The coexistence of glaucoma and cataract is a common occurrence that requires insight into the diagnosis and management of both conditions. The presence of a cataract can affect the ability to detect glaucoma, and cataract surgery can affect both intraocular pressure (IOP) control and the effectiveness of previously performed glaucoma surgery.

The management of coexistent glaucoma and cataract is a complex issue with several therapeutic options, and there is currently a dearth of clear guidelines based on evidence from the literature. When deciding how to manage patients with coexisting cataract and glaucoma, we must consider the impact of each condition on the diagnosis and treatment of the other, as well as the indications for a combined surgery versus cataract or glaucoma surgery alone.

THE IMPACT OF CATARACT ON THE EVALUATION OF GLAUCOMA

First, we must consider the extent to which the presence of a cataract may compromise the evaluation of the glaucomatous eye. The development of a cataract worsens the mean deviation across all tests of the visual field, including standard automated perimetry, frequency doubling perimetry, and short-wavelength automated perimetry. For this reason, visual field analyses are not considered reliable in cases of coexisting glaucoma and cataract. Also, some studies have indicated that the presence of a cataract may affect the visual field index or glaucoma progression index and the characterization of scotomas.¹ Therefore, the presentation of a cataract may affect the decision to monitor versus operate on the glaucomatous eye.

The existence of a cataract may also obfuscate the evaluation of optic nerve structures and the retinal nerve fiber layer (RNFL). The use of optical coherence tomography (OCT) in assessing the RNFL of patients with cataract can cause an underestimation of the thickness of the RNFL and may lead to a false detection of progression with OCT due to the cataract. According to Mwanza and colleagues, ² thinning of the peripapillary RNFL that is typically characteristic of glaucomatous progression may, in cases of coexisting cataract, be due instead to artifact from advancing cataract.

In another study, colleagues and I assessed the measurement of the RNFL using the GDx scanning laser perimeter (Carl Zeiss Meditec, Jena, Germany) in patients before and after cataract surgery.³We found a definite increase in the RNFL thickness after cataract surgery, and we concluded that the cataract was retarding the signal and leading to an underestimation of the parameters of the RNFL (Figures 1 and 2). Therefore, in the presence of a cataract, there may be a false underestimation of the thickness of the peripapillary RNFL, primarily due to a decrease in the signal-tonoise ratio.

SURGICAL EFFECTS

Effect of trabeculectomy on cataractogenesis. Another important consideration is the potential effect of trabeculectomy on accelerating cataract development in glaucomatous eyes. The Advanced Glaucoma Intervention Study (AGIS) showed a 78% increased risk of cataract in patients who have undergone trabeculectomy.⁴ This heightened risk was determined in AGIS eyes after adjusting for age and diabetes, and it was especially pronounced in eyes with a shallow anterior chamber or a history of uveitis following cataract surgery.

Effect of cataract surgery on IOP. According to a review by Shrivastava and Singh,⁵ there may be a modest, longlasting decrease in IOP following phacoemulsification in some eyes with open-angle glaucoma and ocular hypertension. We must also consider the impact of cataract surgery on eyes that already have a filtering bleb. The literature shows that phacoemulsification has an adverse effect on bleb function, even in surgeries that avoid the area of the bleb, such as temporal clear corneal phacoemulsification.

SURGICAL OPTIONS AND INDICATIONS

In deciding when to perform combined phacoemulsification and trabeculectomy surgery, it is wise to consider the extent of glaucomatous damage, the type of patient, the surgeon's individual expertise, and the number of topical medicines the patient is taking. When evaluating the severity of glaucoma, the surgeon must look at the target pressure that is required for the individual case (Figure 3). Most glaucoma patients I see present at a moderate-to-advanced stage and require an IOP below 15 mm Hg. I also consider whether the patient is compliant with topical medications and whether his or her geographic location is conducive to a two-phased surgery.

Once I decide to proceed with surgery, I have three options: cataract surgery alone; combined cataract and glaucoma surgery; or two-phased surgery (glaucoma surgery followed by cataract surgery, or vice versa; Figure 4).

Cataract surgery alone may be sufficient in cases of elevated glaucoma or ocular hypertension or when the IOP is well controlled with a single drug. It is important to remember, however, that cataract surgery in such eyes requires pharmacologic control of the IOP postoperatively. To control the IOP in these eyes as well as possible, I thoroughly remove all ophthalmic viscosurgical device (OVD) at the conclusion of cataract surgery, and I administer a drop of timolol maleate immediately after surgery before patching the eye.

