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Editors Selection IGR 9-4

Examination methods: Compensated polarimetry

Comment by Felipe Medeiros on:

14041 Enhanced imaging algorithm for scanning laser polarimetry with variable corneal compensation, Reus NJ; Zhou Q; Lemij HG, Investigative Ophthalmology and Visual Science, 2006; 47: 3870-3877

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Since its introduction as a method for evaluating the retinal nerve fiber layer, scanning laser polarimetry (SLP) has undergone significant changes both in its hardware and software. The introduction of variable corneal compensation in the GDx VCC (Carl-Zeiss Meditec, Inc., Dublin, CA) has resulted in improved diagnostic accuracy compared to earlier versions of the technology, which employed fixed corneal compensation. However, shortly after the introduction of the GDx VCC, several scans showing atypical retardation patterns (ARP) were observed in some patients. These scans show irregular patches of elevated retardation values which do not match the expected retardation based on the RNFL anatomy. ARPs seem to result from poor signal-to-noise ratio as a consequence of light scattering in the eye. In the work by Reus et al. (758), the authors introduce a new imaging algorithm for SLP, which is designed to minimize ARPs. This method can be incorporated into the GDx VCC instrument and does not require changes to the hardware. In ECC, a known birefringence bias is introduced into the measurement beam path to shift the measurement of total retardation into a more sensitive region of the curve of detection of polarization of the instrument. After image acquisition, the birefringent bias is removed mathematically, point-by-point, to yield the RNFL retardation values. The authors tested the method in 177 eyes from 29 healthy subjects, 70 glaucomatous patients and 78 subjects with ocular hypertension. According to their subjective assessment, images obtained with the ECC method showed the expected appearance of the RNFL better than images obtained using the VCC method. They also showed an objective increase in the typical scan score (indicating less ARPs) in ECC images compared with VCC. With regards to the ability to discriminate glaucomatous from healthy eyes, there was an improvement in some of the RNFL parameters with the ECC method. However, glaucoma patients included in their study had severe disease, with an average MD of -12.15dB. It will be important to evaluate the accuracy of the new technique for detecting damage in patients with less severe disease and in glaucoma suspects who do not show evident damage such as visual field loss. Longitudinal studies will also be necessary to evaluate its ability to detect change over time.

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