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Prospective randomized controlled trial to compare the effect on the macula of AquaLase liquefaction and ultrasound phacoemulsification cataract surgery

Barsam, Allon MRCOphth; Chandra, Aman MRCOphth; Bunce, Catey Msc; Whitefield, Laurence A. FRCOphth

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Journal of Cataract & Refractive Surgery: June 2008 - Volume 34 - Issue 6 - p 991-995
doi: 10.1016/j.jcrs.2008.02.017
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Abstract

Cystoid macular edema (CME) after cataract surgery, also known as the Irvine-Gass syndrome, was first described by Irvine in 1953.1 Clinical CME after cataract surgery is one of the most common complications, with a prevalence of 0.1% to 12.0%.2,3 The large difference in reported rates may be due to several factors including the definition of the disease, different surgical techniques and procedures, different rates of surgical complications, methods used for diagnostic assessment, types of patients, and postoperative follow-up criteria.4

The exact pathophysiology of CME after cataract surgery has not been clearly elucidated. However, several factors are known to be associated with an increased risk. If cataract surgery is complicated by posterior capsule rupture and vitreous loss, iris trauma, or vitreous traction, there is a significantly higher incidence of clinically apparent CME.2,3 Cystoid macular edema is a common cause of poor visual acuity after cataract surgery in patients with diabetes.5

The AquaLase liquefaction device is one option available for lens extraction on the Infiniti Vision System (Alcon Laboratories). AquaLase works by propelling short pulses of a balanced saline solution (warmed to 57°C) that liquefy the lens material. Within the handpiece, 4 μL pulses are generated and emerge from an orifice just inside the smooth polymer tip of the instrument. Aspiration of the liquefied lens material occurs through the central or inner lumen of the tip. The balanced saline solution pulses are delivered at a maximum rate of 50 Hz, and the magnitude of the pulses can be linearly controlled by foot-pedal depression.6,7 Potential advantages of the AquaLase system over standard small-incision phacoemulsification for lens extraction is that with the AquaLase, no heat is generated inside the eye and there is no radiating ultrasonic pressure wave. There may also be a reduced risk for posterior capsule rupture.

Optical coherence tomography (OCT), a method of high-resolution cross-sectional imaging of the retina, directly measures changes in the z-plane (depth of the retina).8–11 Optical coherence tomography uses infrared light to detect relative changes in reflection at optical interfaces by the method of low-coherence interferometry; it is a noncontact, noninvasive, safe, and reproducible technique.12,13 Optical coherence tomography can show cystic changes and increased retinal thickness in patients with CME.10,11 Optical coherence tomography has a resolution up to 10 μm and may be able to detect subtle changes in retinal thickness that cannot be seen at clinical examination.11

AquaLase cataract extraction may carry a reduced risk over standard phacoemulsification cataract extraction for the development of postoperative CME. This reduced risk may result from the absence of heat and ultrasonic vibration and consequent iris microtrauma. The purpose of this study was to compare the effect of these 2 cataract surgery techniques on the macula using OCT before surgery and at various times after surgery.

PATIENTS AND METHODS

A randomized controlled trial was performed at the Department of Ophthalmology, Queen Mary's Hospital, United Kingdom. Research ethics committee approval was granted for the study design, and written informed consent was obtained from all patients. Sixty-three patients having cataract surgery were enrolled. One eye of each patient was included as the study eye (ie, eye to have surgery). Measurements were also taken in the fellow eye as a nontreatment control for comparison. Patients were randomized to receive standard ultrasound (US) phacoemulsification cataract extraction and intraocular lens (IOL) insertion (n = 31) or AquaLase cataract extraction and IOL insertion (n = 32).