Trabeculectomy surgery alone is indicated in eyes in which a very low target IOP is desired. The procedure is also appropriate for patients who are poor candidates for combined surgery, including those with:
• advanced glaucomatous optic neuropathy;
• a very high IOP that is not controlled medically;
• a poor prognosis for trabeculectomy, due either to excessive conjunctival scarring or secondary glaucoma such as uveitic or neovascular glaucoma; and
• pseudoexfoliation or a subluxated lens with anticipated vitreous loss. Combined surgery may be warranted for:
• patients with early-to-moderate glaucoma;
• patients with IOP above or at the required target on multiple medications;
• noncompliant patients or those experiencing side effects of medications; and
• patients whose geographic locations preclude returning for a second surgery.

One important factor to consider is the IOP-lowering potential of combined surgery versus trabeculectomy alone. This has been addressed in several studies, which have found that the IOP-lowering capability of trabeculectomy alone is far superior to that of combined phacoemulsification and trabeculectomy. There is an unmet need for a surgical technique for combined cataract and glaucoma surgery with IOP-lowering efficacy similar to trabeculectomy alone. New microsurgical shunts may offer some advantages in terms of lowering complication rates as compared with standard trabeculectomy, but they must be evaluated in terms of long-term IOP control when used in combination with phacoemulsification.

TECHNIQUE RECOMMENDATIONS

According to an evidence-based review,6 there are three important points to remember when considering the surgical technique for glaucomatous eyes:

• In all combined surgery, the use of standard woundmanagement techniques will provide a 2- to 4-mm Hg benefit;
• Two-site surgery is superior to single-site surgery as far as IOP reduction is concerned; and
• Performing cataract surgery after trabeculectomy may compromise bleb function.

My preferred approach for most glaucomatous eyes with cataract is first to perform temporal clear corneal phacoemulsification and strictly monitor the patient postoperatively to prevent and treat any spikes in IOP. After the cataract has been removed, I reevaluate the eye’s IOP and the structure and function of the optic nerve to set a new baseline and try to reach the desired target IOP with topical medical therapy. If the eye’s IOP is not at the desired level despite medical therapy (a maximum of three medications as three eye drops in 24 hours), I plan a secondstage trabeculectomy with my standard wound-management technique. This way, both procedures are standard eye surgeries with predictable outcomes, and I do not need to change my surgical technique. Additionally, performing trabeculectomy is much easier in a pseudophakic eye with a deep anterior chamber.

CONCLUSION

We have learned that trabeculectomy leads to the progression of cataract and a worsening of visual acuity (and therefore patients’ quality of life), and that cataract surgery can complicate the control of IOP after trabeculectomy. Moreover, the IOP-lowering capability of combined surgery is inferior to that of trabeculectomy alone. In most cases, then, it is prudent to perform a staged procedure; perform cataract surgery alone (IOP control with topical ocular hypotensive therapy), establish a new baseline of IOP, verify the structure and function of the optic nerve, and then perform a second-stage trabeculectomy with standard wound-management techniques if required (if the target IOP was not reached with medical therapy or there is evidence of progression).

Tanuj Dada, MD, specializes in glaucoma and phacoemulsification at the RP Centre for Ophthalmic Sciences in the All India Institute of Medical Sciences in New Delhi, India. Dr. Dada states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +91 98 733 36315; email: tanujdada@gmail.com.

1. Ang GS, Shunmugam M, Azuara-Blanco A, et al. Effect of cataract extraction on the glaucoma progression index (GPI) in glaucoma patients. J Glaucoma. 2010;194:275-278.
2. Mwanza JC, Bhorade AM, Sekhon N, et al. Effect of cataract and its removal on signal strength and peripapillary retinal nerve fiber layer optical coherence tomography measurements. J Glaucoma. 2011;20(1):37-43.
3. Dada T, Behera G, Agarwal A, et al. Effect of cataract surgery on retinal nerve fiber layer thickness parameters using scanning laser polarimetry (GDxVCC). Indian J Ophthalmol. 2010;58(5):389-394.
4. Mitchell P, Smith W, Attebo K, Healey PR. Prevalence of open-angle glaucoma in Australia. The Blue Mountains Eye Study. Ophthalmology. 1999;106:2144-2153.
5. Shrivastava A, Singh K. The effect of cataract extraction on intraocular pressure. Curr Opin Ophthalmol. 2010;21(2):118-122.
6. Jampel HD, Friedman DS, Lubomski LH, et al. Effect of technique on intraocular pressure after combined cataract and glaucoma surgery: an evidence-based review. Ophthalmology. 2002;109(12):2215-2224.

TAKE-HOME MESSAGE

• Cataract can lead to an underestimation of the parameters of the retinal nerve fiber layer.
• There is an increased risk for cataract formation after trabeculectomy.
• When deciding whether to combine procedures, the surgeon must consider the extent of the glaucomatous damage, the type of patient, the surgeon’s expertise, and the number of topical medications the patient is taking.