KERATOCONUS AND LASER

A novel treatment paradigm that combines corneal parameters with refractive measurements can be used to deliver safe and effective excimer laser treatments in patients with mild or forme fruste keratoconus, according to Noel Alpins FRANZCO, FRCOphth, FACS.

“Our long-term data shows that patients with forme fruste and mild keratoconus can be safely and effectively treated with photoastigmatic refractive keratectomy (PARK),” Dr Alpins told delegates attending the XXXI ESCRS Congress in Amsterdam. “Eyes treated using vector planning1 were shown to have a stable refraction and corneal topography up to 10 years after surgery, with no incidence of progressing ectasia or increase in corneal irregularity,” he said.

Dr Alpins noted that treating forme fruste and mild keratoconic patients using refraction parameters alone would generate an excess amount of corneal astigmatism on the cornea, resulting in potentially adverse effects on postoperative visual outcomes. “In keratoconus, this is evidenced by the greater than average calculated ocular residual astigmatism (ORA) in any group of eyes having treatment for astigmatism with myopia. The ORA is defined as the vectorial difference between the corneal astigmatism and the refractive cylinder at the corneal plane, and is expressed in dioptres. On the other hand, treating by corneal data alone will attempt to make the cornea more spherical but will leave excess astigmatism measurable in the manifest refraction postoperatively, which is likely to be unacceptable to the patient,” he said.

Combining both approaches gives these patients the best of both worlds, said Dr Alpins. “Using vector planning the refractive cylinder outcome is not compromised as one might perhaps expect with a technique that also takes account of corneal parameters. Using this approach, the corneal astigmatism remaining is less than with using refractive parameters alone, essentially because we are treating somewhere closer to the corneal steep meridian and magnitude rather than treating by refraction alone. The end result is less astigmatism on the cornea and a better quality of vision,” he said. Advances in diagnostic and imaging technologies over the last decade have helped greatly in the identification of potentially suspect keratoconus corneas, said Dr Alpins.

“There has been a real evolution in the terminology of keratoconus since George Waring III first used the term “suspect keratoconus” in 1993. That designation was based mainly on observation whereas today there are many other qualitative and quantitative keratoconic indices to help us identify such corneas. The sensitivity of these new tools is now of such a high calibre that it has sometimes flagged a cornea as potentially keratoconic that we initially did not suspect ourselves on first appearance,” he said.

The results obtained in most of the earlier studies of excimer laser treatment for mild and forme fruste keratoconus patients were not particularly impressive, said Dr Alpins. “However, we need to bear in mind that the treatments were usually based only on refraction or on corneal parameters, not a combination of the two. The outcomes in these studies show only a partial decrease in refractive astigmatism and there was progression of the keratoconus in some patients. Furthermore, most of the early studies did not even report the postoperative corneal astigmatism parameters,” he said. The data presented by Dr Alpins included 45 eyes of patients with mild or forme fruste keratoconus.

“We adhered to very strict criteria and only treated myopic astigmatism patients, over 25 years of age with nonprogressive keratoconus. All patients had to have a stable refractive and corneal status for two years. We excluded patients with mean K-readings of 50.00 D or greater, bestcorrected visual acuity less than 20/40, those with signs of apical thinning, visible ectasia or scarring on slit lamp examination, and those with residual stromal bed less than 300 μm, assuming an epithelial thickness of 60 μm,” he said. Patients were treated with the Visx Star S1 excimer laser (eight eyes until 1997) or S2 excimer laser (37 eyes from 1997 forward), said Dr Alpins. Unlike earlier studies, the technique used vector planning to incorporate both corneal topographical data and refractive astigmatism data in the treatment plan.

Dr Alpins explained that the simulated keratometry value derived from the topography was incorporated into the treatment plan by calculating the ORA. In the study group, all treatments were optimised, directing only part of the neutralisation to the cornea and a theoretical part to the refraction, targeting 0.75 D or less remaining on the cornea and 0.50 D or less in the refraction. In cases in which the ORA was more than 1.50 D, the proportion was selected as 50 per cent in the theoretic manifest refraction and 50 per cent on the cornea, hence targeting greater amounts of remaining astigmatism than with a lower ORA.

Refractive results overall were very good, said Dr Alpins, with uncorrected visual acuity of 20/20 or better in 56 per cent of eyes and 20/40 or better in all eyes. Seven eyes lost best-corrected visual acuity compared to a gain of BCVA in 16 eyes. Moreover the refraction remained stable over time with no evidence of progression of the keratoconus.

1 Alpins NA. New method of targeting vectors to treat astigmatism. J Cataract Refract Surg 1997; 23(1):65- 75.