As the baby boomers reach their 60s and begin to get visually significant cataracts, demand for premium IOLs is increasing … and with that comes high expectations. Corneal topography and accurate Ks will help you to better meet those expectations, both in determining the optimum IOL power as well as in the astigmatism correction. Here are eight pearls for getting the corneal measurements you’ll need.
- Get accurate Ks on all cataract patients. Remember that 90 percent of the average K value is used to determine the IOL power. This means that if the K(ave) is off by 0.5D, it will cause the refractive outcome to be off by 0.45D. Thus, it is highly important to have accurate Ks on all your patients. I check the IOL Master Ks on all my patients to make sure there is less than 0.3D difference between the three readings.
Normal surface regularity index. Image courtesy of DePaiva
Surface regularity index for Sjögren Syndrome. Image
courtesy of DePaiva Ophthalmol 2003;110:1102-9
- Prior to corneal topography and the IOL Master calculations, make sure that all contact lens patients are out of soft lenses for at least a week for spherical correction, and two weeks or longer for toric or RGP lenses.
- Make sure you have an accurate surgical eye history from all of your patients. You’d be surprised how many people forget to mention that they had PRK or LASIK and don’t bother to tell you, unless you specifically ask about it. Obviously, this has a tremendous impact on the IOL calculations.
- If they had prior refractive surgery, you will need to get their pre-op refraction, pre-op keratometry, intended refractive correction and post-op refraction (before they got cataracts). Consider printing the “K card” from the Academy’s website; your patient can give this to their refractive surgeon to fill out so you can more easily obtain this information. You can then go to the ASCRS Post-Refractive Surgery IOL Power Calculator and fill in this information as well as your current topography and IOL Master readings.
The tool will put this data through a battery of formulas and give you a range of IOL powers to choose from. The formulas in the far right column tend to be the most accurate, even if you have the pre-op data.
- If a Humphrey Atlas topographer is available, get the printout that has the axial, tangential, elevation and placido disc images on it. This will tell you if the astigmatism is regular and can be treated with limbal relaxing incisions (LRIs).
- Look for “blank spots” in the data where defects in the tear film or irregularities in the corneal epithelium from basement membrane dystrophy prevented accurate topographic measurement. This can especially be seen in the placido disc image where the rings are not smooth. It will also show up as focal steepenings of the cornea on the axial image.
- If epithelial basement membrane dystrophy is significant enough, this must be treated with corneal scraping and allowed to heal at least one month prior to K readings to allow for sufficient stabilization of the corneal surface.
- Remember, if the corneal astigmatism is greater than 2.0D, you will not be able to treat this sufficiently with corneal relaxing incisions. I would recommend you use a toric IOL, centering the clear corneal wound over the steep axis for maximal correction, and consider additional LRIs for significant residual astigmatism after the refractive error has stabilized, between four and six weeks post-op. Take note that the higher powers of the Acrysof IQ toric IOL became available for use in June 2011 in the United States.
If the patient desires reading vision without correction, you could do monovision with this technique, since they would not be candidates for an accommodative or multifocal IOL because of their high astigmatism. Alternatively, if the patient is motivated enough, they could get an accommodating or multifocal IOL with post-op refractive surgery enhancement.
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About the author: Natasha L. Herz, MD, is a member of the YO Info editorial board. She is a cornea specialist practicing in the Washington, D.C., metro area.