Astigmatism: the final frontier

[caption id="attachment_13437" align="alignleft" width="940"] The corneal topographic astigmatism total (CorT Total), which includes the posterior cornea, is closer to the refractive cylinder than CorT anterior and Simulated Keratometry[/caption] Astigmatism is an issue in all forms of visual intervention, from spectacles and contacts, to corneal surgery, LASIK and lens implantation. EuroTimes talked to Noel Alpins MD, Medical Director, New Vision Clinics, Melbourne, Australia, about ongoing efforts to deal with astigmatism associated with cataract surgery. How are we doing in terms of assessing cataract patients for pre-operative astigmatism, and planning accordingly? Measuring the preoperative corneal astigmatism has certainly become more accurate as the technology has improved. Tomography devices have the ability to measure the posterior cornea and with this and other information quantify a total corneal astigmatism. The parameter known as corneal topographic astigmatism total (CorT Total) first described in 2015 (1) has been shown to provide a more accurate measure of corneal astigmatism that includes the posterior cornea than those that only measure anterior cornea such as simulated keratometry, manual keratometry and automated keratometry. I use the CorT Total when planning toric IOLs to obtain an accurate toricity of the IOL power, when planning limbal-relaxing incisions (LRIs) and when treating astigmatism by Vector Planning in refractive laser surgery. What can we do to reduce or eliminate iatrogenic astigmatism associated with cataract surgery? The surgeon can harness the primary phaco incision to reduce the corneal astigmatism by placing it at the steepest corneal meridian. It is important that the Flattening Effect (FE)(2) is determined from aggregating results from previous surgical cases – that is, the amount of flattening the incision induces at the planned incision meridian. Looking at my own results, a close to astigmatically neutral incision is obtained when I place a 2.2mm incision on the flat meridian of the corneal astigmatism or on the 20 OD/200 OS meridian, ergonomically comfortable for a RH surgeon. (3) I use this when planning a toric IOL procedure and do not want to change the corneal astigmatism after preoperative measurements and calculations have been performed, to avoid any inaccuracies in the planned toric IOL. uPlacing the phaco incision on the steep corneal meridian can also introduce a useful flattening change in the corneal astigmatism with spheric implants. However, a refractive surprise can occur by rotating the astigmatism as a result of the torque effect of SIA, which can often disappoint when alignment is a priority. So if the corneal astigmatism is against-the-rule and a temporal incision is used, be as accurate as possible with placing the incision on the steep meridian and know how much astigmatism you are able to reduce when planning for the best toric IOL choice. Remember the SIA and FE of the incision increase with increasing astigmatism. (4) At the extremes, what are the lowest and highest amounts of astigmatism that can be treated predictably? The minimum toric IOL I would implant would be 1.50D toricity at the IOL plane. With corneal astigmatism of 1.25D or less, I would proceed with an LRI or place a 3mm phaco incision on the step corneal meridian to minimise the astigmatism postoperatively. My preference is for a spherical cornea rather than an astigmatic cornea and a toric IOL which can degrade the visual quality postoperatively. The maximum toricity I have implanted was a ZCT700 (AMO) IOL, which is approximately 4.80D at the corneal plane, but there are implant powers readily available above this amount. How good are toric IOLs now, and how are we doing with issues such as decentration? Toric IOLs have certainly improved in optical quality and range of parameters available in recent times – with good alignment and effective choice of toric power they are very beneficial to the patient's spectacle-free visual outcome after cataract surgery. I find that incorporating iris imaging using CallistoTM or VerionTM is more reliable and accurate than marking the eye. This has been borne out by several studies. The challenge of centration is that when implanting a toric multifocal IOL, the IOL is primarily centred according to the rings to be concentric with the undilated pupil. The secondary priority is that the toricity is aligned to the correct axis – the imaging lines do not necessarily need to overlap the toric marks on the IOL but must at a minimum be parallel to them, giving priority to the multifocal IOL centration. For more detailed information on astigmatism and cataract surgery refer to: Alpins N. Practical Astigmatism: Planning and Analysis. Thorofare, NJ:SLACK:2017 1. Alpins N, Ong JKY, Stamatelatos G. Corneal topographic astigmatism (CorT) to quantify total corneal astigmatism. J Refract Surg 2015; 31(3):182-186. 2. Alpins NA. Vector analysis of astigmatism changes by flattening, steepening, and torque. J Cataract Refract Surg 1997 December;23(10):1503-14. 3. Alpins N, Ong JKY, Stamatelatos G. Asymmetric Corneal Flattening Effect After Small Incision Cataract Surgery. J Refract Surg 2016 December;32(9):598-603. 4. Chang SW, Su TY, Chen YL. Influence of Ocular Features and Incision Width on Surgically Induced Astigmatism After Cataract Surgery. J Refract Surg. 2015; 31(2): 82-88.

Tags: astigmatism, cataract, toric IOLs