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R. M. Saltzmann, J. T. Whitson, M. B. Wax; Pneumatonography in Normal Pressure Glaucoma. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2850.
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To determine whether tonography, which has largely been abandoned as an ancillary tool in clinical glaucoma diagnosis, may have some remaining utility in the initial work-up of normal-pressure glaucoma (NPG). Specifically, we sought to determine whether tonography can reliably distinguish NPG from "burned-out" forms of primary open angle glaucoma (POAG), particularly in unclear cases where non-glaucomatous optic neuropathy may be suspected and further diagnostic work-up is being considered.
A prospective, non-randomized, non-blinded experimental cohort design with a retrospective case-control arm was used. Eyes of volunteer patients with NPG, POAG, and healthy controls meeting study inclusion and exclusion criteria underwent 2-minute pneumatonography. Primary outcome measures were aqueous outflow facility (C) and a related parameter, P0/C. Repeated eyes were assessed for reproducibility. The utility of tonography in the context of any previous diagnostic testing or neuroimaging was evaluated retrospectively.
46 eyes of 27 volunteer patients were included, comprised of 36 eyes with NPG, 4 eyes with POAG, and 6 controls. Among eyes that underwent repeat testing (n=22), the pneumatonometer machine demonstrated modest reproducibility with both primary outcome measures, based upon several indices, including the within-subject coefficient of variability and intraclass correlation coefficient. Measured mean outflow facility (C) for NPG was 0.386±0.03 µl/min/mmHg with a large range, from 0.11-0.68. Mean C for POAG and control eyes was 0.16±0.01 and 0.316±0.06 µl/min/mmHg, respectively. There was a statistically significant difference between groups (p=0.01), accounted for by the difference between NPG and POAG mean outflow facility. Cutoff values of ≤0.18 µl/min/mmHg and ≥64 mmHg2/µl/min for C and P0/C, respectively, were determined by ROC analysis to distinguish eyes in the POAG-range. By these thresholds, up to 6 eyes of 3 patients initially labeled as NPG were suspect to harbor burned-out POAG. Among patients who had undergone previous extensive diagnostic testing, neither C nor P0/C was predictive of the work-up yield.
The pneumatonography device demonstrated only modest reproducibility, but it remains the only practical and readily available way to perform tonography in many clinical settings. Tonography does appear to distinguish between NPG and POAG on average and may serve as an ancillary testing modality in unclear or atypical cases. A POAG-like tonography profile in an eye suspected of NPG or non-glaucomatous optic neuropathy should discourage any further laboratory testing or neuroimaging, which typically have a very low diagnostic yield in this setting.
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