IOL calculations in highly myopic eyes
Do not use the automated biometer printout to determine IOL power.
Most people consider high myopia to be about –10 D or greater, but what about patients who are –20 D or more? For these patients with very long axial lengths and extreme levels of myopia, cataract surgery can be life-changing, but it is also quite challenging. If your patient has an axial length of at least 30 mm or more than 15 D of myopia, be warned that the IOL power calculations, as well as the surgery, will be more challenging, as seen in Figure 1.
The first consideration is to ensure that the posterior segment is free from dangerous pathology that could lead to vision loss. I strongly recommend sending these patients to a retina specialist before attempting cataract surgery. The retinal periphery will be examined closely, any weak areas can be addressed with laser retinopexy, and the macular status can be evaluated. I value the input of my retinal colleagues so much that I also send the patient back for retinal consultation after the successful cataract surgery.
The IOL power estimation is not nearly as accurate in these eyes with extreme axial lengths. Absolutely do not use the printout of third-generation formulae from your biometer. These patients will often have a negative power IOL, and the lens design will be a meniscus type. Our goal is to err on the side of residual myopia, and in this regard, no matter the method you use for calculation, always aim for at least a little postoperative myopia. Taking a patient from –21.5 D to –2 D is a great success, but having this same patient end up at +2 D is not.
I recommend these methods for IOL power estimation for ultra-high myopes:
- Barrett Universal II;
- Wang-Koch modification to Holladay 1;
- Ladas Super Formula 2.0 with artificial intelligence; and
- Hill-RBF method (if parameters are “in bounds”).
Credit must be given to Dr. Graham Barrett for his elucidation of the issues with calculations when it comes to negative power IOLs.
The Wang-Koch group from Baylor University published groundbreaking work a decade ago that fitted a regression formula to highly myopic eyes to come up with an adjustment to the axial length (AL) to improve accuracy. The adjusted AL = 0.83 × measured AL + 4.27, and then this new value is plugged into Holladay 1 for the IOL power.
Using artificial intelligence algorithms, John Ladas, MD, PhD, has come up with the Ladas Super Formula 2.0 with artificial intelligence to better calculate these challenging eyes. This formula can be accessed for free at www.IOLcalc.com and can track your results and personalize your optimization.
Warren Hill, MD, also has an innovative approach using the Hill-RBF method, and if the data you input is “in bounds,” it delivers a good result for a specific IOL. I encourage you to try all of these methods to see which serves you best.
About 10 years ago, Wolfgang Haigis, PhD, described a method to alter the A-constants of these negative-power IOLs to increase accuracy. This method has since fallen out of favor due to the advances such as the Wang-Koch axial length adjustment. Newer methods such as Ladas Super Formula 2.0, Barrett Universal and Hill-RBF 2.0 will all do a good job. In 2018 and beyond, I disagree with using different A-constants for negative power IOLs. Instead, the best advice is to use the methods that are outlined above.
In our example, the axial length is 36 mm and the patient is noted to have –21 D of myopia. Using the Barrett and Ladas AI formulae, we come up with similar results (Figure 2).
We have chosen a three-piece acrylic IOL in a power of –3 D (minus power) for the right eye to give a postop result of mild myopia of about –0.5 D to –1 D, which should be excellent. Note how different this is from the original biometer printout that called for a –7 D IOL, which certainly would have resulted in an undesirable hyperopic postop surprise. After the surgery, our patient ended up with a postop refraction of –0.5 D spherical equivalent using the –3 D IOL. If we had implanted the –7 D IOL as suggested by the biometer printout, the patient would have certainly ended up hyperopic.
Finally, it is important to have a good 5-mm capsulorrhexis that provides overlap of the 6-mm optic in order to keep the IOL in position. Because these highly myopic eyes are larger, the anterior segment dimensions can make it more difficult to accurately judge the capsulorrhexis size. It is important to avoid using the iris or pupil dilation as a guide to the capsulorrhexis size (Figure 3).
In the future when you come across a patient with ultra-high myopia, remember to always choose the higher IOL power so that you err on the side of myopia. We certainly owe a debt of gratitude to the pioneers who have helped in the IOL power calculations for ultra-high myopic eyes: Jack Holladay, MD, Wolfgang Haigis, PhD, Graham Barrett, MD, Warren Hill, MD, Doug Koch, MD, Li Wang, MD, PhD, and John Ladas, MD, PhD.
Full video and further explanation can be found at www.CataractCoach.com.
- For more information:
- Uday Devgan, MD, is in private practice at Devgan Eye Surgery, Chief of Ophthalmology at Olive View UCLA Medical Center and Clinical Professor of Ophthalmology at the Jules Stein Eye Institute, UCLA School of Medicine. He can be reached at 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; email: firstname.lastname@example.org; website: www.CataractCoach.com.
Disclosure: Devgan reports he is a principal at www.IOLcalc.com, which provides free IOL calculations for all ophthalmologists.