The global prevalence of diabetes is estimated to exceed 220 million people by the year 20101 and 300 million by the year 2025.2 Diabetes is the most common risk factor for cataract development in underdeveloped countries.3 Furthermore, diabetic patients suffer lens opacities at an earlier age than individuals without diabetes. The Framingham Eye Study and the Health and Nutrition Examination Survey showed that the rate of cataract was three- to fourfold higher in diabetic patients who were younger than 65 and up to twofold higher in patients older than 65 compared with patients who were not diabetic.3 The Wisconsin Epidemiological Study of Diabetic Retinopathy4 reported that cataract was the principal cause of legal blindness in adult-onset diabetics and the second most common cause of legal blindness after proliferative diabetic retinopathy in those with juvenile-onset diabetes.

Cataract surgery is indicated when a patient's visual function is significantly reduced as a result of lenticular opacity. Surgical intervention may also be indicated if a cataract reduces the ophthalmologist's view of the retina, thus impeding the diagnosis and treatment of diabetic retinopathy.

CONSIDERATIONS IN DIABETIC PATIENTS

Cataract surgery in diabetic patients has been associated with a higher incidence of postoperative complications, including fibrinous uveitis,5 posterior capsular opacification,6 neovascularization of the anterior segment,7 an accelerated progression of retinopathy,8 and macular edema.9 Thus, it is important for the surgeon to address the preoperative management of diabetic retinopathy, the impact of surgery on the disease's progression, the appropriate timing of surgery, and the optimal management of postoperative macular edema.

Before initiating surgery, every effort should be made to stabilize diabetic retinopathy with an appropriate laser treatment. The Early Treatment of Diabetic Retinopathy Study (ETDRS) report No. 25,10 which assessed diabetic patients' visual results after surgical removal of the crystalline lens, found that eyes assigned to early photocoagulation had more favorable outcomes compared with eyes assigned to deferred laser photocoagulation.

Earlier reports have indicated that patients with preexisting diabetic retinopathy who undergo extracapsular cataract extraction have a less favorable visual prognosis due to their developing neovascular glaucoma,7,11,12 vitreous hemorrhage associated with proliferative diabetic retinopathy,11 preexisting macular edema,13 and postoperative macular edema.8,14,15 Patients with clinically significant macular edema (CSME) or proliferative diabetic retinopathy therefore should receive prompt treatment prior to surgery as recommended by the ETDRS16 and Diabetic Retinopathy Study.

CATARACT SURGERY'S EFFECT ON DIABETIC RETINOPATHY

The effect of cataract surgery on the progression of diabetic retinopathy is a somewhat controversial topic. The rate of the pathologic condition's postoperative progression may be influenced by a number of variables, including (1) the severity of the preoperative diabetic retinopathy,5,11,17-20 (2) how long the patient has had diabetes,17 (3) the adequacy of the patient's glycemic control,17 and (4) the cataract surgeon's operative techniques.21

A review of the literature reveals a wide range in the rate of progression for diabetic retinopathy after cataract surgery, from 15% with experienced surgeons as reported by Mittra et al19 to more than 70% according to Jaffe et al.8 In 1992, Jaffe et al8 conducted a prospective, paired-eye study of 19 diabetic patients that showed a high association between cataract extraction and the asymmetric progression of diabetic retinopathy. The ETDRS report No. 25,10 which followed 270 eyes, also suggested a trend toward accelerated retinopathy in operated eyes compared with fellow unoperated eyes.

Two small, prospective, paired-eyed studies by Henricsson et al17 (35 patients) and Squirrell et al22 (50 patients) showed a similar rate of progression for diabetic retinopathy in both operated and unoperated eyes. In both studies, individuals who experienced postoperative progression of the disease had higher levels of hemoglobin A1C. Based on this information, an effective preoperative glycemic control may be important to preventing the postoperative progression of diabetic retinopathy.

The severity of patients' preoperative diabetic retinopathy also appears to contribute to their risk of postoperative progression. Several studies19,23,24 have found that diabetic patients without diabetic retinopathy preoperatively showed no increase in the rate of diabetic retinopathy's progression in their operated eye compared with their unoperated one. Clear evidence from prospective and large retrospective studies showed that the preoperative presence of nonproliferative diabetic retinopathy or proliferative diabetic retinopathy is strongly linked with the postoperative progression of diabetic retinopathy.8,9,15,19,25

CATARACT SURGERY'S EFFECT ON
MACULAR EDEMA

Macular edema is a common cause of poor visual acuity after cataract surgery in diabetics. Because it is difficult to discern between diabetic maculopathy and pseudophakic cystoid macular edema (Irvine-Gass syndrome) in postoperative cataract patients, it is important for ophthalmologists to understand the natural history of macular edema after cataract surgery in order to treat the condition adequately.

