DOUBLE-PASS ABERROMETRY

Double-pass aberrometry performed with a commercially available platform (OQAS, Visiometrics) and specialised software shows promise as an objective method for evaluating tear film quality and understanding the visual complaints of patients affected by dry eye syndrome (DES), said Pierre- Jean Pisella MD, PhD. Speaking at the 2nd EuCornea Congress, Dr Pisella described the association between tear film quality and visual complaints of DES to establish the rationale for using double-pass aberrometry for dynamic analysis of the tear film. He also reported the findings from clinical studies investigating aberrometric evaluation as a diagnostic and monitoring tool for DES that showed the test result differed significantly depending on DES severity. 'This new objective technique may help us to quantify the blurry vision associated with DES and help us to detect and further understand tear film-related patient complaints, especially in individuals with moderate dry eye who may not yet present with clinical signs such as corneal surface staining. Furthermore, the measurements may be a new tool for evaluating the efficacy of new artificial tears and other treatments for DES and other ocular surface diseases,' said Dr Pisella, professor of ophthalmology, Bretonneau Hospital, University Francois Rabelais, Tours, France.

Although the definition of DES issued by the International Dry Eye Workshop in 2007 identifies symptoms of visual disturbance as a consequence of DES, there is not always good correlation between the symptoms of patients with DES and their clinical signs. This mismatch is particularly true in patients with moderately severe DES who may have significant subjective complaints, including visual disturbances, but without significant corneal staining and only moderate changes in tear film breakup time (TBUT). Compounding the difficulty of interpreting the results of clinical evaluation is the fact that some currently used tests are not highly reproducible, said Dr Pisella. 'For example, we know that the results of TBUT testing in the same individual can be different when the test is performed by two different examiners. Considering these issues, a reliable approach for functional evaluation of DES patients would be very helpful.'

The rationale for using double-pass aberrometry for this purpose derives from knowledge that mucin deficiency resulting from a decrease in the goblet cell population is a common feature in patients with DES. Mucin deficiency leads to tear film instability and irregularity that is functionally expressed as visual disturbances. Dr Pisella explained that when the tear film thickness is normal, the refractive power of the cornea is 43.08 D. However, in patients with DES, the tear film thickness becomes irregularly decreased. Consequently, the anterior radius is reduced and the power of the cornea can be changed by up to +1.30 D while high order aberrations can also appear. 'Tear film quality plays a role in the optical quality of the eye, and double-pass aberrometry allows analysis of the changes in both high order aberrations and light scattering associated with an irregular tear film,' Dr Pisella said. In order to use double-pass aberrometry to evaluate tear film quality, special software was developed to quantify scattering and calculate an Ocular Scattering Index (OSI). Forty patients were enrolled in a study, of which 20 had mild DES, 13 had moderate DES, and seven had severe DES based on DEWS categorisation criteria. The testing was performed over a 20-second interval and repeated four times. A mean OSI value as a function of time, between blinks, was calculated for each patient and used to calculate subgroup means for statistical comparison.

The mean OSI value increased with increasing severity of DES, from 1.86 in the mild group to 5.22 in the severe group, and was significantly higher in the severe DES group compared with the mild and moderate groups. Further analyses aimed to control for potential confounding factors associated with abnormalities in other ocular structures, including the cornea, crystalline lens and vitreous and to take into account the effect of blinking, which results in a temporary change in the OSI and is more frequent with increasing dry eye severity. Even with these adjustments, the mean OSI remained significantly higher in the severe DES patients compared to the other two groups. Additional testing provided early evidence supporting use of the aberrometric evaluation for determining the effect of DES treatment. Dr Pisella described two patients, one with a low tear film osmolarity of 284 mosml/L and the other a DES patient with an elevated tear film osmolarity of 326 mosml/L. In the patient with low osmolarity, artificial tear instillation had no effect on OSI comparing measurements obtained at baseline and 15 minutes after the drop was instilled. However, the OSI decreased and was stabilised after instillation of the artificial tear drop in the DES patient.