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IOL calculations in extreme eyes require meticulous attention
Multivariable formulae may be more accurate in calculating IOL power.
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To achieve the goal of refractive cataract surgery, we need to be able to deliver a consistent and predictable result with our lens calculations. The primary factors in lens calculation accuracy are often thought to be primarily due to biometry issues: If the axial length and keratometry values are accurately measured, then we should be able to calculate the best power IOL. This may work well in average eyes that exhibit average healing, but what about extreme eyes?
 Uday Devgan |
Eyes with large degrees of hyperopia or myopia are far less predictable, even with accurate biometry. In these eyes, the final resting position of the IOL, called the effective lens position, is more difficult to determine, which leads to inaccuracies in IOL power calculation. The effective lens position of each IOL tends to be different, depending on lens geometry and construction, and this is reflected in the A-constant. The lower the A-constant, the more anterior the effective lens position – this is why anterior chamber IOLs have A-constants in the 114 range, while posterior chamber IOLs have A-constants in the 118 or 119 range. Some IOLs also have variable lens geometry in which the A-constant varies with the IOL power.
Effective lens position
The Hoffer Q, the Holladay 1 and the SRK/T are two-variable theoretical formulae that indirectly determine the effective lens position. If the eye has a short axial length and a flat K measurement, it is likely to have a shallower anterior chamber, and the IOL is assumed to have a more anterior effective lens position. If the eye has a longer axial length and a steep K measurement, it is likely to have a deeper anterior chamber, and the IOL is assumed to have a more posterior effective lens position (Figure 1).
But what happens in cases in which these assumptions are wrong? We get a postop refractive surprise, often in the direction that we least desire. For example, if a myopic eye has flat Ks but a deep anterior chamber, then the formula may erroneously determine that the effective lens position is more anterior and that a lower power IOL would be good. In the postoperative state, we see that the IOL is, in fact, sitting posterior and its power is too low and the patient is now uncomfortably hyperopic after surgery.
 Figure 1. The effective lens position of the IOL is indirectly determined in the two-variable formulae, and it may result in a postoperative surprise in extreme eyes. |
 Figure 2. Screen shot from the HIC-SOAP software showing IOL calculations in a highly hyperopic eye with a short axial length using the Holladay 2 formula. Images: Devgan U |
To overcome this issue, Jack Holladay, MD, created a new seven-variable formula, the Holladay 2, which takes into account the standard axial length and keratometry measurements, but also incorporates anterior chamber depth, lens thickness, white-to-white size, patient refraction and age. This formula, which tends to be more accurate than the two-variable formulae in extreme eyes, is available as part of the Holladay IOL Consultant/Surgical Outcomes Assessment Program (HIC-SOAP, available at www.hicsoap.com). The HIC-SOAP software (Figure 2) also performs the outcomes assessment required to personalize A-constants and fine-tune personal calculations.
Highly hyperopic eyes
Because of their shorter axial lengths, highly hyperopic eyes are far more sensitive to errors in measurement as well as slight changes in effective lens position. A change in the axial length of just 1 mm, from 20 mm to 21 mm, results in a 5 D change in the calculated IOL power. Using the Holladay 2 and aiming for a slightly myopic postop refraction, such as –0.25 D or –0.5 D, tends to produce more accurate results. In even more extreme cases of hyperopia, in which the calculated IOL power is more than +40 D, piggybacking two IOLs may be necessary. This can also be calculated with the HIC-SOAP software.
Highly myopic eyes
Highly myopic eyes tend to be less sensitive to errors in axial length measurement, so a 1 mm change in axial length from 27 mm to 28 mm results in a 2.5 D change in the IOL power. But the final effective lens position tends to be more posterior than expected, and these patients often end up hyperopic after surgery, even though we aimed for a plano result. Li Wang, MD, PhD, and Doug Koch, MD, have described a formula to adjust the axial length, when it measures 27 mm or more, for more accuracy. In this formula, the revised axial length = (0.8829 × measured axial length) + 2.825, then the IOL calculation is done with the Holladay formula. In addition, aiming for a postop refractive goal of a little residual myopia such as –0.25 D or –0.5 D can help to avoid postoperative hyperopia.
Lens calculations in extreme eyes require more attention to detail, and newer multivariable formulae such as the Holladay 2 may provide a higher level of accuracy. These patients are more likely to need a second procedure to fine-tune the refractive result, but they tend to be the happiest patients because they have gone from having extremely poor vision to finally seeing normally.
- Uday Devgan, MD, FACS, is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills, and Newport Beach, California. Dr. Devgan is Chief of Ophthalmology at Olive View UCLA Medical Center and an Associate Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine. Dr. Devgan can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax: 310-388-3028; e-mail: devgan@gmail.com; Web site: www.DevganEye.com. Dr.Devgan is a consultant to Abbott Medical Optics and Bausch & Lomb, and is a stockholder in Alcon Laboratories and formerly in Advanced Medical Optics.