Eyeworld

EW NEWS & OPINION 22 February 2017 Pulse of ophthalmology: Survey of clinical practices and opinion by Mitchell Gossman, MD –1.00 sphere. You prefer to do a 50% adjustment, thus you wish to select an IOL power for the left eye that targets +0.50 to compensate. An IOL power for the left eye of +22.50 is very close to this target (+0.56), and you choose this +22.50 power. The patient-specific A-constant method could proceed as follows: Either on your biometer or a web- based IOL formula, you determine which A-constant for that specific patient results in the projected postop refraction being equal to what was actually achieved with the IOL that was implanted (or a percentage of that actual refractive outcome if you wish to employ a partial correction). For example, if you wish to do a 50% adjustment for the unintended –1.00 outcome for the first eye, you would deter- mine what A-constant results in an outcome of –0.50 for the +23.50 IOL power. In Figure 2, you can see the A-constant of 118.63 projects that a +23.50 IOL power for the right eye will result in an outcome of –0.50. Using that A-constant for the left eye, a +22.50 lens is projected to produce a refractive error of –0.02, in agreement with the prior method of eyeballing the IOL power report. I can tell you, from personal expe- rience, that this second method is tedious; there is no implementation of a method to derive the A-constant directly, so you have to use trial and error in a search for the A-constant that produces the result you need. The vergence method would go as follows: With your favorite formula for using the vergence equation to determine the IOL power adjustment for piggyback or IOL exchange, run the formula for your desired adjustment (in this case –0.50 D as a 50% adjustment factor applied to the unintended –1.00 result) as the preop refraction and plano as the desired postop refraction. This may be performed by hand with a mathematical formula, by using the very precise but perhaps unavailable Holladay R formula, or using the formula as implemented in Warren Hill, MD's website in an Excel formula. Figure 3 shows the latter method. The preop information is in place—0.50 is the "preop refraction" ("preop" for this formula refers to the preop refrac- tive error before a planned piggy- back or IOL exchange procedure, Part 2 in a series on the complexities of everyday IOL decisions T his is a continuation of our exploration of the subtleties of IOL power selection in various clin- ical scenarios. See Part 1 in the January issue of EyeWorld. This is derived from a survey of 74 ophthalmologists who volunteered to participate from the ranks of the EyeConnect online community and volunteers in North America. Responses are anonymous in order to encourage candor. Totals may not equal 100% due to rounding. In follow-up of questions one and two, you have determined that you wish to adjust the IOL power for the second eye based upon the result from the first eye's unwanted residual refractive error. The third question was, "You indeed wish to adjust your power for the second eye based upon the refractive outcome of the first eye. How do you do this?" The "Estimation" method is the simplest. To take an example from Figure 1, say you implant a +23.50 IOL in the right eye, and rather than achieving a refractive error of 0.00 sphere, the resulting refraction is Art of IOL power selection: Fudge factors continued and monovision planning Estimation from flanking refractive targets on your biometry printout 79% Formal calculation of a patient-specific A-constant 6% Vergence formula or Holladay to determine hypothetical piggyback lens power aiming for desired adjustment and adjust the second eye's IOL power using this 6% Other (please specify) 10% Mitchell Gossman, MD Figure 1. LENSTAR biometry Figure 2. Online Barrett Universal II formula Source: Mitchell Gossman, MD not your patient's cataract surgery preop refraction), and 0.00 D as your targeted postop refraction. This calls for an adjustment of –0.73 for the actual lens power implanted. You then apply this adjustment factor for the second eye. The mathematics work best if you know from your printout what exact IOL power is projected to result in emmetropia or calculate it by interpolation. In this case, interpolation yields, for the left eye, an IOL power of +23.30 to achieve emmetropia. Applying the –0.73 "fudge factor" to this results in +22.57 as the desired IOL power to use after the 50% adjustment from the first eye's result. Again, this is in close agreement with the prior methods. Obviously, these last two meth- ods are a lot of work compared to the estimation method, which is no doubt why most employ that simpler method, including myself in most cases. Another equivalent method is to find a formula, even an older formula, that shows that the first eye is projected to yield a refractive error equal to what was actually obtained (or a percentage of the actual result if you want to use partial correction) and use the power projected to give emmetropia for the second eye. I use this method often as a tie-breaker for the second eye when emmetro- pia falls between two IOL powers. It may not be immediately obvious, but there is a relationship between these issues of "fudge factors" and selecting which eye to operate upon first when planning on monovision. The second question was, "You are planning surgery desiring mono-

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