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Karen Hong, Robert Chang, Ian Wong, Kuldev Singh; Corneal Applanation Velocity as a Risk Factor for Normal Tension Glaucoma. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4241.
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To determine if a novel biomechanical parameter, corneal applanation velocity, may aid in the diagnosis of normal tension glaucoma (NTG).
This prospective, cross-sectional study enrolled 74 NTG and 143 control subjects examined consecutively at Queen Mary Hospital in Hong Kong over an eight-week period (Table 1). Only one eye from each patient was selected. Inclusion criteria for NTG subjects were an existing NTG diagnosis with confirmatory visual field defects and nerve fiber layer loss as well as treatment with selective laser trabeculoplasty and/or topical glaucoma medications. Subjects were excluded if they had corneal disease or prior incisional surgery that may have affected corneal biomechanics. Subjects underwent corneal biomechanical testing with the Corvis ST noncontact pneumotonometer. The parameters measured included intraocular pressure (IOP), central corneal thickness (CCT), deformation amplitude, time of highest concavity, peak distance of deformation, radius, and inward and outward applanation time, length, and velocity. Based upon prior research, the prospectively designated primary outcome measure was inward applanation velocity. Corvis output measurements were adjusted for age and CCT in a logistic regression model.
Mean IOP was 15.0 mmHg for NTG eyes and 15.3 mmHg for control eyes (p=0.45). Mean CCT was 540 μm and 554 um for NTG and control eyes respectively. (p=0.002). Inward applanation velocity was found to be faster in NTG eyes (0.15 m/s ± 0.02) than in control eyes (0.14 m/s ± 0.02) (p=0.003) after adjusting for age and CCT. The unadjusted and adjusted inward applanation velocity odds ratios were 1.20 [1.06-1.36], p=0.003 and 1.21 [1.07-1.38], p=0.003 respectively (Table 2). The adjusted c-statistic was 0.69. None of the secondary descriptors were significantly correlated with a diagnosis of NTG.
NTG subjects demonstrated faster inward applanation velocity than controls when tested with the Corvis ST suggesting that this parameter may serve as a risk factor for glaucomatous disease in those with normal IOP.
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