Corneal astigmatism options
Correction of high astigmatism is now feasible with a variety of corneal refractive techniques
Jesper Hjortdal MD
Surgeons have a number of corneal refractive procedures to choose from that can produce good results in eyes with regular and irregular high astigmatism, Jesper Hjortdal MD told the 36th Congress of the ESCRS in Vienna, Austria.
The general aim in correcting regular astigmatism is to flatten the steep axis of the cornea or compensate for the astigmatism intraocularly with a toric IOL, said Dr Hjortdal, Aarhus University Hospital, Denmark.
Whichever technique is used, accurate determination of the correct axis pre- and intraoperatively is necessary to bring about the desired effect. If the actual alignment axis is off the target alignment axis by 30 degrees, the procedure will have no anti-stigmatic effect at all, he pointed out.
Indications for surgery in eyes with high astigmatism include poor vision with spectacles, intolerance or discomfort with contact lenses and concomitant cataract. The corneal surgical techniques include incisional and photo-ablative varieties, and most recently stromal lenticule excision, he said.
The oldest of the techniques currently in use is paired arcuate keratotomy. The approach involves the creation of almost fully penetrating incisions in the peripheral steep axis of the cornea. It can achieve very good reductions in high astigmatism, although it can leave considerable amount of residual astigmatism in highly astigmatic eyes and there can be considerable scatter in the technique’s predictability.
As an illustration, he cited a study he and his associates conducted 20 years ago involving highly astigmatic post-PK eyes. It showed that arcuate keratotomy reduced mean keratometric cylinder by 50%, from 7.0D to 3.25D. However, the procedure was safe and he noted that the greater the magnitude of preoperative astigmatism, the greater was the anti-astigmatic effect (Hjortdal et al, Acta Ophthalmol. Scand. 1998: 76: 138-141).
In a more recent study arcuate keratotomy incisions produced with a femtosecond laser produced a similar reduction in corneal astigmatism, from 9.45D to 4.64D, and despite the precision of the technique there remained some scatter in the results (Loriaut et al, Cornea 2015:34:1063-1066).
PRK vs LASIK vs SMILE
Photoablative techniques like PRK and LASIK can correct higher amounts of astigmatism and with greater accuracy than incisional techniques and their results appear to be roughly comparable, he noted. In a study comparing the two techniques in eyes with more than 3.0D of astigmatism, there was no statistically significant difference between the efficacy and the two techniques had similar predictability. That is, in the PRK and LASIK groups, 39% and 54%, respectively, had less than 0.5D of astigmatism postoperatively, and 88% and 94% had less than 1.0D (Katz et al, J Refract Surg. 2013;29(12):824-831).
The results with SMILE® appear to be comparable to LASIK in eyes with high myopic astigmatism. In a recent retrospective study, Dr Chan and his associates found no significant between-group difference in uncorrected distance visual acuity and manifest spherical equivalent in patients undergoing the procedures for myopic astigmatism. At three months, 90% and 95.4% of eyes in the SMILE and LASIK groups, respectively, were within ±0.5D of the attempted cylindrical correction (p=0.423) (Chan et al, J Cataract Refract Surg. 2018 Jul;44(7):802-810).
Topography-supported customised laser PRK is another technique that has been used in eyes with irregular astigmatism due to PK or keratoconus. However, early results in a study involving penetrating keratoplasty patients showed significant haze following the procedure.
More recently Dan Reinstein MD, PhD, UK, has introduced transepithelial phototherapeutic keratectomy (TE-PTK). The ablation is based on population epithelial thickness measurements determined using very high-frequency digital ultrasound. Results to date with the technique suggest that TE-PTK can be a safe and effective method of reducing stromal surface irregularities by taking advantage of the natural masking effect of the epithelium.
Last but not least are intracorneal ring segments (ICRS) for the treatment of keratoconus. Several studies confirm safety and efficacy of ICRS. However, predictability remains a key challenge and current nomograms are insufficient to cover all cases.