Orbscan II alternative to infrared pupillometry

By Cheryl Guttman

ORLANDO, FL — The Orbscan II® (Bausch & Lomb) provides a valid alternative to infrared pupillometry for calculating mesopic pupil size in patients undergoing excimer laser refractive surgery, according to a US ophthalmologist.
Speaking at the annual meeting of the American Academy of Ophthalmology, Lewis R. Groden MD and colleagues conducted a study comparing mesopic pupil measurements obtained using the Keeler infrared pupillometer (Fairville Medical Optics) and the Orbscan II. They used both techniques to study 186 eyes of 93 consecutive patients undergoing LASIK.

The Keeler pupillometer measurements were obtained in very dim light conditions and were recorded to the nearest 0.1 mm.
The Orbscan II examination was conducted under normal light and since light is projected during data acquisition, an arbitrary value of 2.5 mm was added to the pupil size reported to account for pupillary constriction occurring under the higher luminance conditions.

Statistical analyses performed using both a two sample T-test and the Wilcoxon rank-sum test showed there was no significant difference in the mean pupil size values determined by adding 2.5 mm to the pupil size obtained with the Orbscan II and the meospic pupil size measurements acquired using the infrared pupillometer.
Interestingly, and somewhat surprisingly, comparison of the standard deviation values for each method showed there was less variability using the Orbscan II. Mean pupil sizes were 6.27 ± 0.53 mm with the Orbscan II and 6.20 ± 0.84 mm with the Keeler pupillometer, Dr Groden reported.
“Refractive surgeons recognise that determining mesopic pupil size is an important measurement in planning LASIK procedures because a mesopic pupil smaller than the ablation area may contribute to the development of glare, halos and other night vision disturbances after LASIK.

“The results of our investigation demonstrate that the Orbscan II can be used to provide pupil measurements comparable to Keeler infrared pupillometry,” Dr Groden said.
He noted that there are potential benefits for measuring pupil size with the Orbscan II if that device is already being used for topography and pachymetry studies.
It avoids the need to acquire an infrared pupillometer, which can cost several thousand euro. Factors such as medications do not affect the accuracy of the measurement obtained with the Orbscan II.

Moreover, patient anxiety levels which can interfere with the accuracy of mesopic pupil size measurements with infrared pupillometry. Furthermore, it offers added convenience since the patient is spared an additional examination.
Although the results of this study indicate surgeons can rely on pupil size data obtained with the Orbscan II, Dr Groden still evaluates pupil size with both techniques in his own practice.

“I continue to routinely measure the mesopic pupil size with the Keeler infrared pupillometer as well as the Orbscan II because I think having two readings is an important enough piece of surgical planning data,” he said.
Dr Groden also reported that in the statistical evaluation of the data obtained with the two methods of measuring pupil size, a significant linear regression correlation became evident and a simple linear regression model was developed that allows even more precise calculation of pupil size with one technique when knowing the value of the other.

According to the linear regression model Keeler mesopic pupil size equals 3.77 mm + 0.40 mm and Orbscan II pupil size is + 2.5 mm.
“Adding 2.5 mm to the Orbscan II value provides a mesopic pupil size which is statistically equivalent to the measurement obtained with the infrared pupillometer.
“To be more precise, you could calculate the value with this linear regression model, although I don’t use it in everyday practice and I expect most other surgeons would also simply add 2.5 mm to the Orbscan II data,” Dr Groden said.

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