employed ultrasound energy.9
Recently, the introduction of femtosecond laser technology, based on the photodisruption of lens tissue, has provided significant advantages in terms of accuracy in cataract surgery.10 However, it is not yet well studied how the process, which includes the formation of plasma and cavitation bubbles, can affect the biochemical composition of the aqueous humor. This study aims to find out if the carbonic acid gas bubbles created by femtosecond laser photodisruption may cause changes in the pH levels of the aqueous humor.
Patients and Methods
A prospective clinical trial was performed at Vista Monza Eye Clinic, Monza, Italy, to evaluate differences in aqueous humor pH during cataract surgery with femtosecond laser and untreated eyes of patients with cataract. The study received the approval of the local ethics committee and the patients provided their informed consent. All procedures followed the tenets of the Declaration of Helsinki.
The study included 29 eyes of 29 patients with cataract surgery performed by a single surgeon between April and June 2014. Inclusion criteria for the study were significant sclerosis of the crystalline lens, absence of concurrent eye disease, no preexisting eye abnormalities, no history of previous eye surgery or trauma, and no systemic disease or pharmacological therapy, especially with topical antibiotics, nonsteroidal anti-inflammatories, or steroidal treatments in the past 3 months.
All patients were randomly assigned to one of two groups. The femtosecond laser group included 15 eyes of 15 patients (9 male and 6 female) with a mean age of 72.3 ± 4.2 years (range: 60 to 82 years) who underwent cataract surgery using the Catalys femtosecond laser platform (Abbott Medical Optics, Santa Ana, CA). This group was subdivided into three subgroups of low, medium, or high according to the surgeon’s impression about the formation of cavitation gas bubbles after docking and the femtosecond laser surgical procedure. The phacoemulsification group included 14 eyes of 14 patients (7 male and 7 female) with a mean age of 73.4 ± 5.3 years (range: 63 to 85 years) who underwent conventional phacoemulsification surgery. All eyes were preoperatively dilated with tropicamide 0.28 mg and phenylephrine hydrochloride 5.4 mg and topical anesthesia was applied in all cases with benoxinate 4 mg/ mL and lidocaine hydrochloride 4 mg/mL.
Patients included in the femtosecond laser group underwent the laser procedure outside the operating room. Predefined surgeon templates were used for selection of the anterior capsulotomy and fragmentation pattern that consisted of a 5.0-mm diameter capsulotomy, 2.2-mm corneal frontal main access, and two bilateral 1.0-mm accesses. The Liquid Optics Interface (Abbott Medical Optics) was placed onto the eye, with a suction ring placed onto the patient’s sclera. The cornea was entirely inside the hollow optic that was filled with balanced salt solution prior to docking. After ensuring that the eye was fixed, anterior capsulotomy, fragmentation, and paracentesis began. Patients who underwent conventional cataract surgery had continuous curvilinear capsulorhexis, hydrodissection, and phacoemulsification following the standard procedure. After successful removal of the lens cortex, an intraocular lens was inserted into the capsular bag of all patients from both groups. No drops were administered after laser surgery in the femtosecond laser group.
Aqueous Humor Sample Collection
At the beginning of the surgical procedure, after lens fragmentation in the femtosecond laser group and before the injection of the ophthalmic viscoelastic device in the phacoemulsification group, a sample of 0.10 to 0.15 mL of aqueous humor was aspirated through the paracentesis from the anterior eye chamber with a 25-gauge needle connected to a tuberculin syringe and placed into an electronic pH meter (HI98128; HANNA Instruments, Villafranca Padovana, Italy).
Statistical analysis was performed using the SPSS 19.0 software package (SPSS, Inc., Chicago, IL). Data were presented as mean ± standard deviation. Differences between groups were tested using the Wilcoxon test. Correlation analysis was performed by calculating the Spearman coefficient. A P value less than .05 was considered significant.
