EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/119916
64 EW RESIDENTS April 2013 Measure continued from page 63 peaks sharp and the baseline smooth? Compare the Ks to those found via the autorefractor. The Lenstar (Haag Streit, Mason, Ohio) uses similar technology to the IOLMaster, but may have more accurate Ks since more points on the cornea are tested simultaneously. Most axial lengths will be nearly identical for the two systems. Neither has an advantage in the case of dense cataracts, in my opinion. The one disadvantage of the Lenstar is a relative lack of information on accuracy; there is no signal to noise ratio given as there is in the IOLMaster. Though the A-scan plot can be printed, it may not appear by default, compared to the IOLMaster, which gives you the information up front. Again the goal is a low standard deviation (0.05). Examine the peaks and baseline for consistency. Compare the Ks to those found via the autorefractor (although trust the Lenstar Ks more). We've got you covered INSURE UP TO $3,000,000* UNDERWRITING REQUIREMENTS Medical: Paramed Exam, Full Blood Profile, Urinalysis BUSINESS OVERHEAD EXPENSE Covers daily business expenses up to $250,000 per month 916.746.7888 ZIMMERMANWEALTHSTRATEGIES.COM ZIMMERMAN1111@MSN.COM *65% of your earned income If all else fails, turn to manual A-scan for axial length or for confirmation of a discrepancy. Evaluate the baseline and the peaks for chatter. Assure confidence that the measurements were taken with the patient fixating straight ahead and the probe perpendicular to the cornea. Check the standard deviation as in the other modalities. As a general rule, the shortest axial lengths will be achieved with the contact A-scan since there is some axial compression of the eye and the sound waves stop at the vitreoretinal interface. Immersion will be slightly longer since there is no contact. The IOLMaster and Lenstar are non-contact methods and measure back to the RPE, so these will result in the longest and probably physiologically most accurate numbers. Reputedly, the most accurate formula to use is the Holladay 2, but it requires an accurate refraction Robin R. Vann, MD Chief, comprehensive ophthalmology Assistant professor of ophthalmology Duke University School of Medicine Durham, N.C. Successful outcomes in cataract surgery often begin well before the operating room procedure. A systematic approach to biometry and IOL calculations is critical to ensure the success of surgery. I strongly encourage using optical biometry for precise and accurate axial length and keratometric measurements. It is important to take a large sample of axial length measures with very little variation between each measure (0.04 mm for four measures ideally) and good signal capture of retinal pigment epithelial layer for each measure. For keratometric measurements, review the photographic capture of each keratometric measurement and look at the quality of the image dots that are projected onto the cornea surface. Try to obtain all keratometric measures within 0.25 D of each other for each meridian measurement of the eye before acceptance. Don't be afraid to throw out poor measures and repeat measurements until you are satisfied. In addition to these single eye capture refinements, it is useful to apply a data screening validation tool to ensure the accuracy of the measure- that may be difficult to obtain. It also uses lens thickness, horizontal white to white that influence the ELP. SRK/T is the best for long axial lengths and Hoffer Q best for short. Putting non-contact method axial lengths into a calculating spreadsheet designed for ultrasound-obtained measurements may result in surprises. Most doctors choose a target after a careful discussion with the patient. A safe bet is to shoot for –0.25 or at most –0.5 in order to avoid the dreaded hyperopic outcome, but trying to balance the two eyes in the case of a unilateral cataract must also be considered. Most patients tolerate only 2-3 D of anisometropia. Monovision is a viable choice for many, but should be carefully explained in detail. It is ideal for those who have tried it already with contact lenses, but a trial with contacts prior to surgery can help for those who are unsure. ments. Based on a large review of optical biometry measurements in the U.K., Knox Cartwright et al. found that it was worth repeating measurements by a second observer if: 1. Axial length <21.30 or >26.60 mm 2. Average corneal power <41.00 or >47.00 D and cylinder >2.50 D 3. Between eyes: asymmetry of AL >0.70 mm 4. Between eyes: mean K >0.90 D Once you have good eye measurements, you need to choose an appropriate formula for a good refractive outcome. In eyes with an axial length within 22 to 24.5 mm (72%), the current third generation formulas (SRK/T, Holladay 1, Hoffer Q) all accurately determine the estimated intraocular lens positions (ELP) for a good refractive outcome. In mild axial myopes, defined as eyes between 24.5 and 26 mm in length (15% of eyes), the Holladay 1 formula is very accurate, with the SRK/T formula a close second. In moderate to extreme axial myopia (> 26 mm; 7% of eyes), optical biometers inaccurately measure the true axial length due to refractive index changes and require correction of the axial length value by formula. Wang and colleagues have published their findings as follows: • Holladay 1 2-center optimized AL = 0.8814 x IOLMaster AL + 2.8701 • Haigis 2-center optimized AL = 0.9621 x IOLMaster AL + 0.6763 • SRK/T 2-center optimized AL = 0.8981 x IOLMaster AL + 2.5637 • Hoffer Q 2-center optimized AL = 0.8776 x IOLMaster AL + 2.9269 In hyperopic eyes (<22 mm; 8%), the Haigis (optimized on optical biometry for one constant) or Hoffer Q formulas are the most accurate. With these tips, you can begin tracking your outcomes and optimize continued on page 66