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Unusual cases alter accuracy of IOL power calculations
The surgeon should be aware of a patient’s history, review the biometric information in detail and recheck calculations as needed.
For average eyes, IOL power estimation calculations are reasonably accurate because the assumptions that are made by the formulae all hold true: The anterior chamber is an average depth, the corneal curvature and axial length are accurately measured by our machines, and the future position of the IOL is accurately predicted. However, there are some clinical situations in which the IOL calculation is not as accurate, and we can end up with a refractive surprise.
Anisometropia
Anisometropia is an unusual condition in which the refraction of one eye is significantly different from the other eye. Anisometropia in children can lead to amblyopia in the eye with the higher hyperopic refraction or the eye with a higher degree of astigmatism. In patients with long axial lengths, it is possible to have one eye with a few diopters more myopia than the other eye. But for most of the population, both eyes should have about the same spherical equivalent refraction.
Similarly, when looking at the IOL calculations for most patients, the calculated IOL power to achieve plano should be about the same in each eye. A variation of more than 1 D is unusual and should be checked against old records. If the patient has an old pair of glasses from before development of cataracts with the same degree of anisometropia as the current IOL calculations, then there is more certainty in the accuracy. However, if the patient has never had a history of anisometropia but the IOL calculations show a difference of 1 D or more between the eyes, they should be rechecked for accuracy. Note that the development of nuclear cataract changes in one eye more than the other can induce a myopic shift and anisometropia in the refraction, but because the IOL calculations are based primarily on the keratometry and axial length values, there should be no anisometropia in the ideal lens power for plano in each eye.
Unusual anterior segment dimensions
The third-generation IOL calculation formulae (Holladay 1, SRK/T, Hoffer Q) use the keratometry and axial length values to estimate the effective lens position, which is the position in the eye where the IOL is expected to sit. A more anterior position of the IOL will require less power for the same postop refraction, whereas a more posterior position will require a higher IOL power for the same postop refraction. The fourth-generation formulae such as the Holladay 2 and Haigis take into account the measured anterior chamber depth to further hone the effective lens position, and this can give a more accurate result, particularly in eyes that are not average.
In eyes with a short axial length, a small change in the effective lens position can make a large change in the IOL power calculation. In a short eye with a small anterior segment and relatively shallow anterior chamber, the IOL calculations may err on the side of residual postop myopia because the IOL may sit more anterior than predicted. This is not a bad thing because myopia has the benefit of providing excellent near vision, and should the patient desire a postop refraction of plano, a myopic keratorefractive procedure is accurate.
However, in short eyes with a normal anterior segment and a deep anterior chamber, IOL calculations tend to be far more inaccurate. The closer that the IOL is to the retina, the higher in power it needs to be. If we do not accurately predict this more posterior IOL position, then we end up implanting an IOL with a power that is too low and the patient ends up with a hyperopic surprise in the postop period, which is not as easy to treat with keratorefractive surgery.
Figure 1. This eye has a deep anterior chamber of 3.3 mm but a very short axial length of just 20.1 mm, which makes the IOL power calculation far less accurate. The IOL will sit more posterior in this eye and will need to be a higher power than most of our formulae will determine in order to avoid a postop hyperopic surprise.
Image: Devgan U
Another situation in which the IOL can sit more posteriorly than predicted is in eyes with a prior pars plana vitrectomy for retinal disease. In these eyes, I find it helpful to aim for a bit more postoperative myopia in order to produce the desired plano postop result.
Prior LASIK produced plano result
It is helpful to remember that prior keratorefractive surgery such as LASIK is most often used to give a plano refraction to ametropic eyes. Myopic patients are typically that way from axial length elongation so that the cornea and lens produce too much refracting power, and minus power glasses or contact lenses are needed to return the eye to plano. The LASIK procedure takes dioptric power away from the cornea and accomplishes the same goal of a plano refraction. But the crystalline lens in the eye does not change with LASIK, so it should have about the same power as before.
An old rule of thumb from the days when IOL calculation methods were being developed was that an average person has an IOL power of about 20 D or 21 D. If we do IOL power determination for a patient who previously achieved a plano result from prior LASIK, then I would expect the IOL calculations would show that now an IOL power of about +20 or +21 would be needed to return to a plano refraction. If the IOL calculations show a power of +17 is required for plano, it makes me wonder why: Was this patient left myopic after the prior LASIK? If not, then I would expect the IOL power to be closer to +20, and I may need to go back, get more accurate keratometry and repeat the calculations.
The exception to this would be prior radial keratotomy surgery, which may have produced a plano result years ago, but because the effect of RK tends to continue to progress over the years, these patients are often significantly hyperopic at the time of cataract surgery. In these eyes, it is common to need an IOL power of +24, +25 or more to achieve plano.
Our lens power determination methods have improved over the past 2 decades; however, they are still more estimations than exact calculations. While the printout from our biometry machines is pretty good for most patients, the surgeon should review the information in detail and note that unusual eyes will need an adjustment to these numbers in order to produce our desired postop refractive outcomes and happy patients.