Manbir Nagra

Shape of the posterior vitreous chamber in human emmetropia and myopia

PURPOSE To compare posterior vitreous chamber shape in myopia to that in emmetropia. METHODS Both eyes of 55 adult subjects were studied, 27 with emmetropia (mean spherical error [MSE] ≥ -0.55; <+0.75 D; mean +0.09 ± 0.36 D) and 28 with myopia (MSE -5.87 ± 2.31 D). Cycloplegic refraction was measured with a Shin Nippon autorefractor and anterior chamber depth and axial length with a Zeiss IOLMaster. Posterior vitreous chamber shapes were determined from T2-weighted magnetic resonance imaging (3.0-T) using procedures previously reported by our laboratory. Three-dimensional surface model coordinates were assigned to nasal, temporal, superior, and inferior quadrants and plotted in two dimensions to illustrate the composite shape of respective quadrants posterior to the second nodal point. Spherical analogues of chamber shape were constructed to compare relative sphericity between refractive groups and quadrants. RESULTS Differences in shape occurred in the region posterior to points of maximum globe width and were thus in general accord with an equatorial model of myopic expansion. Shape in emmetropia is categorized distinctly as that of an oblate ellipse and in myopia as an oblate ellipse of significantly less degree such that it approximates to a sphere. There was concordance between shape and retinotopic projection of respective quadrants into right, left, superior, and inferior visual fields. CONCLUSIONS Prolate ellipse posterior chamber shapes were rarely found in myopia, and we propose that spherical shape in myopia may constitute a biomechanical limitation on further axial elongation. Synchronization of quadrant shapes with retinotopic projection suggests that binocular growth is coordinated by processes that operate beyond the optic chiasm.

Ocular biometric correlates of ciliary muscle thickness in human myopia

Anterior segment optical coherent tomography (AS‐OCT) is used to further examine previous reports that ciliary muscle thickness (CMT) is increased in myopic eyes. With reference to temporal and nasal CMT, interrelationships between biometric and morphological characteristics of anterior and posterior segments are analysed for British‐White and British‐South‐Asian adults with and without myopia.

Estimation of ocular volume from axial length

Background/aims To determine which biometric parameters provide optimum predictive power for ocular volume. Methods Sixty-seven adult subjects were scanned with a Siemens 3-T MRI scanner. Mean spherical error (MSE) (D) was measured with a Shin-Nippon autorefractor and a Zeiss IOLMaster used to measure (mm) axial length (AL), anterior chamber depth (ACD) and corneal radius (CR). Total ocular volume (TOV) was calculated from T2-weighted MRIs (voxel size 1.0 mm3) using an automatic voxel counting and shading algorithm. Each MR slice was subsequently edited manually in the axial, sagittal and coronal plane, the latter enabling location of the posterior pole of the crystalline lens and partitioning of TOV into anterior (AV) and posterior volume (PV) regions. Results Mean values (±SD) for MSE (D), AL (mm), ACD (mm) and CR (mm) were −2.62±3.83, 24.51±1.47, 3.55±0.34 and 7.75±0.28, respectively. Mean values (±SD) for TOV, AV and PV (mm3) were 8168.21±1141.86, 1099.40±139.24 and 7068.82±1134.05, respectively. TOV showed significant correlation with MSE, AL, PV (all p<0.001), CR (p=0.043) and ACD (p=0.024). Bar CR, the correlations were shown to be wholly attributable to variation in PV. Multiple linear regression indicated that the combination of AL and CR provided optimum R2 values of 79.4% for TOV. Conclusion Clinically useful estimations of ocular volume can be obtained from measurement of AL and CR.

Determination of retinal surface area

Previous attempts at determining retinal surface area and surface area of the whole eye have been based upon mathematical calculations derived from retinal photographs, schematic eyes and retinal biopsies of donor eyes. 3‐dimensional (3‐D) ocular magnetic resonance imaging (MRI) allows a more direct measurement, it can be used to image the eye in vivo, and there is no risk of tissue shrinkage. The primary purpose of this study is to compare, using T2‐weighted 3D MRI, retinal surface areas for superior‐temporal (ST), inferior‐temporal (IT), superior‐nasal (SN) and inferior‐nasal (IN) retinal quadrants. An ancillary aim is to examine whether inter‐quadrant variations in area are concordant with reported inter‐quadrant patterns of susceptibility to retinal breaks associated with posterior vitreous detachment (PVD). Seventy‐three adult participants presenting without retinal pathology (mean age 26.25 ± 6.06 years) were scanned using a Siemens 3‐Tesla MRI scanner to provide T2‐weighted MR images that demarcate fluid‐filled internal structures for the whole eye and provide high‐contrast delineation of the vitreous‐retina interface. Integrated MRI software generated total internal ocular surface area (TSA). The second nodal point was used to demarcate the origin of the peripheral retina in order to calculate total retinal surface area (RSA) and quadrant retinal surface areas (QRSA) for ST, IT, SN, and IN quadrants. Mean spherical error (MSE) was −2.50 ± 4.03D and mean axial length (AL) 24.51 ± 1.57 mm. Mean TSA and RSA for the RE were 2058 ± 189 and 1363 ± 160 mm2, respectively. Repeated measures anova for QRSA data indicated a significant difference within‐quadrants (P < 0.01) which, contrasted with ST (365 ± 43 mm2), was significant for IT (340 ± 40 mm2 P < 0.01), SN (337 ± 40 mm2 P < 0.01) and IN (321 ± 39 mm2 P < 0.01) quadrants. For all quadrants, QRSA was significantly correlated with AL (P < 0.01) and exhibited equivalent increases in retinal area/mm increase in AL. Although the differences between QRSAs are relatively small, there was evidence of concordance with reported inter‐quadrant patterns of susceptibility to retinal breaks associated with PVD. The data allow AL to be converted to QRSAs, which will assist further work on inter‐quadrant structural variation.