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

Purpose/Aim: The optic nerve (ON) becomes taut during adduction beyond similar to 26 degrees 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 (similar to 23-25 degrees) and larger (similar to 30-31 degrees) 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 similar to 26 degrees, transfering stress to the optic disc that could contribute to progressive neuropathy during repeated eye movements.