Minimizing Endothelial Damage
Before a surgeon can master the techniques to minimize endothelial damage, he must learn to identify and describe the factors involved in endothelial cell loss after cataract surgery. These parameters include the presence of preoperative risk factors for endothelial decompensation, cataract removal technique (ie, extracapsular cataract extraction vs phacoemulsification), phacoemulsification technique (ie, coaxial vs biaxial), nuclear fractioning technique (ie, divide and conquer vs chopping), incision site, ultrasound time, and the ophthalmic viscoelastic device (OVD) and irrigating solution used.
Endothelial cell loss after cataract surgery may vary from 4% to 25% in most cases.1 In patients followed for 10 years after cataract surgery, Bourne et al2 found that endothelial cell loss continued at a rate of 2.5% each year—four times the physiological annual rate.
PREOPERATIVE RISK FACTORS, INTRAOPERATIVE COMPLICATIONS
In patients with a coexisting disease or a procedure that compromises the endothelium (eg, Fuchs dystrophy, penetrating keratoplasty), accurate preoperative assessment is crucial. These examinations provide the appropriate patient information needed to identify the risks for postoperative endothelial decompensation. Preoperative assessments are also highly important for older patients, who naturally have a decreased endothelial pump function, as well as patients with a shallow anterior chamber, history of diabetes, previous ocular trauma or surgery, or inflammation. The preoperative factors mentioned above all pose a risk to the endothelium.
When any indication of compromised endothelial cell function is present, it is mandatory to modify patient expectations. I have found that pachymetry and slit-lamp biomicroscopy provide not only an accurate assessment of the risk of endothelial decompensation but also a resource for surgical planning. The slit lamp helps to visualize any epithelial edema or stromal folds and to grade the density of the cataract.
Complications during cataract surgery may also predispose a patient to endothelial cell loss. Adverse effects may occur if the capsule breaks or if too much vitreous is lost during surgery. Sterilization problems or detergent residue in the cannula may also cause toxic endothelial cell destruction3,4 and toxic anterior segment syndrome.5
TOOLS TO MINIMIZE DAMAGE
Irrigating solutions. A well-balanced and safe irrigating solution is an essential component during cataract surgery. The formula should resemble the natural chemical composition of the aqueous humor. Edelhauser et al6 were the first to show that irrigating solution is more important to endothelial survival than irrigation time. Choosing a solution with a pH between 6.7 and 8.1 and an osmolality between 270 and 350 mOsm will help maintain the endothelial ultrastructure. Keep in mind that the chosen irrigation solution should have an adequate buffer and substances to maintain the endothelial junctional integrity. An alternative to Ringer's lactate solution, which causes corneal swelling, is balanced salt solution (BSS Plus; Alcon Laboratories, Inc., Fort Worth, Texas). BSS Plus provides superior protection to the endothelium (Figure 1).7 Used in prolonged surgeries or in patients with a compromised cornea, it may also decrease the risk of corneal damage.8
OVDs. How well an OVD performs depends on its viscosity, concentration, and molecular weight. OVDs with a viscosity 305 mOsmol/kg or greater (ie, cohesive OVDs) are preferred in patients with a compromised corneal endothelium.9 Cohesive OVDs are useful for manipulating tissue and maintaining space. OVDs with a low viscosity (ie, dispersive OVDs) adhere to the corneal endothelium10 and provide protection against damage caused by air bubbles.11,12
Arshinoff et al13 suggested that the combination of sodium hyaluronate plus chondroitin sulfate better protect the endothelium compared with sodium hyaluronate alone. In the soft-shell technique, a dispersive OVD is injected next to the endothelium, followed by injection of a cohesive OVD (Figure 2), which pushes the dispersive OVD against the endothelium. To continually protect the endothelium during surgery, reinjection of the dispersive OVD may be necessary. Use of the soft-shell technique in grades 3 and 4 cataracts reduced the amount of endothelial cell loss at 2 and 3 months compared with use of a single OVD (12.2% vs 19.6% and 6.4% vs 16.3%, respectively).14,15 A combination of cohesive and dispersive OVDs provides longer retention time with use of high flow rates and better anterior chamber maintenance compared with a dispersive OVD alone.
