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Editors Selection IGR 8-3

Examination methods: Laser scanning: Enhanced corneal compensation

Christopher Bowd

Comment by Christopher Bowd on:

18177 Structure-function relationship is stronger with enhanced corneal compensation than with variable corneal compensation in scanning laser polarimetry, Mai TA; Reus NJ; Lemij HG, Investigative Ophthalmology and Visual Science, 2007; 48: 1651-1658


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The release of scanning laser polarimetry (SLP) with enhanced corneal compensation (ECC), a technology developed to boost SNR and eliminate previously described atypical birefringence patterns (ABPs), has resulted in several papers comparing the diagnostic accuracy and the association with visual function observed with this updated SLP technology with that of the previous variable corneal compensation (VCC) version. Mai et al. (588)

ABPs are considerably less frequent using ECC compared to VCC, indicating that the ECC technology is effective. Structurefunction associations are almost always stronger using ECC compared to VCC. Structurefunction associations using VCC are strengthened when scans with ABPs are excluded from the analysis
compare (R2) the structure-function relationships between GDx-derived retinal nerve fiber layer (RNFL) thickness measurements (using both ECC and VCC) and standard automated perimetry-derived visual function measurements in 33 healthy eyes and 68 glaucoma eyes.

First, the results show that ABPs (somewhat arbitrarily defined) are considerably less frequent using ECC (2/101) compared to VCC (41/101),indicating that the ECC technology is effective.Second (and most important for this study), results indicate that structure-function associations are almost always stronger using ECC compared to VCC (e.g., difference in R2 up to 0.22 in the inferotemporal RNFL) and that structure-function associations using VCC are strengthened when scans with ABPs are excluded from the analyses. While these results (and results from other studies, all of which are in agreement1,2) are not surprising (scans with severe ABPs are primarily noise), they are important because they provide evidence that ECC images provide a better cross-sectional representation of visual function and therefore are likely a more accurate representation of anatomical reality, compared to VCC images. The results of Mai and colleagues, and those of others,1-7 suggest that SLP images with apparent ABPs likely should not be used for glaucoma diagnosis. However, it is possible (yet undetermined) that these images remain valuable for monitoring glaucomatous change over time.

References

  1. Bowd C, et al. Ophthalmology 2007; 114: 1259-65.
  2. Sehi XXX, et al. Am J Ophthalmol 2007; 143: 272-9.
  3. Bowd C, et al. IOVS 2007; 48: 223-7.
  4. Da Pozzo XXX, et al. Eye 2006; 20: 769-75.
  5. Mai TA, Reus NJ, Lemij HG. Ophthalmology 2007: Currently online only.
  6. Medeiros F, et al. IOVS 2007; 48: 3146-53.
  7. Reus XXX, et al. IOVS 2006; 47: 3870-7.

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