According to Dr Edward E. Manche (Stanford University School of Medicine, California, USA), "There is more vision fluctuation
in photorefractive keratometry (PRK) compared to laser assisted in-situ keratomileusis (LASIK) in the early postoperative period because of epithelial remodelling." This was revealed in his recent
study comparing postop signs and symptoms of dry eye, dry eye severity, foreign body sensation and vision fluctuations in
myopic patients undergoing PRK and LASIK.
"There was some anecdotal evidence that PRK surgery might produce a lower incidence of dry eye compared to LASIK surgery.
I think it is important for us to get a better understanding of the incidence of dry eye and whether or not there are any
advantages of one keratorefractive procedure versus another," he said.
Delivering precise results
The investigation consisted of 68 consecutive eyes of 34 myopic patients randomized to receive LASIK in one eye and PRK in
the other. Dr Manche added, "We used a previously validated study questionnaire to prospectively compare changes in dry eye
symptoms and severity between the two groups. The contralateral study design is extremely useful in identifying differences."
The questionnaire has been used in previous contralateral eye assessments and was completed preoperatively and at 1, 3, 6
and 12 months postoperatively. Questions included: "Do you have problems with dry eyes?" and "If you do have problems with
dry eyes, how severe is this problem?" Patients were also asked to grade the severity of halos, rings or starbursts of light.
Dr Manche is forward thinking in his approach to the differences in self-reported dry eye symptoms in LASIK and PRK patients.
While some believe there are differences between the surgical techniques, Dr Manche explained, "It was always my clinical
impression that there were no substantive differences between selfreported dry eye symptoms and severity in my patients undergoing
LASIK and PRK surgery. Therefore, I was not surprised by the results of the study."
After one month the uncorrected visual acuity (UCVA), best spectaclecorrected visual acuity (BSCVA), 5% and 25% contrast sensitivity,
induction of higher-order aberrations (HOAs), subjective symptoms of vision clarity, vision fluctuation and general selfassessment
of vision were all worse in the PRK eyes. However, by three months the differences between the PRK and LASIK eyes were resolved.
"All of the epithelial layers have been replaced by one month in most patients following PRK surgery," Dr Manche stated. "There
is often a visible epithelial fusion line on slit lamp exam in the early postoperative period. The irregular central epithelium
often creates central irregularity that is visualized on computerized videokeratography. These epithelial changes resolve
in the majority of cases by the three month postoperative examination. This is the cause of the vision fluctuations in PRK
After one year mean spherical equivalent was reduced by 94% to –0.27 ± 0.31 D in the LASIK group. In the PRK group it was
reduced by 96% to –0.17 ± 0.41 D. In the LASIK group 91% of eyes were within ±0.50 D and 97% were within ±1.0 D in the PRK
group. Apart from the first postoperative month where LASIK was more effective, both LASIK and PRK are equally safe and effective
in reducing myopia in the long run.
Bigger sample sizes
Although there were not many factors that could have affected the results, Dr Manche claims he would have preferred to increase
the amount of patients in the study. "One of the limitations of our study was the relatively small sample size. I found that
recruiting patients for this type of study was more difficult than usual for other studies such as my original dry eye study
in patients undergoing LASIK with a mechanical keratome in one eye and a femtosecond laser in their fellow eye. I would like
to see additional larger multicentre studies to validate our findings," he continued. "Another limitation of our study was
that we only looked at self-reported symptoms of dry eye. It would be useful in future studies to gather and analyse physical
findings including Schirmer testing, corneal staining with vital dyes, corneal sensation and tear break-up time."
Dr Manche revealed that he is working on a number of studies to assess the factors effecting the incidence of dry eye. "This
is an important area of research. I have one ongoing study comparing two femtosecond lasers used in LASIK surgery for flap
creation," he said. "We are looking at self-reported dry eye findings as well as objective finding such as corneal sensation
between the two groups. We plan to present the findings of these studies in some upcoming scientific meetings."
Both LASIK and PRK cause an increase in the signs and symptoms of dry eye and dry eye severity, however, in the early post-op
stages PRK eyes experience larger fluctuations in vision. "I don't believe that we should advise patients with symptomatic
dry eye to have PRK over LASIK. If you feel that a patient is not a candidate for LASIK due to dry eye, then I don't think
that the patient is a candidate for PRK," concluded Dr Manche.