In the femtosecond laser group, the mean time between docking and aqueous humor sample pH measurement was 7.75 ± 0.60 minutes (range: 7 to 9 minutes). This time included: centration and docking approximately 40 seconds, optical coherence tomography image acquisition and program approximately 30 seconds, capsulorhexis approximately 3 seconds, nucleus fragmentation approximately 70 seconds, and paracentesis approximately 25 seconds. Then patients were moved to a different operating room, which took approximately 1 to 2 minutes. Disinfection of periocular skin, sterile sheet application, and disinfection of conjunctival space with povidone iodine (Oftasteril solution; Alfaintes, Napoli, Italy) and aqueous humor sample acquisition and measurement took an additional 3 to 4 minutes. The mean pH value in this group was 6.53 ± 0.09 (range: 6.42 to 6.70) and thus was more acidic compared to reference values of the human aqueous humor pH. The mean pH value in the phacoemulsification group was 7.42 ± 0.07 (range: 7.28 to 7.48) and thus within the normal range of the human aqueous humor pH. Differences in pH between groups were statistically significant (P < .001). Results are summarized in Figure 1.
Box plot diagram showing the mean values of aqueous humor pH in patients undergoing surgery with the femtosecond laser-assisted platform (group 1) and conventional phacoemulsification (group 2). The boxes represent the interquartile range (difference between the upper 75% and lower quartile 25%); the thick black lines, the median; the whiskers, the highest and lowest values that were not outliers or extreme values.
In the femtosecond laser group, no significant differences were observed between the three subgroups defined according to the surgeon’s impression on the formation of cavitation gas bubbles after docking and the femtosecond laser surgical procedure. Mean pH values were 6.55 ± 0.09 (range: 6.53 to 6.57), 6.54 ± 0.07 (range: 6.52 to 6.55), and 6.42 ± 0.00 (range: 6.40 to 6.44) in the low (7 eyes), medium (6 eyes), and high (2 eyes) subgroups, respectively. Likewise, there were no significant differences between subgroups in the docking time. No significant correlation was found in this group between the docking and aqueous humor sample measurement time and the pH (r = 0.460, P = .085).
The pH is a key regulator of the enzymatic activity and different cellular processes, such as the transportation of ions and fluids across nervous system and muscle cellular membranes.11,12 The aqueous humor has a physiologic pH of approximately 7.38.13
Ocular diseases such as glaucoma, uveitis, retinoblastoma, and cataracts show significant changes in the biochemical composition of the aqueous humor.14 Specifically, the progression of cataract provokes changes in the aqueous humor lipid peroxidation markers, superoxide dismutase, proteins, and antioxidant levels related to the leakage of molecules from the lens capsule.15,16 The damage in the trabecular meshwork in glaucoma may be the origin of the release of proteinic byproducts of the apoptosis activation mechanisms to the aqueous humor.17
Femtosecond laser-assisted cataract surgery has been developed to avoid or minimize complications associated with conventional phacoemulsification, specifically to achieve a more precise capsulotomy to provoke less damage to the corneal endothelium because the effective phacoemulsification time is lower and consequently causes less anterior chamber inflammation.18 However, femtosecond laser-assisted cataract surgery produces cavitation and carbon dioxide gas bubbles as a consequence of the photodisruption process of the crystalline lens tissue.
Recent studies note that femtosecond laser-assisted cataract surgery increases prostaglandin levels in the aqueous humor, mainly during capsulotomy, that may cause intraoperative miosis.19,20 We tried to study chemical effects of carbon dioxide production during femtosecond laser procedures. The behavior of the carbon dioxide in the aqueous humor is determined by Henry’s law, which explains, in this case, the chemical interactions between carbon dioxide and aqueous humor at constant pressure and temperature: “at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.” According to Henry’s law, carbon dioxide becomes carbonic acid, and water + carbon dioxide becomes carbonic acid that dissolves itself in hydrogen and bicarbonate, lowering pH. The conversion from carbon dioxide to carbonic acid is directly proportional to carbon dioxide pressure and time until a steady state is achieved. Ocular carbon dioxide pressure is not clinically relevant and so we only estimated time between the entire femtosecond laser procedure and aqueous sample measurement.
