Welcome to Eurotimes

SAN DIEGO, CA - Pupil size plays a small but important role in determining the quality of vision after LASIK, according to the findings of a new study.

Steven C Schallhorn MD and colleagues at the Naval Medical Centre in San Diego analysed patient responses on a comprehensive quality-of-vision questionnaire completed preoperatively and at various intervals to better understand the relationship between pupil size and development of scotopic symptoms after LASIK.

The results showed individuals with large mesopic pupils (>6.0 mm) had a higher incidence of moderate to severe vision problems after refractive surgery compared to their small pupil (<5.5 mm) counterparts three months or less postoperatively.

But multiregression analyses considering a number of demographic, treatment and outcome parameters revealed underlying etiology of the disabilities was multifactorial.
Overall, pupil size made a relatively small contribution, accounting for <10% of the variability in responses.

Other factors
Other factors found to independently predict vision complaints included preoperative myopia and postoperative refractive and UCVA outcomes. All of the predictors combined explained no more than 50% of the variability, reported Dr Schallhorn, Director of the Cornea and Refractive Surgery Service.

"Theoretically, based on optical models, one would readily expect pupil size to significantly affect the quality of vision after LASIK. This issue has received a lot of attention in the press and in previous studies.

"Our investigation was designed to further refine our understanding of that relationship and its results indicate pupil size is a relatively small factor in determining quality of vision after LASIK.

Nevertheless, because it does play a role and is a factor that we can control, it is something that should be taken into account preoperatively in terms of selecting optical zone size and cautioning patients appropriately about their risk for night time vision symptoms," Dr Schallhorn explained.

The study was an Institutional Review Board--approved trial enrolling 100 subjects who underwent sequential bilateral myopic LASIK with one month delay between treatment of the first and second eyes.

The cohort included 72 males and 28 females who had an average age of 36 years and average preoperative manifest spherical equivalent (MSE) of - 4.79 D.
Surgery was performed with the Hansatome microkeratome and the VISX S2 laser using a 6 mm optical zone with no transition zone. Follow-up visits were scheduled at one, three and six months after treatment of the first eye; the one month assessments were made prior to treatment of the second eye.

Refractive and visual outcomes showed about 80% of patients achieved 20/20 UCVA at the various postoperative visits, and mean MSE was within about 0.10 D of plano.

Questionnaire on quality of vision
Quality of vision was assessed with a questionnaire administered preoperatively based on vision with glasses and/or contacts and again at the various follow-up visits.
The one month postoperative questionnaire allowed a comparison of vision complaints in the treated and untreated eye.

The comprehensive survey instrument was designed based on findings from initial assessments performed in focus groups of LASIK patients and presented the study subjects with three sets of seven questions each asking them to rate night time problems with glare, halos and haze using a scale of 1 (never) to 5 (severe).
For each patient and for each symptom, an index score was calculated to categorise the overall level of symptom severity.

Of the 100 patients treated, 93 completed the preoperative vision quality questionnaire.
The follow-up rates for postoperative questionnaire completion were relatively high, and further analyses found the clinical outcomes similar among patients who did and did not fill out the questionnaire postoperatively.

Mesopic pupil size was measured in each patient using the Colvard pupillometer to evaluate correlations between pupil size and quality of vision. Those studies showed pupil size for the 93 patients ranged from 4.0 mm to 8.5 mm; 29 patients were categorised as having "small" pupils (<5.5 mm), 44 as having "medium" pupils (5.5 mm to 6.0 mm) and 20 patients formed the "large" (>6.0 mm) pupil group.

The questionnaire results showed complaints of glare, haze, and halo were present even preoperatively while patients were wearing glasses or contact lenses.
At the one month postoperative visit, the average severity score for all three symptoms was increased significantly from the baseline value, but remained <2.0 (mild).

At three months when both eyes had been treated, the average score for all three symptoms was reduced toward the preoperative level, although the average halo severity score was still significantly elevated compared to pretreatment.
By six months, the increases in symptom complaints had generally resolved.

Focusing on glare complaints to characterise the effect of pupil size, the investigators found no relationship between pupil size and the frequency of vision problems before refractive surgery.

But at one month, treated eyes with large pupils had significantly more complaints than those with small pupils. A similar trend was noted at three months, although the difference between the large and small pupil groups did not achieve statistical significance.

Further data analysis focusing on patients with moderate or severe glare disability showed the proportion of patients with those levels of glare responses was significantly increased from baseline to one month among patients with large pupils.

Within the large pupil group, 20% of patients had moderate to severe glare responses preoperatively compared to over 50% at one month.
"Consistent with that data, there was a remarkable difference in vision quality between the treated and untreated eyes at one month after surgery among the large pupil patients," Dr Schallhorn observed.

In contrast, the proportion of patients with small pupils experiencing moderate to severe glare complaints remained essentially unchanged preoperatively to one month postoperatively.

Nevertheless, within each pupil size category, there was a wide distribution in the severity of glare complaints - some patients with large pupils had no problems with glare while some with smaller pupils had significant complaints.

"This data highlights that using pupil size as the only gauge of vision complaints after LASIK does not enable us to fully encompass why patients experience these problems after their surgery," Dr Schallhorn said.

To examine that issue further, forward and backward multiple regression analyses were conducted. Those analyses confirmed the role of pupil size as a marker of vision complaints in the early postoperative time period after LASIK, but demonstrated it plays a minor role overall.

The researchers further identified the level of preoperative myopia and postoperative cylinder and UCVA outcome as additional independent predictors.
For example, quality of vision problems increased two-fold for every 4.0 D increase in preoperative MSE.

All of the predictors combined, however, accounted for no more than 50% of the variability in the responses.

"Perhaps looking at higher order aberrations or other parameters, such as some measure of patient adaptability, may help explain the rest of the variability in these vision complaints," Dr Schallhorn noted.

Top