Dear Editor

The paper by Munkvitz et al. deals with the interpretation of the Nerve Fiber Analyzer (NFA) printout in a sample of healthy and advanced or early glaucomatous eyes. Three independent readers, at different levels of clinical experience, classified GDx printouts while being masked with respect to the eye condition and optic disc pictures. A questionnaire was used by readers to determine diagnosis but no additional information about this procedure is provided. To the best of our knowledge there is no consensus regarding a standardized procedure to evaluate the GDx printout. Furthermore, other than color-coded RNFL thickness map, absolute values of GDx parameters and their levels of probability ( <_10 or="or" _5="_5" criteria="criteria" used="used" to="to" classify="classify" examined="examined" eyes="eyes" are="are" not="not" defined.="defined." an="an" even="even" greater="greater" flaw="flaw" is="is" anterior="anterior" segment="segment" birefringence="birefringence" compensation="compensation" and="and" its="its" evaluation="evaluation" by="by" means="means" of="of" macular="macular" area="area" imaging="imaging" _1.="_1." the="the" authors="authors" employed="employed" nfa="nfa" with="with" fixed="fixed" corneal="corneal" whose="whose" limited="limited" ability="ability" remove="remove" unwanted="unwanted" in="in" most="most" has="has" been="been" demonstrated="demonstrated" previously="previously" _23="_23" but="but" they="they" did="did" image="image" area.="area." thus="thus" rnfl="rnfl" thickness="thickness" may="may" have="have" overestimated="overestimated" up="up" _20="_20" a="a" certain="certain" portion="portion" studied="studied" _3.="_3." apart="apart" from="from" significantly="significantly" reduced="reduced" sensitivity="sensitivity" this="this" severely="severely" affect="affect" gdx="gdx" printout="printout" separate="separate" healthy="healthy" glaucomatous="glaucomatous" _45="_45" due="due" artificially="artificially" thick="thick" rnfl.="rnfl." spite="spite" values="values" were="were" _100="_100" _90="_90" for="for" reader="reader" _1="_1" _2="_2" respectively.="respectively." at="at" least="least" advanced="advanced" glaucoma="glaucoma" these="these" extremely="extremely" high="high" could="could" be="be" related="related" mean="mean" defect="defect" mentioned="mentioned" text="text" it="it" quite="quite" surprising="surprising" that="that" early="early" nfb="nfb" defects="defects" correctly="correctly" identified="identified" classified="classified" all="all" cases.="cases." methodological="methodological" problems="problems" limit="limit" significance="significance" impact="impact" otherwise="otherwise" interesting="interesting" paper.="paper." p="p">References

(1) Greenfield DS, Knighton RW, Huang XR. Effect of corneal polarization axis on assessment of retinal nerve fiber layer thickness by scanning laser polarimetry. Am J Ophthalmol 2000;129:715-22.

(2) Weinreb RN, Bowd C, Greenfield DS, Zangwill LM. Measurement of the magnitude and axis of corneal polarization with scanning laser polarimetry. Arch Ophthalmol 2002;120:901-6.

(3) Choplin NT, Zhou Q, Knighton RW. Effect of individualized compensation for anterior segment birefringence on retinal nerve fiber layer assessment as determined by scanning laser polarimetry. Ophthalmology 2003;110:719-25.

(4) Greenfield DS, Knighton RW, Feuer WJ, Schiffmann JC, Zangwill LM, Weinreb RN. Correction for corneal polarization axis improves the discriminating power of scanning laser polarimetry. Am J Ophthalmol 2002;134:27-33.

(5) Bowd C, Zangwill LM, Berry CC, Blumenthal EZ, Vasile C, Sanchez- Galeana C et al. Detecting early glaucoma by assessment of retinal nerve fiber layer thickness and visual function. Invest Ophthalmol Vis Sci 2001;42:1993-2003

Dear Editor

Recently, Munkwitz et al. reported excellent sensitivities and specificities of scanning laser polarimetry in detecting glaucoma when printouts were subjectively evaluated by experts having at least nine years experience with the device [1].

The authors clearly defined a major difference with previously published studies, in that they evaluated GDx printouts of both eyes, although 23 of the glaucoma patients' eyes demonstrated no detectable nerve fiber bundle defecton red free photography. Although potentially causing someselection bias, this resembles a "real-life" situation, where patients afflicted with glaucoma often present with asymmetric damage. In a recent study we conducted using the Heidelberg Retina Tomograph, information from both eyes in the form of an asymmetry ratio detected and correlated with disease severity [2], and thus asymmetry may explain in part some of the authors success in subjectively detecting glaucoma.

Although the authors' conclusion that "training in interpreting GDx printouts can greatly improve sensitivity" has yet to be proven, perhaps the experts may share with their readers some of the clinical pearls that were used in their subjective assessment so as to improve our level of clinical care.

References

(1) Munkwitz S, Funk J, Loeffler KU et al, Sensitivity and specificity of scanning laser polarimetry using the GDx., Br J Ophthalmol. 2004 Sep;88(9):1142-5.

(2) Harasymowycz P, Davis B, Xu G et al, The use of RADAAR (ratio of rim area to disc area asymmetry) in detecting glaucoma and its severity., Can J Ophthalmol. 2004 Apr;39(3):240-4.