Monitoring glaucoma progression with SLP

SLP has excellent reproducibility and may be useful in monitoring glaucoma progression.

Scanning laser polarimetry (SLP), which is used as part of the process to diagnose glaucoma, measures the thickness of the retinal nerve fibre layer (RNFL). It takes this measurement from the change of polarization that occurs when light passes through birefringent tissue, such as the microtubules of the cornea and the RNFL: the amount of light retardance exhibited by the RNFL is proportional to its thickness and it is this retardance that produces the measurement data.

The anterior segment of the eye is also composed of birefringent tissue and the most recently available commercial version of the SLP instrument (the GDx) is the GDx-ECC. This improves the diagnostic precision of the images by using a special technique with enhanced corneal compensation to eliminate (at least in part) corneal birefringence.

Dr Mirian Ara Gabarre, Hospital Universitario Miguel Servet (HUMS), Zaragoza, Spain, clarified, "We conducted a study to determine the reproducibility and repeatability of peripalliary RNFL parameters measured using this SLP with enhanced corneal compensation (GDx-ECC) in glaucomatous eyes."

Recruitment criteria for the study

"The subjects in our study were aged from 18 to 80 years (mean age: 58.27 ± 8.95 years; p=0.09) and were eligible for inclusion if they had best corrected visual acuity (BCVA) of a minimum of 20/30. All participants underwent a comprehensive eye examination and at least one reliable standard automated perimetry test (Humphrey, 24-2 SITA Standard)," explained Dr Gabarre. "Eyes were classified as glaucomatous if they had an intraocular pressure (IOP) of greater than 21 mmHg; ‘abnormal visual field (VF)’ was defined as pattern standard deviation outside of the 95% normal confidence limit."

Prospective study subjects were then excluded based on the following criteria:

• A history of intraocular surgery;
• Spherical refraction of greater than 5 dioptres;
• Cylinder correction of more than 3 dioptres; and/or
• Presence of any other ocular systemic diseases that would affect vision.

"Patients were also excluded if the GDx images that had been obtained were of poor quality," added Dr Gabarre. "Informed consent was obtained from all subjects, and only one eye per patient was randomly included in the statistical analysis."

Methods of gaining assessment data

From the initially recruited subjects, 54 consecutive glaucomatous subjects were prospectively selected. To reduce factors that may decrease reproducibility and repeatability, all images were obtained by a single, experienced operator, who used the GDxPRO (Carl Zeiss Meditec, Jena, Germany) to measure the thickness of the peripapillary retinal nerve fibre layers three times on the same day, and then three more times on separate days in a 5-week period.

Mean deviation (MD) and standard deviation (SD) were used for descriptive statistics. Intraclass correlation coefficient (ICC), coefficient of variation (COV) and test–retest variability were calculated for all SLP parameters. These were later calculated using SPSA software.

Divergence from clinical practice

"There were a number of deviations from normal clinical practice in this study setting," Dr Gabarre revealed. "For example, although the manufacturer of the GDxPRO recommends that measurements are taken from the average of three separate readings to produce an accurate examination report in clinical practice, for this study we trusted the reproducibility of single measurements."

"Additionally, although we used only a single technician to obtain all of the images, this operator had been made aware that the images were to be used in a study," continued Dr Gabarre. "It is therefore likely that the SLP measurements that were taken had greater accuracy (and therefore greater reproducibility and repeatability) than would possibly be obtained in normal clinical daily routine."

If further studies in this area were to be performed, Dr Gabarre advised bringing the methodology closer in line with clinical practice, to produce results that could be repeated in a real-world environment.

Results of the study: reproducibility is excellent

"Mean deviation of standard automated perimetry was -7.04 ± 7.04 dB," reported Dr Gabarre. "While we established that reproducibility for all SLP parameters is good (with ICCs of higher than 0.91 for all parameters), the nerve fibre indicator (NFI) was the parameter that presented the most variability, by showing the highest ICC in the intratest sequence (0.982; 95% confidence interval: 0.972-0.989; p

She added, "In this study, we did not assess any correlation between GDx and visual field, but this may be an area suitable for further study in the future."

Conclusions

"Based on the findings from this prospective study, the GDx-ECC technique creates more accurate images compared with earlier GDx instruments," Dr Gabarre said, "Additionally, the GDxPRO has an excellent intravisit and intervisit reproducibility in glaucoma patients, which indicates that SLP is an imaging technology that may be useful in monitoring glaucoma progression as well as in diagnosing the condition."