Randomization was performed using a computer to generate a random sequence of numbers. Allocation was concealed with numbers inside a sealed envelope. The patients and OCT observers were masked. Inclusion criteria were age 45 or older, age-related or diabetic cataract, and the competence and capacity to give informed consent. Exclusion criteria were dense cataract greater than or equal to nuclear color or opalescence 4 on the Lens Opacities Classification System III grading system, previous intraocular inflammation, vitreomacular traction syndrome, active proliferative diabetic retinopathy, argon laser panretinal photocoagulation or focal macular laser within 1 year in either eye, previous intraocular surgery in the study eye, and previous intraocular surgery in the fellow eye within 1 year. The primary outcomes were best corrected visual acuity (BCVA) and OCT central macular thickness. Secondary outcomes were OCT macular volume and perioperative and postoperative complications.

Surgical Technique

All surgery was performed by the same surgeon (L.A.W.) using topical and intracameral anesthesia with lignocaine 1%. After a 3.2 mm clear corneal incision was made and an ophthalmic viscosurgical device (OVD) injected, continuous curvilinear capsulorhexis and hydrodissection were performed. Nuclear removal was by a stop-and-chop technique using US phacoemulsification or AquaLase on the Infiniti Vision System. Phacoemulsification settings for sculpting were as follows: 50% power linear, 40 pulses/s, 45% “on” time, 85 mm Hg vacuum, 25 cc/min aspiration flow rate, and 80 cm bottle height. Phacoemulsification settings for chopping were as follows: burst mode, 50% power linear, 30 ms “on” time, 60 ms “off” time, 400 mm Hg vacuum, 35 cc/min aspiration flow rate, and 100 cm bottle height. Soft lens matter was aspirated using an automated bimanual technique. A foldable MA60AC IOL (Alcon) was inserted and the OVD removed. Perioperative complications were recorded. All patients were given a 4-week tapered dose of dexamethasone 0.1% drops.

Assessment of Macular Thickness

All patients had slitlamp examination of anterior segments and dilated fundoscopy preoperatively and postoperatively at 2 and 6 weeks. At the examinations, the BCVA was measured. Bilateral OCT was performed using the fast macula program of the Zeiss 3.0 OCT machine 2 hours before surgery and 2 and 6 weeks postoperatively (Figure 1, A and B). Values were obtained for central foveal thickness and overall macular volume within a 3.0 mm radius from the center of the fovea.

Figure 1
Figure 1:
A: Optical coherence tomography data generated preoperatively. B: Optical coherence tomography data of the same patient generated 6 weeks after phacoemulsification in the right eye shows macular thickening.
Figure 1
Figure 1:
(continued)

Statistical Analysis

A power calculation was performed for the sample size. The calculation showed that a sample size of 30 in each group would have a 99% power to detect a difference between means of 10.0 with a significance level (α) of 0.05 (2 tailed).

Statistical analysis was performed using the rank sum test. A P value less than 0.05 was considered statistically significant. The data were not normally distributed; thus, the median and interquartile range (IQR) were used rather than the mean and standard deviation.

RESULTS

The AquaLase comprised 17 women (53%) and 15 men (47%) with a mean age of 70.3 years ± 11.4 (SD) (range 45 to 89 years). The phacoemulsification group comprised 16 women (52%) and 15 men (48%) with a mean age of 73.8 ± 9.7 years (range 49 to 87 years). Seventeen patients with type 2 diabetes mellitus, 8 (25%) in the AquaLase group and 9 (29%) in the phacoemulsification group, were included in the study. The mean age of diabetic patients was 66.0 ± 8.1 years in the AquaLase group and 73.7 ± 11.4 years in the phacoemulsification group.

Functional Results

In the AquaLase group, the median BCVA was 0.21 logMAR (IQR 0.18 to 0.38 logMAR) preoperatively and 0.02 logMAR (IQR −0.40 to 0.18 logMAR) 6 weeks postoperatively. In the phacoemulsification group, the median BCVA was 0.26 logMAR (IQR 0.18 to 0.48 logMAR) preoperatively and 0.07 logMAR (IQR 0.02 to 0.20 logMAR) 6 weeks postoperatively.

