EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/119916
April 2013 EW RESIDENTS 63 Cataract tips from the teachers Measure twice, cut once Sherleen Chen, MD Assistant professor of ophthalmology Harvard Medical School Director of Cataract and Comprehensive Ophthalmology Massachusetts Eye and Ear Infirmary Roberto Pineda, MD Assistant professor of ophthalmology Harvard Medical School Director of Refractive Surgery Massachusetts Eye and Ear Infirmary S afe, successful, and accurate cataract outcomes require careful preoperative evaluation and planning. Determining the proper intraocular lens implant for each patient is a crucial aspect of cataract surgery. Technological advances in biometry have improved our refractive outcomes, but with increasing patient expectations and new technology lenses that require precise refractive results for maximal benefit, this remains an area of ongoing work and improvement. For beginning surgeons, this topic may receive less attention than the technical aspects of cataract surgery itself, but it is nonetheless a key component to the larger goal of performing successful cataract surgery. With this in mind, we asked three experts to discuss the key points that they teach trainees about how to assess the quality and adequacy of biometric data and the important process of deciding intraocular lens power. James T. Banta, MD Associate professor of clinical ophthalmology Bascom Palmer Eye Institute University of Miami Miller School of Medicine Refractive accuracy is the most important factor to patient satisfaction following cataract surgery and is paramount to building a successful practice. Although there is no perfect formula and a certain level of potential inaccuracy is inherent in all known formulae, there are many factors to take into consideration when performing and interpreting biometry. First, one must determine the refractive target for the patient. These preop discussions are mandatory to avoid miscommunication and dissatisfied patients. Most patients prefer distance vision correction in both eyes. A thorough discussion regarding the need for postop reading glasses is important; patients do not like to be surprised. Monovision is a highly successful means to provide simultaneous distance and reading correction, but patient selection is crucial. The easiest decision comes with the patients who have previously used monovision with contact lenses. The refractive target for the reading eye is typically between –1.5 and –2.5, depending on the patient's needs. Assessing the patient's visual needs Sherleen Chen, MD, and Roberto Pineda, MD Matt Gardiner, MD Comprehensive ophthalmology Director, emergency ophthalmology services Massachusetts Eye and Ear Infirmary Harvard Medical School, Boston (computer work vs. book reading) will aid the decision. I often aim for around –1.75 to –2.00. I find this allows excellent computer vision and above-average reading vision (especially in good light). Multifocal and accommodative lenses are another option. Laser interferometry has revolutionized IOL calculations. The ease of use and repeatability of measurements make it the first choice for most calculations. I typically utilize the SRK/T and Holladay 1 formulas, often comparing them for similarities. In the patient with average axial length, both are quite accurate. Multiple other formulae have been found accurate in clinical trials (Holladay 2, Haigis, Hoffer Q, etc.). Laser interferometry does have several pitfalls. Posterior subcapsular cataracts and densely brunescent or mature cataracts can prevent accurate measurements. The signal to noise ratio (SNR) must be examined on each and every exam to make certain the results are valid. If the SNR is low, the results should only be used if the retinal spike is clear and the measurements repeatable. If results are not attainable or the SNR is too low with no clear retinal spike, A-scan biometry should be performed. A-scan biometry remains an excellent means to measure axial length, but is much more operator dependent than laser interferometry machines and is more difficult to perform in patients with silicone oil or posterior staphyloma. Major items to consider with biometry: 1. History of refractive surgery a. This simply cannot be missed given its impact on refractive accuracy. b. Special formulae must be utilized to make the appropriate adjustments to IOL power. 2. Axial length symmetry with the fellow eye a. It is uncommon to have a large difference (>0.5 mm) between the eyes without a previous history of anisometropia, amblyopia, or scleral buckle procedure. 3. Unusual results a. Very flat (average K<40) or very steep (average K>46) keratometry readings should be readily explainable. b. High levels of astigmatism should be readily explainable (e.g., pterygium, corneal scar). 4. Astigmatism The correction of astigmatism must be considered to maximize uncorrected visual acuity. The use of toric IOLs or limbal relaxing incisions should be considered with astigmatism ≥1.0 diopter. A pterygium that induces significant corneal astigmatism (especially if irregular) should be removed prior to cataract surgery to promote refractive accuracy. 5. Especially long or short eyes Particularly short (<20 mm) or long (>26 mm) eyes require special consideration. Axial length adjustments or aiming for more myopic outcomes as the axial length increases will help offset errors. When in doubt, repeat, repeat, repeat. You will never regret it. Utilize other ancillary tests to confirm unusual results (e.g., topography). Repeated measurements will allow you the peace of mind to proceed in unusual case The most important consideration in any measurement system for patients being evaluated for cataract surgery is accuracy. The refractive outcome is directly related to the quality of the data you put into the calculations. Any of the modalities used for measuring axial length or keratometry can be assessed for data quality—some directly and others indirectly. Keep in mind the formula 2.5xAL – 0.9xK; any error in the axial length makes a much more substantial difference than mistakes made in the keratometry. The IOLMaster (Carl Zeiss Meditec, Dublin, Calif.) has several means of evaluating accuracy. First compare the inter-eye difference— any difference in axial length between the two eyes more than 0.5 mm or certainly 1.0 mm should be confirmed with another testing modality. Next, check the signal to noise ratio—the higher the better. Anything less than 20 or 30 may be questionable. Check the standard deviation of all the measurements taken—this should be 0.05 or better. Check the A-scan tracing: Are the continued on page 64