Wednesday May 12 2010
The research was very small as it involved only 132 participants
“A new method of correcting short sight could be better and safer than laser eye surgery,” The Independent reported.
This news story is based on a systematic review of studies comparing laser eye surgery to phakic intraocular lenses, which are surgically implanted lenses in the eye that work similarly to contact lenses.
The two techniques were found to be equally successful, both resulting in similar proportions of people with 20/20 vision a year after surgery. Those given phakic lenses were also less likely to have diminished ‘best spectacle corrected visual acuity’ (BSCVA), an important safety standard in eye surgery.
The main drawback of this review is the limited amount of research currently available. Only three trials treating 228 eyes were included. This reduces its statistical power for detecting differences between the treatments.
This is a well-conducted review, but the question of which treatment is the safest and most effective will need to be established in further, longer-term trials.
Where did the story come from?
This research was a Cochrane Review written by clinicians from the Moorfield Eye Hospital in London, and published in The Cochrane Library.
The news stories have accurately reflected these research findings, but have not considered the limitations surrounding the small body of evidence that is currently available.
What kind of research was this?
This was a systematic review of the two main forms of corrective surgery for moderate to severe short-sightedness (myopia).
Myopia is a problem of vision that causes distant objects to appear blurred, while close objects can still be seen clearly.
This is because light rays are focusing in front of the retina (the back of the eye), rather than directly on the retina, which is necessary to produce a clear image.
Myopia occurs when the eye is too long from front to back, or the cornea (the front of the eye) is too steeply curved. As a result, there is a mismatch between the length of the eye and its focusing power.
The two compared techniques were excimer laser refractive surgery and phakic intraocular lenses (IOLs), which work in slightly different ways.
- Laser surgery alters the cornea, which reduces its refractive power (ability to bend light). This enables visual images to reach the back of the retina.
- Phakic lenses are surgically implanted either in front of the iris (the coloured part of the eye) or just behind it. This extra lens works by spreading out light rays so that they do not fall short of the retina, similar to the way a contact lens or a pair of glasses would.
A systematic review of randomised controlled trials (RCT) is the most reliable way of assessing the efficacy and safety of treatments. However, when combining findings from different trials, there is usually some unavoidable limitation due to the differences in methods between the trials.
What did the research involve?
The researchers carried out an extensive search of medical and scientific databases to identify randomised controlled trials (RCTs) that had compared laser surgery with phakic lenses. To be eligible, trials had to have been in adults between the ages of 21 and 60 with moderate to severe myopia of more than -6.0 diopters (a measure of how well the eye’s lens can focus) and who had no eye disease or other reason for short-sightedness (e.g. cataracts).
The main outcome of interest was the percentage of people who had 20/20 vision or better after 12 months after surgery. Various other secondary outcomes were considered, including the percentage of eyes that were within a 0.5 to 1.0 target diopter at 6 or 12 months.
The researchers were also interested in the incidence of complications, ranging from minor (glare, dry eyes) to severe (significant permanent visual loss that got worse after treatment). The two authors independently assessed the studies for quality and eligibility.
What were the basic results?
The researchers identified three eligible studies in a total of 132 patients and 228 eyes. Myopia ranged from moderate to severe (-6.0 to -20.0 diopters), and included up to 4.0 diopters of astigmatism (when the eye is not the usual symmetrical spherical shape but is longer in one direction than the other, causing extra focusing problems). All patients had stable vision without deterioration in the 12 months before the trial.
Two studies compared LASIK laser surgery (laser assisted stromal in-situ keratomileusis) with phakic lenses (the standard lens). One study compared PRK laser surgery (photorefractive keratectomy) with a different type of lens implant - a toric lens (which has the additional power to correct astigmatism).
In total, 166 eyes provided data for the primary outcome of the percentage of eyes with 20/20 vision or better at 12 months after surgery (i.e. only two of the three studies looked at the primary outcome). Both techniques had the same success rate, and there was no difference in the proportion who achieved this outcome with laser compared to phakic lenses (odds ratio 1.33, 95% confidence interval [CI] 0.08 to 22.55).
Phakic lens surgery had fewer side effects than laser surgery in that fewer people lost their ‘best spectacle corrected visual acuity’ (BSCVA) 12 months after surgery (i.e. fewer people's vision deteriorated with phakic lenses: OR 0.35, 95% confidence interval [CI] 0.19 to 0.66; data from 216 eyes). BSCVA is a measure of how good a person’s sight is on a visual chart with the most appropriate spectacle prescription. For this research, deterioration in BSCVA was considered to be a loss of two or more lines on the visual chart.
Phakic lenses were also associated with better contrast sensitivity than laser surgery, and better satisfaction on patient questionnaires. However, two patients developed cataracts after phakic IOL.
How did the researchers interpret the results?
The reviewers conclude that phakic lenses are safer than eximer laser surgical correction for moderate to severe short-sightedness, and that phakic lenses are preferred by patients. The researchers say that although phakic lenses are usually considered only for short-sightedness above -7.0 diopters, they may also be considered preferable to laser for more moderate short-sightedness.
This is a well-conducted Cochrane review, which carried out a thorough search for all suitable trials comparing laser eye surgery with phakic intraocular lenses for moderate to severe short-sightedness.
Both techniques achieved the same success rate for the proportion of people who had 20/20 vision 12 months after surgery. People given phakic lenses were less likely to have a loss in their ‘best spectacle corrected visual acuity’ after treatment. However, on two occasions, cataracts developed after phakic lens surgery.
The main drawback to this review is that there is limited research currently available, and the reviewers could only include three trials, treating 228 eyes. This reduces the statistical power for detecting accurate differences between the treatments, particularly when looking at secondary outcomes, such as rare adverse effects. The statistical power was then further reduced as not all of the trials reported on the same outcomes.
The small numbers also mean that accurate comparisons cannot be carried out between the different populations (such as severity of short-sightedness, presence of astigmatism) or treatments (such as type of laser surgery or lens). As the researchers say, further RCTs are needed to detect differences between subgroups, and to determine the most suitable short-sightedness range for inserting phakic lenses. Follow-up of a larger range of people will also be needed to identify any rarer and potentially more serious adverse effects.
Corrective eye surgery is already established as a treatment for myopia. This is a good review, but the question of which treatment is the safest and most effective will need to be answered in further, longer-term trials.