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June 2003
IN THIS ISSUE

Corneal pachymetry proves key to glaucoma diagnosis

Probing physiology behind accommodative lens implants
Intralase cuts enhancement rates by 30% after LASIK
‘Quality of vision’ in sharp focus as four Main Symposia frame XXI ESCRS Congress
Allegretto laser works well for both hyperopia
and myopia correction, says FDA trial data
Innovative impulse device enables tongue to ‘see’ by processing sensory data to the brain
Increased precision of eye tracking module vital for customised ablations of large corneal areas
New adaptive optics system reduces higher order aberrations and previews custom ablation outcomes
High-resolution WASCA system shows good refractive outcomes for customised ablation
Results of prevalence studies casts link between ocular pressure and glaucoma in new light
New phakic IOL ‘gives good refractive outcome and is very well tolerated’, says specialist
Myopes are more likely to develop vitreoretinal complications than hyperopes after lens exchange
Preoperative myopia proves a good outcome predictor for LASIK surgery
Broad beam laser with Gaussian delivery obviates need for eye tracker in LASEK procedures
Modified approach needed for IOL power readings in post-RK eyes to cut risk of hyperopic outcome
Block excision therapy of choice for epithelial in-growth
CLAPIKS offers novel pharmacological approach for treatment the hyperopia after LASIK surgey
Study shows LASIK could provide long-term savings to patients despite initial costs
Theories take shape to unravel mystery of presbyopia development in the human eye
Retinal detachment risk in high myopes unaltered by excimer laser vision correction procedure
Ocular surgery patients advised to avoid risk of infection by staying away from swimming pools
Personalised iris prosthesis comes a shade closer to the ideal coloured iris solution
FEATURES
From The Editor
Guest Editorial
Reflections on Refractive Surgery
Bio-Ophthalmology
In Your Good Books
Bio-ophthalmology
Digital Opthalmologist
Regulatory Matters


Modified approach needed for IOL power readings in post-RK eyes to cut risk of hyperopic outcome

Cheryl Guttman in Los Angeles, California

CATARACT surgeons need to use a modified approach for pseudophakic IOL power calculation in post-radial keratotomy (RK) eyes to minimise the risk of a hyperopic refractive outcome, according to the results of a new clinical study.
Investigators analysed data from a series of 24 post-RK eyes which underwent cataract surgery with IOL implantation, including nine eyes for which both measured and calculated keratometry values were available.

Based on that research Mark J Mannis MD and colleagues recommend aiming for -1.5 to -2.0 D of myopia rather than emmetropia, as well as calculating corneal power with the contact lens over-refraction method in all eyes, along with the historical method when possible. They suggest using the flatter of those two K values as input for the IOL power calculation formula.

"Standard keratometry usually gives a falsely steep corneal power reading in post-RK eyes because the refractive surgery procedure alters corneal shape from prolate to oblate. Performing standard IOL power calculations with those overestimated K values is likely to result in a hyperopic overcorrection, which can be large enough in some patients to be a refractive disaster," Dr Mannis said.

"The approach we’ve suggested appears useful for significantly reducing the risk of hyperopia after cataract surgery. However, it does not eliminate it. Therefore, we further recommend that surgery in post-RK patients should be done by operating on the non-dominant eye first if bilateral procedures are necessary. We suggest thorough preoperative counselling to inform patients fully about the risk of a refractive complication," Dr Mannis said.

In the 24 eyes studied, target refraction ranged from plano to -3.0 D, with a mean -1.51 D. Final refraction was myopic (<-0.50 D) in 29% of eyes, emmetropic (±0.5 D of plano) in 29% and hyperopic (>+0.5 D) in 42%. Four (17%) eyes had a final refraction of greater than +1.5 D.

The investigators determined that if IOL power had been selected in all eyes to achieve a plano result, the rate of hyperopia would have doubled, with 20 (83%) of the 24 eyes winding up with a refraction of greater than +0.5 D.
The researchers used data from the nine eyes with measured and calculated K values to investigate the effect of using the flatter calculated K together with a myopic refractive target.

Interestingly, comparisons of the flatter measured K and flatter calculated K values for each of the nine eyes showed the variability between those results in both magnitude and direction.

All nine eyes had been evaluated preoperatively with keratometry. Seven of those had a K value from topography, six had K calculated by clinical history and four of those individuals as well as three others underwent hard contact lens over-refraction.
The comparisons between the flatter measured and calculated K values showed that the measured K was steeper, ranging between +0.63 and +2.80 D), compared with the calculated K in seven eyes. In the two remaining eyes, the measured K was flatter by -0.35 and -3.87 D respectively.

"Although some investigators have proposed correcting the measured K in post-RK eyes by the simple subtraction of 1.0 D, our findings demonstrate that method will not eliminate the inaccuracy of the keratometry-derived K," Dr Mannis said.
Calculations of theoretical postoperative refractions assuming the use of the flatter K and a plano target showed that two of the nine eyes would have been myopic, three eyes would have been emmetropic and four would have had a hyperopic outcome, with final refractions ranging from +2.13 D to +3.54 D.

Substituting a target refraction of -1.5 D and using the flatter K resulted in a larger proportion of myopes (56%); the remaining 44% of patients would still have been hyperopic, but with relatively lower amounts of hyperopia ranging between +0.83 D to +2.35 D.

"That is still a high proportion of patients winding up with hyperopia and our calculations suggest it may be necessary to aim for even more than -1.5 D of myopia in some patients. Overall, these results highlight the point that there is no nomogram surgeons can use reliably to calculate IOL power in post-RK eyes.

"To reduce their percentage of hyperopic outcomes in this population, surgeons must take each patient as an individual and reconstruct the optics," Dr Mannis said.
The research appeared in the Journal of Cataract and Refractive Surgery (Chen L, et al. JCRS 2003;29:65-70).

Mark J Mannis MD
University of California, Davis, California, US
Email: mjmannis@ucdavis.edu

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