In 1992, Pollack et al9 conducted the first prospective, controlled trial to evaluate the natural history of macular edema in diabetics after cataract surgery. Twenty-two of the 44 eyes developed macular edema. Half of the eyes that developed macular edema improved without laser treatment, and only five eyes required laser treatments. In 1999, Dowler et al26 reported a 56% incidence of new, clinically detectable macular edema during the first year after surgery, with the peak incidence occurring at week 6. Spontaneous resolution occurred in 50% of the affected eyes within 6 months and in 75% within 1 year after the surgery.

Furthermore, the ETDRS report No. 25 found no long-term increase in macular edema after cataract surgery in diabetics after 4 to 9 years of follow-up.10 Because these studies suggest that most cases of postoperative macular edema follow a relatively benign course, early laser treatment for all diabetics with postoperative macular edema may not be necessary. Some investigators propose delaying laser treatment for up to 6 months to allow Irvine-Gass syndrome to resolve.20,26

The natural history study of macular edema after cataract surgery by Dowler et al26 showed that eyes with CSME at the time of cataract surgery are more likely to develop persistent CSME throughout the first postoperative year. In these eyes, the angiographic macular fluorescence that developed after the surgery was less likely to resolve spontaneously, and it significantly compromised visual acuity after 1 year.

For eyes with CSME in which lenticular opacity precludes adequate preoperative treatment, the surgeon must examine the retina as soon as he can view it after surgery. Eyes with any CSME present should undergo treatment immediately; conservative management (expecting it to resolve spontaneously) may be inappropriate. Randomized, controlled trials are currently underway to evaluate whether visual acuity may improve from macular laser treatment applied earlier in the postoperative course.

THE TIMING OF CATARACT SURGERY

Studies conducted in the early 1990s recommended a conservative approach to cataract surgery in diabetics due to their limited visual improvement.8,15,27 In the study by Jaffe et al,8 only 52% of patients achieved a UCVA of 20/50 or better, and 14% achieved 20/25 or better. Just five of 44 eyes achieved a final UCVA that was more than two lines better than their fellow eye. Pollack et al15 reported that only 31% of patients achieved a postoperative UCVA of better than 20/40, and the investigators recommended against cataract extraction in patients with diabetic retinopathy until their vision had deteriorated to between 20/100 and 20/200. Schatz et al27 reported that only 9% of eyes achieved a postoperative UCVA that was better than 20/40. They suggested that patients with diabetes, especially those with any retinopathy present preoperatively, might prefer to defer cataract surgery given these markedly poor postoperative results.

Most recent studies of the postoperative visual outcome of diabetics after cataract surgery report a lower incidence of complications and better visual results compared with the older studies just described.17,19,21,26,28 Recent evidence that earlier cataract extraction yields fewer complications and better visual outcomes in patients with diabetes may have various explanations, including (1) better preoperative management of retinopathy using the guidelines provided by the ETDRS, (2) improved operative techniques that reduce surgical time and associated complications, and (3) better management of systemic factors such as hyperglycemia and hypertension, as suggested by the Diabetes Control and Complication Study and the UK Prospective Diabetes Study.28-31

CONCLUSION

Increasing evidence indicates that the preoperative presence of CSME is the greatest risk factor for poor visual acuity after cataract surgery.21,26 It therefore may not be appropriate to delay surgery until it is impossible to identify or adequately treat CSME preoperatively. To improve visual outcomes, there is a growing trend among ophthalmologists to intervene with cataract surgery before lenticular opacity prevents their recognition and treatment of retinal thickening. After cataract surgery, many physicians have recommended monitoring diabetics closely for at least 6 months in order to intervene with laser photocoagulation as required to prevent visual loss from diabetic maculopathy and other detrimental sequelae of proliferative diabetic retinopathy.22,32 

Sam S. Yang, MD, practices at the California Pacific Medical Center in San Francisco. Dr. Yang may be reached at (415) 972-4600; syang2020@yahoo.com.
Everett Ai, MD, practices at the California Pacific Medical Center in San Francisco. Dr. Ai may be reached at (415) 972-4600; eai@westcoastretina.com.
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