Adduction-Induced Strain on the Optic Nerve in Primary Open Angle Glaucoma at Normal Intraocular Pressure

Abstract

Purpose/aim: The optic nerve (ON) becomes taut during adduction beyond ~26° in healthy people and patients with primary open angle glaucoma (POAG), but only retracts the globe in POAG. We used magnetic resonance imaging (MRI) to investigate this difference.

Materials and methods: MRI was obtained in 2-mm quasi-coronal planes in central gaze, and smaller (~23-25°) and larger (~30-31°) adduction and abduction in 21 controls and 12 POAG subjects whose intraocular pressure never exceeded 21 mmHg. ON cross-sections were analyzed from the globe to 10 mm posteriorly. Area centroids were used to calculate ON path lengths and changes in cross-sections to calculate elongation assuming volume conservation.

Results: For both groups, ON path was nearly straight (<102.5% of minimum path) in smaller adduction, with minimal further straightening in larger adduction. ON length was redundant in abduction, exceeding 103% of minimum path for both groups. For normals, the ON elongated 0.4 ± 0.5 mm from central gaze to smaller adduction, and 0.4 ± 0.5 mm further from smaller to larger adduction. For POAG subjects, the ON did not elongate on average from central gaze to smaller adduction and only 0.2 ± 0.4 mm from smaller to larger adduction (P = .045 vs normals). Both groups demonstrated minimal ON elongation not exceeding 0.25 mm from central gaze to smaller and larger abduction. The globe retracted significantly more during large adduction in POAG subjects than normals (0.6 ± 0.7 mm vs 0.2 ± 0.5 mm, P = .027), without appreciable retraction in abduction. For each mm increase in globe axial length, ON elongation in large adduction similarly increased by 0.2 mm in each group.

Conclusions: The normal ON stretches to absorb force and avert globe retraction in adduction. In POAG with mild to severe visual field loss, the relatively inelastic ON tethers and retracts the globe during adduction beyond ~26°, transfering stress to the optic disc that could contribute to progressive neuropathy during repeated eye movements.

Keywords: Biomechanics; normal tension glaucoma; optic nerve; optic nerve strain.

Fig. 1.

MRI in normal subject. A. In central gaze (left), this optic nerve (ON) already exhibited minimal redundancy. During adduction (right), the posterior globe rotated laterally. Quasi-coronal magnetic resonance imaging was in planes perpendicular to the long axis of the orbit (dashed white lines). Optic nerve (ON) cross-sections were analyzed in the five image planes posterior to the globe-ON junction (white arrows). B. For these five 2-mm thick image planes arranged from anterior to posterior beginning at the globe at left, the optic nerve (white dots) and inner border of its sheath were outlined. Note that the optic nerve cross-section is smaller and more temporally eccentric within its sheath during adduction (bottom) than in central gaze (top).

Fig. 1.

MRI in normal subject. A. In central gaze (left), this optic nerve (ON) already exhibited minimal redundancy. During adduction (right), the posterior globe rotated laterally. Quasi-coronal magnetic resonance imaging was in planes perpendicular to the long axis of the orbit (dashed white lines). Optic nerve (ON) cross-sections were analyzed in the five image planes posterior to the globe-ON junction (white arrows). B. For these five 2-mm thick image planes arranged from anterior to posterior beginning at the globe at left, the optic nerve (white dots) and inner border of its sheath were outlined. Note that the optic nerve cross-section is smaller and more temporally eccentric within its sheath during adduction (bottom) than in central gaze (top).

Fig. 2.

Mean duction angles achieved during MRI in control subjects, and subjects with primary open angle glaucoma (POAG). Dots represent individual measurements. Angles are corrected for angle kappa so that initial position is nominally zero. Values noted on columns differ significantly for each nominal gaze position in both groups, but not between groups for any nominal gaze position. SD – standard deviation.

Fig. 3.

Actual optic nerve (ON) path length as a percentage of minimum distance from orbital apex to the globe for control subjects and subjects with primary open angle glaucoma (POAG). Dots represent individual measurements. Path redundancy was similarly minimal for small and large adduction for both groups, confirming that the ON was straight in both adduction angles. However, ON path in POAG was significantly more redundant than normal in central gaze and abduction (P<0.005) SD – standard deviation.

Fig. 4.

Mean optic nerve elongation from central gaze to large adduction in each adjacent 2 mm thick interslice analysis segment. Control subjects demonstrated significant elongation, while subjects with primary open angle glaucoma (POAG) did not. Error bars represent standard deviation. Abscissa values are offset slightly to avoid overlap.

Fig. 5.

Optic nerve elongation during gaze shifts from central to small and large abduction and adduction in control subjects and in primary open angle glaucoma (POAG). Symbols represent data for individual orbits. ON elongation for POAG was not significantly different from zero, but was highly significantly so for adduction but not abduction in controls. SD – standard deviation.

Fig. 6.

Subarachnoid space volume was quantitatively similar in control subjects and subjects with primary open angle glaucoma (POAG), and decreased similarly from central gaze to small adduction, but did not further decrease with larger adduction. In both groups, there was a smaller decrease in large abduction. SD – standard deviation.

Fig. 7.

Globe translation during horizontal duction. A. Posterior globe translation in adduction was greater in POAG than in controls. B. Medial globe translation in adduction was similar in POAG and controls.

Fig. 8.

Linear regressions demonstrate effects of globe axial length (AL) on optic nerve elongation in large adduction. Regression slopes were significantly non-zero but did not differ between groups, showing a significant effect of AL (P<0.05).