As expected, our work found a statistically significant acidic shift in the patients in the femtosecond laser group, but this shift was not significantly greater in eyes in which a higher level of gas bubbles was visible after the photodisruption during the surgical procedure. A small number of eyes were included in three gas bubble level groups. An additional relative limitation of our study was the modification of the real aqueous humor pH due to the previous application of mydriatic agents in both groups. Potentially, acidosis may affect essential cellular processes, such as cellular migration and the flow of ions and fluids, and inhibit mitochondrial function, leading to the generation of free radicals.21
Femtosecond laser-assisted cataract surgery leads to acidification of the aqueous humor pH compared to control eyes with cataract. The level of gas bubbles visible after the laser photodisruption does not seem to correlate with this pH shift. It remains unclear whether this acidic shift may significantly affect the function of the adjacent structures and the interaction with topical eye medication such as prostaglandins. In our opinion, this acid shift has no clinical effects because of the short time between acidification and filling the anterior chamber with balanced salt solution during femtosecond laser surgery, and because chronic acidification (eg, pH values of approximately 5.3 pH during therapies with dorzolamide) has no clinical side effects. Further research with larger samples of patients is needed to confirm these results.
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- Sharma RG, Mishra YC, Verma GL, Lal K. A clinical evaluation of oxygen and carbon dioxide values and pH in the human aqueous humor in normal and cataractous eyes. Indian J Ophthalmol. 1983;31:525–527.
- Dias PL. Postinflammatory and malignant protein patterns in aqueous humor. Br J Ophthalmol. 1979;63:161–164. doi:10.1136/bjo.63.3.161 [CrossRef]
- Tripathi RC, Borisuth NS, Tripathi BJ, Gotsis SS. Quantitative and qualitative analysis of transferrin in aqueous humor from patients with primary and secondary glaucoma. Invest Ophthalmol Vis Sci. 1992;33:2866–2873.
- Bito IZ, Davson H, Snider N. The effects of autonomic drugs on mitosis and DNA synthesis in the lens epithelium and on the composition of the aqueous humour. Exp Eye Res. 1965;4:54–61. doi:10.1016/S0014-4835(65)80010-0 [CrossRef]
- Reyes MR, Cheng Q, Chuang PY, Lee DA. The effect of antiglaucoma drugs on rabbit aqueous humor proteins determined by gel electrophoresis. J Ocul Pharmacol Ther. 1998;14;229–237. doi:10.1089/jop.1998.14.229 [CrossRef]
- Osher RH, Injev VP. Thermal study of bare tips with various system parameters and incision sizes. J Cataract Refract Surg. 2006;32:867–872. doi:10.1016/j.jcrs.2005.06.054 [CrossRef]
- Suzuki H, Oki K, Igarashi T, Shiwa T, Takahashi H. Temperature in the anterior chamber during phacoemulsification. J Cataract Refract Surg. 2014;40:805–810. doi:10.1016/j.jcrs.2013.08.063 [CrossRef]
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- Edelhauser HF, Ubels JL. Cornea and sclera. In: Kaufman PL, Alm A, eds. Adler’s Physiology of the Eye, 10th ed. St. Louis, MO: Mosby; 2003:89.
- Hadjistilianou T, Giglioni S, Micheli L, et al. Analysis of aqueous humor proteins in patients with retinoblastoma. Clin Experiment Ophthalmol. 2012;40:e8–e15. doi:10.1111/j.1442-9071.2011.02711.x [CrossRef]
- Miric DJ, Kisic BM, Zoric LD, Miric BM, Mirkovic M, Mitic R. Influence of cataract maturity on aqueous humor lipid peroxidation markers and antioxidant enzymes. Eye (Lond). 2014;28:72–77. doi:10.1038/eye.2013.207 [CrossRef]
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- Abell RG, Allen PL, Vote BJ. Anterior chamber flare after femtosecond laser-assisted cataract surgery. J Cataract Refract Surg. 2013;39:1321–1326. doi:10.1016/j.jcrs.2013.06.009 [CrossRef]
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- Schultz T, Joachim SC, Stellbogen M, Dick HB. Prostaglandin release during femtosecond laser-assisted cataract surgery: main inducer. J Refract Surg. 2015;31:78–81. doi:10.3928/1081597X-20150122-01 [CrossRef]