Glaucoma is the second leading cause of blindness in the Western world and is generally more prevalent in the elderly, blacks, and Hispanics. It is estimated that by next year, 60 million people worldwide will have some form of glaucoma.¹ Many of these patients will develop cataract because of advancing age or glaucoma treatment.¹ Therefore, it is paramount that cataract surgeons be familiar with how to treat patients with preexisting glaucoma.

In the Advanced Glaucoma Intervention Study, cataract was noted in 46% to 48% of eyes,² and trabeculectomy increased the overall risk of cataract by 78%.³ In patients with concomitant cataract and glaucoma, the cataract may influence visual field testing, which is crucial in monitoring the progression of optic nerve damage. Most studies have shown that cataract extraction with IOL insertion results in improvement of mean deviation of visual fields in normal and glaucomatous eyes.^4-6 In addition to influencing visual fields, cataract may impair high-contrast visual acuity and contrast sensitivity⁷ and induce spherical and other higherorder aberrations (HOAs). Cataract surgery has changed significantly over the past few decades, with the introduction of reduced incision sizes and more sophisticated means of measuring functional outcomes. There have been tremendous advances in IOL design and safety with the aims of maximizing visual acuity, improving contrast acuity, and reducing spherical and other HOAs.

There is a limited amount of published literature about IOL choices in patients with glaucoma. This article aims to evaluate the current evidence that influences IOL choice in patients with glaucoma. We emphasize the effect of IOL choice on color vision and contrast acuity and the effect of cataract surgery on the management of coexisting glaucoma. However, we must first understand the changes in visual function induced by the glaucomatous process, onto which further alterations are induced by cataract formation.

VISUAL FUNCTION CHANGES
Contrast sensitivity. People with glaucoma and glaucoma suspects experience changes in contrast sensitivity. These changes monitor the stability or progression of the disease, particularly in the absence of cataract, age-related macular degeneration (AMD), or other ocular comorbidity. The gradual decrease in contrast sensitivity affects the patient's visual function, despite normal or near-normal Snellen visual acuity. Contrast sensitivity is preferentially affected by glaucoma and correlated with increased visual field loss in patients with 20/40 visual acuity or better.^8,9 A developing cataract further reduces contrast sensitivity.¹⁰ Hence, there is a combined impact of glaucoma and cataract progression on contrast sensitivity.

Color vision. Color vision defects have been noted in primary open-angle glaucoma (POAG). The prevalence of normal color discrimination in POAG is 20% to 40%; it is between 30% and 50% for blue-yellow defects, 5% for redgreen defects, and 20% to 30% for a general loss of chromatic discrimination.^11,12 Color vision defects have been noted in primary open-angle glaucoma (POAG). The prevalence of normal color discrimination in POAG is 20% to 40%; it is between 30% and 50% for blue-yellow defects, 5% for redgreen defects, and 20% to 30% for a general loss of chromatic discrimination.¹³ It is important to remember that cataract affects color discrimination, particularly with the color yellow; yellowing in the crystalline lens increases with age.¹⁴

IOLS, CONTRAST SENSITIVITY, AND COLOR VISION
Aspheric IOLs. Currently, monofocal, multifocal (diffractive, refractive, and bifocal), aspheric, and accommodating lens implants are available. Multifocal refractive IOLs include the ReZoom (Abbott Medical Optics Inc., Santa Ana, California). Diffractive multifocal IOLs include the AcrySof Restor (Alcon Laboratories, Inc., Fort Worth, Texas) and the Tecnis Multifocal (Abbott Medical Optics Inc.). A large range of aspheric IOLs are available; common examples include the Tecnis and AcrySof IQ IOLs, both of which have negative spherical aberration to compensate for the average positive spherical aberration of the cornea. Another approach to IOL aspericity is an aberration-free design, which leaves existing corneal aberration unaltered. IOLs in this group include the SofPort AO and Akreos Adapt AO (both from Bausch & Lomb, Rochester, New York).

IOLs that affect contrast sensitivity adversely impact visual function in the real world and disease monitoring in the hospital environment. It is known that conventional monofocal, planoconvex, and biconvex IOLs can introduce spherical aberration and thus decrease image quality.¹⁵

The most significant difference between aspheric and spherical IOLs is contrast sensitivity, particularly at mesopic levels; aspheric IOLs provide superior contrast sensitivity. Experience with aspheric IOLs in patients with concurrent eye diseases is accumulating in the literature, particularly in patients with glaucoma. These publications show that aspheric IOLs provide better contrast sensitivity and may be the most suitable for patients with glaucoma and cataract compared with multifocal or standard monofocal IOLs.

Blue-blocking IOLs. Blue-light–filtering IOLs may negatively affect contrast acuity and blue/yellow foveal threshold when compared with UV-only–filtering IOLs in patients without concomitant eye diseases.¹⁶ It is therefore reasonable to assume that these IOLs may negatively affect eyes with concomitant eye diseases, such as glaucoma or AMD.

