LETTER Comment on the role of 3T diffusion tensor imaging in glaucoma We appreciate Dr Thakur ’s interest in our work, 1 and we are grateful to him for giving us the opportunity to clarify certain aspects of our findings. As stated in the Methods section, the visual-field criteria used for diagnosis of glau- coma consisted mainly of ‘deficits typical of glaucoma’, in particular ‘(1) corrected pattern SD less that 5% and/or glaucoma hemifield test results exceeding normal limits (2) absence of other disease that would explain the visual field abnormality (3) fixation loss rate of 25% or less and (4) false positive and false negative rates below 15%’. Criterion 1 has been used in numerous studies where the presence of at least one of the findings listed was considered sufficient (see eg, refs. 2 and 3). As for criterion 2, the absence of other disease that would explain the visual field abnormality was obviously a fundamental criterion for exclusion during enrolment as stated in the Methods section. The fact that it also appears among the criteria for diagnosis of glaucoma is, admit- tedly, potentially confusing, but we merely wanted to stress that the visual field deficits were not caused by systemic disease or by eye disease other than glaucoma. As far as the fixation loss rate (criterion 3) is concerned, it is true that cut-offs as high as 33% have been used in some studies (eg, ref. 4). However, the cut-off we chose (25%) has also been used in others (eg, refs. 3 and 5). Similar considerations apply to the false posi- tive/false negative rates (criterion 4): a cut-off of 33% is indeed used by some investigators’ studies (eg, ref. 4). We deliberately chose a more restrictive cut-off for these parameters to increase the reliability of the diagnostic criteria based on visual field testing. The significance threshold used for the single correlation coefficient matrix was p<0.004 because the conventional value of p<0.05 was subjected to Bonferroni correction (for the study: 0.05/12=0.004). Bonferroni cor- rection is not applicable when the single vari- ables are included in a multiple regression analysis. In this case, findings reported as sig- nificant with a p<0.01 or p=0.01 referred to the multiple regression analysis, where the universally accepted significance threshold is p<0.05. We are also grateful to Dr Thakur for pointing out some typographical errors in our article: the asymmetrical vertical cup/ disc ratio cut-off should indeed be 0.2 (not 0.5 as stated); Corrected Pattern SD should read Pattern SD, and the reference relative to our group’s DTI study should be no. 11 (ref. 6 listed below) instead of no. 10. It is important to stress that our aim in this study was simply to see how diffusion tensor-MRI findings at the level of the optic nerve correlate with those of scanning laser polarimetry (GDx-VCC), optical coherence tomography (Stratus-OCT) and Heidelberg retina tomograph (HRT-III) in patients with glaucoma. Consequently, our findings shed no light on the question of whether or not DTI can be used to distinguish between axonal loss caused by glaucoma, and that caused by other diseases. This would have required at the very least a group of patients with different types of non-glaucomatous optic nerve disease. Dr Thakur ’s question cor- rectly highlights the need for additional research on these aspects. Carlo Nucci, 1 Raffaele Mancino, 1 Alessio Martucci, 1 Francesca Bolacchi, 2 Guglielmo Manenti, 2 Claudio Cedrone, 1 Franco Culasso, 3 Roberto Floris, 2 Luciano Cerulli, 1 Francesco Giuseppe Garaci 2,4 1 Ophthalmology Unit, Department of Experimental Medicine and Surgery, Universityof Rome Tor Vergata, Rome, Italy; 2 Department of Diagnostic Imaging, Molecular Imaging, Interventional Radiology and Radiotherapy, University of Rome Tor Vergata, Rome, Italy; 3 Health Statistics, Department of Experimental Medicine, University of Rome La Sapienza, Rome, Italy; 4 IRCCS San Raffaele, Rome, Italy Correspondence to Professor Carlo Nucci, Ophthalmology Unit, Department of Experimental Medicine and Surgery, University of Rome Tor Vergata, Via Montpellier 1, Rome 00133, Italy; nucci@med.uniroma2.it Contributors This letter has been conceptualised, designed and drafted by the equal contribution of all the authors. Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors. Competing interests None. Provenance and peer review Not commissioned; internally peer reviewed. Accepted 11 August 2012 Br J Ophthalmol 2012;00:1. doi:10.1136/bjophthalmol-2012-302472 REFERENCES 1. Nucci C, Mancino R, Martucci A, et al. 3T Diffusion tensor imaging of the optic nerve in subjects with glaucoma: correlation with GDx-VCC, HRT-III and stratus optical coherence tomography findings. Br J Ophthalmol 2012;96:976–80. 2. Bowd C, Tafreshi A, Zangwill LM, et al. Pattern electroretinogram association with spectral domain-OCT structural measurements in glaucoma. Eye 2011;25:224–32. 3. Bowd C, Balasubramanian M, Weinreb RN, et al. Performance of confocal scanning laser tomograph Topographic Change Analysis (TCA) for assessing glaucomatous progression. Invest Ophthalmol Vis Sci 2009;50:691–701. 4. Sakata K, Sakata LM, Sakata VM, et al. Prevalence of glaucoma in a South brazilian population: Projeto Glaucoma. Invest Ophthalmol Vis Sci 2007;48:4974–9. 5. Reus NJ, Lemij HG. Relationships between standard automated perimetry, HRT confocal scanning laser ophthalmoscopy, and GDxVCC scanning laser polarimetry. Invest Ophthalmol Vis Sci 2005;46:4182–8. 6. Garaci FG, Bolacchi F, Cerulli A, et al. Optic nerve and optic radiation neurodegeneration in patients with glaucoma: in vivo analysis with 3-T diffusion-tensor MR imaging. Radiology 2009;252:496–501. Br J Ophthalmol Month 2012 Vol 00 No 00 1 PostScript BJO Online First, published on September 8, 2012 as 10.1136/bjophthalmol-2012-302472 Copyright Article author (or their employer) 2012. Produced by BMJ Publishing Group Ltd under licence. group.bmj.com on November 5, 2012 - Published by bjo.bmj.com Downloaded from