Abstract and Introduction
Abstract
Patients expect their vision to improve after cataract surgery and, increasingly, they expect to be free of the encumbrance of glasses and contact lenses. These expectations are driven by technological advancements throughout the ophthalmic industry and the experiences of family and friends. To meet or exceed patient expectations, cataract surgeons must pay increasing attention to the management of pre-existing corneal astigmatism at the time of cataract surgery. Pre-existing corneal astigmatism is a significant component of preoperative ametropia. To achieve satisfactory postoperative refractive results, it is important to correct pre-existing spherical errors by accurate biometry and intraocular lens (IOL) power calculation, and to manage preoperative corneal cylinder errors by a suitable method. Guided by preoperative corneal topography, patients with varying degrees of corneal astigmatism can be placed into a variety of dioptric categories. These categories dictate a systematic 'stepladder' approach to guide the optimum approach to surgical correction. For patients with less than 1 D of corneal astigmatism, the preferred approach is to place the phacoemulsification incision on the steep corneal meridian. For 1–3 D of corneal astigmatism, single or paired peripheral corneal-relaxing incisions may be used. Alternately, for astigmatic errors of 1–4 D, toric IOLs can be implanted. Astigmatism of 4.5–7 D may be addressed using a combined approach of toric IOL implantation and peripheral corneal relaxing incision placement in the steep meridian of corneal cylinder. Moreover, high-powered toric IOLs, currently available in select European and Asian markets and not currently approved by the US FDA, may be implanted in the USA if an ophthalmologist obtains a compassionate use exemption from the FDA and local Institutional Review Board approval. If visually significant refractive errors are present after cataract surgery, surgical management options include piggyback IOL implantation, IOL exchange, toric IOL rotation, photorefractive keratectomy and laser-assisted in situ keratomileusis. Of these options, photorefractive keratectomy and laser-assisted in situ keratomileusis usually provide the most simple solution to manage suboptimal refractive outcomes. This simple stepladder approach to astigmatism management, which is based on preoperative corneal topography, enables cataract surgeons to achieve excellent unaided visual acuity in the absence of vision-limiting ocular comorbidities.
Introduction
The 'classic' low-order refractive errors in clinical optics are sphere and cylinder. The corneal cylinder is a significant component of preoperative ametropia across the population. Among patients older than 60 years of age, approximately 50% exhibit more than 1.0 D of astigmatism. Hoffer reported that 1770 eyes (23.6%), in a series of 7500 undergoing cataract surgery, exhibited >1.5 D of corneal astigmatism. Ninn-Pedersen et al. have shown that up to 22% of cataract surgery candidates exhibit pre-existing corneal astigmatism of 1.5 D or more. Hoffmann and Hütz demonstrated that, among 23,239 eyes, 8% exhibited >2.0 D of corneal astigmatism and 2.6% exhibited >3.0 D. Residual postoperative corneal astigmatism of 0.75 D or greater may reduce uncorrected visual acuity and induce symptomatic blur, ghosting of images, and halos. To meet patients' expectations for good, unaided visual acuity after cataract surgery, it is important to address sphere and cylinder errors that may affect final refractive outcomes.
Preoperative assessment for cataract surgery should be followed by comprehensive counseling that focuses on a patient's individual vision needs. The refractive part of the discussion should include a review of surgical options, including astigmatism management with peripheral corneal-relaxing incisions (PCRIs) and toric intraocular lens (IOLs), presbyopic multifocal and accommodative IOLs, and the possibility of postoperative laser refractive surgery. It is important to estimate the likelihood of spectacle independence that can realistically be achieved with a given surgical option. Videos or computer-based illustrations may be a useful and effective way of explaining the different approaches. Although some surgeons prefer to use laser-based keratorefractive procedures to correct astigmatism after cataract surgery, we prefer to achieve it at the time of cataract surgery. In our experience, laser refractive surgery is reserved for the <1% of patients with symptomatic residual refractive errors.
Preoperative evaluation of the cataract patient should include a manifest refraction, slit-lamp biomicroscopy, tonometry, fundus examination, ocular biometry, IOL power calculation and computerized corneal topography. Spherical refractive errors are managed by accurate biometry and IOL power calculations. The target refraction is emmetropia in most cases, so that only corneal astigmatic error and 'central adaptive astigmatism' remain. Central adaptive astigmatism is our term for the residual cylindrical correction that a patient may request after corneal and lenticular astigmatism have been surgically eliminated.
Phacoemulsification eliminates the lenticular component of astigmatism. To eliminate the corneal component, the surgeon must determine the amount and meridian of corneal astigmatism and factor in the surgically induced astigmatism (SIA) that will be generated by the cataract incision. The importance of SIA should not be overlooked, especially if the pre-existing astigmatism is low.
Relying on traditional or 'manual' keratometry to characterize a patient's corneal curvature is insufficient. Significant cylindrical errors may exist inside and outside the central 3.2 mm optical zone measured by keratometry. In addition, meridian changes over the entrance pupil and irregular astigmatism may be missed. Using the IOLMaster® (Carl Zeiss Meditec, CA, USA) for corneal curvature measurements can result in similar mistakes. For these reasons, computerized corneal topography is the current standard of care. Topography accurately measures the global corneal astigmatism, quantifies the nature of the astigmatism (i.e., symmetric 'bow tie' vs asymmetric or 'irregular' astigmatism) and identifies the steep meridian of astigmatism.
In the authors' experience, patients implanted with monofocal IOLs usually achieve 20/20 uncorrected visual acuity if the spherical error approaches emmetropia and residual manifest astigmatism is less than 1 D. For patients implanted with multifocal IOLs, the postoperative refractive goal is to achieve less than 0.75 D of residual corneal astigmatism. Final results are also dependent on pupil size and the amount of residual spherical error. Overcorrections and large rotations in the axis of residual astigmatism should be avoided.