Optical Coherence Tomography Results

Over the 6-week study period, the median increase in foveal thickness in the study eye compared with that in the fellow eye was 11 μm (IQR −21 to 23 μm) in the AquaLase group and 17 μm (IQR −11 to 33 μm) the phacoemulsification group (P = .229). The median increase in macular volume in the study eye versus that in the fellow eye was 0.17 mm3 (IQR −0.04 to 0.34 mm3) and 0.20 mm3 (IQR −0.05 to 0.42 mm3), respectively (P = .426).

Diabetic Subgroup Analysis

Over the 6-week study period, the median increase in foveal thickness in the study eye compared with that in the fellow eye was 2 μm (IQR −14 to 23 μm) in the AquaLase group and 29 μm (IQR 11 to 41 μm) in the phacoemulsification group (P = .07) (Figure 2). The median increase in macular volume in the study eye versus the fellow eye was −0.04 mm3 (IQR −0.21 to 0.33 mm3) and 0.25 mm3 (IQR −0.05 to 0.34 mm3), respectively (P = .37).

Figure 2
Figure 2:
Box plot of 6-week difference in foveal thickness (μm) between the treated eye and fellow eye of diabetic patients.

Perioperative and Postoperative Complications

There were no perioperative or postoperative complications in the phacoemulsification group. In the AquaLase group, 1 patient (3.1%) had a posterior capsule rupture without vitreous loss that occurred at the irrigation and aspiration phase after the nucleus had been removed with AquaLase. There were no other postoperative complications in the AquaLase group. The mean volume of irrigating fluid used during surgery was 143 ± 57.8 mL in the phacoemulsification group and 145 ± 51.2 mL in the AquaLase group.

DISCUSSION

Cystoid macular edema is a common cause of reduced visual acuity after cataract surgery. Visual loss is normally self-limiting; however, in some patients CME can become chronic (>6 months duration) as well as visually significant (Snellen visual acuity 20/40 or worse).2,4

Optical coherence tomography is a safe alternative to fluorescein angiography for the diagnosis of CME. Optical coherence tomography has been found to be useful for objectively monitoring retinal thickness in patients with CME with a high degree of reproducibility and repeatability.9–12 Several studies show that the extent of macular edema can change with variation in arterial pressure, glycemic control, and circadian cycles.14–16 These are effectively systemic changes that tend to affect both eyes to a similar extent. It is therefore important in this study to use a fellow nontreated control eye and compare the difference between that eye and the treated eye over the study period to reduce the effects of systemic factors.

In this study, there was a greater increase in macular thickness and macular volume in the phacoemulsification group than in the AquaLase group; however, the difference was not statistically significant. A subgroup analysis of diabetic patients also showed a greater increase in macular thickness and macular volume in the phacoemulsification group; again, the difference was not statistically significant. One patient in the AquaLase group had posterior capsule rupture without vitreous loss that occurred during irrigation and aspiration of soft lens matter; the rupture was presumed not to be related to AquaLase liquefaction.

Limitations of this study include a relatively small sample size. However, the repeatability, accuracy, and precision of OCT in the determination of macular thickness suggest that in our initial power calculation, a large sample size was not required. Another limitation of the study is the relatively short follow-up period. However, we know from previous studies that macular thickness measured by OCT can accurately determine the presence of CME10,11 and that OCT macular thickness peaks 4 to 6 weeks after surgery.12,13 Strengths of our study include the randomization and double-masking process.

The results in this study suggest that both methods of cataract extraction are equally safe in terms of the effect on the macula. AquaLase cataract extraction may carry a reduced risk over standard US phacoemulsification cataract extraction for postoperative CME. This reduced risk may result from the absence of heat and ultrasonic vibration and consequent iris microtrauma. This may be particularly evident in those at most risk for developing postoperative CME (eg, diabetics). Further randomized controlled studies with greater numbers of patients that evaluate exclusively diabetic patients or other groups of patients particularly at risk for postoperative CME are necessary.

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