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IOL calculations and IOL selection in small eyes
IOL selection
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The three variables in IOL calculations are the power of the existing cornea, the axial length of the eye and the effective lens position.
It is a challenge in any lens power calculation to predict the final resting position of the IOL based on preoperative measurements. In an eye with a long axial length, the effective lens position (ELP) is not as critical because a small movement of the IOL in the anterior-posterior dimension, coupled with the lower power IOL used in these axial myopes, only mildly alters the final refractive outcome. However, in an eye with a short axial length, the IOL power is typically higher and even a slight change in the ELP can have a significant effect on the refractive results.
Another important consideration is the relative size of the anterior chamber compared to the axial length. In a short eye with a proportionately small anterior chamber, the lens power calculations tend to be more accurate than in an eye with a deep anterior chamber. In a large study done by Jack Holladay, MD, when looking at eyes with short axial lengths, about 20% of these eyes had a small anterior segment size and were classified as nanophthalmic. The remaining 80% of these short-axial length eyes have a normal anterior segment size. The eyes with a shallow anterior chamber depth tend to have IOL powers of +30 D or less, whereas the eyes with the normal anterior chamber depths can have IOL powers of more than +40 D.
The best formulas for small eyes are the ones that most accurately predict ELP. The third generation formulas use just two input variables, the keratometric power and the axial length, to determine the ELP and the power calculation. Of these, the HofferQ tends to be the most accurate when the axial length is less than 22 mm. Fourth generation formulas use multiple input variables in addition to the keratometry and axial length measurement. The Haigis formula requires anterior chamber depth (ACD) as a variable, while the Holladay 2 formula requires ACD as well as white-to-white measurement, lens thickness, refraction and age. These fourth generation formulas, particularly the Holladay 2, are preferred when calculating lens power in small eyes.
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