IOL power calculation remains problematic in small eyes

This study provides a stark reminder that refractive outcomes in small eyes remain scarcely predictable, even when gold-standard optical biometry and latest-generation IOL power prediction formulas are used. We should discuss with patients before cataract surgery the difficulty of achieving target refraction in small eyes to limit patient frustration and dissatisfaction.

The authors of this retrospective review assessed the accuracy of the Hoffer Q formula for standard cataract surgery in small adult eyes when the IOLMaster (version 5.4) was used to obtain axial length measurement and corneal power assessment. They then evaluated whether the adoption of a different IOL power prediction formula would have yielded better outcomes.

The Hoffer Q formula led to good or fair refractive outcomes in less than two-thirds of the cases. Had the Holladay 1 or 2 or Haigis formula been used, outcomes would have been similar. The SRK formulas yielded less accurate predictions.  

Of all the possible factors influencing the accuracy of refractive prediction, the authors underscored the need for a different IOL labeling system for the highly powerful implants required in very small eyes. IOLs manufactured in the most commonly used power ranges (≥15.0 D to ≤25.0 D) have a true dioptric IOL power within ±0.40 D of the labeled power, with the permitted IOL tolerance becoming as great as ±1.00 D for powers greater than 30 D.

To complicate matters, only a few intraocular implants are available in high powers, and they are often provided in 1 D increments for powers higher than 30 D, which further limits the possibility of getting close to emmetropia.

The author believe that labeling the IOLs with the true dioptric power rather than with the nominal dioptric power could allow more meaningful analysis of the obtained outcomes and further refinement of the employed formulas or algorithms in cataract surgery for microphthalmic eyes. This might also lead to better refractive results for a category of patients who generally undergo cataract surgery earlier in life and are, therefore, more demanding.

For the time being, and until such data are obtained, they offer a practical suggestion: calculate the required IOL power with the Hoffer Q, Haigis and Holladay 2 formulas and choose the most sensible option.