SLO-MP shows higher structure/function correlation compared with standard perimetry
The thickness of the retinal nerve fiber layer at the macula is a good predictor of reductions in retinal sensitivity in advanced glaucoma.
By Lynda Charters; Reviewed by Sadhana V. Kulkarni, MD
Ottawa, Canada-Thinning of the retinal nerve fiber layer (RNFL) with corresponding reduction in retinal sensitivity are good predictors of progression of glaucoma.
However, there seems to be a higher structure/function correlation when using scanning laser ophthalmoscopy microperimetry (SLO-MP) compared with the Humphrey 10-2 (H-10) standard perimetry in making these inferences.
“Patients with advanced glaucoma likely have dense central 10° field defects that split fixation and make monitoring of progression inaccurate,” said Sadhana V. Kulkarni, MD, who is affiliated with the University of Ottawa Eye Institute and the Ottawa Hospital Research Institute, Ottawa, Canada.
“It has been shown that the correlation between structure and function is a better diagnostic tool than following visual fields alone,” Dr. Kulkarni said. “Evaluating the RNFL sensitivity in areas with RNFL defects might help detect progression earlier in advanced glaucoma.”
Microperimetry may have some advantages over the Humphrey perimetry, she explained, in that the same retinal locus is tested by tracking the blood vessels, there is precise localization of each stimulus with real-time tracking of fixation and better reliability indices, and therefore, a good structure/function correlation.
Dr. Kulkarni and colleagues conducted a single-center, nonrandomized, prospective, longitudinal study in which they compared the efficacy of standard automated perimetry (SAP) with that of SLO-MP for predicting glaucomatous progression at 3-years in 12 patients (18 eyes) with advanced primary angle-closure glaucoma (POAG) who had central 10° visual field defects and to correlate the changes with corresponding sectoral RNFL loss measured by SLO-optical coherence tomography (OCT).
The investigators used a modified 52-points 10-2 SLO-MP grid to evaluate patients within 3 months after their last reliable SAP H-10 test. A Goldmann size III, 200-millisecond stimulus at 1-second intervals was used to administer the SLO-MP test. Ambient illumination was photometrically standardized with the Gossen Starlite instrument to justify the absence of a bowl in SLO-MP test. A standard 3.5-mm CSME (clinically significant macular edema) grid was superimposed on the SLO-MP data and the 52 points were divided into three rings that were 1, 2, and 3 mm, respectively from the center of the macula. The same CSME grid was used to divide the 68 points on the H-10 raw scale into three rings and then compared with the SLO-MP rings by flipping it over the horizontal meridian to maintain retinal topography, Dr. Kulkarni explained.
The primary outcome measure was the correlation of the retinal sensitivity to the mean baseline macular RNFL thickness; the secondary outcome measure was the comparison of the decline in the mean retinal sensitivities of the SLO-MP and Humphrey-10 at 3 years.
The results showed that for ring segment 1 there was no correlation between retinal sensitivity and the mean baseline macular RNFL thickness using the two instruments. However, Dr. Kulkarni noted that with rings 2 and 3 there were significant correlations between the two parameters for both instruments (ring 2, SLO-MP, p <0.04 and H-10, p <0.05; ring 3, SLO-MP, p <0.12 and H-10, p <0.03).
Interestingly, she showed that the decline in overall threshold macular sensitivities measured with the SLO-MP instrument was “impressively more” compared with the H-10 (SLO-MP = -6.12 ± 2.0, H-10= -1.87 ± 2.23), although the decrease with both instruments reached significance.
Since 98% of eyes with advanced glaucoma have perimacular RNFL changes, Dr. Kulkarni commented, early diagnosis of progression in these advanced glaucoma patients through evaluation of the ganglion cell complex, which constitute 35% of the retinal thickness at the macula, rather than the peripapillary RNFL makes sense.
Explaining the results further, she said that no correlations were found between the retinal sensitivity and the baseline RNFL thickness in ring 1 by either instrument probably because even though the retinal ganglion cell thickness correlates well with H-10 loss for the central 7.2° from the fovea; there is a great deal of ganglion cell displacement within the 1-millimeter of the center of the fovea. However, correlations were significant in rings 2 and 3 with SLO-MP than H-10 as SLO-MP detects glaucomatous change better in areas with advanced RNFL thinning that can go undetected by H-10.
The clinical implications of early detection of progression are “huge,” she noted.
“Early detection of progression in these advanced cases could help preserve the severely compromised visual fields,” Dr. Kulkarni emphasized. “If the exact location of the preferred retinal locus could be identified, precise residual visual field mapping could aid in the development of low -vision aids.”
The thickness of the retinal nerve fiber layer by SLO-OCT and retinal sensitivity measurements obtained with SLO-MP together are good predictors for progression of glaucoma compared with the Humphrey 10-2 perimetry evaluation. There is a trend toward higher structure/function correlation with SLO-OCT and SLO-MP, she concluded.
Sadhana V. Kulkarni, MD
Dr. Kulkarni has no financial interest in the subject matter. This article is adapted from Dr. Kulkarni’s presentation at the annual meeting of the American Academy of Ophthalmology.