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J. Wang, M. R. Lesk; Estimation of Ocular Rigidity Using Ocular Pulse Amplitude and Pulsatile Choroidal Blood Flowmetry in Glaucoma. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4901.
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Ocular rigidity likely plays an important role in pathogenesis of various posterior segment diseases such as glaucoma and possibly age-related macular degeneration. Our goal is to develop a non-invasive method of estimating ocular rigidity in vivo. Our hypotheses include that glaucoma patients have lower ocular rigidity compared to normal and ocular hypertensive (OHT) patients and that ocular rigidity correlates with the degree of glaucoma damage and with the axial length.
A cohort of subjects with diagnoses ranging from normal (N), glaucoma suspect (GS), mild to advanced open-angle glaucoma (OAG) and OHT were recruited. Subjects with incisional ocular surgery were excluded. For each subject, we measured, in one single session, the ocular pulse amplitude (OPA) using the Pascal dynamic contour tonometer and the pulsatility of the sub-foveolar choroidal blood volume (chBV) using laser Doppler flowmetry (Oculix). Based on the Friedenwald equation (E = Δ IOP / Δ V), the coefficient of ocular rigidity (E) is calculated by relating OPA (as ΔIOP) to chBV (as ΔV). A higher E value signifies a more rigid eye and a lower E, a more elastic eye. Visual field mean defects (MD) and axial length (AL) were recorded for correlation analysis. For statistical analysis (SPSS 16), we compared the value of E between the four diagnostic groups and correlated the value of E to MD and AL.
261 patients were included (53 N; 51 OHT; 51 GS; 106 OAG). The mean E value was lowest in OAG group (10.65±5.25) and highest in the OHT group (15.94±8.39). The difference between the two groups was statistically significant (p = 0.004). The E value for the N and GS groups were in between those of the OHT and OAG groups (N: 12.21±7.01; GS: 12.53 ±6.44). E correlated positively with MD across the four groups (R = 0.17; p = 0.01) and this positive correlation was stronger in the OHT group (R = 0.37; p = 0.01). E also correlated negatively with AL across the four groups (R = -0.324; p < 0.001).
The present study showed that OAG patients had the lowest ocular rigidity and that OHT patients had the highest, with N and GS patients in between the two extremes. This result is compatible with biomechanical studies that show that more elastic eyes have greater mechanical stress and strain at the optic nerve head, and might therefore be more susceptible to glaucoma damage. The positive correlation of MD with E and its negative correlation with AL suggested that more elastic eyes are associated with greater visual field damage and that a longer eye is more elastic. Estimating ocular rigidity using OPA and chBV is a promising avenue and further validating studies are warranted.
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