Multifocal IOLs.
Multifocal IOL use in patients with concurrent eye disease has not been studied extensively. Kamath et al¹⁷ studied 11 eyes with coexisting glaucoma and six with ocular hypertension that were implanted with the Array multifocal IOL (Abbott Medical Optics Inc.; no longer available). In a control group, 12 eyes with glaucoma or ocular hypertension received standard monofocal implants. There were no differences in the outcomes of the multifocal and monofocal IOL groups in patients with glaucoma or ocular hypertension; however UCVA was better in eyes that received the multifocal IOL.

Pieh et al¹⁸ compared tritan color contrast sensitivity, with and without glare, in patients receiving the refractive multifocal Array or a similar monofocal IOL (SA 40N and SI 40NB, respectively). The multifocal IOL influenced the tritan color contrast sensitivity compared with the monofocal.

The data available on the use of multifocal IOLs in patients with concurrent eye diseases, particularly glaucoma, although limited, suggest that this lens type tends to affect contrast sensitivity. However, patients' vision for daily activities is only marginally affected.

Our own investigation¹⁹ concluded that, due to a lack of large studies, firm recommendations could not be made on the use of multifocal IOLs in patients with glaucoma. Future randomized, controlled trials are awaited in this regard.

Some modern multifocal IOL designs incorporate asphericity into the optic; some of the contrast sensitivity loss inherent in multifocality may be counteracted by the better performance of the aspheric optic, thus reducing concerns about the use of these IOLs in glaucomatous eyes.

Accommodating IOLs. Accommodating IOLs offer an attractive option for patients who want spectacle independence, as the photic phenomena associated with multifocal lenses are not an issue. Some doubts remain about the effectiveness of current accommodative designs, but the technology is rapidly improving. Most available accommodating IOLs, including the 1CU (HumanOptics AG, Erlangen, Germany) and Tetraflex (Lenstec, St. Petersburg, Florida) do not have aspheric optics; however, aspheric versions such as the Crystalens AO (Bausch & Lomb) are being developed and should be available for use shortly. These may prove to be the best choices for patients with glaucoma once aspheric optics are incorporated into their design.

VISUAL FIELD TESTING AND DIAGNOSTICS
Glaucoma is monitored with intraocular pressure, visual field, and disc assessment. Visual fields can be affected by cataract, particularly the mean standard deviation on a Humphrey visual field test.²⁰ The correct monitoring parameter is to review the pattern and corrected pattern standard deviation. The literature shows that patients with an aspheric IOL perform better than those with multifocal or standard IOLs. Other forms of assessment such as scanning laser topometry and scanning laser polarimetry may not be affected by cataract surgery and IOL implantation.²¹

CONCLUSIONS
Cataract and glaucoma are concomitant in many patients. This is likely to increase with the aging of the population, especially in the industrialized world. Cataract has an effect on visual field monitoring in glaucoma due to depression of automated threshold parameters, as well as effects on color vision and contrast sensitivity. Removal of cataract is an ideal option for treatment, particularly in patients with stable glaucoma. IOLs have different merits and drawbacks in these patients depending on their individual designs. Because of the limited amount of published data about the use of specific IOL designs in eyes with glaucoma, choice of IOL must be made on an individual basis. Discussion with the patient must include personal requirements, including spectacle independence. Consideration of factors including the stability of the disease, the need for future disease monitoring, and the potential impact on the patient's contrast sensitivity and color vision will allow the surgeon to make appropriate recommendations for each individual.

Based on the available evidence, aspheric IOLs seem to perform better than existing standard monofocal, multifocal, or accommodating IOL designs in patients with stable glaucoma. If spectacle independence is a high priority for the patient, a multifocal IOL with an aspheric design can be considered in the presence of stable glaucoma. In the future, another possibility will be an accommodating IOL with aspheric optics. It is important to bear in mind that baseline parameters may change after cataract surgery. Thus, we recommend that a full glaucoma assessment, including visual fields and other tests such as nerve fiber layer analysis, be repeated a few weeks after cataract surgery. This will benefit long-term disease monitoring by acting as the new baseline.

Balakrishna Vineeth Kumar, FRCSEd(Ophth), practices in the Department of Ophthalmology, Wirral University Teaching Hospital NHS Trust, Arrowe Park Hospital, Wirral, United Kingdom. Dr. Kumar states that he no financial interest in the products or companies mentioned. He may be reached at tel: +33 1516047717; e-mail: drvineeth@gmail.com.

Som Prasad, MS, FRCS, FACS, FRCOphth, practices in the Department of Ophthalmology, Wirral University Teaching Hospital NHS Trust, Arrowe Park Hospital, Wirral, United Kingdom. Dr. Prasad states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +44 1516047193; e-mail: sprasad@rcsed.ac.uk.

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