High to Extreme Axial Myopia

Axial length correction for high to extreme axial myopia.

The way we perform IOL power calculations for high to extreme axial myopes has been evolving, and for some surgeons and their staff, this is an area of confusion. There are currently four approaches in use among surgeons worldwide.

Best Practices.

The one unifying theme for IOL power calculations in the setting of high to extreme axial myopia is that some amount of unanticipated hyperopia appears to occur when using older, 2-variable formulas without taking any special measures.

One highly successful approach to selecting IOL power for these patients is based on a landmark Journal of Cataract and Refractive Surgery paper by Li Wang and Doug Koch at Baylor University. It describes a method for adjusting the axial length for the high to extreme axial myope when the axial length is measured by optical biometry. This would apply to both the Lenstar and the IOLMaster, as the approach of both instruments is almost identical at present.

Note: This approach was recently updated for the Holladay 1 and SRK/T formulas.

One working theory is that optical biometry may exhibit a systematic error in measuring axial length that increases linearly with the measurement. This is because optical biometry assigns a single, global index of refraction to all eyes, regardless of their axial length. The vitreous cavity for the high to extreme axial myope dominates the axial measurement, and the greater the amount of vitreous, the more its index of refraction contributes to the overall measurement. In this setting, optical biometry will overstate the axial length. In other words, the longer the eye, the greater the error. However, others believe that the error is the result of older formulas that are less suited to this task. A definitive answer remains to be established.

In personal communications, Doug Koch suggests using the axial length adjustment below, combined with the Holladay 1 formula, and the regular optical biometry lens constant for the IOL to be used. Select the IOL power that yields the least amount of minus power for the refractive target. For me, the Holladay 1 Surgeon Factor for the MN60MA would be 1.87, and for the SN60WF, it would be 1.80. To use this adjustment, you must apply the Holladay 1 formula. Suppose the calculated IOL power is greater than -5.00 D. In that case, there are foldable IOLs that go down to -10.00 D, and you would use the regular optical biometry Holladay 1 lens constant for that IOL model.

This is what I am presently using with the Holladay 1 formula:

You can prove this to yourself by taking the last high-to-extreme axial myope case you did (AL greater than 26 mm) by optical biometry and re-running the calculation with the above axial length correction. Doug Koch and Li Wang consistently deliver outstanding work, and they are to be commended for this important insight.

Another approach gaining popularity is the use of the Barrett Universal II formula. This is one of the most accurate theoretical formulas currently available. It is a resident of the Haag-Streit Lenstar and is also a free offering on the website of the Asia Pacific Association of Cataract and Refractive Surgeons. It can be accessed at:

When using the Barrett Universal II formula, no axial length adjustment is required, and standard optical biometry lens constants are used. I highly recommend this approach.

The Hill-RBF method also works very well for high axial myopia and requires no adjustment of axial length. In its current form, it will return an “in-bounds” indication for IOL powers as low as -5.00 D.

Performing ultrasound-based biometry for high to extreme axial myopes has its own set of pitfalls and peculiarities. It is well known that the incidence of a peripapillary staphyloma increases significantly with increasing axial length. One problem with ultrasound is that the operator has no way of knowing exactly where the sound beam is measuring. Typically, ultrasound measures the anatomic axial length (from the corneal vertex to the most posterior portion of the macular region) rather than the refractive axial length (from the corneal vertex to the foveal center), resulting in an IOL power that is too low. This is explained in detail on my website.

For these patients, when optical biometry is not possible due to media opacity, I generally perform vector A/B Biometry. By this approach, the intersection of the vector A-scan is with the location of the fovea and can be controlled via direct observation using a horizontal B-scan. Suppose a standard immersion A-scan is done for the high to extreme axial myope in the presence of a peripapillary staphyloma. In that case, there is a good chance for an axial length over-measurement and an unanticipated hyperopic result.