High to Extreme Axial Myopia - IOL Power Calculations.
Axial length correction for high to extreme axial myopia.
The way in which we do IOL power calculations for the high to extreme axial myope has been evolving and, for some surgeons and their staff, this is an area of some confusion. There are currently four approaches in use by surgeons around the world.
- Target a moderate amount of myopia. (What many surgeons do and is not recommended.)
- Adjust the lens constants as recommended by Dr. Haigis. (Those unusual lens constants that used to be listed on this website.)
- Adjust the optical biometry axial length as recommended by Wang and Koch in the JCRS. (Recommended)
- Use the Barrett Universal II formula with optical biometry, which is well suited to this task. (Recommended)
The one unifying theme for IOL power calculations in the setting of high to extreme axial myopia is that some amount of unanticipated hyperopia seems to occur without doing something special.
One very successful way of handling the selection of IOL power for these patients is based on an 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 carried out by optical biometry. This would apply to both the Lenstar and the IOLMaster, as the approach of both instruments is almost identical at the present time.
Wang L, et al. Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm. JCRS 2011; 37:2018-2027.
One working theory is that optical biometry may exhibit a systematic error in the measurement of axial length that increases in a linear fashion. This is due to the fact that optical biometry assigns a single, global index of refraction to all eyes, no matter what the 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 over-state 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. Pick the IOL power that gives the least amount of minus for the refractive target. For me, for the MN60MA Holladay 1 Surgeon Factor would be 1.87 and for the SN60WF would be 1.80. In order to use this adjustment, you need to use the Holladay 1 formula. If the calculated IOL power is greater than -5.00 D, 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:
Optimized Optical Biometry AL = (0.8289 x measured AL) + 4.2663
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. This approach is relatively new, but the science looks solid and I have had great results for more than 20 such cases. Doug Koch and Li Wang always do fabulous work and they are to be congratulated for this important insight.
Note: This method is not used for patients with prior refractive surgery as the calculation algorithms have already been optimized for long axial lengths and adding this correction will give a myopic result. DO NOT ADJUST THE AXIAL LENGTH IN THE SETTING OF PRIOR ALK, RK, LASIK and PRK.
BTW, those unusual lens constants (dramatically different ones for the + and - MN60MA) that used to be on my website are for when the axial length has not been corrected. This approach pre-dates the Wang-Koch article. I have since removed this information from this website.
Another approach that is gaining in popularity is the use of the Barrett Universal II formula. This is one of the most accurate theoretical formulas currently available. It is resident of the Haag-Streit Lenstar and is also a free offering on the web site of the Asia Pacific Association of Cataract and Refractive Surgeons web site. 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.
Doing ultrasound-based biometry for the high to extreme axial myope has its own set of pitfalls and peculiarities. And although not published, I have begun adjusting the axial length and with good results. However, additional work in this area will be required before a standard is established. 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 to where the sound beam is measuring. Typically, ultrasound gives the anatomic axial length (corneal vertex to the most posterior portion of the macular region) rather than the refractive axial length (corneal vertex to 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 do 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. If a standard immersion A-scan is done for the high to extreme axial myope, in the presence of a peripapillary staphyloma, there is a good chance for an axial length over-measurement and an unanticipated hyperopic result.