Phacoemulsification. Techniques that require lower energy cause less endothelial cell loss. According to Fine et al,16 the most important predictor of corneal clarity on postoperative day 1 is the amount of energy used. All current phacoemulsification platforms use power modulations that decrease the amount of ultrasound energy micropulses (ie, cycling the energy on and off) or microbursts (ie, delivery of identical ultrashort bursts of energy). Both methods of energy reduction decrease the heat generated at the phaco tip and more efficiently cut the cataract.
Recently, microincision cataract surgery (MICS) has become a popular mode of surgery; however, it has not been shown to induce less endothelial cell loss compared with conventional coaxial phacoemulsification.17-21 Torsional ultrasound (Ozil; Alcon Laboratories, Inc.) is another relatively new tool that may actually decrease the amount of endothelial damage created by phacoemulsification. In a randomized study of conventional coaxial phacoemulsification versus torsional phaco, lower ultrasound energy levels and time of phacoemulsification were noted in the torsional group.22 Liu et al22 also noted that endothelial cell loss was lower and visual recovery was faster with torsional ultrasound.
SURGICAL TIPS
Toxic endothelial cell destruction. Profound corneal edema may occur less than 24 hours after surgery if toxic endothelial cell destruction—which may be caused by topical antiseptic solutions, intraocular medications, preservatives, detergent residues, or a faulty sterilization procedure—is present. Surgeons should remember that complications may arise from an inadequate cleaning and sterilization process. One consideration is to restrict the use of reusable cannulas, as small-gauge cannulas retain toxic detergents and cleaners more easily. With use of viscoelastics, the retention of a toxic substance inside the cannula is more frequent. If reusuable cannulas are necessary, recommended cleaning procedures should be followed.
Surgical factors. In a prospective randomized study of 500 patients who underwent cataract surgery with either phacoemulsification or extracapsular cataract extraction,23 mean endothelial cell loss rates were virtually the same (10%) at 12 months. Endothelial cell loss among patients with harder cataracts, however, was higher in the phaco group (18.9% vs 11.8%). Factors associated with greater than 15% of endothelial cell loss in all patients included firmness and size of the nucleus, age, capsular break, vitreous loss, and a greater infusion volume.
Following is a list of surgical considerations to potentially minimize endothelial cell damage:
- Scleral tunnels produce lower endothelial cell loss at the incision site compared with clear corneal incisions;24
- Larger capsulorrhexes in hard cataracts facilitate adequate manipulation for nucleus removal;
- Visibility is increased when trypan blue is used in dense cataracts;
- The nucleus should be completely hydrodissected before initiating rotational procedures;
- Prior to endocapsular phaco, mechanical cleavage of the nucleus will reduce the amount of ultrasound energy required;
- Phaco in the iris plane may induce as much as 10% to 35% endothelial cell damage; and
- Prolapse of the nucleus into the anterior chamber may harm the endothelium because of the mechanical contact between the nuclear fragments and the endothelium. This is the principal cause of endothelial injury.
CONCLUSION
Minimizing endothelial cell damage during cataract surgery is challenging; however, with careful surgical planning, it is attainable. Surgeons should remember to perform preoperative assessment and note any predisposing factors for endothelial decompensation before deciding upon the surgical technique. Discuss all risk factors with the patient before surgery. During the procedure, use well-balanced irrigating solutions and protective OVDs to prevent further damage. In compromised corneas, the soft-shell technique is preferred.
Rudy M.M.A. Nuijts, MD, PhD, is an Associate Professor of Ophthalmology in the Department of Ophthalmology at Academic Hospital Maastricht, Netherlands. He states that he has no financial interest in the products or companies mentioned. Dr. Nuijts may be reached at tel: +31 43 3875343; fax: +31 43 3877346; rnu@soog.azm.nl.
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