EyeWorld Asia-Pacific September 2015 Issue

September 2015 28 EWAP FEATURE Dr. Henderson discusses biometry for toric IOLs at the ASCRS Side X Side meeting in Aventura, Fla. Source: EyeWorld How to interpret your toric IOL biometry by Stefanie Petrou Binder, MD EyeWorld Contributing Writer Expert explains variability encountered in toric IOL biometry T oric IOLs offer a high standard of care to patients with cataracts and corneal astigmatism by providing excellent visual outcomes. Knowing which instruments provide the most reliable measurements for each individual patient is crucial to help surgeons make the right preoperative planning choices. EyeWorld spoke to Bonnie An Henderson, MD , clinical professor of ophthalmology, Tufts University School of Medicine, Waltham, Mass., who discussed biometry for toric IOLs at the ASCRS Side X Side meeting in Aventura, Fla. In a video interview, Dr. Henderson explained that measurements taken by different devices and operated by different operators do not always correlate. The surgeon needs to be able to interpret mixed messages and know which measurements he or she can trust. “There is no right answer, no perfect algorithm, when it comes to trusting the right measurement. I have a system where I rely on certain instruments for certain variables,” Dr. Henderson said. The variables In toric IOL biometry, there are two main variables, according to Dr. Henderson: the steep axis and the magnitude of the astigmatism. To determine the axis, she opts for machines that give objective data. When considering measurements from devices like the manual keratometer, for which the surgeon relies on the operator to run it and properly determine the axis, Dr. Henderson thinks that variability can be high. She prefers to implement the autokeratometer or non-contact biometry for a more objective determination of what the cornea looks like. “Also, I always look at my topography to see what type of astigmatism it is, regular, irregular, or if there is a lot of drop out,” she said. “If the topography looks very unusual, then I look for other things I may have missed like subtle corneal anterior basement membrane dystrophy or dry eyes, which could lead to differing measurements from the various machines.” Risk of overtreatment Flipping an axis by overtreating a patient is not an ideal situation. The problem is overcorrecting the astigmatism can lead to unwanted postoperative cylinder in the opposite meridian. To make the right judgment where there is a risk of overtreatment in toric IOL implantation, Dr. Henderson said to choose a slightly lower amount of cylindrical correction. “In most cases the preoperative manifest refraction is not used to determine whether a toric lens should be used because the manifest refraction looks at the overall astigmatism of the eye prior to surgery—including the cataract lens. Once the cataract is removed, the far majority, if not all, of the astigmatism is just in the cornea. That is why corneal astigmatism is relied upon more,” she said. However, patients whose spectacles have corrected their astigmatism for many years may represent a difficult situation, according to Dr. Henderson. Their brain is accustomed to compensating for a certain amount of elongation in a particular meridian or a certain magnitude of astigmatism, and they may have trouble with the change in image once the toric IOL neutralizes the astigmatism. “In these cases, it may be beneficial to consider the patient’s manifest refraction and choose a toric IOL that is cognizant of the preoperative astigmatism. For example, if choosing between two toric powers, and the surgeon has to decide to either slightly overcorrect, in which case the axis could be flipped, or undercorrect and leave the patient with a little residual astigmatism in the same preoperative axis, the better choice may be to leave the patient a little undercorrected,” Dr. Henderson said. Toric calculations Recent evidence suggests the need to weigh the contribution of the posterior corneal curvature in toric calculations. An eye surgeon has a number of resources available to him to calculate the contribution of the posterior corneal curvature, for instance using IOL calculation formulas found on the ASCRS website or the calculators provided by the individual lens manufacturers, such as the AcrySof Toric IOL (Alcon, Fort Worth, Texas) and the Tecnis Toric IOL (Abbott Medical Optics, Abbott Park, Ill.). Dr. Henderson factors in the contribution of the posterior cornea by evaluating whether the steep axis is in the vertical or horizontal meridian. “If the patient has with-the-rule astigmatism (i.e., in the vertical meridian), I tend to use a little bit less toric power compared to if the steep axis is against the rule in the horizontal meridian. So if I am trying to decide between two toric powers and the patient is steep at 90 degrees, I will tend to go with a lower toric power; conversely, if they are steep horizontally, I will choose a slightly higher toric power. Although this is not a perfect method of addressing posterior corneal astigmatism that has been reported by Douglas Koch, MD, and Li Wang, MD, PhD, a more accurate approach would be to utilize a machine that measures the actual posterior cornea, like the Galilei [Ziemer Ophthalmic Systems, Port, Switzerland],” she said. EWAP Editors’ note: Dr. Henderson has financial interests with Abbott Medical Optics, Alcon, and Bausch + Lomb (Bridgewater, NJ). Contact information Henderson : bahenderson@eyeboston.com